Node.js v19.5.0 documentation

Table of contents

About this documentation#

Welcome to the official API reference documentation for Node.js!

Node.js is a JavaScript runtime built on the V8 JavaScript engine.

Contributing#

Report errors in this documentation in the issue tracker. See the contributing guide for directions on how to submit pull requests.

Stability index#

Throughout the documentation are indications of a section's stability. Some APIs are so proven and so relied upon that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous.

The stability indices are as follows:

Stability: 0 - Deprecated. The feature may emit warnings. Backward compatibility is not guaranteed.

Stability: 1 - Experimental. The feature is not subject to semantic versioning rules. Non-backward compatible changes or removal may occur in any future release. Use of the feature is not recommended in production environments.

Stability: 2 - Stable. Compatibility with the npm ecosystem is a high priority.

Stability: 3 - Legacy. Although this feature is unlikely to be removed and is still covered by semantic versioning guarantees, it is no longer actively maintained, and other alternatives are available.

Features are marked as legacy rather than being deprecated if their use does no harm, and they are widely relied upon within the npm ecosystem. Bugs found in legacy features are unlikely to be fixed.

Use caution when making use of Experimental features, particularly within modules. Users may not be aware that experimental features are being used. Bugs or behavior changes may surprise users when Experimental API modifications occur. To avoid surprises, use of an Experimental feature may need a command-line flag. Experimental features may also emit a warning.

Stability overview#

JSON output#

Added in: v0.6.12

Every .html document has a corresponding .json document. This is for IDEs and other utilities that consume the documentation.

System calls and man pages#

Node.js functions which wrap a system call will document that. The docs link to the corresponding man pages which describe how the system call works.

Most Unix system calls have Windows analogues. Still, behavior differences may be unavoidable.

Usage and example#

Usage#

node [options] [V8 options] [script.js | -e "script" | - ] [arguments]

Please see the Command-line options document for more information.

Example#

An example of a web server written with Node.js which responds with 'Hello, World!':

Commands in this document start with $ or > to replicate how they would appear in a user's terminal. Do not include the $ and > characters. They are there to show the start of each command.

Lines that don't start with $ or > character show the output of the previous command.

First, make sure to have downloaded and installed Node.js. See Installing Node.js via package manager for further install information.

Now, create an empty project folder called projects, then navigate into it.

Linux and Mac:

$ mkdir ~/projects
$ cd ~/projects

Windows CMD:

> mkdir %USERPROFILE%\projects
> cd %USERPROFILE%\projects

Windows PowerShell:

> mkdir $env:USERPROFILE\projects
> cd $env:USERPROFILE\projects

Next, create a new source file in the projects folder and call it hello-world.js.

Open hello-world.js in any preferred text editor and paste in the following content:

const http = require('node:http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello, World!\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

Save the file, go back to the terminal window, and enter the following command:

$ node hello-world.js

Output like this should appear in the terminal:

Server running at http://127.0.0.1:3000/

Now, open any preferred web browser and visit http://127.0.0.1:3000.

If the browser displays the string Hello, World!, that indicates the server is working.

Assert#

Stability: 2 - Stable

Source Code: lib/assert.js

The node:assert module provides a set of assertion functions for verifying invariants.

Strict assertion mode#

History
VersionChanges
v15.0.0

Exposed as require('node:assert/strict').

v13.9.0, v12.16.2

Changed "strict mode" to "strict assertion mode" and "legacy mode" to "legacy assertion mode" to avoid confusion with the more usual meaning of "strict mode".

v9.9.0

Added error diffs to the strict assertion mode.

v9.9.0

Added strict assertion mode to the assert module.

v9.9.0

Added in: v9.9.0

In strict assertion mode, non-strict methods behave like their corresponding strict methods. For example, assert.deepEqual() will behave like assert.deepStrictEqual().

In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.

To use strict assertion mode:

import { strict as assert } from 'node:assert';const assert = require('node:assert').strict;
import assert from 'node:assert/strict';const assert = require('node:assert/strict');

Example error diff:

import { strict as assert } from 'node:assert';

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
//   [
//     [
// ...
//       2,
// +     3
// -     '3'
//     ],
// ...
//     5
//   ]const assert = require('node:assert/strict');

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
//   [
//     [
// ...
//       2,
// +     3
// -     '3'
//     ],
// ...
//     5
//   ]

To deactivate the colors, use the NO_COLOR or NODE_DISABLE_COLORS environment variables. This will also deactivate the colors in the REPL. For more on color support in terminal environments, read the tty getColorDepth() documentation.

Legacy assertion mode#

Legacy assertion mode uses the == operator in:

To use legacy assertion mode:

import assert from 'node:assert';const assert = require('node:assert');

Legacy assertion mode may have surprising results, especially when using assert.deepEqual():

// WARNING: This does not throw an AssertionError in legacy assertion mode!
assert.deepEqual(/a/gi, new Date());

Class: assert.AssertionError[src]#

Indicates the failure of an assertion. All errors thrown by the node:assert module will be instances of the AssertionError class.

new assert.AssertionError(options)#

Added in: v0.1.21
  • options <Object>
    • message <string> If provided, the error message is set to this value.
    • actual <any> The actual property on the error instance.
    • expected <any> The expected property on the error instance.
    • operator <string> The operator property on the error instance.
    • stackStartFn <Function> If provided, the generated stack trace omits frames before this function.

A subclass of Error that indicates the failure of an assertion.

All instances contain the built-in Error properties (message and name) and:

  • actual <any> Set to the actual argument for methods such as assert.strictEqual().
  • expected <any> Set to the expected value for methods such as assert.strictEqual().
  • generatedMessage <boolean> Indicates if the message was auto-generated (true) or not.
  • code <string> Value is always ERR_ASSERTION to show that the error is an assertion error.
  • operator <string> Set to the passed in operator value.
import assert from 'node:assert';

// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
  actual: 1,
  expected: 2,
  operator: 'strictEqual',
});

// Verify error output:
try {
  assert.strictEqual(1, 2);
} catch (err) {
  assert(err instanceof assert.AssertionError);
  assert.strictEqual(err.message, message);
  assert.strictEqual(err.name, 'AssertionError');
  assert.strictEqual(err.actual, 1);
  assert.strictEqual(err.expected, 2);
  assert.strictEqual(err.code, 'ERR_ASSERTION');
  assert.strictEqual(err.operator, 'strictEqual');
  assert.strictEqual(err.generatedMessage, true);
}const assert = require('node:assert');

// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
  actual: 1,
  expected: 2,
  operator: 'strictEqual',
});

// Verify error output:
try {
  assert.strictEqual(1, 2);
} catch (err) {
  assert(err instanceof assert.AssertionError);
  assert.strictEqual(err.message, message);
  assert.strictEqual(err.name, 'AssertionError');
  assert.strictEqual(err.actual, 1);
  assert.strictEqual(err.expected, 2);
  assert.strictEqual(err.code, 'ERR_ASSERTION');
  assert.strictEqual(err.operator, 'strictEqual');
  assert.strictEqual(err.generatedMessage, true);
}

Class: assert.CallTracker#

Added in: v14.2.0, v12.19.0

Stability: 1 - Experimental

This feature is currently experimental and behavior might still change.

new assert.CallTracker()#

Added in: v14.2.0, v12.19.0

Creates a new CallTracker object which can be used to track if functions were called a specific number of times. The tracker.verify() must be called for the verification to take place. The usual pattern would be to call it in a process.on('exit') handler.

import assert from 'node:assert';
import process from 'node:process';

const tracker = new assert.CallTracker();

function func() {}

// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);

callsfunc();

// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
  tracker.verify();
});const assert = require('node:assert');

const tracker = new assert.CallTracker();

function func() {}

// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);

callsfunc();

// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
  tracker.verify();
});

tracker.calls([fn][, exact])#

Added in: v14.2.0, v12.19.0

The wrapper function is expected to be called exactly exact times. If the function has not been called exactly exact times when tracker.verify() is called, then tracker.verify() will throw an error.

import assert from 'node:assert';

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);const assert = require('node:assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);

tracker.getCalls(fn)#

Added in: v18.8.0, v16.18.0 
import assert from 'node:assert';

const tracker = new assert.CallTracker();

function func() {}
const callsfunc = tracker.calls(func);
callsfunc(1, 2, 3);

assert.deepStrictEqual(tracker.getCalls(callsfunc),
                       [{ thisArg: this, arguments: [1, 2, 3 ] }]);const assert = require('node:assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}
const callsfunc = tracker.calls(func);
callsfunc(1, 2, 3);

assert.deepStrictEqual(tracker.getCalls(callsfunc),
                       [{ thisArg: this, arguments: [1, 2, 3 ] }]);

tracker.report()#

Added in: v14.2.0, v12.19.0
  • Returns: <Array> of objects containing information about the wrapper functions returned by tracker.calls().
  • Object <Object>
    • message <string>
    • actual <number> The actual number of times the function was called.
    • expected <number> The number of times the function was expected to be called.
    • operator <string> The name of the function that is wrapped.
    • stack <Object> A stack trace of the function.

The arrays contains information about the expected and actual number of calls of the functions that have not been called the expected number of times.

import assert from 'node:assert';

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

// Returns an array containing information on callsfunc()
tracker.report();
// [
//  {
//    message: 'Expected the func function to be executed 2 time(s) but was
//    executed 0 time(s).',
//    actual: 0,
//    expected: 2,
//    operator: 'func',
//    stack: stack trace
//  }
// ]const assert = require('node:assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

// Returns an array containing information on callsfunc()
tracker.report();
// [
//  {
//    message: 'Expected the func function to be executed 2 time(s) but was
//    executed 0 time(s).',
//    actual: 0,
//    expected: 2,
//    operator: 'func',
//    stack: stack trace
//  }
// ]

tracker.reset([fn])#

Added in: v18.8.0, v16.18.0

reset calls of the call tracker. if a tracked function is passed as an argument, the calls will be reset for it. if no arguments are passed, all tracked functions will be reset

import assert from 'node:assert';

const tracker = new assert.CallTracker();

function func() {}
const callsfunc = tracker.calls(func);

callsfunc();
// Tracker was called once
tracker.getCalls(callsfunc).length === 1;

tracker.reset(callsfunc);
tracker.getCalls(callsfunc).length === 0;const assert = require('node:assert');

function func() {}
const callsfunc = tracker.calls(func);

callsfunc();
// Tracker was called once
tracker.getCalls(callsfunc).length === 1;

tracker.reset(callsfunc);
tracker.getCalls(callsfunc).length === 0;

tracker.verify()#

Added in: v14.2.0, v12.19.0

Iterates through the list of functions passed to tracker.calls() and will throw an error for functions that have not been called the expected number of times.

import assert from 'node:assert';

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

callsfunc();

// Will throw an error since callsfunc() was only called once.
tracker.verify();const assert = require('node:assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

callsfunc();

// Will throw an error since callsfunc() was only called once.
tracker.verify();

assert(value[, message])#

Added in: v0.5.9

An alias of assert.ok().

assert.deepEqual(actual, expected[, message])#

History
VersionChanges
v18.0.0

Regular expressions lastIndex property is now compared as well.

v16.0.0, v14.18.0

In Legacy assertion mode, changed status from Deprecated to Legacy.

v14.0.0

NaN is now treated as being identical if both sides are NaN.

v12.0.0

The type tags are now properly compared and there are a couple minor comparison adjustments to make the check less surprising.

v9.0.0

The Error names and messages are now properly compared.

v8.0.0

The Set and Map content is also compared.

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0, v4.5.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.deepStrictEqual().

Legacy assertion mode

Stability: 3 - Legacy: Use assert.deepStrictEqual() instead.

Tests for deep equality between the actual and expected parameters. Consider using assert.deepStrictEqual() instead. assert.deepEqual() can have surprising results.

Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.

Comparison details#

  • Primitive values are compared with the == operator, with the exception of NaN. It is treated as being identical in case both sides are NaN.
  • Type tags of objects should be the same.
  • Only enumerable "own" properties are considered.
  • Error names and messages are always compared, even if these are not enumerable properties.
  • Object wrappers are compared both as objects and unwrapped values.
  • Object properties are compared unordered.
  • Map keys and Set items are compared unordered.
  • Recursion stops when both sides differ or both sides encounter a circular reference.
  • Implementation does not test the [[Prototype]] of objects.
  • Symbol properties are not compared.
  • WeakMap and WeakSet comparison does not rely on their values.
  • RegExp lastIndex, flags, and source are always compared, even if these are not enumerable properties.

The following example does not throw an AssertionError because the primitives are compared using the == operator.

import assert from 'node:assert';
// WARNING: This does not throw an AssertionError!

assert.deepEqual('+00000000', false);const assert = require('node:assert');
// WARNING: This does not throw an AssertionError!

assert.deepEqual('+00000000', false);

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

import assert from 'node:assert';

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: obj1 };

assert.deepEqual(obj1, obj1);
// OK

// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

assert.deepEqual(obj1, obj3);
// OK

// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}const assert = require('node:assert');

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: obj1 };

assert.deepEqual(obj1, obj1);
// OK

// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

assert.deepEqual(obj1, obj3);
// OK

// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.deepStrictEqual(actual, expected[, message])#

History
VersionChanges
v18.0.0

Regular expressions lastIndex property is now compared as well.

v9.0.0

Enumerable symbol properties are now compared.

v9.0.0

The NaN is now compared using the SameValueZero comparison.

v8.5.0

The Error names and messages are now properly compared.

v8.0.0

The Set and Map content is also compared.

v6.1.0

Objects with circular references can be used as inputs now.

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

Tests for deep equality between the actual and expected parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules.

Comparison details#

  • Primitive values are compared using Object.is().
  • Type tags of objects should be the same.
  • [[Prototype]] of objects are compared using the === operator.
  • Only enumerable "own" properties are considered.
  • Error names and messages are always compared, even if these are not enumerable properties.
  • Enumerable own Symbol properties are compared as well.
  • Object wrappers are compared both as objects and unwrapped values.
  • Object properties are compared unordered.
  • Map keys and Set items are compared unordered.
  • Recursion stops when both sides differ or both sides encounter a circular reference.
  • WeakMap and WeakSet comparison does not rely on their values. See below for further details.
  • RegExp lastIndex, flags, and source are always compared, even if these are not enumerable properties.
import assert from 'node:assert/strict';

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   [Symbol()]: 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;

assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries

// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   WeakMap {
// +   [items unknown]
// -   [items unknown],
// -   unequal: true
//   }const assert = require('node:assert/strict');

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   [Symbol()]: 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;

assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries

// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   WeakMap {
// +   [items unknown]
// -   [items unknown],
// -   unequal: true
//   }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.doesNotMatch(string, regexp[, message])#

History
VersionChanges
v16.0.0

This API is no longer experimental.

v13.6.0, v12.16.0

Added in: v13.6.0, v12.16.0

Expects the string input not to match the regular expression.

import assert from 'node:assert/strict';

assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.doesNotMatch('I will pass', /different/);
// OKconst assert = require('node:assert/strict');

assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.doesNotMatch('I will pass', /different/);
// OK

If the values do match, or if the string argument is of another type than string, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.doesNotReject(asyncFn[, error][, message])#

Added in: v10.0.0

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected.

If asyncFn is a function and it throws an error synchronously, assert.doesNotReject() will return a rejected Promise with that error. If the function does not return a promise, assert.doesNotReject() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Using assert.doesNotReject() is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible.

If specified, error can be a Class, RegExp, or a validation function. See assert.throws() for more details.

Besides the async nature to await the completion behaves identically to assert.doesNotThrow().

import assert from 'node:assert/strict';

await assert.doesNotReject(
  async () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError,
);const assert = require('node:assert/strict');

(async () => {
  await assert.doesNotReject(
    async () => {
      throw new TypeError('Wrong value');
    },
    SyntaxError,
  );
})();
import assert from 'node:assert/strict';

assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
  .then(() => {
    // ...
  });const assert = require('node:assert/strict');

assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
  .then(() => {
    // ...
  });

assert.doesNotThrow(fn[, error][, message])#

History
VersionChanges
v5.11.0, v4.4.5

The message parameter is respected now.

v4.2.0

The error parameter can now be an arrow function.

v0.1.21

Added in: v0.1.21

Asserts that the function fn does not throw an error.

Using assert.doesNotThrow() is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible.

When assert.doesNotThrow() is called, it will immediately call the fn function.

If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

If specified, error can be a Class, RegExp, or a validation function. See assert.throws() for more details.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

import assert from 'node:assert/strict';

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError,
);const assert = require('node:assert/strict');

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError,
);

However, the following will result in an AssertionError with the message 'Got unwanted exception...':

import assert from 'node:assert/strict';

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
);const assert = require('node:assert/strict');

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
);

If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message:

import assert from 'node:assert/strict';

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  /Wrong value/,
  'Whoops',
);
// Throws: AssertionError: Got unwanted exception: Whoopsconst assert = require('node:assert/strict');

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  /Wrong value/,
  'Whoops',
);
// Throws: AssertionError: Got unwanted exception: Whoops

assert.equal(actual, expected[, message])#

History
VersionChanges
v16.0.0, v14.18.0

In Legacy assertion mode, changed status from Deprecated to Legacy.

v14.0.0

NaN is now treated as being identical if both sides are NaN.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.strictEqual().

Legacy assertion mode

Stability: 3 - Legacy: Use assert.strictEqual() instead.

Tests shallow, coercive equality between the actual and expected parameters using the == operator. NaN is specially handled and treated as being identical if both sides are NaN.

import assert from 'node:assert';

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }const assert = require('node:assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.fail([message])#

Added in: v0.1.21

Throws an AssertionError with the provided error message or a default error message. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

import assert from 'node:assert/strict';

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

assert.fail(new TypeError('need array'));
// TypeError: need arrayconst assert = require('node:assert/strict');

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

assert.fail(new TypeError('need array'));
// TypeError: need array

Using assert.fail() with more than two arguments is possible but deprecated. See below for further details.

assert.fail(actual, expected[, message[, operator[, stackStartFn]]])#

History
VersionChanges
v10.0.0

Calling assert.fail() with more than one argument is deprecated and emits a warning.

v0.1.21

Added in: v0.1.21

Stability: 0 - Deprecated: Use assert.fail([message]) or other assert functions instead.

If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. If just the two actual and expected arguments are provided, operator will default to '!='. If message is provided as third argument it will be used as the error message and the other arguments will be stored as properties on the thrown object. If stackStartFn is provided, all stack frames above that function will be removed from stacktrace (see Error.captureStackTrace). If no arguments are given, the default message Failed will be used.

import assert from 'node:assert/strict';

assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'

assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops

assert.fail(1, 2, new TypeError('need array'));
// TypeError: need arrayconst assert = require('node:assert/strict');

assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'

assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops

assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array

In the last three cases actual, expected, and operator have no influence on the error message.

Example use of stackStartFn for truncating the exception's stacktrace:

import assert from 'node:assert/strict';

function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...const assert = require('node:assert/strict');

function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...

assert.ifError(value)#

History
VersionChanges
v10.0.0

Instead of throwing the original error it is now wrapped into an [AssertionError][] that contains the full stack trace.

v10.0.0

Value may now only be undefined or null. Before all falsy values were handled the same as null and did not throw.

v0.1.97

Added in: v0.1.97

Throws value if value is not undefined or null. This is useful when testing the error argument in callbacks. The stack trace contains all frames from the error passed to ifError() including the potential new frames for ifError() itself.

import assert from 'node:assert/strict';

assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

// Create some random error frames.
let err;
(function errorFrame() {
  err = new Error('test error');
})();

(function ifErrorFrame() {
  assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
//     at ifErrorFrame
//     at errorFrameconst assert = require('node:assert/strict');

assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

// Create some random error frames.
let err;
(function errorFrame() {
  err = new Error('test error');
})();

(function ifErrorFrame() {
  assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
//     at ifErrorFrame
//     at errorFrame

assert.match(string, regexp[, message])#

History
VersionChanges
v16.0.0

This API is no longer experimental.

v13.6.0, v12.16.0

Added in: v13.6.0, v12.16.0

Expects the string input to match the regular expression.

import assert from 'node:assert/strict';

assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...

assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.match('I will pass', /pass/);
// OKconst assert = require('node:assert/strict');

assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...

assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.match('I will pass', /pass/);
// OK

If the values do not match, or if the string argument is of another type than string, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notDeepEqual(actual, expected[, message])#

History
VersionChanges
v16.0.0, v14.18.0

In Legacy assertion mode, changed status from Deprecated to Legacy.

v14.0.0

NaN is now treated as being identical if both sides are NaN.

v9.0.0

The Error names and messages are now properly compared.

v8.0.0

The Set and Map content is also compared.

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0, v4.5.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.notDeepStrictEqual().

Legacy assertion mode

Stability: 3 - Legacy: Use assert.notDeepStrictEqual() instead.

Tests for any deep inequality. Opposite of assert.deepEqual().

import assert from 'node:assert';

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: obj1 };

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OKconst assert = require('node:assert');

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: obj1 };

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK

If the values are deeply equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notDeepStrictEqual(actual, expected[, message])#

History
VersionChanges
v9.0.0

The -0 and +0 are not considered equal anymore.

v9.0.0

The NaN is now compared using the SameValueZero comparison.

v9.0.0

The Error names and messages are now properly compared.

v8.0.0

The Set and Map content is also compared.

v6.1.0

Objects with circular references can be used as inputs now.

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

import assert from 'node:assert/strict';

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OKconst assert = require('node:assert/strict');

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK

If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notEqual(actual, expected[, message])#

History
VersionChanges
v16.0.0, v14.18.0

In Legacy assertion mode, changed status from Deprecated to Legacy.

v14.0.0

NaN is now treated as being identical if both sides are NaN.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.notStrictEqual().

Legacy assertion mode

Stability: 3 - Legacy: Use assert.notStrictEqual() instead.

Tests shallow, coercive inequality with the != operator. NaN is specially handled and treated as being identical if both sides are NaN.

import assert from 'node:assert';

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'const assert = require('node:assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notStrictEqual(actual, expected[, message])#

History
VersionChanges
v10.0.0

Used comparison changed from Strict Equality to Object.is().

v0.1.21

Added in: v0.1.21

Tests strict inequality between the actual and expected parameters as determined by Object.is().

import assert from 'node:assert/strict';

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1

assert.notStrictEqual(1, '1');
// OKconst assert = require('node:assert/strict');

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.ok(value[, message])#

History
VersionChanges
v10.0.0

The assert.ok() (no arguments) will now use a predefined error message.

v0.1.21

Added in: v0.1.21

Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message).

If value is not truthy, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError. If no arguments are passed in at all message will be set to the string: 'No value argument passed to `assert.ok()`'.

Be aware that in the repl the error message will be different to the one thrown in a file! See below for further details.

import assert from 'node:assert/strict';

assert.ok(true);
// OK
assert.ok(1);
// OK

assert.ok();
// AssertionError: No value argument passed to `assert.ok()`

assert.ok(false, 'it\'s false');
// AssertionError: it's false

// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true

// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(typeof 123 === 'string')

assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(false)

assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)const assert = require('node:assert/strict');

assert.ok(true);
// OK
assert.ok(1);
// OK

assert.ok();
// AssertionError: No value argument passed to `assert.ok()`

assert.ok(false, 'it\'s false');
// AssertionError: it's false

// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true

// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(typeof 123 === 'string')

assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(false)

assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)
import assert from 'node:assert/strict';

// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(0)const assert = require('node:assert');

// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(0)

assert.rejects(asyncFn[, error][, message])#

Added in: v10.0.0

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected.

If asyncFn is a function and it throws an error synchronously, assert.rejects() will return a rejected Promise with that error. If the function does not return a promise, assert.rejects() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Besides the async nature to await the completion behaves identically to assert.throws().

If specified, error can be a Class, RegExp, a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable message and name properties.

If specified, message will be the message provided by the AssertionError if the asyncFn fails to reject.

import assert from 'node:assert/strict';

await assert.rejects(
  async () => {
    throw new TypeError('Wrong value');
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
  },
);const assert = require('node:assert/strict');

(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    {
      name: 'TypeError',
      message: 'Wrong value',
    },
  );
})();
import assert from 'node:assert/strict';

await assert.rejects(
  async () => {
    throw new TypeError('Wrong value');
  },
  (err) => {
    assert.strictEqual(err.name, 'TypeError');
    assert.strictEqual(err.message, 'Wrong value');
    return true;
  },
);const assert = require('node:assert/strict');

(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    (err) => {
      assert.strictEqual(err.name, 'TypeError');
      assert.strictEqual(err.message, 'Wrong value');
      return true;
    },
  );
})();
import assert from 'node:assert/strict';

assert.rejects(
  Promise.reject(new Error('Wrong value')),
  Error,
).then(() => {
  // ...
});const assert = require('node:assert/strict');

assert.rejects(
  Promise.reject(new Error('Wrong value')),
  Error,
).then(() => {
  // ...
});

error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Please read the example in assert.throws() carefully if using a string as the second argument gets considered.

assert.strictEqual(actual, expected[, message])#

History
VersionChanges
v10.0.0

Used comparison changed from Strict Equality to Object.is().

v0.1.21

Added in: v0.1.21

Tests strict equality between the actual and expected parameters as determined by Object.is().

import assert from 'node:assert/strict';

assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
//          ^

const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identicalconst assert = require('node:assert/strict');

assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
//          ^

const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identical

If the values are not strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.throws(fn[, error][, message])#

History
VersionChanges
v10.2.0

The error parameter can be an object containing regular expressions now.

v9.9.0

The error parameter can now be an object as well.

v4.2.0

The error parameter can now be an arrow function.

v0.1.21

Added in: v0.1.21

Expects the function fn to throw an error.

If specified, error can be a Class, RegExp, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable message and name properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

If specified, message will be appended to the message provided by the AssertionError if the fn call fails to throw or in case the error validation fails.

Custom validation object/error instance:

import assert from 'node:assert/strict';

const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
  nested: true,
  baz: 'text',
};
err.reg = /abc/i;

assert.throws(
  () => {
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
    info: {
      nested: true,
      baz: 'text',
    },
    // Only properties on the validation object will be tested for.
    // Using nested objects requires all properties to be present. Otherwise
    // the validation is going to fail.
  },
);

// Using regular expressions to validate error properties:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text',
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i,
  },
);

// Fails due to the different `message` and `name` properties:
assert.throws(
  () => {
    const otherErr = new Error('Not found');
    // Copy all enumerable properties from `err` to `otherErr`.
    for (const [key, value] of Object.entries(err)) {
      otherErr[key] = value;
    }
    throw otherErr;
  },
  // The error's `message` and `name` properties will also be checked when using
  // an error as validation object.
  err,
);const assert = require('node:assert/strict');

const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
  nested: true,
  baz: 'text',
};
err.reg = /abc/i;

assert.throws(
  () => {
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
    info: {
      nested: true,
      baz: 'text',
    },
    // Only properties on the validation object will be tested for.
    // Using nested objects requires all properties to be present. Otherwise
    // the validation is going to fail.
  },
);

// Using regular expressions to validate error properties:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text',
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i,
  },
);

// Fails due to the different `message` and `name` properties:
assert.throws(
  () => {
    const otherErr = new Error('Not found');
    // Copy all enumerable properties from `err` to `otherErr`.
    for (const [key, value] of Object.entries(err)) {
      otherErr[key] = value;
    }
    throw otherErr;
  },
  // The error's `message` and `name` properties will also be checked when using
  // an error as validation object.
  err,
);

Validate instanceof using constructor:

import assert from 'node:assert/strict';

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error,
);const assert = require('node:assert/strict');

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error,
);

Validate error message using RegExp:

Using a regular expression runs .toString on the error object, and will therefore also include the error name.

import assert from 'node:assert/strict';

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /^Error: Wrong value$/,
);const assert = require('node:assert/strict');

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /^Error: Wrong value$/,
);

Custom error validation:

The function must return true to indicate all internal validations passed. It will otherwise fail with an AssertionError.

import assert from 'node:assert/strict';

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  (err) => {
    assert(err instanceof Error);
    assert(/value/.test(err));
    // Avoid returning anything from validation functions besides `true`.
    // Otherwise, it's not clear what part of the validation failed. Instead,
    // throw an error about the specific validation that failed (as done in this
    // example) and add as much helpful debugging information to that error as
    // possible.
    return true;
  },
  'unexpected error',
);const assert = require('node:assert/strict');

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  (err) => {
    assert(err instanceof Error);
    assert(/value/.test(err));
    // Avoid returning anything from validation functions besides `true`.
    // Otherwise, it's not clear what part of the validation failed. Instead,
    // throw an error about the specific validation that failed (as done in this
    // example) and add as much helpful debugging information to that error as
    // possible.
    return true;
  },
  'unexpected error',
);

error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an ERR_AMBIGUOUS_ARGUMENT error. Please read the example below carefully if using a string as the second argument gets considered:

import assert from 'node:assert/strict';

function throwingFirst() {
  throw new Error('First');
}

function throwingSecond() {
  throw new Error('Second');
}

function notThrowing() {}

// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]

// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second

// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);

// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]const assert = require('node:assert/strict');

function throwingFirst() {
  throw new Error('First');
}

function throwingSecond() {
  throw new Error('Second');
}

function notThrowing() {}

// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]

// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second

// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);

// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]

Due to the confusing error-prone notation, avoid a string as the second argument.

Asynchronous context tracking#

Stability: 2 - Stable

Source Code: lib/async_hooks.js

Introduction#

These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.

The AsyncLocalStorage and AsyncResource classes are part of the node:async_hooks module:

import { AsyncLocalStorage, AsyncResource } from 'node:async_hooks';const { AsyncLocalStorage, AsyncResource } = require('node:async_hooks');

Class: AsyncLocalStorage#

History
VersionChanges
v16.4.0

AsyncLocalStorage is now Stable. Previously, it had been Experimental.

v13.10.0, v12.17.0

Added in: v13.10.0, v12.17.0

This class creates stores that stay coherent through asynchronous operations.

While you can create your own implementation on top of the node:async_hooks module, AsyncLocalStorage should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement.

The following example uses AsyncLocalStorage to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request.

import http from 'node:http';
import { AsyncLocalStorage } from 'node:async_hooks';

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
  const id = asyncLocalStorage.getStore();
  console.log(`${id !== undefined ? id : '-'}:`, msg);
}

let idSeq = 0;
http.createServer((req, res) => {
  asyncLocalStorage.run(idSeq++, () => {
    logWithId('start');
    // Imagine any chain of async operations here
    setImmediate(() => {
      logWithId('finish');
      res.end();
    });
  });
}).listen(8080);

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
//   0: start
//   1: start
//   0: finish
//   1: finishconst http = require('node:http');
const { AsyncLocalStorage } = require('node:async_hooks');

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
  const id = asyncLocalStorage.getStore();
  console.log(`${id !== undefined ? id : '-'}:`, msg);
}

let idSeq = 0;
http.createServer((req, res) => {
  asyncLocalStorage.run(idSeq++, () => {
    logWithId('start');
    // Imagine any chain of async operations here
    setImmediate(() => {
      logWithId('finish');
      res.end();
    });
  });
}).listen(8080);

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
//   0: start
//   1: start
//   0: finish
//   1: finish

Each instance of AsyncLocalStorage maintains an independent storage context. Multiple instances can safely exist simultaneously without risk of interfering with each other's data.

new AsyncLocalStorage([options])#

History
VersionChanges
v19.2.0

Add option onPropagate.

v13.10.0, v12.17.0

Added in: v13.10.0, v12.17.0

Stability: 1 - options.onPropagate is experimental.

  • options <Object>
    • onPropagate <Function> Optional callback invoked before a store is propagated to a new async resource. Returning true allows propagation, returning false avoids it. Default is to propagate always.

Creates a new instance of AsyncLocalStorage. Store is only provided within a run() call or after an enterWith() call.

The onPropagate is called during creation of an async resource. Throwing at this time will print the stack trace and exit. See async_hooks Error handling for details.

Creating an async resource within the onPropagate callback will result in a recursive call to onPropagate.

asyncLocalStorage.disable()#

Added in: v13.10.0, v12.17.0

Stability: 1 - Experimental

Disables the instance of AsyncLocalStorage. All subsequent calls to asyncLocalStorage.getStore() will return undefined until asyncLocalStorage.run() or asyncLocalStorage.enterWith() is called again.

When calling asyncLocalStorage.disable(), all current contexts linked to the instance will be exited.

Calling asyncLocalStorage.disable() is required before the asyncLocalStorage can be garbage collected. This does not apply to stores provided by the asyncLocalStorage, as those objects are garbage collected along with the corresponding async resources.

Use this method when the asyncLocalStorage is not in use anymore in the current process.

asyncLocalStorage.getStore()#

Added in: v13.10.0, v12.17.0

Returns the current store. If called outside of an asynchronous context initialized by calling asyncLocalStorage.run() or asyncLocalStorage.enterWith(), it returns undefined.

asyncLocalStorage.enterWith(store)#

Added in: v13.11.0, v12.17.0

Stability: 1 - Experimental

Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls.

Example:

const store = { id: 1 };
// Replaces previous store with the given store object
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
  asyncLocalStorage.getStore(); // Returns the same object
});

This transition will continue for the entire synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an AsyncResource. That is why run() should be preferred over enterWith() unless there are strong reasons to use the latter method.

const store = { id: 1 };

emitter.on('my-event', () => {
  asyncLocalStorage.enterWith(store);
});
emitter.on('my-event', () => {
  asyncLocalStorage.getStore(); // Returns the same object
});

asyncLocalStorage.getStore(); // Returns undefined
emitter.emit('my-event');
asyncLocalStorage.getStore(); // Returns the same object

asyncLocalStorage.run(store, callback[, ...args])#

Added in: v13.10.0, v12.17.0

Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.

The optional args are passed to the callback function.

If the callback function throws an error, the error is thrown by run() too. The stacktrace is not impacted by this call and the context is exited.

Example:

const store = { id: 2 };
try {
  asyncLocalStorage.run(store, () => {
    asyncLocalStorage.getStore(); // Returns the store object
    setTimeout(() => {
      asyncLocalStorage.getStore(); // Returns the store object
    }, 200);
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns undefined
  // The error will be caught here
}

asyncLocalStorage.exit(callback[, ...args])#

Added in: v13.10.0, v12.17.0

Stability: 1 - Experimental

Runs a function synchronously outside of a context and returns its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback. Any getStore() call done within the callback function will always return undefined.

The optional args are passed to the callback function.

If the callback function throws an error, the error is thrown by exit() too. The stacktrace is not impacted by this call and the context is re-entered.

Example:

// Within a call to run
try {
  asyncLocalStorage.getStore(); // Returns the store object or value
  asyncLocalStorage.exit(() => {
    asyncLocalStorage.getStore(); // Returns undefined
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns the same object or value
  // The error will be caught here
}

Usage with async/await#

If, within an async function, only one await call is to run within a context, the following pattern should be used:

async function fn() {
  await asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('key', value);
    return foo(); // The return value of foo will be awaited
  });
}

In this example, the store is only available in the callback function and the functions called by foo. Outside of run, calling getStore will return undefined.

Troubleshooting: Context loss#

In most cases, AsyncLocalStorage works without issues. In rare situations, the current store is lost in one of the asynchronous operations.

If your code is callback-based, it is enough to promisify it with util.promisify() so it starts working with native promises.

If you need to use a callback-based API or your code assumes a custom thenable implementation, use the AsyncResource class to associate the asynchronous operation with the correct execution context. Find the function call responsible for the context loss by logging the content of asyncLocalStorage.getStore() after the calls you suspect are responsible for the loss. When the code logs undefined, the last callback called is probably responsible for the context loss.

Class: AsyncResource#

History
VersionChanges
v16.4.0

AsyncResource is now Stable. Previously, it had been Experimental.

The class AsyncResource is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources.

The init hook will trigger when an AsyncResource is instantiated.

The following is an overview of the AsyncResource API.

import { AsyncResource, executionAsyncId } from 'node:async_hooks';

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
  type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
);

// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);

// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();

// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();

// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();const { AsyncResource, executionAsyncId } = require('node:async_hooks');

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
  type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
);

// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);

// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();

// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();

// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();

new AsyncResource(type[, options])#

  • type <string> The type of async event.
  • options <Object>
    • triggerAsyncId <number> The ID of the execution context that created this async event. Default: executionAsyncId().
    • requireManualDestroy <boolean> If set to true, disables emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. When set to false, the emitDestroy call on garbage collection will only take place if there is at least one active destroy hook. Default: false.

Example usage:

class DBQuery extends AsyncResource {
  constructor(db) {
    super('DBQuery');
    this.db = db;
  }

  getInfo(query, callback) {
    this.db.get(query, (err, data) => {
      this.runInAsyncScope(callback, null, err, data);
    });
  }

  close() {
    this.db = null;
    this.emitDestroy();
  }
}

Static method: AsyncResource.bind(fn[, type[, thisArg]])#

History
VersionChanges
v17.8.0, v16.15.0

Changed the default when thisArg is undefined to use this from the caller.

v16.0.0

Added optional thisArg.

v14.8.0, v12.19.0

Added in: v14.8.0, v12.19.0

  • fn <Function> The function to bind to the current execution context.
  • type <string> An optional name to associate with the underlying AsyncResource.
  • thisArg <any>

Binds the given function to the current execution context.

The returned function will have an asyncResource property referencing the AsyncResource to which the function is bound.

asyncResource.bind(fn[, thisArg])#

History
VersionChanges
v17.8.0, v16.15.0

Changed the default when thisArg is undefined to use this from the caller.

v16.0.0

Added optional thisArg.

v14.8.0, v12.19.0

Added in: v14.8.0, v12.19.0

  • fn <Function> The function to bind to the current AsyncResource.
  • thisArg <any>

Binds the given function to execute to this AsyncResource's scope.

The returned function will have an asyncResource property referencing the AsyncResource to which the function is bound.

asyncResource.runInAsyncScope(fn[, thisArg, ...args])#

Added in: v9.6.0
  • fn <Function> The function to call in the execution context of this async resource.
  • thisArg <any> The receiver to be used for the function call.
  • ...args <any> Optional arguments to pass to the function.

Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.

asyncResource.emitDestroy()#

Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.

asyncResource.asyncId()#

  • Returns: <number> The unique asyncId assigned to the resource.

asyncResource.triggerAsyncId()#

  • Returns: <number> The same triggerAsyncId that is passed to the AsyncResource constructor.

Using AsyncResource for a Worker thread pool#

The following example shows how to use the AsyncResource class to properly provide async tracking for a Worker pool. Other resource pools, such as database connection pools, can follow a similar model.

Assuming that the task is adding two numbers, using a file named task_processor.js with the following content:

import { parentPort } from 'node:worker_threads';
parentPort.on('message', (task) => {
  parentPort.postMessage(task.a + task.b);
});const { parentPort } = require('node:worker_threads');
parentPort.on('message', (task) => {
  parentPort.postMessage(task.a + task.b);
});

a Worker pool around it could use the following structure:

import { AsyncResource } from 'node:async_hooks';
import { EventEmitter } from 'node:events';
import path from 'node:path';
import { Worker } from 'node:worker_threads';

const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

class WorkerPoolTaskInfo extends AsyncResource {
  constructor(callback) {
    super('WorkerPoolTaskInfo');
    this.callback = callback;
  }

  done(err, result) {
    this.runInAsyncScope(this.callback, null, err, result);
    this.emitDestroy();  // `TaskInfo`s are used only once.
  }
}

export default class WorkerPool extends EventEmitter {
  constructor(numThreads) {
    super();
    this.numThreads = numThreads;
    this.workers = [];
    this.freeWorkers = [];
    this.tasks = [];

    for (let i = 0; i < numThreads; i++)
      this.addNewWorker();

    // Any time the kWorkerFreedEvent is emitted, dispatch
    // the next task pending in the queue, if any.
    this.on(kWorkerFreedEvent, () => {
      if (this.tasks.length > 0) {
        const { task, callback } = this.tasks.shift();
        this.runTask(task, callback);
      }
    });
  }

  addNewWorker() {
    const worker = new Worker(new URL('task_processor.js', import.meta.url));
    worker.on('message', (result) => {
      // In case of success: Call the callback that was passed to `runTask`,
      // remove the `TaskInfo` associated with the Worker, and mark it as free
      // again.
      worker[kTaskInfo].done(null, result);
      worker[kTaskInfo] = null;
      this.freeWorkers.push(worker);
      this.emit(kWorkerFreedEvent);
    });
    worker.on('error', (err) => {
      // In case of an uncaught exception: Call the callback that was passed to
      // `runTask` with the error.
      if (worker[kTaskInfo])
        worker[kTaskInfo].done(err, null);
      else
        this.emit('error', err);
      // Remove the worker from the list and start a new Worker to replace the
      // current one.
      this.workers.splice(this.workers.indexOf(worker), 1);
      this.addNewWorker();
    });
    this.workers.push(worker);
    this.freeWorkers.push(worker);
    this.emit(kWorkerFreedEvent);
  }

  runTask(task, callback) {
    if (this.freeWorkers.length === 0) {
      // No free threads, wait until a worker thread becomes free.
      this.tasks.push({ task, callback });
      return;
    }

    const worker = this.freeWorkers.pop();
    worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
    worker.postMessage(task);
  }

  close() {
    for (const worker of this.workers) worker.terminate();
  }
}const { AsyncResource } = require('node:async_hooks');
const { EventEmitter } = require('node:events');
const path = require('node:path');
const { Worker } = require('node:worker_threads');

const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

class WorkerPoolTaskInfo extends AsyncResource {
  constructor(callback) {
    super('WorkerPoolTaskInfo');
    this.callback = callback;
  }

  done(err, result) {
    this.runInAsyncScope(this.callback, null, err, result);
    this.emitDestroy();  // `TaskInfo`s are used only once.
  }
}

class WorkerPool extends EventEmitter {
  constructor(numThreads) {
    super();
    this.numThreads = numThreads;
    this.workers = [];
    this.freeWorkers = [];
    this.tasks = [];

    for (let i = 0; i < numThreads; i++)
      this.addNewWorker();

    // Any time the kWorkerFreedEvent is emitted, dispatch
    // the next task pending in the queue, if any.
    this.on(kWorkerFreedEvent, () => {
      if (this.tasks.length > 0) {
        const { task, callback } = this.tasks.shift();
        this.runTask(task, callback);
      }
    });
  }

  addNewWorker() {
    const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));
    worker.on('message', (result) => {
      // In case of success: Call the callback that was passed to `runTask`,
      // remove the `TaskInfo` associated with the Worker, and mark it as free
      // again.
      worker[kTaskInfo].done(null, result);
      worker[kTaskInfo] = null;
      this.freeWorkers.push(worker);
      this.emit(kWorkerFreedEvent);
    });
    worker.on('error', (err) => {
      // In case of an uncaught exception: Call the callback that was passed to
      // `runTask` with the error.
      if (worker[kTaskInfo])
        worker[kTaskInfo].done(err, null);
      else
        this.emit('error', err);
      // Remove the worker from the list and start a new Worker to replace the
      // current one.
      this.workers.splice(this.workers.indexOf(worker), 1);
      this.addNewWorker();
    });
    this.workers.push(worker);
    this.freeWorkers.push(worker);
    this.emit(kWorkerFreedEvent);
  }

  runTask(task, callback) {
    if (this.freeWorkers.length === 0) {
      // No free threads, wait until a worker thread becomes free.
      this.tasks.push({ task, callback });
      return;
    }

    const worker = this.freeWorkers.pop();
    worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
    worker.postMessage(task);
  }

  close() {
    for (const worker of this.workers) worker.terminate();
  }
}

module.exports = WorkerPool;

Without the explicit tracking added by the WorkerPoolTaskInfo objects, it would appear that the callbacks are associated with the individual Worker objects. However, the creation of the Workers is not associated with the creation of the tasks and does not provide information about when tasks were scheduled.

This pool could be used as follows:

import WorkerPool from './worker_pool.js';
import os from 'node:os';

const pool = new WorkerPool(os.availableParallelism());

let finished = 0;
for (let i = 0; i < 10; i++) {
  pool.runTask({ a: 42, b: 100 }, (err, result) => {
    console.log(i, err, result);
    if (++finished === 10)
      pool.close();
  });
}const WorkerPool = require('./worker_pool.js');
const os = require('node:os');

const pool = new WorkerPool(os.availableParallelism());

let finished = 0;
for (let i = 0; i < 10; i++) {
  pool.runTask({ a: 42, b: 100 }, (err, result) => {
    console.log(i, err, result);
    if (++finished === 10)
      pool.close();
  });
}

Integrating AsyncResource with EventEmitter#

Event listeners triggered by an EventEmitter may be run in a different execution context than the one that was active when eventEmitter.on() was called.

The following example shows how to use the AsyncResource class to properly associate an event listener with the correct execution context. The same approach can be applied to a Stream or a similar event-driven class.

import { createServer } from 'node:http';
import { AsyncResource, executionAsyncId } from 'node:async_hooks';

const server = createServer((req, res) => {
  req.on('close', AsyncResource.bind(() => {
    // Execution context is bound to the current outer scope.
  }));
  req.on('close', () => {
    // Execution context is bound to the scope that caused 'close' to emit.
  });
  res.end();
}).listen(3000);const { createServer } = require('node:http');
const { AsyncResource, executionAsyncId } = require('node:async_hooks');

const server = createServer((req, res) => {
  req.on('close', AsyncResource.bind(() => {
    // Execution context is bound to the current outer scope.
  }));
  req.on('close', () => {
    // Execution context is bound to the scope that caused 'close' to emit.
  });
  res.end();
}).listen(3000);

Async hooks#

Stability: 1 - Experimental. Please migrate away from this API, if you can. We do not recommend using the createHook, AsyncHook, and executionAsyncResource APIs as they have usability issues, safety risks, and performance implications. Async context tracking use cases are better served by the stable AsyncLocalStorage API. If you have a use case for createHook, AsyncHook, or executionAsyncResource beyond the context tracking need solved by AsyncLocalStorage or diagnostics data currently provided by Diagnostics Channel, please open an issue at https://github.com/nodejs/node/issues describing your use case so we can create a more purpose-focused API.

Source Code: lib/async_hooks.js

We strongly discourage the use of the async_hooks API. Other APIs that can cover most of its use cases include:

The node:async_hooks module provides an API to track asynchronous resources. It can be accessed using:

import async_hooks from 'node:async_hooks';const async_hooks = require('node:async_hooks');

Terminology#

An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, such as the 'connection' event in net.createServer(), or just a single time like in fs.open(). A resource can also be closed before the callback is called. AsyncHook does not explicitly distinguish between these different cases but will represent them as the abstract concept that is a resource.

If Workers are used, each thread has an independent async_hooks interface, and each thread will use a new set of async IDs.

Overview#

Following is a simple overview of the public API.

import async_hooks from 'node:async_hooks';

// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();

// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();

// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
    async_hooks.createHook({ init, before, after, destroy, promiseResolve });

// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();

// Disable listening for new asynchronous events.
asyncHook.disable();

//
// The following are the callbacks that can be passed to createHook().
//

// init() is called during object construction. The resource may not have
// completed construction when this callback runs. Therefore, all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }

// before() is called just before the resource's callback is called. It can be
// called 0-N times for handles (such as TCPWrap), and will be called exactly 1
// time for requests (such as FSReqCallback).
function before(asyncId) { }

// after() is called just after the resource's callback has finished.
function after(asyncId) { }

// destroy() is called when the resource is destroyed.
function destroy(asyncId) { }

// promiseResolve() is called only for promise resources, when the
// resolve() function passed to the Promise constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }const async_hooks = require('node:async_hooks');

// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();

// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();

// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
    async_hooks.createHook({ init, before, after, destroy, promiseResolve });

// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();

// Disable listening for new asynchronous events.
asyncHook.disable();

//
// The following are the callbacks that can be passed to createHook().
//

// init() is called during object construction. The resource may not have
// completed construction when this callback runs. Therefore, all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }

// before() is called just before the resource's callback is called. It can be
// called 0-N times for handles (such as TCPWrap), and will be called exactly 1
// time for requests (such as FSReqCallback).
function before(asyncId) { }

// after() is called just after the resource's callback has finished.
function after(asyncId) { }

// destroy() is called when the resource is destroyed.
function destroy(asyncId) { }

// promiseResolve() is called only for promise resources, when the
// resolve() function passed to the Promise constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }

async_hooks.createHook(callbacks)#

Added in: v8.1.0

Registers functions to be called for different lifetime events of each async operation.

The callbacks init()/before()/after()/destroy() are called for the respective asynchronous event during a resource's lifetime.

All callbacks are optional. For example, if only resource cleanup needs to be tracked, then only the destroy callback needs to be passed. The specifics of all functions that can be passed to callbacks is in the Hook Callbacks section.

import { createHook } from 'node:async_hooks';

const asyncHook = createHook({
  init(asyncId, type, triggerAsyncId, resource) { },
  destroy(asyncId) { },
});const async_hooks = require('node:async_hooks');

const asyncHook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) { },
  destroy(asyncId) { },
});

The callbacks will be inherited via the prototype chain:

class MyAsyncCallbacks {
  init(asyncId, type, triggerAsyncId, resource) { }
  destroy(asyncId) {}
}

class MyAddedCallbacks extends MyAsyncCallbacks {
  before(asyncId) { }
  after(asyncId) { }
}

const asyncHook = async_hooks.createHook(new MyAddedCallbacks());

Because promises are asynchronous resources whose lifecycle is tracked via the async hooks mechanism, the init(), before(), after(), and destroy() callbacks must not be async functions that return promises.

Error handling#

If any AsyncHook callbacks throw, the application will print the stack trace and exit. The exit path does follow that of an uncaught exception, but all 'uncaughtException' listeners are removed, thus forcing the process to exit. The 'exit' callbacks will still be called unless the application is run with --abort-on-uncaught-exception, in which case a stack trace will be printed and the application exits, leaving a core file.

The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.

Printing in AsyncHook callbacks#

Because printing to the console is an asynchronous operation, console.log() will cause AsyncHook callbacks to be called. Using console.log() or similar asynchronous operations inside an AsyncHook callback function will cause an infinite recursion. An easy solution to this when debugging is to use a synchronous logging operation such as fs.writeFileSync(file, msg, flag). This will print to the file and will not invoke AsyncHook recursively because it is synchronous.

import { writeFileSync } from 'node:fs';
import { format } from 'node:util';

function debug(...args) {
  // Use a function like this one when debugging inside an AsyncHook callback
  writeFileSync('log.out', `${format(...args)}\n`, { flag: 'a' });
}const fs = require('node:fs');
const util = require('node:util');

function debug(...args) {
  // Use a function like this one when debugging inside an AsyncHook callback
  fs.writeFileSync('log.out', `${util.format(...args)}\n`, { flag: 'a' });
}

If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by AsyncHook itself. The logging should then be skipped when it was the logging itself that caused the AsyncHook callback to be called. By doing this, the otherwise infinite recursion is broken.

Class: AsyncHook#

The class AsyncHook exposes an interface for tracking lifetime events of asynchronous operations.

asyncHook.enable()#

Enable the callbacks for a given AsyncHook instance. If no callbacks are provided, enabling is a no-op.

The AsyncHook instance is disabled by default. If the AsyncHook instance should be enabled immediately after creation, the following pattern can be used.

import { createHook } from 'node:async_hooks';

const hook = createHook(callbacks).enable();const async_hooks = require('node:async_hooks');

const hook = async_hooks.createHook(callbacks).enable();

asyncHook.disable()#

Disable the callbacks for a given AsyncHook instance from the global pool of AsyncHook callbacks to be executed. Once a hook has been disabled it will not be called again until enabled.

For API consistency disable() also returns the AsyncHook instance.

Hook callbacks#

Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destroyed.

init(asyncId, type, triggerAsyncId, resource)#
  • asyncId <number> A unique ID for the async resource.
  • type <string> The type of the async resource.
  • triggerAsyncId <number> The unique ID of the async resource in whose execution context this async resource was created.
  • resource <Object> Reference to the resource representing the async operation, needs to be released during destroy.

Called when a class is constructed that has the possibility to emit an asynchronous event. This does not mean the instance must call before/after before destroy is called, only that the possibility exists.

This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.

import { createServer } from 'node:net';

createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));require('node:net').createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));

Every new resource is assigned an ID that is unique within the scope of the current Node.js instance.

type#

The type is a string identifying the type of resource that caused init to be called. Generally, it will correspond to the name of the resource's constructor.

The type of resources created by Node.js itself can change in any Node.js release. Valid values include TLSWRAP, TCPWRAP, TCPSERVERWRAP, GETADDRINFOREQWRAP, FSREQCALLBACK, Microtask, and Timeout. Inspect the source code of the Node.js version used to get the full list.

Furthermore users of AsyncResource create async resources independent of Node.js itself.

There is also the PROMISE resource type, which is used to track Promise instances and asynchronous work scheduled by them.

Users are able to define their own type when using the public embedder API.

It is possible to have type name collisions. Embedders are encouraged to use unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.

triggerAsyncId#

triggerAsyncId is the asyncId of the resource that caused (or "triggered") the new resource to initialize and that caused init to call. This is different from async_hooks.executionAsyncId() that only shows when a resource was created, while triggerAsyncId shows why a resource was created.

The following is a simple demonstration of triggerAsyncId:

import { createHook, executionAsyncId } from 'node:async_hooks';
import { stdout } from 'node:process';
import net from 'node:net';

createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = executionAsyncId();
    fs.writeSync(
      stdout.fd,
      `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
}).enable();

net.createServer((conn) => {}).listen(8080);const { createHook, executionAsyncId } = require('node:async_hooks');
const { stdout } = require('node:process');
const net = require('node:net');

createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = executionAsyncId();
    fs.writeSync(
      stdout.fd,
      `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
}).enable();

net.createServer((conn) => {}).listen(8080);

Output when hitting the server with nc localhost 8080:

TCPSERVERWRAP(5): trigger: 1 execution: 1
TCPWRAP(7): trigger: 5 execution: 0

The TCPSERVERWRAP is the server which receives the connections.

The TCPWRAP is the new connection from the client. When a new connection is made, the TCPWrap instance is immediately constructed. This happens outside of any JavaScript stack. (An executionAsyncId() of 0 means that it is being executed from C++ with no JavaScript stack above it.) With only that information, it would be impossible to link resources together in terms of what caused them to be created, so triggerAsyncId is given the task of propagating what resource is responsible for the new resource's existence.

resource#

resource is an object that represents the actual async resource that has been initialized. The API to access the object may be specified by the creator of the resource. Resources created by Node.js itself are internal and may change at any time. Therefore no API is specified for these.

In some cases the resource object is reused for performance reasons, it is thus not safe to use it as a key in a WeakMap or add properties to it.

Asynchronous context example#

The context tracking use case is covered by the stable API AsyncLocalStorage. This example only illustrates async hooks operation but AsyncLocalStorage fits better to this use case.

The following is an example with additional information about the calls to init between the before and after calls, specifically what the callback to listen() will look like. The output formatting is slightly more elaborate to make calling context easier to see.

const async_hooks = require('node:async_hooks');
const fs = require('node:fs');
const net = require('node:net');
const { fd } = process.stdout;

let indent = 0;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      fd,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
    indent += 2;
  },
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
  },
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
  },
}).enable();

net.createServer(() => {}).listen(8080, () => {
  // Let's wait 10ms before logging the server started.
  setTimeout(() => {
    console.log('>>>', async_hooks.executionAsyncId());
  }, 10);
});

Output from only starting the server:

TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before:  6
  Timeout(7): trigger: 6 execution: 6
after:   6
destroy: 6
before:  7
>>> 7
  TickObject(8): trigger: 7 execution: 7
after:   7
before:  8
after:   8

As illustrated in the example, executionAsyncId() and execution each specify the value of the current execution context; which is delineated by calls to before and after.

Only using execution to graph resource allocation results in the following:

  root(1)
     ^
     |
TickObject(6)
     ^
     |
 Timeout(7)

The TCPSERVERWRAP is not part of this graph, even though it was the reason for console.log() being called. This is because binding to a port without a host name is a synchronous operation, but to maintain a completely asynchronous API the user's callback is placed in a process.nextTick(). Which is why TickObject is present in the output and is a 'parent' for .listen() callback.

The graph only shows when a resource was created, not why, so to track the why use triggerAsyncId. Which can be represented with the following graph:

 bootstrap(1)
     |
     ˅
TCPSERVERWRAP(5)
     |
     ˅
 TickObject(6)
     |
     ˅
  Timeout(7)
before(asyncId)#

When an asynchronous operation is initiated (such as a TCP server receiving a new connection) or completes (such as writing data to disk) a callback is called to notify the user. The before callback is called just before said callback is executed. asyncId is the unique identifier assigned to the resource about to execute the callback.

The before callback will be called 0 to N times. The before callback will typically be called 0 times if the asynchronous operation was cancelled or, for example, if no connections are received by a TCP server. Persistent asynchronous resources like a TCP server will typically call the before callback multiple times, while other operations like fs.open() will call it only once.

after(asyncId)#

Called immediately after the callback specified in before is completed.

If an uncaught exception occurs during execution of the callback, then after will run after the 'uncaughtException' event is emitted or a domain's handler runs.

destroy(asyncId)#

Called after the resource corresponding to asyncId is destroyed. It is also called asynchronously from the embedder API emitDestroy().

Some resources depend on garbage collection for cleanup, so if a reference is made to the resource object passed to init it is possible that destroy will never be called, causing a memory leak in the application. If the resource does not depend on garbage collection, then this will not be an issue.

Using the destroy hook results in additional overhead because it enables tracking of Promise instances via the garbage collector.

promiseResolve(asyncId)#
Added in: v8.6.0

Called when the resolve function passed to the Promise constructor is invoked (either directly or through other means of resolving a promise).

resolve() does not do any observable synchronous work.

The Promise is not necessarily fulfilled or rejected at this point if the Promise was resolved by assuming the state of another Promise.

new Promise((resolve) => resolve(true)).then((a) => {});

calls the following callbacks:

init for PROMISE with id 5, trigger id: 1
  promise resolve 5      # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5  # the Promise returned by then()
  before 6               # the then() callback is entered
  promise resolve 6      # the then() callback resolves the promise by returning
  after 6

async_hooks.executionAsyncResource()#

Added in: v13.9.0, v12.17.0
  • Returns: <Object> The resource representing the current execution. Useful to store data within the resource.

Resource objects returned by executionAsyncResource() are most often internal Node.js handle objects with undocumented APIs. Using any functions or properties on the object is likely to crash your application and should be avoided.

Using executionAsyncResource() in the top-level execution context will return an empty object as there is no handle or request object to use, but having an object representing the top-level can be helpful.

import { open } from 'node:fs';
import { executionAsyncId, executionAsyncResource } from 'node:async_hooks';

console.log(executionAsyncId(), executionAsyncResource());  // 1 {}
open(new URL(import.meta.url), 'r', (err, fd) => {
  console.log(executionAsyncId(), executionAsyncResource());  // 7 FSReqWrap
});const { open } = require('node:fs');
const { executionAsyncId, executionAsyncResource } = require('node:async_hooks');

console.log(executionAsyncId(), executionAsyncResource());  // 1 {}
open(__filename, 'r', (err, fd) => {
  console.log(executionAsyncId(), executionAsyncResource());  // 7 FSReqWrap
});

This can be used to implement continuation local storage without the use of a tracking Map to store the metadata:

import { createServer } from 'node:http';
import {
  executionAsyncId,
  executionAsyncResource,
  createHook,
} from 'async_hooks';
const sym = Symbol('state'); // Private symbol to avoid pollution

createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    const cr = executionAsyncResource();
    if (cr) {
      resource[sym] = cr[sym];
    }
  },
}).enable();

const server = createServer((req, res) => {
  executionAsyncResource()[sym] = { state: req.url };
  setTimeout(function() {
    res.end(JSON.stringify(executionAsyncResource()[sym]));
  }, 100);
}).listen(3000);const { createServer } = require('node:http');
const {
  executionAsyncId,
  executionAsyncResource,
  createHook,
} = require('node:async_hooks');
const sym = Symbol('state'); // Private symbol to avoid pollution

createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    const cr = executionAsyncResource();
    if (cr) {
      resource[sym] = cr[sym];
    }
  },
}).enable();

const server = createServer((req, res) => {
  executionAsyncResource()[sym] = { state: req.url };
  setTimeout(function() {
    res.end(JSON.stringify(executionAsyncResource()[sym]));
  }, 100);
}).listen(3000);

async_hooks.executionAsyncId()#

History
VersionChanges
v8.2.0

Renamed from currentId.

v8.1.0

Added in: v8.1.0

  • Returns: <number> The asyncId of the current execution context. Useful to track when something calls.
import { executionAsyncId } from 'node:async_hooks';

console.log(executionAsyncId());  // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
  console.log(executionAsyncId());  // 6 - open()
});const async_hooks = require('node:async_hooks');

console.log(async_hooks.executionAsyncId());  // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
  console.log(async_hooks.executionAsyncId());  // 6 - open()
});

The ID returned from executionAsyncId() is related to execution timing, not causality (which is covered by triggerAsyncId()):

const server = net.createServer((conn) => {
  // Returns the ID of the server, not of the new connection, because the
  // callback runs in the execution scope of the server's MakeCallback().
  async_hooks.executionAsyncId();

}).listen(port, () => {
  // Returns the ID of a TickObject (process.nextTick()) because all
  // callbacks passed to .listen() are wrapped in a nextTick().
  async_hooks.executionAsyncId();
});

Promise contexts may not get precise executionAsyncIds by default. See the section on promise execution tracking.

async_hooks.triggerAsyncId()#

  • Returns: <number> The ID of the resource responsible for calling the callback that is currently being executed.
const server = net.createServer((conn) => {
  // The resource that caused (or triggered) this callback to be called
  // was that of the new connection. Thus the return value of triggerAsyncId()
  // is the asyncId of "conn".
  async_hooks.triggerAsyncId();

}).listen(port, () => {
  // Even though all callbacks passed to .listen() are wrapped in a nextTick()
  // the callback itself exists because the call to the server's .listen()
  // was made. So the return value would be the ID of the server.
  async_hooks.triggerAsyncId();
});

Promise contexts may not get valid triggerAsyncIds by default. See the section on promise execution tracking.

async_hooks.asyncWrapProviders#

Added in: v17.2.0, v16.14.0
  • Returns: A map of provider types to the corresponding numeric id. This map contains all the event types that might be emitted by the async_hooks.init() event.

This feature suppresses the deprecated usage of process.binding('async_wrap').Providers. See: DEP0111

Promise execution tracking#

By default, promise executions are not assigned asyncIds due to the relatively expensive nature of the promise introspection API provided by V8. This means that programs using promises or async/await will not get correct execution and trigger ids for promise callback contexts by default.

import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

Promise.resolve(1729).then(() => {
  console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');

Promise.resolve(1729).then(() => {
  console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0

Observe that the then() callback claims to have executed in the context of the outer scope even though there was an asynchronous hop involved. Also, the triggerAsyncId value is 0, which means that we are missing context about the resource that caused (triggered) the then() callback to be executed.

Installing async hooks via async_hooks.createHook enables promise execution tracking:

import { createHook, executionAsyncId, triggerAsyncId } from 'node:async_hooks';
createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
  console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6const { createHook, executionAsyncId, triggerAsyncId } = require('node:async_hooks');

createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
  console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6

In this example, adding any actual hook function enabled the tracking of promises. There are two promises in the example above; the promise created by Promise.resolve() and the promise returned by the call to then(). In the example above, the first promise got the asyncId 6 and the latter got asyncId 7. During the execution of the then() callback, we are executing in the context of promise with asyncId 7. This promise was triggered by async resource 6.

Another subtlety with promises is that before and after callbacks are run only on chained promises. That means promises not created by then()/catch() will not have the before and after callbacks fired on them. For more details see the details of the V8 PromiseHooks API.

JavaScript embedder API#

Library developers that handle their own asynchronous resources performing tasks like I/O, connection pooling, or managing callback queues may use the AsyncResource JavaScript API so that all the appropriate callbacks are called.

Class: AsyncResource#

The documentation for this class has moved AsyncResource.

Class: AsyncLocalStorage#

The documentation for this class has moved AsyncLocalStorage.

Buffer#

Stability: 2 - Stable

Source Code: lib/buffer.js

Buffer objects are used to represent a fixed-length sequence of bytes. Many Node.js APIs support Buffers.

The Buffer class is a subclass of JavaScript's Uint8Array class and extends it with methods that cover additional use cases. Node.js APIs accept plain Uint8Arrays wherever Buffers are supported as well.

While the Buffer class is available within the global scope, it is still recommended to explicitly reference it via an import or require statement.

import { Buffer } from 'node:buffer';

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);

// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');

// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');const { Buffer } = require('node:buffer');

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);

// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');

// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');

Buffers and character encodings#

History
VersionChanges
v15.7.0, v14.18.0

Introduced base64url encoding.

v6.4.0

Introduced latin1 as an alias for binary.

v5.0.0

Removed the deprecated raw and raws encodings.

When converting between Buffers and strings, a character encoding may be specified. If no character encoding is specified, UTF-8 will be used as the default.

import { Buffer } from 'node:buffer';

const buf = Buffer.from('hello world', 'utf8');

console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=

console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>const { Buffer } = require('node:buffer');

const buf = Buffer.from('hello world', 'utf8');

console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=

console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>

Node.js buffers accept all case variations of encoding strings that they receive. For example, UTF-8 can be specified as 'utf8', 'UTF8', or 'uTf8'.

The character encodings currently supported by Node.js are the following:

  • 'utf8' (alias: 'utf-8'): Multi-byte encoded Unicode characters. Many web pages and other document formats use UTF-8. This is the default character encoding. When decoding a Buffer into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement character U+FFFD � will be used to represent those errors.

  • 'utf16le' (alias: 'utf-16le'): Multi-byte encoded Unicode characters. Unlike 'utf8', each character in the string will be encoded using either 2 or 4 bytes. Node.js only supports the little-endian variant of UTF-16.

  • 'latin1': Latin-1 stands for ISO-8859-1. This character encoding only supports the Unicode characters from U+0000 to U+00FF. Each character is encoded using a single byte. Characters that do not fit into that range are truncated and will be mapped to characters in that range.

Converting a Buffer into a string using one of the above is referred to as decoding, and converting a string into a Buffer is referred to as encoding.

Node.js also supports the following binary-to-text encodings. For binary-to-text encodings, the naming convention is reversed: Converting a Buffer into a string is typically referred to as encoding, and converting a string into a Buffer as decoding.

  • 'base64': Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC 4648, Section 5. Whitespace characters such as spaces, tabs, and new lines contained within the base64-encoded string are ignored.

  • 'base64url': base64url encoding as specified in RFC 4648, Section 5. When creating a Buffer from a string, this encoding will also correctly accept regular base64-encoded strings. When encoding a Buffer to a string, this encoding will omit padding.

  • 'hex': Encode each byte as two hexadecimal characters. Data truncation may occur when decoding strings that do not exclusively consist of an even number of hexadecimal characters. See below for an example.

The following legacy character encodings are also supported:

  • 'ascii': For 7-bit ASCII data only. When encoding a string into a Buffer, this is equivalent to using 'latin1'. When decoding a Buffer into a string, using this encoding will additionally unset the highest bit of each byte before decoding as 'latin1'. Generally, there should be no reason to use this encoding, as 'utf8' (or, if the data is known to always be ASCII-only, 'latin1') will be a better choice when encoding or decoding ASCII-only text. It is only provided for legacy compatibility.

  • 'binary': Alias for 'latin1'. See binary strings for more background on this topic. The name of this encoding can be very misleading, as all of the encodings listed here convert between strings and binary data. For converting between strings and Buffers, typically 'utf8' is the right choice.

  • 'ucs2', 'ucs-2': Aliases of 'utf16le'. UCS-2 used to refer to a variant of UTF-16 that did not support characters that had code points larger than U+FFFF. In Node.js, these code points are always supported.

import { Buffer } from 'node:buffer';

Buffer.from('1ag123', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.

Buffer.from('1a7', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.const { Buffer } = require('node:buffer');

Buffer.from('1ag123', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.

Buffer.from('1a7', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.

Modern Web browsers follow the WHATWG Encoding Standard which aliases both 'latin1' and 'ISO-8859-1' to 'win-1252'. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned 'win-1252'-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArrays#

History
VersionChanges
v3.0.0

The Buffers class now inherits from Uint8Array.

Buffer instances are also JavaScript Uint8Array and TypedArray instances. All TypedArray methods are available on Buffers. There are, however, subtle incompatibilities between the Buffer API and the TypedArray API.

In particular:

There are two ways to create new TypedArray instances from a Buffer:

  • Passing a Buffer to a TypedArray constructor will copy the Buffers contents, interpreted as an array of integers, and not as a byte sequence of the target type.
import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);

console.log(uint32array);

// Prints: Uint32Array(4) [ 1, 2, 3, 4 ]const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);

console.log(uint32array);

// Prints: Uint32Array(4) [ 1, 2, 3, 4 ]
  • Passing the Buffers underlying ArrayBuffer will create a TypedArray that shares its memory with the Buffer.
import { Buffer } from 'node:buffer';

const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
  buf.buffer,
  buf.byteOffset,
  buf.length / Uint16Array.BYTES_PER_ELEMENT);

console.log(uint16array);

// Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]const { Buffer } = require('node:buffer');

const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
  buf.buffer,
  buf.byteOffset,
  buf.length / Uint16Array.BYTES_PER_ELEMENT);

console.log(uint16array);

// Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypedArray object's .buffer property in the same way. Buffer.from() behaves like new Uint8Array() in this context.

import { Buffer } from 'node:buffer';

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);

// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>const { Buffer } = require('node:buffer');

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);

// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>

When creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

import { Buffer } from 'node:buffer';

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// Prints: 16const { Buffer } = require('node:buffer');

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// Prints: 16

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

  • TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

Buffers and iteration#

Buffer instances can be iterated over using for..of syntax:

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}
// Prints:
//   1
//   2
//   3const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}
// Prints:
//   1
//   2
//   3

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Blob#

History
VersionChanges
v18.0.0, v16.17.0

No longer experimental.

v15.7.0, v14.18.0

Added in: v15.7.0, v14.18.0

A Blob encapsulates immutable, raw data that can be safely shared across multiple worker threads.

new buffer.Blob([sources[, options]])#

History
VersionChanges
v16.7.0

Added the standard endings option to replace line-endings, and removed the non-standard encoding option.

v15.7.0, v14.18.0

Added in: v15.7.0, v14.18.0

Creates a new Blob object containing a concatenation of the given sources.

<ArrayBuffer>, <TypedArray>, <DataView>, and <Buffer> sources are copied into the 'Blob' and can therefore be safely modified after the 'Blob' is created.

String sources are encoded as UTF-8 byte sequences and copied into the Blob. Unmatched surrogate pairs within each string part will be replaced by Unicode U+FFFD replacement characters.

blob.arrayBuffer()#

Added in: v15.7.0, v14.18.0

Returns a promise that fulfills with an <ArrayBuffer> containing a copy of the Blob data.

blob.size#

Added in: v15.7.0, v14.18.0

The total size of the Blob in bytes.

blob.slice([start[, end[, type]]])#

Added in: v15.7.0, v14.18.0

Creates and returns a new Blob containing a subset of this Blob objects data. The original Blob is not altered.

blob.stream()#

Added in: v16.7.0

Returns a new ReadableStream that allows the content of the Blob to be read.

blob.text()#

Added in: v15.7.0, v14.18.0

Returns a promise that fulfills with the contents of the Blob decoded as a UTF-8 string.

blob.type#

Added in: v15.7.0, v14.18.0

The content-type of the Blob.

Blob objects and MessageChannel#

Once a <Blob> object is created, it can be sent via MessagePort to multiple destinations without transferring or immediately copying the data. The data contained by the Blob is copied only when the arrayBuffer() or text() methods are called.

import { Blob, Buffer } from 'node:buffer';
import { setTimeout as delay } from 'node:timers/promises';

const blob = new Blob(['hello there']);

const mc1 = new MessageChannel();
const mc2 = new MessageChannel();

mc1.port1.onmessage = async ({ data }) => {
  console.log(await data.arrayBuffer());
  mc1.port1.close();
};

mc2.port1.onmessage = async ({ data }) => {
  await delay(1000);
  console.log(await data.arrayBuffer());
  mc2.port1.close();
};

mc1.port2.postMessage(blob);
mc2.port2.postMessage(blob);

// The Blob is still usable after posting.
blob.text().then(console.log);const { Blob, Buffer } = require('node:buffer');
const { setTimeout: delay } = require('node:timers/promises');

const blob = new Blob(['hello there']);

const mc1 = new MessageChannel();
const mc2 = new MessageChannel();

mc1.port1.onmessage = async ({ data }) => {
  console.log(await data.arrayBuffer());
  mc1.port1.close();
};

mc2.port1.onmessage = async ({ data }) => {
  await delay(1000);
  console.log(await data.arrayBuffer());
  mc2.port1.close();
};

mc1.port2.postMessage(blob);
mc2.port2.postMessage(blob);

// The Blob is still usable after posting.
blob.text().then(console.log);

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

Static method: Buffer.alloc(size[, fill[, encoding]])#

History
VersionChanges
v15.0.0

Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.

v10.0.0

Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.

v10.0.0

Specifying an invalid string for fill triggers a thrown exception.

v8.9.3

Specifying an invalid string for fill now results in a zero-filled buffer.

v5.10.0

Added in: v5.10.0

Allocates a new Buffer of size bytes. If fill is undefined, the Buffer will be zero-filled.

import { Buffer } from 'node:buffer';

const buf = Buffer.alloc(5);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00>const { Buffer } = require('node:buffer');

const buf = Buffer.alloc(5);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00>

If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_ARG_VALUE is thrown.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill).

import { Buffer } from 'node:buffer';

const buf = Buffer.alloc(5, 'a');

console.log(buf);
// Prints: <Buffer 61 61 61 61 61>const { Buffer } = require('node:buffer');

const buf = Buffer.alloc(5, 'a');

console.log(buf);
// Prints: <Buffer 61 61 61 61 61>

If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding).

import { Buffer } from 'node:buffer';

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>const { Buffer } = require('node:buffer');

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>

Calling Buffer.alloc() can be measurably slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data from previous allocations, including data that might not have been allocated for Buffers.

A TypeError will be thrown if size is not a number.

Static method: Buffer.allocUnsafe(size)#

History
VersionChanges
v15.0.0

Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.

v7.0.0

Passing a negative size will now throw an error.

v5.10.0

Added in: v5.10.0

  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_ARG_VALUE is thrown.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances with zeroes.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(10);

console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

buf.fill(0);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(10);

console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

buf.fill(0);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>

A TypeError will be thrown if size is not a number.

The Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe(), Buffer.from(array), Buffer.concat(), and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill). Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize. The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides.

Static method: Buffer.allocUnsafeSlow(size)#

History
VersionChanges
v15.0.0

Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.

v5.12.0

Added in: v5.12.0

  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_ARG_VALUE is thrown. A zero-length Buffer is created if size is 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances with zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4 KiB are sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and clean up as many individual ArrayBuffer objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() and then copying out the relevant bits.

import { Buffer } from 'node:buffer';

// Need to keep around a few small chunks of memory.
const store = [];

socket.on('readable', () => {
  let data;
  while (null !== (data = readable.read())) {
    // Allocate for retained data.
    const sb = Buffer.allocUnsafeSlow(10);

    // Copy the data into the new allocation.
    data.copy(sb, 0, 0, 10);

    store.push(sb);
  }
});const { Buffer } = require('node:buffer');

// Need to keep around a few small chunks of memory.
const store = [];

socket.on('readable', () => {
  let data;
  while (null !== (data = readable.read())) {
    // Allocate for retained data.
    const sb = Buffer.allocUnsafeSlow(10);

    // Copy the data into the new allocation.
    data.copy(sb, 0, 0, 10);

    store.push(sb);
  }
});

A TypeError will be thrown if size is not a number.

Static method: Buffer.byteLength(string[, encoding])#

History
VersionChanges
v7.0.0

Passing invalid input will now throw an error.

v5.10.0

The string parameter can now be any TypedArray, DataView or ArrayBuffer.

v0.1.90

Added in: v0.1.90

Returns the byte length of a string when encoded using encoding. This is not the same as String.prototype.length, which does not account for the encoding that is used to convert the string into bytes.

For 'base64', 'base64url', and 'hex', this function assumes valid input. For strings that contain non-base64/hex-encoded data (e.g. whitespace), the return value might be greater than the length of a Buffer created from the string.

import { Buffer } from 'node:buffer';

const str = '\u00bd + \u00bc = \u00be';

console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytesconst { Buffer } = require('node:buffer');

const str = '\u00bd + \u00bc = \u00be';

console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes

When string is a Buffer/DataView/TypedArray/ArrayBuffer/ SharedArrayBuffer, the byte length as reported by .byteLength is returned.

Static method: Buffer.compare(buf1, buf2)#

History
VersionChanges
v8.0.0

The arguments can now be Uint8Arrays.

v0.11.13

Added in: v0.11.13

Compares buf1 to buf2, typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2).

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)const { Buffer } = require('node:buffer');

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)

Static method: Buffer.concat(list[, totalLength])#

History
VersionChanges
v8.0.0

The elements of list can now be Uint8Arrays.

v0.7.11

Added in: v0.7.11

Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together.

If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned.

If totalLength is not provided, it is calculated from the Buffer instances in list by adding their lengths.

If totalLength is provided, it is coerced to an unsigned integer. If the combined length of the Buffers in list exceeds totalLength, the result is truncated to totalLength.

import { Buffer } from 'node:buffer';

// Create a single `Buffer` from a list of three `Buffer` instances.

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

console.log(totalLength);
// Prints: 42

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42const { Buffer } = require('node:buffer');

// Create a single `Buffer` from a list of three `Buffer` instances.

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

console.log(totalLength);
// Prints: 42

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42

Buffer.concat() may also use the internal Buffer pool like Buffer.allocUnsafe() does.

Static method: Buffer.from(array)#

Added in: v5.10.0

Allocates a new Buffer using an array of bytes in the range 0 – 255. Array entries outside that range will be truncated to fit into it.

import { Buffer } from 'node:buffer';

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);const { Buffer } = require('node:buffer');

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

A TypeError will be thrown if array is not an Array or another type appropriate for Buffer.from() variants.

Buffer.from(array) and Buffer.from(string) may also use the internal Buffer pool like Buffer.allocUnsafe() does.

Static method: Buffer.from(arrayBuffer[, byteOffset[, length]])#

Added in: v5.10.0

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray's underlying ArrayBuffer.

import { Buffer } from 'node:buffer';

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);

console.log(buf);
// Prints: <Buffer 88 13 a0 0f>

// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;

console.log(buf);
// Prints: <Buffer 88 13 70 17>const { Buffer } = require('node:buffer');

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);

console.log(buf);
// Prints: <Buffer 88 13 a0 0f>

// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;

console.log(buf);
// Prints: <Buffer 88 13 70 17>

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

import { Buffer } from 'node:buffer';

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

console.log(buf.length);
// Prints: 2const { Buffer } = require('node:buffer');

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

console.log(buf.length);
// Prints: 2

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer or a SharedArrayBuffer or another type appropriate for Buffer.from() variants.

It is important to remember that a backing ArrayBuffer can cover a range of memory that extends beyond the bounds of a TypedArray view. A new Buffer created using the buffer property of a TypedArray may extend beyond the range of the TypedArray:

import { Buffer } from 'node:buffer';

const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
console.log(arrA.buffer === arrB.buffer); // true

const buf = Buffer.from(arrB.buffer);
console.log(buf);
// Prints: <Buffer 63 64 65 66>const { Buffer } = require('node:buffer');

const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
console.log(arrA.buffer === arrB.buffer); // true

const buf = Buffer.from(arrB.buffer);
console.log(buf);
// Prints: <Buffer 63 64 65 66>

Static method: Buffer.from(buffer)#

Added in: v5.10.0

Copies the passed buffer data onto a new Buffer instance.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: bufferconst { Buffer } = require('node:buffer');

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer

A TypeError will be thrown if buffer is not a Buffer or another type appropriate for Buffer.from() variants.

Static method: Buffer.from(object[, offsetOrEncoding[, length]])#

Added in: v8.2.0

For objects whose valueOf() function returns a value not strictly equal to object, returns Buffer.from(object.valueOf(), offsetOrEncoding, length).

import { Buffer } from 'node:buffer';

const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>const { Buffer } = require('node:buffer');

const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

For objects that support Symbol.toPrimitive, returns Buffer.from(object[Symbol.toPrimitive]('string'), offsetOrEncoding).

import { Buffer } from 'node:buffer';

class Foo {
  [Symbol.toPrimitive]() {
    return 'this is a test';
  }
}

const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>const { Buffer } = require('node:buffer');

class Foo {
  [Symbol.toPrimitive]() {
    return 'this is a test';
  }
}

const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

A TypeError will be thrown if object does not have the mentioned methods or is not of another type appropriate for Buffer.from() variants.

Static method: Buffer.from(string[, encoding])#

Added in: v5.10.0
  • string <string> A string to encode.
  • encoding <string> The encoding of string. Default: 'utf8'.

Creates a new Buffer containing string. The encoding parameter identifies the character encoding to be used when converting string into bytes.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a téstconst { Buffer } = require('node:buffer');

const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a tést

A TypeError will be thrown if string is not a string or another type appropriate for Buffer.from() variants.

Static method: Buffer.isBuffer(obj)#

Added in: v0.1.101

Returns true if obj is a Buffer, false otherwise.

import { Buffer } from 'node:buffer';

Buffer.isBuffer(Buffer.alloc(10)); // true
Buffer.isBuffer(Buffer.from('foo')); // true
Buffer.isBuffer('a string'); // false
Buffer.isBuffer([]); // false
Buffer.isBuffer(new Uint8Array(1024)); // falseconst { Buffer } = require('node:buffer');

Buffer.isBuffer(Buffer.alloc(10)); // true
Buffer.isBuffer(Buffer.from('foo')); // true
Buffer.isBuffer('a string'); // false
Buffer.isBuffer([]); // false
Buffer.isBuffer(new Uint8Array(1024)); // false

Static method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if encoding is the name of a supported character encoding, or false otherwise.

import { Buffer } from 'node:buffer';

console.log(Buffer.isEncoding('utf8'));
// Prints: true

console.log(Buffer.isEncoding('hex'));
// Prints: true

console.log(Buffer.isEncoding('utf/8'));
// Prints: false

console.log(Buffer.isEncoding(''));
// Prints: falseconst { Buffer } = require('node:buffer');

console.log(Buffer.isEncoding('utf8'));
// Prints: true

console.log(Buffer.isEncoding('hex'));
// Prints: true

console.log(Buffer.isEncoding('utf/8'));
// Prints: false

console.log(Buffer.isEncoding(''));
// Prints: false

Class property: Buffer.poolSize#

Added in: v0.11.3

This is the size (in bytes) of pre-allocated internal Buffer instances used for pooling. This value may be modified.

buf[index]#

The index operator [index] can be used to get and set the octet at position index in buf. The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal).

This operator is inherited from Uint8Array, so its behavior on out-of-bounds access is the same as Uint8Array. In other words, buf[index] returns undefined when index is negative or greater or equal to buf.length, and buf[index] = value does not modify the buffer if index is negative or >= buf.length.

import { Buffer } from 'node:buffer';

// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length; i++) {
  buf[i] = str.charCodeAt(i);
}

console.log(buf.toString('utf8'));
// Prints: Node.jsconst { Buffer } = require('node:buffer');

// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length; i++) {
  buf[i] = str.charCodeAt(i);
}

console.log(buf.toString('utf8'));
// Prints: Node.js

buf.buffer#

  • <ArrayBuffer> The underlying ArrayBuffer object based on which this Buffer object is created.

This ArrayBuffer is not guaranteed to correspond exactly to the original Buffer. See the notes on buf.byteOffset for details.

import { Buffer } from 'node:buffer';

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: trueconst { Buffer } = require('node:buffer');

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: true

buf.byteOffset#

  • <integer> The byteOffset of the Buffers underlying ArrayBuffer object.

When setting byteOffset in Buffer.from(ArrayBuffer, byteOffset, length), or sometimes when allocating a Buffer smaller than Buffer.poolSize, the buffer does not start from a zero offset on the underlying ArrayBuffer.

This can cause problems when accessing the underlying ArrayBuffer directly using buf.buffer, as other parts of the ArrayBuffer may be unrelated to the Buffer object itself.

A common issue when creating a TypedArray object that shares its memory with a Buffer is that in this case one needs to specify the byteOffset correctly:

import { Buffer } from 'node:buffer';

// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);const { Buffer } = require('node:buffer');

// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);

buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#

History
VersionChanges
v8.0.0

The target parameter can now be a Uint8Array.

v5.11.0

Additional parameters for specifying offsets are supported now.

v0.11.13

Added in: v0.11.13

  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array with which to compare buf.
  • targetStart <integer> The offset within target at which to begin comparison. Default: 0.
  • targetEnd <integer> The offset within target at which to end comparison (not inclusive). Default: target.length.
  • sourceStart <integer> The offset within buf at which to begin comparison. Default: 0.
  • sourceEnd <integer> The offset within buf at which to end comparison (not inclusive). Default: buf.length.
  • Returns: <integer>

Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer.

  • 0 is returned if target is the same as buf
  • 1 is returned if target should come before buf when sorted.
  • -1 is returned if target should come after buf when sorted.
import { Buffer } from 'node:buffer';

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)const { Buffer } = require('node:buffer');

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)

The optional targetStart, targetEnd, sourceStart, and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1const { Buffer } = require('node:buffer');

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1

ERR_OUT_OF_RANGE is thrown if targetStart < 0, sourceStart < 0, targetEnd > target.byteLength, or sourceEnd > source.byteLength.

buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#

Added in: v0.1.90
  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into.
  • targetStart <integer> The offset within target at which to begin writing. Default: 0.
  • sourceStart <integer> The offset within buf from which to begin copying. Default: 0.
  • sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length.
  • Returns: <integer> The number of bytes copied.

Copies data from a region of buf to a region in target, even if the target memory region overlaps with buf.

TypedArray.prototype.set() performs the same operation, and is available for all TypedArrays, including Node.js Buffers, although it takes different function arguments.

import { Buffer } from 'node:buffer';

// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!const { Buffer } = require('node:buffer');

// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
import { Buffer } from 'node:buffer';

// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyzconst { Buffer } = require('node:buffer');

// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz

buf.entries()#

Added in: v1.1.0

Creates and returns an iterator of [index, byte] pairs from the contents of buf.

import { Buffer } from 'node:buffer';

// Log the entire contents of a `Buffer`.

const buf = Buffer.from('buffer');

for (const pair of buf.entries()) {
  console.log(pair);
}
// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]const { Buffer } = require('node:buffer');

// Log the entire contents of a `Buffer`.

const buf = Buffer.from('buffer');

for (const pair of buf.entries()) {
  console.log(pair);
}
// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]

buf.equals(otherBuffer)#

History
VersionChanges
v8.0.0

The arguments can now be Uint8Arrays.

v0.11.13

Added in: v0.11.13

Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise. Equivalent to buf.compare(otherBuffer) === 0.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: falseconst { Buffer } = require('node:buffer');

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: false

buf.fill(value[, offset[, end]][, encoding])#

History
VersionChanges
v11.0.0

Throws ERR_OUT_OF_RANGE instead of ERR_INDEX_OUT_OF_RANGE.

v10.0.0

Negative end values throw an ERR_INDEX_OUT_OF_RANGE error.

v10.0.0

Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.

v10.0.0

Specifying an invalid string for value triggers a thrown exception.

v5.7.0

The encoding parameter is supported now.

v0.5.0

Added in: v0.5.0

  • value <string> | <Buffer> | <Uint8Array> | <integer> The value with which to fill buf. Empty value (string, Uint8Array, Buffer) is coerced to 0.
  • offset <integer> Number of bytes to skip before starting to fill buf. Default: 0.
  • end <integer> Where to stop filling buf (not inclusive). Default: buf.length.
  • encoding <string> The encoding for value if value is a string. Default: 'utf8'.
  • Returns: <Buffer> A reference to buf.

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled:

import { Buffer } from 'node:buffer';

// Fill a `Buffer` with the ASCII character 'h'.

const b = Buffer.allocUnsafe(50).fill('h');

console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

// Fill a buffer with empty string
const c = Buffer.allocUnsafe(5).fill('');

console.log(c.fill(''));
// Prints: <Buffer 00 00 00 00 00>const { Buffer } = require('node:buffer');

// Fill a `Buffer` with the ASCII character 'h'.

const b = Buffer.allocUnsafe(50).fill('h');

console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

// Fill a buffer with empty string
const c = Buffer.allocUnsafe(5).fill('');

console.log(c.fill(''));
// Prints: <Buffer 00 00 00 00 00>

value is coerced to a uint32 value if it is not a string, Buffer, or integer. If the resulting integer is greater than 255 (decimal), buf will be filled with value & 255.

If the final write of a fill() operation falls on a multi-byte character, then only the bytes of that character that fit into buf are written:

import { Buffer } from 'node:buffer';

// Fill a `Buffer` with character that takes up two bytes in UTF-8.

console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>const { Buffer } = require('node:buffer');

// Fill a `Buffer` with character that takes up two bytes in UTF-8.

console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>

If value contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown:

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(5);

console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(5);

console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.

buf.includes(value[, byteOffset][, encoding])#

Added in: v5.3.0
  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
  • encoding <string> If value is a string, this is its encoding. Default: 'utf8'.
  • Returns: <boolean> true if value was found in buf, false otherwise.

Equivalent to buf.indexOf() !== -1.

import { Buffer } from 'node:buffer';

const buf = Buffer.from('this is a buffer');

console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: falseconst { Buffer } = require('node:buffer');

const buf = Buffer.from('this is a buffer');

console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: false

buf.indexOf(value[, byteOffset][, encoding])#

History
VersionChanges
v8.0.0

The value can now be a Uint8Array.

v5.7.0, v4.4.0

When encoding is being passed, the byteOffset parameter is no longer required.

v1.5.0

Added in: v1.5.0

  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
  • encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
  • Returns: <integer> The index of the first occurrence of value in buf, or -1 if buf does not contain value.

If value is:

  • a string, value is interpreted according to the character encoding in encoding.
  • a Buffer or Uint8Array, value will be used in its entirety. To compare a partial Buffer, use buf.subarray.
  • a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.
import { Buffer } from 'node:buffer';

const buf = Buffer.from('this is a buffer');

console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6const { Buffer } = require('node:buffer');

const buf = Buffer.from('this is a buffer');

console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. If the result of coercion is NaN or 0, then the entire buffer will be searched. This behavior matches String.prototype.indexOf().

import { Buffer } from 'node:buffer';

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));const { Buffer } = require('node:buffer');

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));

If value is an empty string or empty Buffer and byteOffset is less than buf.length, byteOffset will be returned. If value is empty and byteOffset is at least buf.length, buf.length will be returned.

buf.keys()#

Added in: v1.1.0

Creates and returns an iterator of buf keys (indices).

import { Buffer } from 'node:buffer';

const buf = Buffer.from('buffer');

for (const key of buf.keys()) {
  console.log(key);
}
// Prints:
//   0
//   1
//   2
//   3
//   4
//   5const { Buffer } = require('node:buffer');

const buf = Buffer.from('buffer');

for (const key of buf.keys()) {
  console.log(key);
}
// Prints:
//   0
//   1
//   2
//   3
//   4
//   5

buf.lastIndexOf(value[, byteOffset][, encoding])#

History
VersionChanges
v8.0.0

The value can now be a Uint8Array.

v6.0.0

Added in: v6.0.0

  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: buf.length - 1.
  • encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
  • Returns: <integer> The index of the last occurrence of value in buf, or -1 if buf does not contain value.

Identical to buf.indexOf(), except the last occurrence of value is found rather than the first occurrence.

import { Buffer } from 'node:buffer';

const buf = Buffer.from('this buffer is a buffer');

console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4const { Buffer } = require('node:buffer');

const buf = Buffer.from('this buffer is a buffer');

console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN, like {} or undefined, will search the whole buffer. This behavior matches String.prototype.lastIndexOf().

import { Buffer } from 'node:buffer';

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));const { Buffer } = require('node:buffer');

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));

If value is an empty string or empty Buffer, byteOffset will be returned.

buf.length#

Added in: v0.1.90

Returns the number of bytes in buf.

import { Buffer } from 'node:buffer';

// Create a `Buffer` and write a shorter string to it using UTF-8.

const buf = Buffer.alloc(1234);

console.log(buf.length);
// Prints: 1234

buf.write('some string', 0, 'utf8');

console.log(buf.length);
// Prints: 1234const { Buffer } = require('node:buffer');

// Create a `Buffer` and write a shorter string to it using UTF-8.

const buf = Buffer.alloc(1234);

console.log(buf.length);
// Prints: 1234

buf.write('some string', 0, 'utf8');

console.log(buf.length);
// Prints: 1234

buf.parent#

Deprecated since: v8.0.0

Stability: 0 - Deprecated: Use buf.buffer instead.

The buf.parent property is a deprecated alias for buf.buffer.

buf.readBigInt64BE([offset])#

Added in: v12.0.0, v10.20.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads a signed, big-endian 64-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

buf.readBigInt64LE([offset])#

Added in: v12.0.0, v10.20.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads a signed, little-endian 64-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

buf.readBigUInt64BE([offset])#

History
VersionChanges
v14.10.0, v12.19.0

This function is also available as buf.readBigUint64BE().

v12.0.0, v10.20.0

Added in: v12.0.0, v10.20.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads an unsigned, big-endian 64-bit integer from buf at the specified offset.

This function is also available under the readBigUint64BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295nconst { Buffer } = require('node:buffer');

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295n

buf.readBigUInt64LE([offset])#

History
VersionChanges
v14.10.0, v12.19.0

This function is also available as buf.readBigUint64LE().

v12.0.0, v10.20.0

Added in: v12.0.0, v10.20.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads an unsigned, little-endian 64-bit integer from buf at the specified offset.

This function is also available under the readBigUint64LE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320nconst { Buffer } = require('node:buffer');

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320n

buf.readDoubleBE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <number>

Reads a 64-bit, big-endian double from buf at the specified offset.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304

buf.readDoubleLE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <number>

Reads a 64-bit, little-endian double from buf at the specified offset.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readFloatBE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <number>

Reads a 32-bit, big-endian float from buf at the specified offset.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38

buf.readFloatLE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <number>

Reads a 32-bit, little-endian float from buf at the specified offset.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt8([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer>

Reads a signed 8-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([-1, 5]);

console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([-1, 5]);

console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readInt16BE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads a signed, big-endian 16-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16BE(0));
// Prints: 5const { Buffer } = require('node:buffer');

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16BE(0));
// Prints: 5

buf.readInt16LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads a signed, little-endian 16-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt32BE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads a signed, big-endian 32-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32BE(0));
// Prints: 5const { Buffer } = require('node:buffer');

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32BE(0));
// Prints: 5

buf.readInt32LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads a signed, little-endian 32-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readIntBE(offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a big-endian, two's complement signed value supporting up to 48 bits of accuracy.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readIntLE(offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a little-endian, two's complement signed value supporting up to 48 bits of accuracy.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbeeconst { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee

buf.readUInt8([offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUint8().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

This function is also available under the readUint8 alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([1, -2]);

console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([1, -2]);

console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt16BE([offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUint16BE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads an unsigned, big-endian 16-bit integer from buf at the specified offset.

This function is also available under the readUint16BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456

buf.readUInt16LE([offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUint16LE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads an unsigned, little-endian 16-bit integer from buf at the specified offset.

This function is also available under the readUint16LE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt32BE([offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUint32BE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads an unsigned, big-endian 32-bit integer from buf at the specified offset.

This function is also available under the readUint32BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678

buf.readUInt32LE([offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUint32LE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads an unsigned, little-endian 32-bit integer from buf at the specified offset.

This function is also available under the readUint32LE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUIntBE(offset, byteLength)#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUintBE().

v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned big-endian integer supporting up to 48 bits of accuracy.

This function is also available under the readUintBE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUIntLE(offset, byteLength)#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.readUintLE().

v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned, little-endian integer supporting up to 48 bits of accuracy.

This function is also available under the readUintLE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412

buf.subarray([start[, end]])#

Added in: v3.0.0

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

Specifying end greater than buf.length will return the same result as that of end equal to buf.length.

This method is inherited from TypedArray.prototype.subarray().

Modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

import { Buffer } from 'node:buffer';

// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

const buf2 = buf1.subarray(0, 3);

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc

buf1[0] = 33;

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bcconst { Buffer } = require('node:buffer');

// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

const buf2 = buf1.subarray(0, 3);

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc

buf1[0] = 33;

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bc

Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning.

import { Buffer } from 'node:buffer';

const buf = Buffer.from('buffer');

console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)

console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)

console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)const { Buffer } = require('node:buffer');

const buf = Buffer.from('buffer');

console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)

console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)

console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)

buf.slice([start[, end]])#

History
VersionChanges
v17.5.0, v16.15.0

The buf.slice() method has been deprecated.

v7.0.0

All offsets are now coerced to integers before doing any calculations with them.

v7.1.0, v6.9.2

Coercing the offsets to integers now handles values outside the 32-bit integer range properly.

v0.3.0

Added in: v0.3.0

Stability: 0 - Deprecated: Use buf.subarray instead.

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

This method is not compatible with the Uint8Array.prototype.slice(), which is a superclass of Buffer. To copy the slice, use Uint8Array.prototype.slice().

import { Buffer } from 'node:buffer';

const buf = Buffer.from('buffer');

const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer

console.log(buf.toString());
// Prints: buffer

// With buf.slice(), the original buffer is modified.
const notReallyCopiedBuf = buf.slice();
notReallyCopiedBuf[0]++;
console.log(notReallyCopiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Also prints: cuffer (!)const { Buffer } = require('node:buffer');

const buf = Buffer.from('buffer');

const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer

console.log(buf.toString());
// Prints: buffer

// With buf.slice(), the original buffer is modified.
const notReallyCopiedBuf = buf.slice();
notReallyCopiedBuf[0]++;
console.log(notReallyCopiedBuf.toString());
// Prints: cuffer
console.log(buf.toString());
// Also prints: cuffer (!)

buf.swap16()#

Added in: v5.10.0

Interprets buf as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 2.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap16();

console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.const { Buffer } = require('node:buffer');

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap16();

console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.

One convenient use of buf.swap16() is to perform a fast in-place conversion between UTF-16 little-endian and UTF-16 big-endian:

import { Buffer } from 'node:buffer';

const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.const { Buffer } = require('node:buffer');

const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.

buf.swap32()#

Added in: v5.10.0

Interprets buf as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 4.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap32();

console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.const { Buffer } = require('node:buffer');

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap32();

console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.swap64()#

Added in: v6.3.0

Interprets buf as an array of 64-bit numbers and swaps byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 8.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap64();

console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.const { Buffer } = require('node:buffer');

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap64();

console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.toJSON()#

Added in: v0.9.2

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

Buffer.from() accepts objects in the format returned from this method. In particular, Buffer.from(buf.toJSON()) works like Buffer.from(buf).

import { Buffer } from 'node:buffer';

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer' ?
    Buffer.from(value) :
    value;
});

console.log(copy);
// Prints: <Buffer 01 02 03 04 05>const { Buffer } = require('node:buffer');

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer' ?
    Buffer.from(value) :
    value;
});

console.log(copy);
// Prints: <Buffer 01 02 03 04 05>

buf.toString([encoding[, start[, end]]])#

Added in: v0.1.90
  • encoding <string> The character encoding to use. Default: 'utf8'.
  • start <integer> The byte offset to start decoding at. Default: 0.
  • end <integer> The byte offset to stop decoding at (not inclusive). Default: buf.length.
  • Returns: <string>

Decodes buf to a string according to the specified character encoding in encoding. start and end may be passed to decode only a subset of buf.

If encoding is 'utf8' and a byte sequence in the input is not valid UTF-8, then each invalid byte is replaced with the replacement character U+FFFD.

The maximum length of a string instance (in UTF-16 code units) is available as buffer.constants.MAX_STRING_LENGTH.

import { Buffer } from 'node:buffer';

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde

const buf2 = Buffer.from('tést');

console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: téconst { Buffer } = require('node:buffer');

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde

const buf2 = Buffer.from('tést');

console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: té

buf.values()#

Added in: v1.1.0

Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement.

import { Buffer } from 'node:buffer';

const buf = Buffer.from('buffer');

for (const value of buf.values()) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

for (const value of buf) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114const { Buffer } = require('node:buffer');

const buf = Buffer.from('buffer');

for (const value of buf.values()) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

for (const value of buf) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

buf.write(string[, offset[, length]][, encoding])#

Added in: v0.1.90
  • string <string> String to write to buf.
  • offset <integer> Number of bytes to skip before starting to write string. Default: 0.
  • length <integer> Maximum number of bytes to write (written bytes will not exceed buf.length - offset). Default: buf.length - offset.
  • encoding <string> The character encoding of string. Default: 'utf8'.
  • Returns: <integer> Number of bytes written.

Writes string to buf at offset according to the character encoding in encoding. The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only part of string will be written. However, partially encoded characters will not be written.

import { Buffer } from 'node:buffer';

const buf = Buffer.alloc(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾

const buffer = Buffer.alloc(10);

const length = buffer.write('abcd', 8);

console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : abconst { Buffer } = require('node:buffer');

const buf = Buffer.alloc(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾

const buffer = Buffer.alloc(10);

const length = buffer.write('abcd', 8);

console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : ab

buf.writeBigInt64BE(value[, offset])#

Added in: v12.0.0, v10.20.0
  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian.

value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64BE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64BE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf.writeBigInt64LE(value[, offset])#

Added in: v12.0.0, v10.20.0
  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian.

value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64LE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 08 07 06 05 04 03 02 01>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64LE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 08 07 06 05 04 03 02 01>

buf.writeBigUInt64BE(value[, offset])#

History
VersionChanges
v14.10.0, v12.19.0

This function is also available as buf.writeBigUint64BE().

v12.0.0, v10.20.0

Added in: v12.0.0, v10.20.0

  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian.

This function is also available under the writeBigUint64BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64BE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de ca fa fe ca ce fa de>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64BE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de ca fa fe ca ce fa de>

buf.writeBigUInt64LE(value[, offset])#

History
VersionChanges
v14.10.0, v12.19.0

This function is also available as buf.writeBigUint64LE().

v12.0.0, v10.20.0

Added in: v12.0.0, v10.20.0

  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>

This function is also available under the writeBigUint64LE alias.

buf.writeDoubleBE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. The value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(123.456, 0);

console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(123.456, 0);

console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>

buf.writeDoubleLE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. The value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleLE(123.456, 0);

console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleLE(123.456, 0);

console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>

buf.writeFloatBE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. Behavior is undefined when value is anything other than a JavaScript number.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer 4f 4a fe bb>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer 4f 4a fe bb>

buf.writeFloatLE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. Behavior is undefined when value is anything other than a JavaScript number.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeFloatLE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer bb fe 4a 4f>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeFloatLE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer bb fe 4a 4f>

buf.writeInt8(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer.

value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

console.log(buf);
// Prints: <Buffer 02 fe>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

console.log(buf);
// Prints: <Buffer 02 fe>

buf.writeInt16BE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. The value must be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

The value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(2);

buf.writeInt16BE(0x0102, 0);

console.log(buf);
// Prints: <Buffer 01 02>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(2);

buf.writeInt16BE(0x0102, 0);

console.log(buf);
// Prints: <Buffer 01 02>

buf.writeInt16LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. The value must be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

The value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(2);

buf.writeInt16LE(0x0304, 0);

console.log(buf);
// Prints: <Buffer 04 03>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(2);

buf.writeInt16LE(0x0304, 0);

console.log(buf);
// Prints: <Buffer 04 03>

buf.writeInt32BE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. The value must be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

The value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeInt32BE(0x01020304, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeInt32BE(0x01020304, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04>

buf.writeInt32LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. The value must be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

The value is interpreted and written as a two's complement signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeInt32LE(0x05060708, 0);

console.log(buf);
// Prints: <Buffer 08 07 06 05>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeInt32LE(0x05060708, 0);

console.log(buf);
// Prints: <Buffer 08 07 06 05>

buf.writeIntBE(value, offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeIntLE(value, offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(6);

buf.writeIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(6);

buf.writeIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

buf.writeUInt8(value[, offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUint8().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

This function is also available under the writeUint8 alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

console.log(buf);
// Prints: <Buffer 03 04 23 42>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

console.log(buf);
// Prints: <Buffer 03 04 23 42>

buf.writeUInt16BE(value[, offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUint16BE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. The value must be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

This function is also available under the writeUint16BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer de ad be ef>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer de ad be ef>

buf.writeUInt16LE(value[, offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUint16LE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. The value must be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

This function is also available under the writeUint16LE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer ad de ef be>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer ad de ef be>

buf.writeUInt32BE(value[, offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUint32BE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as big-endian. The value must be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

This function is also available under the writeUint32BE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer fe ed fa ce>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer fe ed fa ce>

buf.writeUInt32LE(value[, offset])#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUint32LE().

v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset as little-endian. The value must be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

This function is also available under the writeUint32LE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32LE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer ce fa ed fe>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32LE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer ce fa ed fe>

buf.writeUIntBE(value, offset, byteLength)#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUintBE().

v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

This function is also available under the writeUintBE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeUIntLE(value, offset, byteLength)#

History
VersionChanges
v14.9.0, v12.19.0

This function is also available as buf.writeUintLE().

v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

This function is also available under the writeUintLE alias.

import { Buffer } from 'node:buffer';

const buf = Buffer.allocUnsafe(6);

buf.writeUIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>const { Buffer } = require('node:buffer');

const buf = Buffer.allocUnsafe(6);

buf.writeUIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

new Buffer(array)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(array) instead.

See Buffer.from(array).

new Buffer(arrayBuffer[, byteOffset[, length]])#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

The byteOffset and length parameters are supported now.

v6.0.0

Deprecated since: v6.0.0

v3.0.0

Added in: v3.0.0

Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset[, length]]) instead.

See Buffer.from(arrayBuffer[, byteOffset[, length]]).

new Buffer(buffer)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(buffer) instead.

See Buffer.from(buffer).

new Buffer(size)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v8.0.0

The new Buffer(size) will return zero-filled memory by default.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see Buffer.allocUnsafe()).

  • size <integer> The desired length of the new Buffer.

See Buffer.alloc() and Buffer.allocUnsafe(). This variant of the constructor is equivalent to Buffer.alloc().

new Buffer(string[, encoding])#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(string[, encoding]) instead.

  • string <string> String to encode.
  • encoding <string> The encoding of string. Default: 'utf8'.

See Buffer.from(string[, encoding]).

Class: File#

Added in: v19.2.0

Stability: 1 - Experimental

A File provides information about files.

new buffer.File(sources, fileName[, options])#

Added in: v19.2.0

file.name#

Added in: v19.2.0

The name of the File.

file.lastModified#

Added in: v19.2.0

The last modified date of the File.

node:buffer module APIs#

While, the Buffer object is available as a global, there are additional Buffer-related APIs that are available only via the node:buffer module accessed using require('node:buffer').

buffer.atob(data)#

Added in: v15.13.0, v14.17.0

Stability: 3 - Legacy. Use Buffer.from(data, 'base64') instead.

  • data <any> The Base64-encoded input string.

Decodes a string of Base64-encoded data into bytes, and encodes those bytes into a string using Latin-1 (ISO-8859-1).

The data may be any JavaScript-value that can be coerced into a string.

This function is only provided for compatibility with legacy web platform APIs and should never be used in new code, because they use strings to represent binary data and predate the introduction of typed arrays in JavaScript. For code running using Node.js APIs, converting between base64-encoded strings and binary data should be performed using Buffer.from(str, 'base64') and buf.toString('base64').

buffer.btoa(data)#

Added in: v15.13.0, v14.17.0

Stability: 3 - Legacy. Use buf.toString('base64') instead.

  • data <any> An ASCII (Latin1) string.

Decodes a string into bytes using Latin-1 (ISO-8859), and encodes those bytes into a string using Base64.

The data may be any JavaScript-value that can be coerced into a string.

This function is only provided for compatibility with legacy web platform APIs and should never be used in new code, because they use strings to represent binary data and predate the introduction of typed arrays in JavaScript. For code running using Node.js APIs, converting between base64-encoded strings and binary data should be performed using Buffer.from(str, 'base64') and buf.toString('base64').

buffer.isUtf8(input)#

Added in: v19.4.0

This function returns true if input contains only valid UTF-8-encoded data, including the case in which input is empty.

Throws if the input is a detached array buffer.

buffer.INSPECT_MAX_BYTES#

Added in: v0.5.4

Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior.

buffer.kMaxLength#

Added in: v3.0.0
  • <integer> The largest size allowed for a single Buffer instance.

An alias for buffer.constants.MAX_LENGTH.

buffer.kStringMaxLength#

Added in: v3.0.0
  • <integer> The largest length allowed for a single string instance.

An alias for buffer.constants.MAX_STRING_LENGTH.

buffer.resolveObjectURL(id)#

Added in: v16.7.0

Stability: 1 - Experimental

  • id <string> A 'blob:nodedata:... URL string returned by a prior call to URL.createObjectURL().
  • Returns: <Blob>

Resolves a 'blob:nodedata:...' an associated <Blob> object registered using a prior call to URL.createObjectURL().

buffer.transcode(source, fromEnc, toEnc)#

History
VersionChanges
v8.0.0

The source parameter can now be a Uint8Array.

v7.1.0

Added in: v7.1.0

Re-encodes the given Buffer or Uint8Array instance from one character encoding to another. Returns a new Buffer instance.

Throws if the fromEnc or toEnc specify invalid character encodings or if conversion from fromEnc to toEnc is not permitted.

Encodings supported by buffer.transcode() are: 'ascii', 'utf8', 'utf16le', 'ucs2', 'latin1', and 'binary'.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

import { Buffer, transcode } from 'node:buffer';

const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'const { Buffer, transcode } = require('node:buffer');

const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'

Because the Euro (€) sign is not representable in US-ASCII, it is replaced with ? in the transcoded Buffer.

Class: SlowBuffer#

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

See Buffer.allocUnsafeSlow(). This was never a class in the sense that the constructor always returned a Buffer instance, rather than a SlowBuffer instance.

new SlowBuffer(size)#
Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

  • size <integer> The desired length of the new SlowBuffer.

See Buffer.allocUnsafeSlow().

Buffer constants#

Added in: v8.2.0
buffer.constants.MAX_LENGTH#
History
VersionChanges
v15.0.0

Value is changed to 232 on 64-bit architectures.

v14.0.0

Value is changed from 231 - 1 to 232 - 1 on 64-bit architectures.

v8.2.0

Added in: v8.2.0

  • <integer> The largest size allowed for a single Buffer instance.

On 32-bit architectures, this value currently is 230 - 1 (about 1 GiB).

On 64-bit architectures, this value currently is 232 (about 4 GiB).

It reflects v8::TypedArray::kMaxLength under the hood.

This value is also available as buffer.kMaxLength.

buffer.constants.MAX_STRING_LENGTH#
Added in: v8.2.0
  • <integer> The largest length allowed for a single string instance.

Represents the largest length that a string primitive can have, counted in UTF-16 code units.

This value may depend on the JS engine that is being used.

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to 6.0.0, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

  • Passing a number as the first argument to Buffer() (e.g. new Buffer(10)) allocates a new Buffer object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be subsequently initialized by using either buf.fill(0) or by writing to the entire Buffer before reading data from the Buffer. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer. Since Node.js 8.0.0, Buffer(num) and new Buffer(num) return a Buffer with initialized memory.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer.

Because the behavior of new Buffer() is different depending on the type of the first argument, security and reliability issues can be inadvertently introduced into applications when argument validation or Buffer initialization is not performed.

For example, if an attacker can cause an application to receive a number where a string is expected, the application may call new Buffer(100) instead of new Buffer("100"), leading it to allocate a 100 byte buffer instead of allocating a 3 byte buffer with content "100". This is commonly possible using JSON API calls. Since JSON distinguishes between numeric and string types, it allows injection of numbers where a naively written application that does not validate its input sufficiently might expect to always receive a string. Before Node.js 8.0.0, the 100 byte buffer might contain arbitrary pre-existing in-memory data, so may be used to expose in-memory secrets to a remote attacker. Since Node.js 8.0.0, exposure of memory cannot occur because the data is zero-filled. However, other attacks are still possible, such as causing very large buffers to be allocated by the server, leading to performance degradation or crashing on memory exhaustion.

To make the creation of Buffer instances more reliable and less error-prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe() and Buffer.from(array) may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command-line option#

Added in: v5.10.0

Node.js can be started using the --zero-fill-buffers command-line option to cause all newly-allocated Buffer instances to be zero-filled upon creation by default. Without the option, buffers created with Buffer.allocUnsafe(), Buffer.allocUnsafeSlow(), and new SlowBuffer(size) are not zero-filled. Use of this flag can have a measurable negative impact on performance. Use the --zero-fill-buffers option only when necessary to enforce that newly allocated Buffer instances cannot contain old data that is potentially sensitive.

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.

C++ addons#

Addons are dynamically-linked shared objects written in C++. The require() function can load addons as ordinary Node.js modules. Addons provide an interface between JavaScript and C/C++ libraries.

There are three options for implementing addons: Node-API, nan, or direct use of internal V8, libuv, and Node.js libraries. Unless there is a need for direct access to functionality which is not exposed by Node-API, use Node-API. Refer to C/C++ addons with Node-API for more information on Node-API.

When not using Node-API, implementing addons is complicated, involving knowledge of several components and APIs:

  • V8: the C++ library Node.js uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node.js source tree), which is also available online.

  • libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the file system, sockets, timers, and system events. libuv also provides a threading abstraction similar to POSIX threads for more sophisticated asynchronous addons that need to move beyond the standard event loop. Addon authors should avoid blocking the event loop with I/O or other time-intensive tasks by offloading work via libuv to non-blocking system operations, worker threads, or a custom use of libuv threads.

  • Internal Node.js libraries. Node.js itself exports C++ APIs that addons can use, the most important of which is the node::ObjectWrap class.

  • Node.js includes other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the libuv, OpenSSL, V8, and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by addons. See Linking to libraries included with Node.js for additional information.

All of the following examples are available for download and may be used as the starting-point for an addon.

Hello world#

This "Hello world" example is a simple addon, written in C++, that is the equivalent of the following JavaScript code:

module.exports.hello = () => 'world';

First, create the file hello.cc:

// hello.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void Method(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(
      isolate, "world").ToLocalChecked());
}

void Initialize(Local<Object> exports) {
  NODE_SET_METHOD(exports, "hello", Method);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)

}  // namespace demo

All Node.js addons must export an initialization function following the pattern:

void Initialize(Local<Object> exports);
NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)

There is no semi-colon after NODE_MODULE as it's not a function (see node.h).

The module_name must match the filename of the final binary (excluding the .node suffix).

In the hello.cc example, then, the initialization function is Initialize and the addon module name is addon.

When building addons with node-gyp, using the macro NODE_GYP_MODULE_NAME as the first parameter of NODE_MODULE() will ensure that the name of the final binary will be passed to NODE_MODULE().

Addons defined with NODE_MODULE() can not be loaded in multiple contexts or multiple threads at the same time.

Context-aware addons#

There are environments in which Node.js addons may need to be loaded multiple times in multiple contexts. For example, the Electron runtime runs multiple instances of Node.js in a single process. Each instance will have its own require() cache, and thus each instance will need a native addon to behave correctly when loaded via require(). This means that the addon must support multiple initializations.

A context-aware addon can be constructed by using the macro NODE_MODULE_INITIALIZER, which expands to the name of a function which Node.js will expect to find when it loads an addon. An addon can thus be initialized as in the following example:

using namespace v8;

extern "C" NODE_MODULE_EXPORT void
NODE_MODULE_INITIALIZER(Local<Object> exports,
                        Local<Value> module,
                        Local<Context> context) {
  /* Perform addon initialization steps here. */
}

Another option is to use the macro NODE_MODULE_INIT(), which will also construct a context-aware addon. Unlike NODE_MODULE(), which is used to construct an addon around a given addon initializer function, NODE_MODULE_INIT() serves as the declaration of such an initializer to be followed by a function body.

The following three variables may be used inside the function body following an invocation of NODE_MODULE_INIT():

  • Local<Object> exports,
  • Local<Value> module, and
  • Local<Context> context

The choice to build a context-aware addon carries with it the responsibility of carefully managing global static data. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created.

The context-aware addon can be structured to avoid global static data by performing the following steps:

  • Define a class which will hold per-addon-instance data and which has a static member of the form 
    static void DeleteInstance(void* data) {
  // Cast `data` to an instance of the class and delete it.
}
  • Heap-allocate an instance of this class in the addon initializer. This can be accomplished using the new keyword.
  • Call node::AddEnvironmentCleanupHook(), passing it the above-created instance and a pointer to DeleteInstance(). This will ensure the instance is deleted when the environment is torn down.
  • Store the instance of the class in a v8::External, and
  • Pass the v8::External to all methods exposed to JavaScript by passing it to v8::FunctionTemplate::New() or v8::Function::New() which creates the native-backed JavaScript functions. The third parameter of v8::FunctionTemplate::New() or v8::Function::New() accepts the v8::External and makes it available in the native callback using the v8::FunctionCallbackInfo::Data() method.

This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create.

The following example illustrates the implementation of a context-aware addon:

#include <node.h>

using namespace v8;

class AddonData {
 public:
  explicit AddonData(Isolate* isolate):
      call_count(0) {
    // Ensure this per-addon-instance data is deleted at environment cleanup.
    node::AddEnvironmentCleanupHook(isolate, DeleteInstance, this);
  }

  // Per-addon data.
  int call_count;

  static void DeleteInstance(void* data) {
    delete static_cast<AddonData*>(data);
  }
};

static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
  // Retrieve the per-addon-instance data.
  AddonData* data =
      reinterpret_cast<AddonData*>(info.Data().As<External>()->Value());
  data->call_count++;
  info.GetReturnValue().Set((double)data->call_count);
}

// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
  Isolate* isolate = context->GetIsolate();

  // Create a new instance of `AddonData` for this instance of the addon and
  // tie its life cycle to that of the Node.js environment.
  AddonData* data = new AddonData(isolate);

  // Wrap the data in a `v8::External` so we can pass it to the method we
  // expose.
  Local<External> external = External::New(isolate, data);

  // Expose the method `Method` to JavaScript, and make sure it receives the
  // per-addon-instance data we created above by passing `external` as the
  // third parameter to the `FunctionTemplate` constructor.
  exports->Set(context,
               String::NewFromUtf8(isolate, "method").ToLocalChecked(),
               FunctionTemplate::New(isolate, Method, external)
                  ->GetFunction(context).ToLocalChecked()).FromJust();
}
Worker support#
History
VersionChanges
v14.8.0, v12.19.0

Cleanup hooks may now be asynchronous.

In order to be loaded from multiple Node.js environments, such as a main thread and a Worker thread, an add-on needs to either:

  • Be an Node-API addon, or
  • Be declared as context-aware using NODE_MODULE_INIT() as described above

In order to support Worker threads, addons need to clean up any resources they may have allocated when such a thread exists. This can be achieved through the usage of the AddEnvironmentCleanupHook() function:

void AddEnvironmentCleanupHook(v8::Isolate* isolate,
                               void (*fun)(void* arg),
                               void* arg);

This function adds a hook that will run before a given Node.js instance shuts down. If necessary, such hooks can be removed before they are run using RemoveEnvironmentCleanupHook(), which has the same signature. Callbacks are run in last-in first-out order.

If necessary, there is an additional pair of AddEnvironmentCleanupHook() and RemoveEnvironmentCleanupHook() overloads, where the cleanup hook takes a callback function. This can be used for shutting down asynchronous resources, such as any libuv handles registered by the addon.

The following addon.cc uses AddEnvironmentCleanupHook:

// addon.cc
#include <node.h>
#include <assert.h>
#include <stdlib.h>

using node::AddEnvironmentCleanupHook;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;

// Note: In a real-world application, do not rely on static/global data.
static char cookie[] = "yum yum";
static int cleanup_cb1_called = 0;
static int cleanup_cb2_called = 0;

static void cleanup_cb1(void* arg) {
  Isolate* isolate = static_cast<Isolate*>(arg);
  HandleScope scope(isolate);
  Local<Object> obj = Object::New(isolate);
  assert(!obj.IsEmpty());  // assert VM is still alive
  assert(obj->IsObject());
  cleanup_cb1_called++;
}

static void cleanup_cb2(void* arg) {
  assert(arg == static_cast<void*>(cookie));
  cleanup_cb2_called++;
}

static void sanity_check(void*) {
  assert(cleanup_cb1_called == 1);
  assert(cleanup_cb2_called == 1);
}

// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
  Isolate* isolate = context->GetIsolate();

  AddEnvironmentCleanupHook(isolate, sanity_check, nullptr);
  AddEnvironmentCleanupHook(isolate, cleanup_cb2, cookie);
  AddEnvironmentCleanupHook(isolate, cleanup_cb1, isolate);
}

Test in JavaScript by running:

// test.js
require('./build/Release/addon');

Building#

Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of the module using a JSON-like format. This file is used by node-gyp, a tool written specifically to compile Node.js addons.

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "hello.cc" ]
    }
  ]
}

A version of the node-gyp utility is bundled and distributed with Node.js as part of npm. This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp. See the node-gyp installation instructions for more information, including platform-specific requirements.

Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory.

Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory.

When using npm install to install a Node.js addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the addon for the user's platform on demand.

Once built, the binary addon can be used from within Node.js by pointing require() to the built addon.node module:

// hello.js
const addon = require('./build/Release/addon');

console.log(addon.hello());
// Prints: 'world'

Because the exact path to the compiled addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/), addons can use the bindings package to load the compiled module.

While the bindings package implementation is more sophisticated in how it locates addon modules, it is essentially using a try…catch pattern similar to:

try {
  return require('./build/Release/addon.node');
} catch (err) {
  return require('./build/Debug/addon.node');
}

Linking to libraries included with Node.js#

Node.js uses statically linked libraries such as V8, libuv, and OpenSSL. All addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h>) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

  • When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

  • node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the addon will have access to the full set of dependencies.

Loading addons using require()#

The filename extension of the compiled addon binary is .node (as opposed to .dll or .so). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries.

When calling require(), the .node extension can usually be omitted and Node.js will still find and initialize the addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node, then require('addon') will give precedence to the addon.js file and load it instead.

Native abstractions for Node.js#

Each of the examples illustrated in this document directly use the Node.js and V8 APIs for implementing addons. The V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or nan) provide a set of tools that addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used.

Node-API#

Stability: 2 - Stable

Node-API is an API for building native addons. It is independent from the underlying JavaScript runtime (e.g. V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the Node-API are used.

Creating and maintaining an addon that benefits from the ABI stability provided by Node-API carries with it certain implementation considerations.

To use Node-API in the above "Hello world" example, replace the content of hello.cc with the following. All other instructions remain the same.

// hello.cc using Node-API
#include <node_api.h>

namespace demo {

napi_value Method(napi_env env, napi_callback_info args) {
  napi_value greeting;
  napi_status status;

  status = napi_create_string_utf8(env, "world", NAPI_AUTO_LENGTH, &greeting);
  if (status != napi_ok) return nullptr;
  return greeting;
}

napi_value init(napi_env env, napi_value exports) {
  napi_status status;
  napi_value fn;

  status = napi_create_function(env, nullptr, 0, Method, nullptr, &fn);
  if (status != napi_ok) return nullptr;

  status = napi_set_named_property(env, exports, "hello", fn);
  if (status != napi_ok) return nullptr;
  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, init)

}  // namespace demo

The functions available and how to use them are documented in C/C++ addons with Node-API.

Addon examples#

Following are some example addons intended to help developers get started. The examples use the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples using the following binding.gyp file:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "addon.cc" ]
    }
  ]
}

In cases where there is more than one .cc file, simply add the additional filename to the sources array:

"sources": ["addon.cc", "myexample.cc"]

Once the binding.gyp file is ready, the example addons can be configured and built using node-gyp:

$ node-gyp configure build

Function arguments#

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

// addon.cc
#include <node.h>

namespace demo {

using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    // Throw an Error that is passed back to JavaScript
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong number of arguments").ToLocalChecked()));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong arguments").ToLocalChecked()));
    return;
  }

  // Perform the operation
  double value =
      args[0].As<Number>()->Value() + args[1].As<Number>()->Value();
  Local<Number> num = Number::New(isolate, value);

  // Set the return value (using the passed in
  // FunctionCallbackInfo<Value>&)
  args.GetReturnValue().Set(num);
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

Once compiled, the example addon can be required and used from within Node.js:

// test.js
const addon = require('./build/Release/addon');

console.log('This should be eight:', addon.add(3, 5));

Callbacks#

It is common practice within addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;

void RunCallback(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();
  Local<Function> cb = Local<Function>::Cast(args[0]);
  const unsigned argc = 1;
  Local<Value> argv[argc] = {
      String::NewFromUtf8(isolate,
                          "hello world").ToLocalChecked() };
  cb->Call(context, Null(isolate), argc, argv).ToLocalChecked();
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", RunCallback);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

This example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the addon to completely overwrite exports with a single function instead of adding the function as a property of exports.

To test it, run the following JavaScript:

// test.js
const addon = require('./build/Release/addon');

addon((msg) => {
  console.log(msg);
// Prints: 'hello world'
});

In this example, the callback function is invoked synchronously.

Object factory#

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject():

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  Local<Object> obj = Object::New(isolate);
  obj->Set(context,
           String::NewFromUtf8(isolate,
                               "msg").ToLocalChecked(),
                               args[0]->ToString(context).ToLocalChecked())
           .FromJust();

  args.GetReturnValue().Set(obj);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

To test it in JavaScript:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'

Function factory#

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void MyFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(
      isolate, "hello world").ToLocalChecked());
}

void CreateFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<Context> context = isolate->GetCurrentContext();
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
  Local<Function> fn = tpl->GetFunction(context).ToLocalChecked();

  // omit this to make it anonymous
  fn->SetName(String::NewFromUtf8(
      isolate, "theFunction").ToLocalChecked());

  args.GetReturnValue().Set(fn);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateFunction);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

To test:

// test.js
const addon = require('./build/Release/addon');

const fn = addon();
console.log(fn());
// Prints: 'hello world'

Wrapping C++ objects#

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Local;
using v8::Object;

void InitAll(Local<Object> exports) {
  MyObject::Init(exports);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

Then, in myobject.h, the wrapper class inherits from node::ObjectWrap:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Local<v8::Object> exports);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);

  double value_;
};

}  // namespace demo

#endif

In myobject.cc, implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype:

// myobject.cc
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::ObjectTemplate;
using v8::String;
using v8::Value;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Local<Object> exports) {
  Isolate* isolate = exports->GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  Local<ObjectTemplate> addon_data_tpl = ObjectTemplate::New(isolate);
  addon_data_tpl->SetInternalFieldCount(1);  // 1 field for the MyObject::New()
  Local<Object> addon_data =
      addon_data_tpl->NewInstance(context).ToLocalChecked();

  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New, addon_data);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  Local<Function> constructor = tpl->GetFunction(context).ToLocalChecked();
  addon_data->SetInternalField(0, constructor);
  exports->Set(context, String::NewFromUtf8(
      isolate, "MyObject").ToLocalChecked(),
      constructor).FromJust();
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons =
        args.Data().As<Object>()->GetInternalField(0).As<Function>();
    Local<Object> result =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(result);
  }
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

To build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing.

During shutdown of the process or worker threads destructors are not called by the JS engine. Therefore it's the responsibility of the user to track these objects and ensure proper destruction to avoid resource leaks.

Factory of wrapped objects#

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator:

const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();

First, the createObject() method is implemented in addon.cc:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void InitAll(Local<Object> exports, Local<Object> module) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

In myobject.h, the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Global<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation in myobject.cc is similar to the previous example:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  Local<Context> context = isolate->GetCurrentContext();
  constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

  AddEnvironmentCleanupHook(isolate, [](void*) {
    constructor.Reset();
  }, nullptr);
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

Once again, to build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const createObject = require('./build/Release/addon');

const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23

Passing wrapped objects around#

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap. The following examples shows a function add() that can take two MyObject objects as input arguments:

// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
      args[0]->ToObject(context).ToLocalChecked());
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
      args[1]->ToObject(context).ToLocalChecked());

  double sum = obj1->value() + obj2->value();
  args.GetReturnValue().Set(Number::New(isolate, sum));
}

void InitAll(Local<Object> exports) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(exports, "createObject", CreateObject);
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

In myobject.h, a new public method is added to allow access to private values after unwrapping the object.

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
  inline double value() const { return value_; }

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Global<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation of myobject.cc is similar to before:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  Local<Context> context = isolate->GetCurrentContext();
  constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

  AddEnvironmentCleanupHook(isolate, [](void*) {
    constructor.Reset();
  }, nullptr);
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

}  // namespace demo

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);

console.log(result);
// Prints: 30

Node-API#

Stability: 2 - Stable

Node-API (formerly N-API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (for example, V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The ABI Stability guide provides a more in-depth explanation.

Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in Node-API are used.

APIs exposed by Node-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA-262 Language Specification. The APIs have the following properties:

  • All Node-API calls return a status code of type napi_status. This status indicates whether the API call succeeded or failed.
  • The API's return value is passed via an out parameter.
  • All JavaScript values are abstracted behind an opaque type named napi_value.
  • In case of an error status code, additional information can be obtained using napi_get_last_error_info. More information can be found in the error handling section Error handling.

Node-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. A C++ API can be easier to use. To support using C++, the project maintains a C++ wrapper module called node-addon-api. This wrapper provides an inlineable C++ API. Binaries built with node-addon-api will depend on the symbols for the Node-API C-based functions exported by Node.js. node-addon-api is a more efficient way to write code that calls Node-API. Take, for example, the following node-addon-api code. The first section shows the node-addon-api code and the second section shows what actually gets used in the addon.

Object obj = Object::New(env);
obj["foo"] = String::New(env, "bar");
napi_status status;
napi_value object, string;
status = napi_create_object(env, &object);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_set_named_property(env, object, "foo", string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

The end result is that the addon only uses the exported C APIs. As a result, it still gets the benefits of the ABI stability provided by the C API.

When using node-addon-api instead of the C APIs, start with the API docs for node-addon-api.

The Node-API Resource offers an excellent orientation and tips for developers just getting started with Node-API and node-addon-api. Additional media resources can be found on the Node-API Media page.

Implications of ABI stability#

Although Node-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:

  • the Node.js C++ APIs available via any of

    #include <node.h>
#include <node_buffer.h>
#include <node_version.h>
#include <node_object_wrap.h>

  • the libuv APIs which are also included with Node.js and available via

    #include <uv.h>

  • the V8 API available via

    #include <v8.h>

Thus, for an addon to remain ABI-compatible across Node.js major versions, it must use Node-API exclusively by restricting itself to using

#include <node_api.h>

and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to Node-API.

Building#

Unlike modules written in JavaScript, developing and deploying Node.js native addons using Node-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.

For Linux developers, the necessary C/C++ toolchain packages are readily available. GCC is widely used in the Node.js community to build and test across a variety of platforms. For many developers, the LLVM compiler infrastructure is also a good choice.

For Mac developers, Xcode offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:

xcode-select --install

For Windows developers, Visual Studio offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:

npm install --global windows-build-tools

The sections below describe the additional tools available for developing and deploying Node.js native addons.

Build tools#

Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.

node-gyp#

node-gyp is a build system based on the gyp-next fork of Google's GYP tool and comes bundled with npm. GYP, and therefore node-gyp, requires that Python be installed.

Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.

CMake.js#

CMake.js is an alternative build system based on CMake.

CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp.

Uploading precompiled binaries#

The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like Travis CI and AppVeyor to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.

node-pre-gyp#

node-pre-gyp is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.

prebuild#

prebuild is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to GitHub releases. prebuild is a good choice for GitHub projects using CMake.js.

prebuildify#

prebuildify is a tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native addon when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native addon is installed.

Usage#

In order to use the Node-API functions, include the file node_api.h which is located in the src directory in the node development tree:

#include <node_api.h>

This will opt into the default NAPI_VERSION for the given release of Node.js. In order to ensure compatibility with specific versions of Node-API, the version can be specified explicitly when including the header:

#define NAPI_VERSION 3
#include <node_api.h>

This restricts the Node-API surface to just the functionality that was available in the specified (and earlier) versions.

Some of the Node-API surface is experimental and requires explicit opt-in:

#define NAPI_EXPERIMENTAL
#include <node_api.h>

In this case the entire API surface, including any experimental APIs, will be available to the module code.

Node-API version matrix#

Node-API versions are additive and versioned independently from Node.js. Version 4 is an extension to version 3 in that it has all of the APIs from version 3 with some additions. This means that it is not necessary to recompile for new versions of Node.js which are listed as supporting a later version.

1 2 3
v6.x v6.14.2*
v8.x v8.6.0** v8.10.0* v8.11.2
v9.x v9.0.0* v9.3.0* v9.11.0*
≥ v10.x all releases all releases all releases
4 5 6 7 8
v10.x v10.16.0 v10.17.0 v10.20.0 v10.23.0
v11.x v11.8.0
v12.x v12.0.0 v12.11.0 v12.17.0 v12.19.0 v12.22.0
v13.x v13.0.0 v13.0.0
v14.x v14.0.0 v14.0.0 v14.0.0 v14.12.0 v14.17.0
v15.x v15.0.0 v15.0.0 v15.0.0 v15.0.0 v15.12.0
v16.x v16.0.0 v16.0.0 v16.0.0 v16.0.0 v16.0.0

* Node-API was experimental.

** Node.js 8.0.0 included Node-API as experimental. It was released as Node-API version 1 but continued to evolve until Node.js 8.6.0. The API is different in versions prior to Node.js 8.6.0. We recommend Node-API version 3 or later.

Each API documented for Node-API will have a header named added in:, and APIs which are stable will have the additional header Node-API version:. APIs are directly usable when using a Node.js version which supports the Node-API version shown in Node-API version: or higher. When using a Node.js version that does not support the Node-API version: listed or if there is no Node-API version: listed, then the API will only be available if #define NAPI_EXPERIMENTAL precedes the inclusion of node_api.h or js_native_api.h. If an API appears not to be available on a version of Node.js which is later than the one shown in added in: then this is most likely the reason for the apparent absence.

The Node-APIs associated strictly with accessing ECMAScript features from native code can be found separately in js_native_api.h and js_native_api_types.h. The APIs defined in these headers are included in node_api.h and node_api_types.h. The headers are structured in this way in order to allow implementations of Node-API outside of Node.js. For those implementations the Node.js specific APIs may not be applicable.

The Node.js-specific parts of an addon can be separated from the code that exposes the actual functionality to the JavaScript environment so that the latter may be used with multiple implementations of Node-API. In the example below, addon.c and addon.h refer only to js_native_api.h. This ensures that addon.c can be reused to compile against either the Node.js implementation of Node-API or any implementation of Node-API outside of Node.js.

addon_node.c is a separate file that contains the Node.js specific entry point to the addon and which instantiates the addon by calling into addon.c when the addon is loaded into a Node.js environment.

// addon.h
#ifndef _ADDON_H_
#define _ADDON_H_
#include <js_native_api.h>
napi_value create_addon(napi_env env);
#endif  // _ADDON_H_
// addon.c
#include "addon.h"

#define NAPI_CALL(env, call)                                      \
  do {                                                            \
    napi_status status = (call);                                  \
    if (status != napi_ok) {                                      \
      const napi_extended_error_info* error_info = NULL;          \
      napi_get_last_error_info((env), &error_info);               \
      const char* err_message = error_info->error_message;        \
      bool is_pending;                                            \
      napi_is_exception_pending((env), &is_pending);              \
      if (!is_pending) {                                          \
        const char* message = (err_message == NULL)               \
            ? "empty error message"                               \
            : err_message;                                        \
        napi_throw_error((env), NULL, message);                   \
        return NULL;                                              \
      }                                                           \
    }                                                             \
  } while(0)

static napi_value
DoSomethingUseful(napi_env env, napi_callback_info info) {
  // Do something useful.
  return NULL;
}

napi_value create_addon(napi_env env) {
  napi_value result;
  NAPI_CALL(env, napi_create_object(env, &result));

  napi_value exported_function;
  NAPI_CALL(env, napi_create_function(env,
                                      "doSomethingUseful",
                                      NAPI_AUTO_LENGTH,
                                      DoSomethingUseful,
                                      NULL,
                                      &exported_function));

  NAPI_CALL(env, napi_set_named_property(env,
                                         result,
                                         "doSomethingUseful",
                                         exported_function));

  return result;
}
// addon_node.c
#include <node_api.h>
#include "addon.h"

NAPI_MODULE_INIT() {
  // This function body is expected to return a `napi_value`.
  // The variables `napi_env env` and `napi_value exports` may be used within
  // the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
  return create_addon(env);
}

Environment life cycle APIs#

Section 8.7 of the ECMAScript Language Specification defines the concept of an "Agent" as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.

A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as worker threads. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.

From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.

Native addons may need to allocate global state which they use during their entire life cycle such that the state must be unique to each instance of the addon.

To this end, Node-API provides a way to allocate data such that its life cycle is tied to the life cycle of the Agent.

napi_set_instance_data#

Added in: v12.8.0, v10.20.0 N-API version: 6 
napi_status napi_set_instance_data(napi_env env,
                                   void* data,
                                   napi_finalize finalize_cb,
                                   void* finalize_hint);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] data: The data item to make available to bindings of this instance.
  • [in] finalize_cb: The function to call when the environment is being torn down. The function receives data so that it might free it. napi_finalize provides more details.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.

Returns napi_ok if the API succeeded.

This API associates data with the currently running Agent. data can later be retrieved using napi_get_instance_data(). Any existing data associated with the currently running Agent which was set by means of a previous call to napi_set_instance_data() will be overwritten. If a finalize_cb was provided by the previous call, it will not be called.

napi_get_instance_data#

Added in: v12.8.0, v10.20.0 N-API version: 6 
napi_status napi_get_instance_data(napi_env env,
                                   void** data);
  • [in] env: The environment that the Node-API call is invoked under.
  • [out] data: The data item that was previously associated with the currently running Agent by a call to napi_set_instance_data().

Returns napi_ok if the API succeeded.

This API retrieves data that was previously associated with the currently running Agent via napi_set_instance_data(). If no data is set, the call will succeed and data will be set to NULL.

Basic Node-API data types#

Node-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other Node-API calls.

napi_status#

Added in: v8.0.0 N-API version: 1

Integral status code indicating the success or failure of a Node-API call. Currently, the following status codes are supported.

typedef enum {
  napi_ok,
  napi_invalid_arg,
  napi_object_expected,
  napi_string_expected,
  napi_name_expected,
  napi_function_expected,
  napi_number_expected,
  napi_boolean_expected,
  napi_array_expected,
  napi_generic_failure,
  napi_pending_exception,
  napi_cancelled,
  napi_escape_called_twice,
  napi_handle_scope_mismatch,
  napi_callback_scope_mismatch,
  napi_queue_full,
  napi_closing,
  napi_bigint_expected,
  napi_date_expected,
  napi_arraybuffer_expected,
  napi_detachable_arraybuffer_expected,
  napi_would_deadlock,  /* unused */
  napi_no_external_buffers_allowed
} napi_status;

If additional information is required upon an API returning a failed status, it can be obtained by calling napi_get_last_error_info.

napi_extended_error_info#

Added in: v8.0.0 N-API version: 1 
typedef struct {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
} napi_extended_error_info;
  • error_message: UTF8-encoded string containing a VM-neutral description of the error.
  • engine_reserved: Reserved for VM-specific error details. This is currently not implemented for any VM.
  • engine_error_code: VM-specific error code. This is currently not implemented for any VM.
  • error_code: The Node-API status code that originated with the last error.

See the Error handling section for additional information.

napi_env#

napi_env is used to represent a context that the underlying Node-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making Node-API calls. Specifically, the same napi_env that was passed in when the initial native function was called must be passed to any subsequent nested Node-API calls. Caching the napi_env for the purpose of general reuse, and passing the napi_env between instances of the same addon running on different Worker threads is not allowed. The napi_env becomes invalid when an instance of a native addon is unloaded. Notification of this event is delivered through the callbacks given to napi_add_env_cleanup_hook and napi_set_instance_data.

napi_value#

This is an opaque pointer that is used to represent a JavaScript value.

napi_threadsafe_function#

Added in: v10.6.0 N-API version: 4

This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via napi_call_threadsafe_function().

napi_threadsafe_function_release_mode#

Added in: v10.6.0 N-API version: 4

A value to be given to napi_release_threadsafe_function() to indicate whether the thread-safe function is to be closed immediately (napi_tsfn_abort) or merely released (napi_tsfn_release) and thus available for subsequent use via napi_acquire_threadsafe_function() and napi_call_threadsafe_function().

typedef enum {
  napi_tsfn_release,
  napi_tsfn_abort
} napi_threadsafe_function_release_mode;

napi_threadsafe_function_call_mode#

Added in: v10.6.0 N-API version: 4

A value to be given to napi_call_threadsafe_function() to indicate whether the call should block whenever the queue associated with the thread-safe function is full.

typedef enum {
  napi_tsfn_nonblocking,
  napi_tsfn_blocking
} napi_threadsafe_function_call_mode;

Node-API memory management types#

napi_handle_scope#

This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, Node-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, Node-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.

Handle scopes are created using napi_open_handle_scope and are destroyed using napi_close_handle_scope. Closing the scope can indicate to the GC that all napi_values created during the lifetime of the handle scope are no longer referenced from the current stack frame.

For more details, review the Object lifetime management.

napi_escapable_handle_scope#
Added in: v8.0.0 N-API version: 1

Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.

napi_ref#
Added in: v8.0.0 N-API version: 1

This is the abstraction to use to reference a napi_value. This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly.

For more details, review the Object lifetime management.

napi_type_tag#
Added in: v14.8.0, v12.19.0 N-API version: 8

A 128-bit value stored as two unsigned 64-bit integers. It serves as a UUID with which JavaScript objects can be "tagged" in order to ensure that they are of a certain type. This is a stronger check than napi_instanceof, because the latter can report a false positive if the object's prototype has been manipulated. Type-tagging is most useful in conjunction with napi_wrap because it ensures that the pointer retrieved from a wrapped object can be safely cast to the native type corresponding to the type tag that had been previously applied to the JavaScript object.

typedef struct {
  uint64_t lower;
  uint64_t upper;
} napi_type_tag;
napi_async_cleanup_hook_handle#
Added in: v14.10.0, v12.19.0

An opaque value returned by napi_add_async_cleanup_hook. It must be passed to napi_remove_async_cleanup_hook when the chain of asynchronous cleanup events completes.

Node-API callback types#

napi_callback_info#
Added in: v8.0.0 N-API version: 1

Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.

napi_callback#
Added in: v8.0.0 N-API version: 1

Function pointer type for user-provided native functions which are to be exposed to JavaScript via Node-API. Callback functions should satisfy the following signature:

typedef napi_value (*napi_callback)(napi_env, napi_callback_info);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside a napi_callback is not necessary.

napi_finalize#
Added in: v8.0.0 N-API version: 1

Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object with which it was associated with, has been garbage-collected. The user must provide a function satisfying the following signature which would get called upon the object's collection. Currently, napi_finalize can be used for finding out when objects that have external data are collected.

typedef void (*napi_finalize)(napi_env env,
                              void* finalize_data,
                              void* finalize_hint);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_async_execute_callback#
Added in: v8.0.0 N-API version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_execute_callback)(napi_env env, void* data);

Implementations of this function must avoid making Node-API calls that execute JavaScript or interact with JavaScript objects. Node-API calls should be in the napi_async_complete_callback instead. Do not use the napi_env parameter as it will likely result in execution of JavaScript.

napi_async_complete_callback#
Added in: v8.0.0 N-API version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_threadsafe_function_call_js#
Added in: v10.6.0 N-API version: 4

Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via napi_call_function, and then make the call into JavaScript.

The data arriving from the secondary thread via the queue is given in the data parameter and the JavaScript function to call is given in the js_callback parameter.

Node-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via napi_call_function rather than via napi_make_callback.

Callback functions must satisfy the following signature:

typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                 napi_value js_callback,
                                                 void* context,
                                                 void* data);
  • [in] env: The environment to use for API calls, or NULL if the thread-safe function is being torn down and data may need to be freed.
  • [in] js_callback: The JavaScript function to call, or NULL if the thread-safe function is being torn down and data may need to be freed. It may also be NULL if the thread-safe function was created without js_callback.
  • [in] context: The optional data with which the thread-safe function was created.
  • [in] data: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with Node-API functions) that can be passed as parameters when js_callback is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_cleanup_hook#
Added in: v19.2.0 N-API version: 3

Function pointer used with napi_add_env_cleanup_hook. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

typedef void (*napi_cleanup_hook)(void* data);
napi_async_cleanup_hook#
Added in: v14.10.0, v12.19.0

Function pointer used with napi_add_async_cleanup_hook. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

typedef void (*napi_async_cleanup_hook)(napi_async_cleanup_hook_handle handle,
                                        void* data);

The body of the function should initiate the asynchronous cleanup actions at the end of which handle must be passed in a call to napi_remove_async_cleanup_hook.

Error handling#

Node-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.

Return values#

All of the Node-API functions share the same error handling pattern. The return type of all API functions is napi_status.

The return value will be napi_ok if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the napi_status value for the error will be returned. If an exception was thrown, and no error occurred, napi_pending_exception will be returned.

In cases where a return value other than napi_ok or napi_pending_exception is returned, napi_is_exception_pending must be called to check if an exception is pending. See the section on exceptions for more details.

The full set of possible napi_status values is defined in napi_api_types.h.

The napi_status return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information.

In order to retrieve this information napi_get_last_error_info is provided which returns a napi_extended_error_info structure. The format of the napi_extended_error_info structure is as follows:

Added in: v8.0.0 N-API version: 1 
typedef struct napi_extended_error_info {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
};
  • error_message: Textual representation of the error that occurred.
  • engine_reserved: Opaque handle reserved for engine use only.
  • engine_error_code: VM specific error code.
  • error_code: Node-API status code for the last error.

napi_get_last_error_info returns the information for the last Node-API call that was made.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

napi_get_last_error_info#
Added in: v8.0.0 N-API version: 1 
napi_status
napi_get_last_error_info(napi_env env,
                         const napi_extended_error_info** result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The napi_extended_error_info structure with more information about the error.

Returns napi_ok if the API succeeded.

This API retrieves a napi_extended_error_info structure with information about the last error that occurred.

The content of the napi_extended_error_info returned is only valid up until a Node-API function is called on the same env. This includes a call to napi_is_exception_pending so it may often be necessary to make a copy of the information so that it can be used later. The pointer returned in error_message points to a statically-defined string so it is safe to use that pointer if you have copied it out of the error_message field (which will be overwritten) before another Node-API function was called.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

This API can be called even if there is a pending JavaScript exception.

Exceptions#

Any Node-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.

If the napi_status returned by a function is napi_ok then no exception is pending and no additional action is required. If the napi_status returned is anything other than napi_ok or napi_pending_exception, in order to try to recover and continue instead of simply returning immediately, napi_is_exception_pending must be called in order to determine if an exception is pending or not.

In many cases when a Node-API function is called and an exception is already pending, the function will return immediately with a napi_status of napi_pending_exception. However, this is not the case for all functions. Node-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, napi_status will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call.

When an exception is pending one of two approaches can be employed.

The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript, the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most Node-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception, so do as little as possible and then return to JavaScript where the exception can be handled.

The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases napi_get_and_clear_last_exception can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript Object thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with napi_throw where error is the JavaScript value to be thrown.

The following utility functions are also available in case native code needs to throw an exception or determine if a napi_value is an instance of a JavaScript Error object: napi_throw_error, napi_throw_type_error, napi_throw_range_error, node_api_throw_syntax_error and napi_is_error.

The following utility functions are also available in case native code needs to create an Error object: napi_create_error, napi_create_type_error, napi_create_range_error and node_api_create_syntax_error, where result is the napi_value that refers to the newly created JavaScript Error object.

The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with Node-API, both in internal functionality and for module specific functionality (as its good practice), the throw_ and create_ functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is NULL then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be:

originalName [code]

where originalName is the original name associated with the error and code is the code that was provided. For example, if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be:

TypeError [ERR_ERROR_1]
napi_throw#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);
  • [in] env: The environment that the API is invoked under.
  • [in] error: The JavaScript value to be thrown.

Returns napi_ok if the API succeeded.

This API throws the JavaScript value provided.

napi_throw_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_error(napi_env env,
                                         const char* code,
                                         const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript Error with the text provided.

napi_throw_type_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_type_error(napi_env env,
                                              const char* code,
                                              const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript TypeError with the text provided.

napi_throw_range_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_range_error(napi_env env,
                                               const char* code,
                                               const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript RangeError with the text provided.

node_api_throw_syntax_error#
Added in: v17.2.0, v16.14.0

Stability: 1 - Experimental

NAPI_EXTERN napi_status node_api_throw_syntax_error(napi_env env,
                                                    const char* code,
                                                    const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript SyntaxError with the text provided.

napi_is_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_is_error(napi_env env,
                                      napi_value value,
                                      bool* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: The napi_value to be checked.
  • [out] result: Boolean value that is set to true if napi_value represents an error, false otherwise.

Returns napi_ok if the API succeeded.

This API queries a napi_value to check if it represents an error object.

napi_create_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_error(napi_env env,
                                          napi_value code,
                                          napi_value msg,
                                          napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript string to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript Error with the text provided.

napi_create_type_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_type_error(napi_env env,
                                               napi_value code,
                                               napi_value msg,
                                               napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript string to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript TypeError with the text provided.

napi_create_range_error#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_range_error(napi_env env,
                                                napi_value code,
                                                napi_value msg,
                                                napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript string to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript RangeError with the text provided.

node_api_create_syntax_error#
Added in: v17.2.0, v16.14.0

Stability: 1 - Experimental

NAPI_EXTERN napi_status node_api_create_syntax_error(napi_env env,
                                                     napi_value code,
                                                     napi_value msg,
                                                     napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript string to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript SyntaxError with the text provided.

napi_get_and_clear_last_exception#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_and_clear_last_exception(napi_env env,
                                              napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The exception if one is pending, NULL otherwise.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_is_exception_pending#
Added in: v8.0.0 N-API version: 1 
napi_status napi_is_exception_pending(napi_env env, bool* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: Boolean value that is set to true if an exception is pending.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_fatal_exception#
Added in: v9.10.0 N-API version: 3 
napi_status napi_fatal_exception(napi_env env, napi_value err);
  • [in] env: The environment that the API is invoked under.
  • [in] err: The error that is passed to 'uncaughtException'.

Trigger an 'uncaughtException' in JavaScript. Useful if an async callback throws an exception with no way to recover.

Fatal errors#

In the event of an unrecoverable error in a native addon, a fatal error can be thrown to immediately terminate the process.

napi_fatal_error#
Added in: v8.2.0 N-API version: 1 
NAPI_NO_RETURN void napi_fatal_error(const char* location,
                                     size_t location_len,
                                     const char* message,
                                     size_t message_len);
  • [in] location: Optional location at which the error occurred.
  • [in] location_len: The length of the location in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] message: The message associated with the error.
  • [in] message_len: The length of the message in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.

The function call does not return, the process will be terminated.

This API can be called even if there is a pending JavaScript exception.

Object lifetime management#

As Node-API calls are made, handles to objects in the heap for the underlying VM may be returned as napi_values. These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected before the native code was finished using them.

As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.

In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the Node-API functions that can be used to change the handle lifespan from the default.

Making handle lifespan shorter than that of the native method#

It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:

for (int i = 0; i < 1000000; i++) {
  napi_value result;
  napi_status status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
}

This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.

To handle this case, Node-API provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be 'closed' and any handles associated with the scope are invalidated. The methods available to open/close scopes are napi_open_handle_scope and napi_close_handle_scope.

Node-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.

Taking the earlier example, adding calls to napi_open_handle_scope and napi_close_handle_scope would ensure that at most a single handle is valid throughout the execution of the loop:

for (int i = 0; i < 1000000; i++) {
  napi_handle_scope scope;
  napi_status status = napi_open_handle_scope(env, &scope);
  if (status != napi_ok) {
    break;
  }
  napi_value result;
  status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
  status = napi_close_handle_scope(env, scope);
  if (status != napi_ok) {
    break;
  }
}

When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. Node-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.

The methods available to open/close escapable scopes are napi_open_escapable_handle_scope and napi_close_escapable_handle_scope.

The request to promote a handle is made through napi_escape_handle which can only be called once.

napi_open_handle_scope#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env,
                                               napi_handle_scope* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API opens a new scope.

napi_close_handle_scope#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env,
                                                napi_handle_scope scope);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_open_escapable_handle_scope#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status
    napi_open_escapable_handle_scope(napi_env env,
                                     napi_handle_scope* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API opens a new scope from which one object can be promoted to the outer scope.

napi_close_escapable_handle_scope#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status
    napi_close_escapable_handle_scope(napi_env env,
                                      napi_handle_scope scope);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_escape_handle#
Added in: v8.0.0 N-API version: 1 
napi_status napi_escape_handle(napi_env env,
                               napi_escapable_handle_scope scope,
                               napi_value escapee,
                               napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the current scope.
  • [in] escapee: napi_value representing the JavaScript Object to be escaped.
  • [out] result: napi_value representing the handle to the escaped Object in the outer scope.

Returns napi_ok if the API succeeded.

This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.

This API can be called even if there is a pending JavaScript exception.

References to objects with a lifespan longer than that of the native method#

In some cases an addon will need to be able to create and reference objects with a lifespan longer than that of a single native method invocation. For example, to create a constructor and later use that constructor in a request to creates instances, it must be possible to reference the constructor object across many different instance creation requests. This would not be possible with a normal handle returned as a napi_value as described in the earlier section. The lifespan of a normal handle is managed by scopes and all scopes must be closed before the end of a native method.

Node-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.

References can be created with an initial reference count. The count can then be modified through napi_reference_ref and napi_reference_unref. If an object is collected while the count for a reference is 0, all subsequent calls to get the object associated with the reference napi_get_reference_value will return NULL for the returned napi_value. An attempt to call napi_reference_ref for a reference whose object has been collected results in an error.

References must be deleted once they are no longer required by the addon. When a reference is deleted, it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference results in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.

There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count. Multiple persistent references to the same object can result in unexpectedly keeping alive native memory. The native structures for a persistent reference must be kept alive until finalizers for the referenced object are executed. If a new persistent reference is created for the same object, the finalizers for that object will not be run and the native memory pointed by the earlier persistent reference will not be freed. This can be avoided by calling napi_delete_reference in addition to napi_reference_unref when possible.

napi_create_reference#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_reference(napi_env env,
                                              napi_value value,
                                              uint32_t initial_refcount,
                                              napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the Object to which we want a reference.
  • [in] initial_refcount: Initial reference count for the new reference.
  • [out] result: napi_ref pointing to the new reference.

Returns napi_ok if the API succeeded.

This API creates a new reference with the specified reference count to the Object passed in.

napi_delete_reference#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref to be deleted.

Returns napi_ok if the API succeeded.

This API deletes the reference passed in.

This API can be called even if there is a pending JavaScript exception.

napi_reference_ref#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_reference_ref(napi_env env,
                                           napi_ref ref,
                                           uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which the reference count will be incremented.
  • [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API increments the reference count for the reference passed in and returns the resulting reference count.

napi_reference_unref#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_reference_unref(napi_env env,
                                             napi_ref ref,
                                             uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which the reference count will be decremented.
  • [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API decrements the reference count for the reference passed in and returns the resulting reference count.

napi_get_reference_value#
Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_get_reference_value(napi_env env,
                                                 napi_ref ref,
                                                 napi_value* result);

the napi_value passed in or out of these methods is a handle to the object to which the reference is related.

  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which we requesting the corresponding Object.
  • [out] result: The napi_value for the Object referenced by the napi_ref.

Returns napi_ok if the API succeeded.

If still valid, this API returns the napi_value representing the JavaScript Object associated with the napi_ref. Otherwise, result will be NULL.

Cleanup on exit of the current Node.js instance#

While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js instance exits.

Node-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up.

napi_add_env_cleanup_hook#
Added in: v10.2.0 N-API version: 3 
NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env,
                                                  napi_cleanup_hook fun,
                                                  void* arg);

Registers fun as a function to be run with the arg parameter once the current Node.js environment exits.

A function can safely be specified multiple times with different arg values. In that case, it will be called multiple times as well. Providing the same fun and arg values multiple times is not allowed and will lead the process to abort.

The hooks will be called in reverse order, i.e. the most recently added one will be called first.

Removing this hook can be done by using napi_remove_env_cleanup_hook. Typically, that happens when the resource for which this hook was added is being torn down anyway.

For asynchronous cleanup, napi_add_async_cleanup_hook is available.

napi_remove_env_cleanup_hook#
Added in: v10.2.0 N-API version: 3 
NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(napi_env env,
                                                     void (*fun)(void* arg),
                                                     void* arg);

Unregisters fun as a function to be run with the arg parameter once the current Node.js environment exits. Both the argument and the function value need to be exact matches.

The function must have originally been registered with napi_add_env_cleanup_hook, otherwise the process will abort.

napi_add_async_cleanup_hook#
History
VersionChanges
v14.10.0, v12.19.0

Changed signature of the hook callback.

v14.8.0, v12.19.0

Added in: v14.8.0, v12.19.0

N-API version: 8 
NAPI_EXTERN napi_status napi_add_async_cleanup_hook(
    napi_env env,
    napi_async_cleanup_hook hook,
    void* arg,
    napi_async_cleanup_hook_handle* remove_handle);
  • [in] env: The environment that the API is invoked under.
  • [in] hook: The function pointer to call at environment teardown.
  • [in] arg: The pointer to pass to hook when it gets called.
  • [out] remove_handle: Optional handle that refers to the asynchronous cleanup hook.

Registers hook, which is a function of type napi_async_cleanup_hook, as a function to be run with the remove_handle and arg parameters once the current Node.js environment exits.

Unlike napi_add_env_cleanup_hook, the hook is allowed to be asynchronous.

Otherwise, behavior generally matches that of napi_add_env_cleanup_hook.

If remove_handle is not NULL, an opaque value will be stored in it that must later be passed to napi_remove_async_cleanup_hook, regardless of whether the hook has already been invoked. Typically, that happens when the resource for which this hook was added is being torn down anyway.

napi_remove_async_cleanup_hook#
History
VersionChanges
v14.10.0, v12.19.0

Removed env parameter.

v14.8.0, v12.19.0

Added in: v14.8.0, v12.19.0

NAPI_EXTERN napi_status napi_remove_async_cleanup_hook(
    napi_async_cleanup_hook_handle remove_handle);

Unregisters the cleanup hook corresponding to remove_handle. This will prevent the hook from being executed, unless it has already started executing. This must be called on any napi_async_cleanup_hook_handle value obtained from napi_add_async_cleanup_hook.

Module registration#

Node-API modules are registered in a manner similar to other modules except that instead of using the NODE_MODULE macro the following is used:

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

The next difference is the signature for the Init method. For a Node-API module it is as follows:

napi_value Init(napi_env env, napi_value exports);

The return value from Init is treated as the exports object for the module. The Init method is passed an empty object via the exports parameter as a convenience. If Init returns NULL, the parameter passed as exports is exported by the module. Node-API modules cannot modify the module object but can specify anything as the exports property of the module.

To add the method hello as a function so that it can be called as a method provided by the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor desc = {
    "hello",
    NULL,
    Method,
    NULL,
    NULL,
    NULL,
    napi_writable | napi_enumerable | napi_configurable,
    NULL
  };
  status = napi_define_properties(env, exports, 1, &desc);
  if (status != napi_ok) return NULL;
  return exports;
}

To set a function to be returned by the require() for the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_value method;
  napi_status status;
  status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
  if (status != napi_ok) return NULL;
  return method;
}

To define a class so that new instances can be created (often used with Object wrap):

// NOTE: partial example, not all referenced code is included
napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor properties[] = {
    { "value", NULL, NULL, GetValue, SetValue, NULL, napi_writable | napi_configurable, NULL },
    DECLARE_NAPI_METHOD("plusOne", PlusOne),
    DECLARE_NAPI_METHOD("multiply", Multiply),
  };

  napi_value cons;
  status =
      napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons);
  if (status != napi_ok) return NULL;

  status = napi_create_reference(env, cons, 1, &constructor);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "MyObject", cons);
  if (status != napi_ok) return NULL;

  return exports;
}

You can also use the NAPI_MODULE_INIT macro, which acts as a shorthand for NAPI_MODULE and defining an Init function:

NAPI_MODULE_INIT() {
  napi_value answer;
  napi_status result;

  status = napi_create_int64(env, 42, &answer);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "answer", answer);
  if (status != napi_ok) return NULL;

  return exports;
}

All Node-API addons are context-aware, meaning they may be loaded multiple times. There are a few design considerations when declaring such a module. The documentation on context-aware addons provides more details.

The variables env and exports will be available inside the function body following the macro invocation.

For more details on setting properties on objects, see the section on Working with JavaScript properties.

For more details on building addon modules in general, refer to the existing API.

Working with JavaScript values#

Node-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.

Fundamentally, these APIs are used to do one of the following:

  1. Create a new JavaScript object
  2. Convert from a primitive C type to a Node-API value
  3. Convert from Node-API value to a primitive C type
  4. Get global instances including undefined and null

Node-API values are represented by the type napi_value. Any Node-API call that requires a JavaScript value takes in a napi_value. In some cases, the API does check the type of the napi_value up-front. However, for better performance, it's better for the caller to make sure that the napi_value in question is of the JavaScript type expected by the API.

Enum types#

napi_key_collection_mode#
Added in: v13.7.0, v12.17.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_include_prototypes,
  napi_key_own_only
} napi_key_collection_mode;

Describes the Keys/Properties filter enums:

napi_key_collection_mode limits the range of collected properties.

napi_key_own_only limits the collected properties to the given object only. napi_key_include_prototypes will include all keys of the objects's prototype chain as well.

napi_key_filter#
Added in: v13.7.0, v12.17.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_all_properties = 0,
  napi_key_writable = 1,
  napi_key_enumerable = 1 << 1,
  napi_key_configurable = 1 << 2,
  napi_key_skip_strings = 1 << 3,
  napi_key_skip_symbols = 1 << 4
} napi_key_filter;

Property filter bits. They can be or'ed to build a composite filter.

napi_key_conversion#
Added in: v13.7.0, v12.17.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_keep_numbers,
  napi_key_numbers_to_strings
} napi_key_conversion;

napi_key_numbers_to_strings will convert integer indices to strings. napi_key_keep_numbers will return numbers for integer indices.

napi_valuetype#
typedef enum {
  // ES6 types (corresponds to typeof)
  napi_undefined,
  napi_null,
  napi_boolean,
  napi_number,
  napi_string,
  napi_symbol,
  napi_object,
  napi_function,
  napi_external,
  napi_bigint,
} napi_valuetype;

Describes the type of a napi_value. This generally corresponds to the types described in Section 6.1 of the ECMAScript Language Specification. In addition to types in that section, napi_valuetype can also represent Functions and Objects with external data.

A JavaScript value of type napi_external appears in JavaScript as a plain object such that no properties can be set on it, and no prototype.

napi_typedarray_type#
typedef enum {
  napi_int8_array,
  napi_uint8_array,
  napi_uint8_clamped_array,
  napi_int16_array,
  napi_uint16_array,
  napi_int32_array,
  napi_uint32_array,
  napi_float32_array,
  napi_float64_array,
  napi_bigint64_array,
  napi_biguint64_array,
} napi_typedarray_type;

This represents the underlying binary scalar datatype of the TypedArray. Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification.

Object creation functions#

napi_create_array#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_array(napi_env env, napi_value* result)
  • [in] env: The environment that the Node-API call is invoked under.
  • [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_array_with_length#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_array_with_length(napi_env env,
                                          size_t length,
                                          napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: The initial length of the Array.
  • [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript Array type. The Array's length property is set to the passed-in length parameter. However, the underlying buffer is not guaranteed to be pre-allocated by the VM when the array is created. That behavior is left to the underlying VM implementation. If the buffer must be a contiguous block of memory that can be directly read and/or written via C, consider using napi_create_external_arraybuffer.

JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_arraybuffer#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_arraybuffer(napi_env env,
                                    size_t byte_length,
                                    void** data,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: The length in bytes of the array buffer to create.
  • [out] data: Pointer to the underlying byte buffer of the ArrayBuffer. data can optionally be ignored by passing NULL.
  • [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript ArrayBuffer. ArrayBuffers are used to represent fixed-length binary data buffers. They are normally used as a backing-buffer for TypedArray objects. The ArrayBuffer allocated will have an underlying byte buffer whose size is determined by the length parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or DataView object would need to be created.

JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_buffer#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_buffer(napi_env env,
                               size_t size,
                               void** data,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] size: Size in bytes of the underlying buffer.
  • [out] data: Raw pointer to the underlying buffer. data can optionally be ignored by passing NULL.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_buffer_copy#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_buffer_copy(napi_env env,
                                    size_t length,
                                    const void* data,
                                    void** result_data,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] size: Size in bytes of the input buffer (should be the same as the size of the new buffer).
  • [in] data: Raw pointer to the underlying buffer to copy from.
  • [out] result_data: Pointer to the new Buffer's underlying data buffer. result_data can optionally be ignored by passing NULL.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object and initializes it with data copied from the passed-in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_date#
Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_create_date(napi_env env,
                             double time,
                             napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] time: ECMAScript time value in milliseconds since 01 January, 1970 UTC.
  • [out] result: A napi_value representing a JavaScript Date.

Returns napi_ok if the API succeeded.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

This API allocates a JavaScript Date object.

JavaScript Date objects are described in Section 20.3 of the ECMAScript Language Specification.

napi_create_external#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_external(napi_env env,
                                 void* data,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] data: Raw pointer to the external data.
  • [in] finalize_cb: Optional callback to call when the external value is being collected. napi_finalize provides more details.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing an external value.

Returns napi_ok if the API succeeded.

This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code using napi_get_value_external.

The API adds a napi_finalize callback which will be called when the JavaScript object just created has been garbage collected.

The created value is not an object, and therefore does not support additional properties. It is considered a distinct value type: calling napi_typeof() with an external value yields napi_external.

napi_create_external_arraybuffer#
Added in: v8.0.0 N-API version: 1 
napi_status
napi_create_external_arraybuffer(napi_env env,
                                 void* external_data,
                                 size_t byte_length,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] external_data: Pointer to the underlying byte buffer of the ArrayBuffer.
  • [in] byte_length: The length in bytes of the underlying buffer.
  • [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected. napi_finalize provides more details.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

Some runtimes other than Node.js have dropped support for external buffers. On runtimes other than Node.js this method may return napi_no_external_buffers_allowed to indicate that external buffers are not supported. One such runtime is Electron as described in this issue electron/issues/35801.

In order to maintain broadest compatibility with all runtimes you may define NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED in your addon before includes for the node-api headers. Doing so will hide the 2 functions that create external buffers. This will ensure a compilation error occurs if you accidentally use one of these methods.

This API returns a Node-API value corresponding to a JavaScript ArrayBuffer. The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.

The API adds a napi_finalize callback which will be called when the JavaScript object just created has been garbage collected.

JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_external_buffer#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_external_buffer(napi_env env,
                                        size_t length,
                                        void* data,
                                        napi_finalize finalize_cb,
                                        void* finalize_hint,
                                        napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: Size in bytes of the input buffer (should be the same as the size of the new buffer).
  • [in] data: Raw pointer to the underlying buffer to expose to JavaScript.
  • [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected. napi_finalize provides more details.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

Some runtimes other than Node.js have dropped support for external buffers. On runtimes other than Node.js this method may return napi_no_external_buffers_allowed to indicate that external buffers are not supported. One such runtime is Electron as described in this issue electron/issues/35801.

In order to maintain broadest compatibility with all runtimes you may define NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED in your addon before includes for the node-api headers. Doing so will hide the 2 functions that create external buffers. This will ensure a compilation error occurs if you accidentally use one of these methods.

This API allocates a node::Buffer object and initializes it with data backed by the passed in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

The API adds a napi_finalize callback which will be called when the JavaScript object just created has been garbage collected.

For Node.js >=4 Buffers are Uint8Arrays.

napi_create_object#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_object(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: A napi_value representing a JavaScript Object.

Returns napi_ok if the API succeeded.

This API allocates a default JavaScript Object. It is the equivalent of doing new Object() in JavaScript.

The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification.

napi_create_symbol#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_symbol(napi_env env,
                               napi_value description,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] description: Optional napi_value which refers to a JavaScript string to be set as the description for the symbol.
  • [out] result: A napi_value representing a JavaScript symbol.

Returns napi_ok if the API succeeded.

This API creates a JavaScript symbol value from a UTF8-encoded C string.

The JavaScript symbol type is described in Section 19.4 of the ECMAScript Language Specification.

node_api_symbol_for#
Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

napi_status node_api_symbol_for(napi_env env,
                                const char* utf8description,
                                size_t length,
                                napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] utf8description: UTF-8 C string representing the text to be used as the description for the symbol.
  • [in] length: The length of the description string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript symbol.

Returns napi_ok if the API succeeded.

This API searches in the global registry for an existing symbol with the given description. If the symbol already exists it will be returned, otherwise a new symbol will be created in the registry.

The JavaScript symbol type is described in Section 19.4 of the ECMAScript Language Specification.

napi_create_typedarray#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_typedarray(napi_env env,
                                   napi_typedarray_type type,
                                   size_t length,
                                   napi_value arraybuffer,
                                   size_t byte_offset,
                                   napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] type: Scalar datatype of the elements within the TypedArray.
  • [in] length: Number of elements in the TypedArray.
  • [in] arraybuffer: ArrayBuffer underlying the typed array.
  • [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the TypedArray.
  • [out] result: A napi_value representing a JavaScript TypedArray.

Returns napi_ok if the API succeeded.

This API creates a JavaScript TypedArray object over an existing ArrayBuffer. TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.

It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript TypedArray objects are described in Section 22.2 of the ECMAScript Language Specification.

napi_create_dataview#
Added in: v8.3.0 N-API version: 1 
napi_status napi_create_dataview(napi_env env,
                                 size_t byte_length,
                                 napi_value arraybuffer,
                                 size_t byte_offset,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: Number of elements in the DataView.
  • [in] arraybuffer: ArrayBuffer underlying the DataView.
  • [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the DataView.
  • [out] result: A napi_value representing a JavaScript DataView.

Returns napi_ok if the API succeeded.

This API creates a JavaScript DataView object over an existing ArrayBuffer. DataView objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the ArrayBuffer.

It is required that byte_length + byte_offset is less than or equal to the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript DataView objects are described in Section 24.3 of the ECMAScript Language Specification.

Functions to convert from C types to Node-API#

napi_create_int32#
Added in: v8.4.0 N-API version: 1 
napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int32_t type to the JavaScript number type.

The JavaScript number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_uint32#
Added in: v8.4.0 N-API version: 1 
napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Unsigned integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C uint32_t type to the JavaScript number type.

The JavaScript number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_int64#
Added in: v8.4.0 N-API version: 1 
napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int64_t type to the JavaScript number type.

The JavaScript number type is described in Section 6.1.6 of the ECMAScript Language Specification. Note the complete range of int64_t cannot be represented with full precision in JavaScript. Integer values outside the range of Number.MIN_SAFE_INTEGER -(2**53 - 1) - Number.MAX_SAFE_INTEGER (2**53 - 1) will lose precision.

napi_create_double#
Added in: v8.4.0 N-API version: 1 
napi_status napi_create_double(napi_env env, double value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Double-precision value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C double type to the JavaScript number type.

The JavaScript number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_bigint_int64#
Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_int64(napi_env env,
                                     int64_t value,
                                     napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C int64_t type to the JavaScript BigInt type.

napi_create_bigint_uint64#
Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_uint64(napi_env env,
                                      uint64_t value,
                                      napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: Unsigned integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C uint64_t type to the JavaScript BigInt type.

napi_create_bigint_words#
Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_words(napi_env env,
                                     int sign_bit,
                                     size_t word_count,
                                     const uint64_t* words,
                                     napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] sign_bit: Determines if the resulting BigInt will be positive or negative.
  • [in] word_count: The length of the words array.
  • [in] words: An array of uint64_t little-endian 64-bit words.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts an array of unsigned 64-bit words into a single BigInt value.

The resulting BigInt is calculated as: (–1)sign_bit (words[0] × (264)0 + words[1] × (264)1 + …)

napi_create_string_latin1#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_latin1(napi_env env,
                                      const char* str,
                                      size_t length,
                                      napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing an ISO-8859-1-encoded string.
  • [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript string.

Returns napi_ok if the API succeeded.

This API creates a JavaScript string value from an ISO-8859-1-encoded C string. The native string is copied.

The JavaScript string type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf16#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_utf16(napi_env env,
                                     const char16_t* str,
                                     size_t length,
                                     napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing a UTF16-LE-encoded string.
  • [in] length: The length of the string in two-byte code units, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript string.

Returns napi_ok if the API succeeded.

This API creates a JavaScript string value from a UTF16-LE-encoded C string. The native string is copied.

The JavaScript string type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf8#
Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_utf8(napi_env env,
                                    const char* str,
                                    size_t length,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing a UTF8-encoded string.
  • [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript string.

Returns napi_ok if the API succeeded.

This API creates a JavaScript string value from a UTF8-encoded C string. The native string is copied.

The JavaScript string type is described in Section 6.1.4 of the ECMAScript Language Specification.

Functions to convert from Node-API to C types#

napi_get_array_length#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_array_length(napi_env env,
                                  napi_value value,
                                  uint32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the JavaScript Array whose length is being queried.
  • [out] result: uint32 representing length of the array.

Returns napi_ok if the API succeeded.

This API returns the length of an array.

Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification.

napi_get_arraybuffer_info#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_arraybuffer_info(napi_env env,
                                      napi_value arraybuffer,
                                      void** data,
                                      size_t* byte_length)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: napi_value representing the ArrayBuffer being queried.
  • [out] data: The underlying data buffer of the ArrayBuffer. If byte_length is 0, this may be NULL or any other pointer value.
  • [out] byte_length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length.

WARNING: Use caution while using this API. The lifetime of the underlying data buffer is managed by the ArrayBuffer even after it's returned. A possible safe way to use this API is in conjunction with napi_create_reference, which can be used to guarantee control over the lifetime of the ArrayBuffer. It's also safe to use the returned data buffer within the same callback as long as there are no calls to other APIs that might trigger a GC.

napi_get_buffer_info#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_buffer_info(napi_env env,
                                 napi_value value,
                                 void** data,
                                 size_t* length)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the node::Buffer being queried.
  • [out] data: The underlying data buffer of the node::Buffer. If length is 0, this may be NULL or any other pointer value.
  • [out] length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of a node::Buffer and its length.

Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.

napi_get_prototype#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_prototype(napi_env env,
                               napi_value object,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] object: napi_value representing JavaScript Object whose prototype to return. This returns the equivalent of Object.getPrototypeOf (which is not the same as the function's prototype property).
  • [out] result: napi_value representing prototype of the given object.

Returns napi_ok if the API succeeded.

napi_get_typedarray_info#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_typedarray_info(napi_env env,
                                     napi_value typedarray,
                                     napi_typedarray_type* type,
                                     size_t* length,
                                     void** data,
                                     napi_value* arraybuffer,
                                     size_t* byte_offset)
  • [in] env: The environment that the API is invoked under.
  • [in] typedarray: napi_value representing the TypedArray whose properties to query.
  • [out] type: Scalar datatype of the elements within the TypedArray.
  • [out] length: The number of elements in the TypedArray.
  • [out] data: The data buffer underlying the TypedArray adjusted by the byte_offset value so that it points to the first element in the TypedArray. If the length of the array is 0, this may be NULL or any other pointer value.
  • [out] arraybuffer: The ArrayBuffer underlying the TypedArray.
  • [out] byte_offset: The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be at data - byte_offset.

Returns napi_ok if the API succeeded.

This API returns various properties of a typed array.

Any of the out parameters may be NULL if that property is unneeded.

Warning: Use caution while using this API since the underlying data buffer is managed by the VM.

napi_get_dataview_info#
Added in: v8.3.0 N-API version: 1 
napi_status napi_get_dataview_info(napi_env env,
                                   napi_value dataview,
                                   size_t* byte_length,
                                   void** data,
                                   napi_value* arraybuffer,
                                   size_t* byte_offset)
  • [in] env: The environment that the API is invoked under.
  • [in] dataview: napi_value representing the DataView whose properties to query.
  • [out] byte_length: Number of bytes in the DataView.
  • [out] data: The data buffer underlying the DataView. If byte_length is 0, this may be NULL or any other pointer value.
  • [out] arraybuffer: ArrayBuffer underlying the DataView.
  • [out] byte_offset: The byte offset within the data buffer from which to start projecting the DataView.

Returns napi_ok if the API succeeded.

Any of the out parameters may be NULL if that property is unneeded.

This API returns various properties of a DataView.

napi_get_date_value#
Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_get_date_value(napi_env env,
                                napi_value value,
                                double* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing a JavaScript Date.
  • [out] result: Time value as a double represented as milliseconds since midnight at the beginning of 01 January, 1970 UTC.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

Returns napi_ok if the API succeeded. If a non-date napi_value is passed in it returns napi_date_expected.

This API returns the C double primitive of time value for the given JavaScript Date.

napi_get_value_bool#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Boolean.
  • [out] result: C boolean primitive equivalent of the given JavaScript Boolean.

Returns napi_ok if the API succeeded. If a non-boolean napi_value is passed in it returns napi_boolean_expected.

This API returns the C boolean primitive equivalent of the given JavaScript Boolean.

napi_get_value_double#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_double(napi_env env,
                                  napi_value value,
                                  double* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript number.
  • [out] result: C double primitive equivalent of the given JavaScript number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C double primitive equivalent of the given JavaScript number.

napi_get_value_bigint_int64#
Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_int64(napi_env env,
                                        napi_value value,
                                        int64_t* result,
                                        bool* lossless);
  • [in] env: The environment that the API is invoked under
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] result: C int64_t primitive equivalent of the given JavaScript BigInt.
  • [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C int64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_uint64#
Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_uint64(napi_env env,
                                        napi_value value,
                                        uint64_t* result,
                                        bool* lossless);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] result: C uint64_t primitive equivalent of the given JavaScript BigInt.
  • [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C uint64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_words#
Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_words(napi_env env,
                                        napi_value value,
                                        int* sign_bit,
                                        size_t* word_count,
                                        uint64_t* words);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] sign_bit: Integer representing if the JavaScript BigInt is positive or negative.
  • [in/out] word_count: Must be initialized to the length of the words array. Upon return, it will be set to the actual number of words that would be needed to store this BigInt.
  • [out] words: Pointer to a pre-allocated 64-bit word array.

Returns napi_ok if the API succeeded.

This API converts a single BigInt value into a sign bit, 64-bit little-endian array, and the number of elements in the array. sign_bit and words may be both set to NULL, in order to get only word_count.

napi_get_value_external#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_external(napi_env env,
                                    napi_value value,
                                    void** result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript external value.
  • [out] result: Pointer to the data wrapped by the JavaScript external value.

Returns napi_ok if the API succeeded. If a non-external napi_value is passed in it returns napi_invalid_arg.

This API retrieves the external data pointer that was previously passed to napi_create_external().

napi_get_value_int32#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_int32(napi_env env,
                                 napi_value value,
                                 int32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript number.
  • [out] result: C int32 primitive equivalent of the given JavaScript number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in napi_number_expected.

This API returns the C int32 primitive equivalent of the given JavaScript number.

If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 231 - 1.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_int64#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_int64(napi_env env,
                                 napi_value value,
                                 int64_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript number.
  • [out] result: C int64 primitive equivalent of the given JavaScript number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C int64 primitive equivalent of the given JavaScript number.

number values outside the range of Number.MIN_SAFE_INTEGER -(2**53 - 1) - Number.MAX_SAFE_INTEGER (2**53 - 1) will lose precision.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_string_latin1#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_latin1(napi_env env,
                                         napi_value value,
                                         char* buf,
                                         size_t bufsize,
                                         size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the ISO-8859-1-encoded string into. If NULL is passed in, the length of the string in bytes and excluding the null terminator is returned in result.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
  • [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-string napi_value is passed in it returns napi_string_expected.

This API returns the ISO-8859-1-encoded string corresponding the value passed in.

napi_get_value_string_utf8#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_utf8(napi_env env,
                                       napi_value value,
                                       char* buf,
                                       size_t bufsize,
                                       size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string in bytes and excluding the null terminator is returned in result.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
  • [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-string napi_value is passed in it returns napi_string_expected.

This API returns the UTF8-encoded string corresponding the value passed in.

napi_get_value_string_utf16#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_utf16(napi_env env,
                                        napi_value value,
                                        char16_t* buf,
                                        size_t bufsize,
                                        size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string in 2-byte code units and excluding the null terminator is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
  • [out] result: Number of 2-byte code units copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-string napi_value is passed in it returns napi_string_expected.

This API returns the UTF16-encoded string corresponding the value passed in.

napi_get_value_uint32#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_uint32(napi_env env,
                                  napi_value value,
                                  uint32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript number.
  • [out] result: C primitive equivalent of the given napi_value as a uint32_t.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C primitive equivalent of the given napi_value as a uint32_t.

Functions to get global instances#

napi_get_boolean#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The value of the boolean to retrieve.
  • [out] result: napi_value representing JavaScript Boolean singleton to retrieve.

Returns napi_ok if the API succeeded.

This API is used to return the JavaScript singleton object that is used to represent the given boolean value.

napi_get_global#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_global(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript global object.

Returns napi_ok if the API succeeded.

This API returns the global object.

napi_get_null#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_null(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript null object.

Returns napi_ok if the API succeeded.

This API returns the null object.

napi_get_undefined#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_undefined(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript Undefined value.

Returns napi_ok if the API succeeded.

This API returns the Undefined object.

Working with JavaScript values and abstract operations#

Node-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.

These APIs support doing one of the following:

  1. Coerce JavaScript values to specific JavaScript types (such as number or string).
  2. Check the type of a JavaScript value.
  3. Check for equality between two JavaScript values.

napi_coerce_to_bool#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_bool(napi_env env,
                                napi_value value,
                                napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript Boolean.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToBoolean() as defined in Section 7.1.2 of the ECMAScript Language Specification.

napi_coerce_to_number#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_number(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript number.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToNumber() as defined in Section 7.1.3 of the ECMAScript Language Specification. This function potentially runs JS code if the passed-in value is an object.

napi_coerce_to_object#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_object(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript Object.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToObject() as defined in Section 7.1.13 of the ECMAScript Language Specification.

napi_coerce_to_string#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_string(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript string.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToString() as defined in Section 7.1.13 of the ECMAScript Language Specification. This function potentially runs JS code if the passed-in value is an object.

napi_typeof#

Added in: v8.0.0 N-API version: 1 
napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value whose type to query.
  • [out] result: The type of the JavaScript value.

Returns napi_ok if the API succeeded.

  • napi_invalid_arg if the type of value is not a known ECMAScript type and value is not an External value.

This API represents behavior similar to invoking the typeof Operator on the object as defined in Section 12.5.5 of the ECMAScript Language Specification. However, there are some differences:

  1. It has support for detecting an External value.
  2. It detects null as a separate type, while ECMAScript typeof would detect object.

If value has a type that is invalid, an error is returned.

napi_instanceof#

Added in: v8.0.0 N-API version: 1 
napi_status napi_instanceof(napi_env env,
                            napi_value object,
                            napi_value constructor,
                            bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] object: The JavaScript value to check.
  • [in] constructor: The JavaScript function object of the constructor function to check against.
  • [out] result: Boolean that is set to true if object instanceof constructor is true.

Returns napi_ok if the API succeeded.

This API represents invoking the instanceof Operator on the object as defined in Section 12.10.4 of the ECMAScript Language Specification.

napi_is_array#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_array(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given object is an array.

Returns napi_ok if the API succeeded.

This API represents invoking the IsArray operation on the object as defined in Section 7.2.2 of the ECMAScript Language Specification.

napi_is_arraybuffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given object is an ArrayBuffer.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an array buffer.

napi_is_buffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a node::Buffer object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a buffer.

napi_is_date#

Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_is_date(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a JavaScript Date object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a date.

napi_is_error#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_error(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents an Error object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an Error.

napi_is_typedarray#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a TypedArray.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a typed array.

napi_is_dataview#

Added in: v8.3.0 N-API version: 1 
napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a DataView.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a DataView.

napi_strict_equals#

Added in: v8.0.0 N-API version: 1 
napi_status napi_strict_equals(napi_env env,
                               napi_value lhs,
                               napi_value rhs,
                               bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] lhs: The JavaScript value to check.
  • [in] rhs: The JavaScript value to check against.
  • [out] result: Whether the two napi_value objects are equal.

Returns napi_ok if the API succeeded.

This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.

napi_detach_arraybuffer#

Added in: v13.0.0, v12.16.0, v10.22.0 N-API version: 7 
napi_status napi_detach_arraybuffer(napi_env env,
                                    napi_value arraybuffer)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: The JavaScript ArrayBuffer to be detached.

Returns napi_ok if the API succeeded. If a non-detachable ArrayBuffer is passed in it returns napi_detachable_arraybuffer_expected.

Generally, an ArrayBuffer is non-detachable if it has been detached before. The engine may impose additional conditions on whether an ArrayBuffer is detachable. For example, V8 requires that the ArrayBuffer be external, that is, created with napi_create_external_arraybuffer.

This API represents the invocation of the ArrayBuffer detach operation as defined in Section 24.1.1.3 of the ECMAScript Language Specification.

napi_is_detached_arraybuffer#

Added in: v13.3.0, v12.16.0, v10.22.0 N-API version: 7 
napi_status napi_is_detached_arraybuffer(napi_env env,
                                         napi_value arraybuffer,
                                         bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: The JavaScript ArrayBuffer to be checked.
  • [out] result: Whether the arraybuffer is detached.

Returns napi_ok if the API succeeded.

The ArrayBuffer is considered detached if its internal data is null.

This API represents the invocation of the ArrayBuffer IsDetachedBuffer operation as defined in Section 24.1.1.2 of the ECMAScript Language Specification.

Working with JavaScript properties#

Node-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.

Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in Node-API can be represented in one of the following forms:

  • Named: a simple UTF8-encoded string
  • Integer-Indexed: an index value represented by uint32_t
  • JavaScript value: these are represented in Node-API by napi_value. This can be a napi_value representing a string, number, or symbol.

Node-API values are represented by the type napi_value. Any Node-API call that requires a JavaScript value takes in a napi_value. However, it's the caller's responsibility to make sure that the napi_value in question is of the JavaScript type expected by the API.

The APIs documented in this section provide a simple interface to get and set properties on arbitrary JavaScript objects represented by napi_value.

For instance, consider the following JavaScript code snippet:

const obj = {};
obj.myProp = 123;

The equivalent can be done using Node-API values with the following snippet:

napi_status status = napi_generic_failure;

// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create a napi_value for 123
status = napi_create_int32(env, 123, &value);
if (status != napi_ok) return status;

// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;

Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:

const arr = [];
arr[123] = 'hello';

The equivalent can be done using Node-API values with the following snippet:

napi_status status = napi_generic_failure;

// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value);
if (status != napi_ok) return status;

// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;

Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:

const arr = [];
const value = arr[123];

The following is the approximate equivalent of the Node-API counterpart:

napi_status status = napi_generic_failure;

// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;

Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:

const obj = {};
Object.defineProperties(obj, {
  'foo': { value: 123, writable: true, configurable: true, enumerable: true },
  'bar': { value: 456, writable: true, configurable: true, enumerable: true },
});

The following is the approximate equivalent of the Node-API counterpart:

napi_status status = napi_status_generic_failure;

// const obj = {};
napi_value obj;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_int32(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_int32(env, 456, &barValue);
if (status != napi_ok) return status;

// Set the properties
napi_property_descriptor descriptors[] = {
  { "foo", NULL, NULL, NULL, NULL, fooValue, napi_writable | napi_configurable, NULL },
  { "bar", NULL, NULL, NULL, NULL, barValue, napi_writable | napi_configurable, NULL }
}
status = napi_define_properties(env,
                                obj,
                                sizeof(descriptors) / sizeof(descriptors[0]),
                                descriptors);
if (status != napi_ok) return status;

Structures#

napi_property_attributes#
History
VersionChanges
v14.12.0

added napi_default_method and napi_default_property.

typedef enum {
  napi_default = 0,
  napi_writable = 1 << 0,
  napi_enumerable = 1 << 1,
  napi_configurable = 1 << 2,

  // Used with napi_define_class to distinguish static properties
  // from instance properties. Ignored by napi_define_properties.
  napi_static = 1 << 10,

  // Default for class methods.
  napi_default_method = napi_writable | napi_configurable,

  // Default for object properties, like in JS obj[prop].
  napi_default_jsproperty = napi_writable |
                          napi_enumerable |
                          napi_configurable,
} napi_property_attributes;

napi_property_attributes are flags used to control the behavior of properties set on a JavaScript object. Other than napi_static they correspond to the attributes listed in Section 6.1.7.1 of the ECMAScript Language Specification. They can be one or more of the following bitflags:

  • napi_default: No explicit attributes are set on the property. By default, a property is read only, not enumerable and not configurable.
  • napi_writable: The property is writable.
  • napi_enumerable: The property is enumerable.
  • napi_configurable: The property is configurable as defined in Section 6.1.7.1 of the ECMAScript Language Specification.
  • napi_static: The property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only by napi_define_class. It is ignored by napi_define_properties.
  • napi_default_method: Like a method in a JS class, the property is configurable and writeable, but not enumerable.
  • napi_default_jsproperty: Like a property set via assignment in JavaScript, the property is writable, enumerable, and configurable.
napi_property_descriptor#
typedef struct {
  // One of utf8name or name should be NULL.
  const char* utf8name;
  napi_value name;

  napi_callback method;
  napi_callback getter;
  napi_callback setter;
  napi_value value;

  napi_property_attributes attributes;
  void* data;
} napi_property_descriptor;
  • utf8name: Optional string describing the key for the property, encoded as UTF8. One of utf8name or name must be provided for the property.
  • name: Optional napi_value that points to a JavaScript string or symbol to be used as the key for the property. One of utf8name or name must be provided for the property.
  • value: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, set getter, setter, method and data to NULL (since these members won't be used).
  • getter: A function to call when a get access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a Node-API call). napi_callback provides more details.
  • setter: A function to call when a set access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a Node-API call). napi_callback provides more details.
  • method: Set this to make the property descriptor object's value property to be a JavaScript function represented by method. If this is passed in, set value, getter and setter to NULL (since these members won't be used). napi_callback provides more details.
  • attributes: The attributes associated with the particular property. See napi_property_attributes.
  • data: The callback data passed into method, getter and setter if this function is invoked.

Functions#

napi_get_property_names#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_property_names(napi_env env,
                                    napi_value object,
                                    napi_value* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate over result using napi_get_array_length and napi_get_element.

Returns napi_ok if the API succeeded.

This API returns the names of the enumerable properties of object as an array of strings. The properties of object whose key is a symbol will not be included.

napi_get_all_property_names#
Added in: v13.7.0, v12.17.0, v10.20.0 N-API version: 6 
napi_get_all_property_names(napi_env env,
                            napi_value object,
                            napi_key_collection_mode key_mode,
                            napi_key_filter key_filter,
                            napi_key_conversion key_conversion,
                            napi_value* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [in] key_mode: Whether to retrieve prototype properties as well.
  • [in] key_filter: Which properties to retrieve (enumerable/readable/writable).
  • [in] key_conversion: Whether to convert numbered property keys to strings.
  • [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. napi_get_array_length and napi_get_element can be used to iterate over result.

Returns napi_ok if the API succeeded.

This API returns an array containing the names of the available properties of this object.

napi_set_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_set_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value value);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object on which to set the property.
  • [in] key: The name of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This API set a property on the Object passed in.

napi_get_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] key: The name of the property to retrieve.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the requested property from the Object passed in.

napi_has_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_has_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named property.

napi_delete_property#
Added in: v8.2.0 N-API version: 1 
napi_status napi_delete_property(napi_env env,
                                 napi_value object,
                                 napi_value key,
                                 bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the property to delete.
  • [out] result: Whether the property deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the key own property from object.

napi_has_own_property#
Added in: v8.2.0 N-API version: 1 
napi_status napi_has_own_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the own property whose existence to check.
  • [out] result: Whether the own property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named own property. key must be a string or a symbol, or an error will be thrown. Node-API will not perform any conversion between data types.

napi_set_named_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_set_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value value);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object on which to set the property.
  • [in] utf8Name: The name of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_set_property with a napi_value created from the string passed in as utf8Name.

napi_get_named_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] utf8Name: The name of the property to get.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_get_property with a napi_value created from the string passed in as utf8Name.

napi_has_named_property#
Added in: v8.0.0 N-API version: 1 
napi_status napi_has_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] utf8Name: The name of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_has_property with a napi_value created from the string passed in as utf8Name.

napi_set_element#
Added in: v8.0.0 N-API version: 1 
napi_status napi_set_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value value);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to set the properties.
  • [in] index: The index of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This API sets an element on the Object passed in.

napi_get_element#
Added in: v8.0.0 N-API version: 1 
napi_status napi_get_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] index: The index of the property to get.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the element at the requested index.

napi_has_element#
Added in: v8.0.0 N-API version: 1 
napi_status napi_has_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] index: The index of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API returns if the Object passed in has an element at the requested index.

napi_delete_element#
Added in: v8.2.0 N-API version: 1 
napi_status napi_delete_element(napi_env env,
                                napi_value object,
                                uint32_t index,
                                bool* result);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to query.
  • [in] index: The index of the property to delete.
  • [out] result: Whether the element deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the specified index from object.

napi_define_properties#
Added in: v8.0.0 N-API version: 1 
napi_status napi_define_properties(napi_env env,
                                   napi_value object,
                                   size_t property_count,
                                   const napi_property_descriptor* properties);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [in] property_count: The number of elements in the properties array.
  • [in] properties: The array of property descriptors.

Returns napi_ok if the API succeeded.

This method allows the efficient definition of multiple properties on a given object. The properties are defined using property descriptors (see napi_property_descriptor). Given an array of such property descriptors, this API will set the properties on the object one at a time, as defined by DefineOwnProperty() (described in Section 9.1.6 of the ECMA-262 specification).

napi_object_freeze#
Added in: v14.14.0, v12.20.0 N-API version: 8 
napi_status napi_object_freeze(napi_env env,
                               napi_value object);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to freeze.

Returns napi_ok if the API succeeded.

This method freezes a given object. This prevents new properties from being added to it, existing properties from being removed, prevents changing the enumerability, configurability, or writability of existing properties, and prevents the values of existing properties from being changed. It also prevents the object's prototype from being changed. This is described in Section 19.1.2.6 of the ECMA-262 specification.

napi_object_seal#
Added in: v14.14.0, v12.20.0 N-API version: 8 
napi_status napi_object_seal(napi_env env,
                             napi_value object);
  • [in] env: The environment that the Node-API call is invoked under.
  • [in] object: The object to seal.

Returns napi_ok if the API succeeded.

This method seals a given object. This prevents new properties from being added to it, as well as marking all existing properties as non-configurable. This is described in Section 19.1.2.20 of the ECMA-262 specification.

Working with JavaScript functions#

Node-API provides a set of APIs that allow JavaScript code to call back into native code. Node-APIs that support calling back into native code take in a callback functions represented by the napi_callback type. When the JavaScript VM calls back to native code, the napi_callback function provided is invoked. The APIs documented in this section allow the callback function to do the following:

  • Get information about the context in which the callback was invoked.
  • Get the arguments passed into the callback.
  • Return a napi_value back from the callback.

Additionally, Node-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.

Any non-NULL data which is passed to this API via the data field of the napi_property_descriptor items can be associated with object and freed whenever object is garbage-collected by passing both object and the data to napi_add_finalizer.

napi_call_function#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_call_function(napi_env env,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] recv: The this value passed to the called function.
  • [in] func: napi_value representing the JavaScript function to be invoked.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of napi_values representing JavaScript values passed in as arguments to the function.
  • [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This is the primary mechanism of calling back from the add-on's native code into JavaScript. For the special case of calling into JavaScript after an async operation, see napi_make_callback.

A sample use case might look as follows. Consider the following JavaScript snippet:

function AddTwo(num) {
  return num + 2;
}
global.AddTwo = AddTwo;

Then, the above function can be invoked from a native add-on using the following code:

// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;

// const arg = 1337
status = napi_create_int32(env, 1337, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;

// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;

napi_create_function#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_function(napi_env env,
                                 const char* utf8name,
                                 size_t length,
                                 napi_callback cb,
                                 void* data,
                                 napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] utf8Name: Optional name of the function encoded as UTF8. This is visible within JavaScript as the new function object's name property.
  • [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] cb: The native function which should be called when this function object is invoked. napi_callback provides more details.
  • [in] data: User-provided data context. This will be passed back into the function when invoked later.
  • [out] result: napi_value representing the JavaScript function object for the newly created function.

Returns napi_ok if the API succeeded.

This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript.

The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.

In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:

napi_value SayHello(napi_env env, napi_callback_info info) {
  printf("Hello\n");
  return NULL;
}

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;

  napi_value fn;
  status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "sayHello", fn);
  if (status != napi_ok) return NULL;

  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

Given the above code, the add-on can be used from JavaScript as follows:

const myaddon = require('./addon');
myaddon.sayHello();

The string passed to require() is the name of the target in binding.gyp responsible for creating the .node file.

Any non-NULL data which is passed to this API via the data parameter can be associated with the resulting JavaScript function (which is returned in the result parameter) and freed whenever the function is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.

napi_get_cb_info#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_cb_info(napi_env env,
                             napi_callback_info cbinfo,
                             size_t* argc,
                             napi_value* argv,
                             napi_value* thisArg,
                             void** data)
  • [in] env: The environment that the API is invoked under.
  • [in] cbinfo: The callback info passed into the callback function.
  • [in-out] argc: Specifies the length of the provided argv array and receives the actual count of arguments. argc can optionally be ignored by passing NULL.
  • [out] argv: C array of napi_values to which the arguments will be copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest of argv is filled with napi_value values that represent undefined. argv can optionally be ignored by passing NULL.
  • [out] thisArg: Receives the JavaScript this argument for the call. thisArg can optionally be ignored by passing NULL.
  • [out] data: Receives the data pointer for the callback. data can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This method is used within a callback function to retrieve details about the call like the arguments and the this pointer from a given callback info.

napi_get_new_target#

Added in: v8.6.0 N-API version: 1 
napi_status napi_get_new_target(napi_env env,
                                napi_callback_info cbinfo,
                                napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] cbinfo: The callback info passed into the callback function.
  • [out] result: The new.target of the constructor call.

Returns napi_ok if the API succeeded.

This API returns the new.target of the constructor call. If the current callback is not a constructor call, the result is NULL.

napi_new_instance#

Added in: v8.0.0 N-API version: 1 
napi_status napi_new_instance(napi_env env,
                              napi_value cons,
                              size_t argc,
                              napi_value* argv,
                              napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] cons: napi_value representing the JavaScript function to be invoked as a constructor.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of JavaScript values as napi_value representing the arguments to the constructor. If argc is zero this parameter may be omitted by passing in NULL.
  • [out] result: napi_value representing the JavaScript object returned, which in this case is the constructed object.

This method is used to instantiate a new JavaScript value using a given napi_value that represents the constructor for the object. For example, consider the following snippet:

function MyObject(param) {
  this.param = param;
}

const arg = 'hello';
const value = new MyObject(arg);

The following can be approximated in Node-API using the following snippet:

// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;

// const arg = "hello"
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);

Returns napi_ok if the API succeeded.

Object wrap#

Node-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.

  1. The napi_define_class API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class.
  2. When JavaScript code invokes the constructor, the constructor callback uses napi_wrap to wrap a new C++ instance in a JavaScript object, then returns the wrapper object.
  3. When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback C++ function is invoked. For an instance callback, napi_unwrap obtains the C++ instance that is the target of the call.

For wrapped objects it may be difficult to distinguish between a function called on a class prototype and a function called on an instance of a class. A common pattern used to address this problem is to save a persistent reference to the class constructor for later instanceof checks.

napi_value MyClass_constructor = NULL;
status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
assert(napi_ok == status);
bool is_instance = false;
status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
assert(napi_ok == status);
if (is_instance) {
  // napi_unwrap() ...
} else {
  // otherwise...
}

The reference must be freed once it is no longer needed.

There are occasions where napi_instanceof() is insufficient for ensuring that a JavaScript object is a wrapper for a certain native type. This is the case especially when wrapped JavaScript objects are passed back into the addon via static methods rather than as the this value of prototype methods. In such cases there is a chance that they may be unwrapped incorrectly.

const myAddon = require('./build/Release/my_addon.node');

// `openDatabase()` returns a JavaScript object that wraps a native database
// handle.
const dbHandle = myAddon.openDatabase();

// `query()` returns a JavaScript object that wraps a native query handle.
const queryHandle = myAddon.query(dbHandle, 'Gimme ALL the things!');

// There is an accidental error in the line below. The first parameter to
// `myAddon.queryHasRecords()` should be the database handle (`dbHandle`), not
// the query handle (`query`), so the correct condition for the while-loop
// should be
//
// myAddon.queryHasRecords(dbHandle, queryHandle)
//
while (myAddon.queryHasRecords(queryHandle, dbHandle)) {
  // retrieve records
}

In the above example myAddon.queryHasRecords() is a method that accepts two arguments. The first is a database handle and the second is a query handle. Internally, it unwraps the first argument and casts the resulting pointer to a native database handle. It then unwraps the second argument and casts the resulting pointer to a query handle. If the arguments are passed in the wrong order, the casts will work, however, there is a good chance that the underlying database operation will fail, or will even cause an invalid memory access.

To ensure that the pointer retrieved from the first argument is indeed a pointer to a database handle and, similarly, that the pointer retrieved from the second argument is indeed a pointer to a query handle, the implementation of queryHasRecords() has to perform a type validation. Retaining the JavaScript class constructor from which the database handle was instantiated and the constructor from which the query handle was instantiated in napi_refs can help, because napi_instanceof() can then be used to ensure that the instances passed into queryHashRecords() are indeed of the correct type.

Unfortunately, napi_instanceof() does not protect against prototype manipulation. For example, the prototype of the database handle instance can be set to the prototype of the constructor for query handle instances. In this case, the database handle instance can appear as a query handle instance, and it will pass the napi_instanceof() test for a query handle instance, while still containing a pointer to a database handle.

To this end, Node-API provides type-tagging capabilities.

A type tag is a 128-bit integer unique to the addon. Node-API provides the napi_type_tag structure for storing a type tag. When such a value is passed along with a JavaScript object stored in a napi_value to napi_type_tag_object(), the JavaScript object will be "marked" with the type tag. The "mark" is invisible on the JavaScript side. When a JavaScript object arrives into a native binding, napi_check_object_type_tag() can be used along with the original type tag to determine whether the JavaScript object was previously "marked" with the type tag. This creates a type-checking capability of a higher fidelity than napi_instanceof() can provide, because such type- tagging survives prototype manipulation and addon unloading/reloading.

Continuing the above example, the following skeleton addon implementation illustrates the use of napi_type_tag_object() and napi_check_object_type_tag().

// This value is the type tag for a database handle. The command
//
//   uuidgen | sed -r -e 's/-//g' -e 's/(.{16})(.*)/0x\1, 0x\2/'
//
// can be used to obtain the two values with which to initialize the structure.
static const napi_type_tag DatabaseHandleTypeTag = {
  0x1edf75a38336451d, 0xa5ed9ce2e4c00c38
};

// This value is the type tag for a query handle.
static const napi_type_tag QueryHandleTypeTag = {
  0x9c73317f9fad44a3, 0x93c3920bf3b0ad6a
};

static napi_value
openDatabase(napi_env env, napi_callback_info info) {
  napi_status status;
  napi_value result;

  // Perform the underlying action which results in a database handle.
  DatabaseHandle* dbHandle = open_database();

  // Create a new, empty JS object.
  status = napi_create_object(env, &result);
  if (status != napi_ok) return NULL;

  // Tag the object to indicate that it holds a pointer to a `DatabaseHandle`.
  status = napi_type_tag_object(env, result, &DatabaseHandleTypeTag);
  if (status != napi_ok) return NULL;

  // Store the pointer to the `DatabaseHandle` structure inside the JS object.
  status = napi_wrap(env, result, dbHandle, NULL, NULL, NULL);
  if (status != napi_ok) return NULL;

  return result;
}

// Later when we receive a JavaScript object purporting to be a database handle
// we can use `napi_check_object_type_tag()` to ensure that it is indeed such a
// handle.

static napi_value
query(napi_env env, napi_callback_info info) {
  napi_status status;
  size_t argc = 2;
  napi_value argv[2];
  bool is_db_handle;

  status = napi_get_cb_info(env, info, &argc, argv, NULL, NULL);
  if (status != napi_ok) return NULL;

  // Check that the object passed as the first parameter has the previously
  // applied tag.
  status = napi_check_object_type_tag(env,
                                      argv[0],
                                      &DatabaseHandleTypeTag,
                                      &is_db_handle);
  if (status != napi_ok) return NULL;

  // Throw a `TypeError` if it doesn't.
  if (!is_db_handle) {
    // Throw a TypeError.
    return NULL;
  }
}

napi_define_class#

Added in: v8.0.0 N-API version: 1 
napi_status napi_define_class(napi_env env,
                              const char* utf8name,
                              size_t length,
                              napi_callback constructor,
                              void* data,
                              size_t property_count,
                              const napi_property_descriptor* properties,
                              napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] utf8name: Name of the JavaScript constructor function; When wrapping a C++ class, we recommend for clarity that this name be the same as that of the C++ class.
  • [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] constructor: Callback function that handles constructing instances of the class. When wrapping a C++ class, this method must be a static member with the napi_callback signature. A C++ class constructor cannot be used. napi_callback provides more details.
  • [in] data: Optional data to be passed to the constructor callback as the data property of the callback info.
  • [in] property_count: Number of items in the properties array argument.
  • [in] properties: Array of property descriptors describing static and instance data properties, accessors, and methods on the class See napi_property_descriptor.
  • [out] result: A napi_value representing the constructor function for the class.

Returns napi_ok if the API succeeded.

Defines a JavaScript class, including:

  • A JavaScript constructor function that has the class name. When wrapping a corresponding C++ class, the callback passed via constructor can be used to instantiate a new C++ class instance, which can then be placed inside the JavaScript object instance being constructed using napi_wrap.
  • Properties on the constructor function whose implementation can call corresponding static data properties, accessors, and methods of the C++ class (defined by property descriptors with the napi_static attribute).
  • Properties on the constructor function's prototype object. When wrapping a C++ class, non-static data properties, accessors, and methods of the C++ class can be called from the static functions given in the property descriptors without the napi_static attribute after retrieving the C++ class instance placed inside the JavaScript object instance by using napi_unwrap.

When wrapping a C++ class, the C++ constructor callback passed via constructor should be a static method on the class that calls the actual class constructor, then wraps the new C++ instance in a JavaScript object, and returns the wrapper object. See napi_wrap for details.

The JavaScript constructor function returned from napi_define_class is often saved and used later to construct new instances of the class from native code, and/or to check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, a strong persistent reference to it can be created using napi_create_reference, ensuring that the reference count is kept >= 1.

Any non-NULL data which is passed to this API via the data parameter or via the data field of the napi_property_descriptor array items can be associated with the resulting JavaScript constructor (which is returned in the result parameter) and freed whenever the class is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

napi_wrap#

Added in: v8.0.0 N-API version: 1 
napi_status napi_wrap(napi_env env,
                      napi_value js_object,
                      void* native_object,
                      napi_finalize finalize_cb,
                      void* finalize_hint,
                      napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object that will be the wrapper for the native object.
  • [in] native_object: The native instance that will be wrapped in the JavaScript object.
  • [in] finalize_cb: Optional native callback that can be used to free the native instance when the JavaScript object has been garbage-collected. napi_finalize provides more details.
  • [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
  • [out] result: Optional reference to the wrapped object.

Returns napi_ok if the API succeeded.

Wraps a native instance in a JavaScript object. The native instance can be retrieved later using napi_unwrap().

When JavaScript code invokes a constructor for a class that was defined using napi_define_class(), the napi_callback for the constructor is invoked. After constructing an instance of the native class, the callback must then call napi_wrap() to wrap the newly constructed instance in the already-created JavaScript object that is the this argument to the constructor callback. (That this object was created from the constructor function's prototype, so it already has definitions of all the instance properties and methods.)

Typically when wrapping a class instance, a finalize callback should be provided that simply deletes the native instance that is received as the data argument to the finalize callback.

The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Finalizer callbacks may be deferred, leaving a window where the object has been garbage collected (and the weak reference is invalid) but the finalizer hasn't been called yet. When using napi_get_reference_value() on weak references returned by napi_wrap(), you should still handle an empty result.

Calling napi_wrap() a second time on an object will return an error. To associate another native instance with the object, use napi_remove_wrap() first.

napi_unwrap#

Added in: v8.0.0 N-API version: 1 
napi_status napi_unwrap(napi_env env,
                        napi_value js_object,
                        void** result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The object associated with the native instance.
  • [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in a JavaScript object using napi_wrap().

When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback is invoked. If the callback is for an instance method or accessor, then the this argument to the callback is the wrapper object; the wrapped C++ instance that is the target of the call can be obtained then by calling napi_unwrap() on the wrapper object.

napi_remove_wrap#

Added in: v8.5.0 N-API version: 1 
napi_status napi_remove_wrap(napi_env env,
                             napi_value js_object,
                             void** result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The object associated with the native instance.
  • [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in the JavaScript object js_object using napi_wrap() and removes the wrapping. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected.

napi_type_tag_object#

Added in: v14.8.0, v12.19.0 N-API version: 8 
napi_status napi_type_tag_object(napi_env env,
                                 napi_value js_object,
                                 const napi_type_tag* type_tag);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object to be marked.
  • [in] type_tag: The tag with which the object is to be marked.

Returns napi_ok if the API succeeded.

Associates the value of the type_tag pointer with the JavaScript object. napi_check_object_type_tag() can then be used to compare the tag that was attached to the object with one owned by the addon to ensure that the object has the right type.

If the object already has an associated type tag, this API will return napi_invalid_arg.

napi_check_object_type_tag#

Added in: v14.8.0, v12.19.0 N-API version: 8 
napi_status napi_check_object_type_tag(napi_env env,
                                       napi_value js_object,
                                       const napi_type_tag* type_tag,
                                       bool* result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object whose type tag to examine.
  • [in] type_tag: The tag with which to compare any tag found on the object.
  • [out] result: Whether the type tag given matched the type tag on the object. false is also returned if no type tag was found on the object.

Returns napi_ok if the API succeeded.

Compares the pointer given as type_tag with any that can be found on js_object. If no tag is found on js_object or, if a tag is found but it does not match type_tag, then result is set to false. If a tag is found and it matches type_tag, then result is set to true.

napi_add_finalizer#

Added in: v8.0.0 N-API version: 5 
napi_status napi_add_finalizer(napi_env env,
                               napi_value js_object,
                               void* finalize_data,
                               napi_finalize finalize_cb,
                               void* finalize_hint,
                               napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object to which the native data will be attached.
  • [in] finalize_data: Optional data to be passed to finalize_cb.
  • [in] finalize_cb: Native callback that will be used to free the native data when the JavaScript object has been garbage-collected. napi_finalize provides more details.
  • [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
  • [out] result: Optional reference to the JavaScript object.

Returns napi_ok if the API succeeded.

Adds a napi_finalize callback which will be called when the JavaScript object in js_object has been garbage-collected.

This API can be called multiple times on a single JavaScript object.

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Simple asynchronous operations#

Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This allows them to avoid blocking overall execution of the Node.js application.

Node-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.

Node-API defines the napi_async_work structure which is used to manage asynchronous workers. Instances are created/deleted with napi_create_async_work and napi_delete_async_work.

The execute and complete callbacks are functions that will be invoked when the executor is ready to execute and when it completes its task respectively.

The execute function should avoid making any Node-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make Node-API calls should be made in complete callback instead. Avoid using the napi_env parameter in the execute callback as it will likely execute JavaScript.

These functions implement the following interfaces:

typedef void (*napi_async_execute_callback)(napi_env env,
                                            void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

When these methods are invoked, the data parameter passed will be the addon-provided void* data that was passed into the napi_create_async_work call.

Once created the async worker can be queued for execution using the napi_queue_async_work function:

napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);

napi_cancel_async_work can be used if the work needs to be cancelled before the work has started execution.

After calling napi_cancel_async_work, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even when it was cancelled.

napi_create_async_work#

History
VersionChanges
v8.6.0

Added async_resource and async_resource_name parameters.

v8.0.0

Added in: v8.0.0

N-API version: 1 
napi_status napi_create_async_work(napi_env env,
                                   napi_value async_resource,
                                   napi_value async_resource_name,
                                   napi_async_execute_callback execute,
                                   napi_async_complete_callback complete,
                                   void* data,
                                   napi_async_work* result);
  • [in] env: The environment that the API is invoked under.
  • [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
  • [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [in] execute: The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread.
  • [in] complete: The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread. napi_async_complete_callback provides more details.
  • [in] data: User-provided data context. This will be passed back into the execute and complete functions.
  • [out] result: napi_async_work* which is the handle to the newly created async work.

Returns napi_ok if the API succeeded.

This API allocates a work object that is used to execute logic asynchronously. It should be freed using napi_delete_async_work once the work is no longer required.

async_resource_name should be a null-terminated, UTF-8-encoded string.

The async_resource_name identifier is provided by the user and should be representative of the type of async work being performed. It is also recommended to apply namespacing to the identifier, e.g. by including the module name. See the async_hooks documentation for more information.

napi_delete_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_delete_async_work(napi_env env,
                                   napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API frees a previously allocated work object.

This API can be called even if there is a pending JavaScript exception.

napi_queue_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API requests that the previously allocated work be scheduled for execution. Once it returns successfully, this API must not be called again with the same napi_async_work item or the result will be undefined.

napi_cancel_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_cancel_async_work(napi_env env,
                                   napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled and napi_generic_failure will be returned. If successful, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even if it has been successfully cancelled.

This API can be called even if there is a pending JavaScript exception.

Custom asynchronous operations#

The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime.

napi_async_init#

Added in: v8.6.0 N-API version: 1 
napi_status napi_async_init(napi_env env,
                            napi_value async_resource,
                            napi_value async_resource_name,
                            napi_async_context* result)
  • [in] env: The environment that the API is invoked under.
  • [in] async_resource: Object associated with the async work that will be passed to possible async_hooks init hooks and can be accessed by async_hooks.executionAsyncResource().
  • [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [out] result: The initialized async context.

Returns napi_ok if the API succeeded.

The async_resource object needs to be kept alive until napi_async_destroy to keep async_hooks related API acts correctly. In order to retain ABI compatibility with previous versions, napi_async_contexts are not maintaining the strong reference to the async_resource objects to avoid introducing causing memory leaks. However, if the async_resource is garbage collected by JavaScript engine before the napi_async_context was destroyed by napi_async_destroy, calling napi_async_context related APIs like napi_open_callback_scope and napi_make_callback can cause problems like loss of async context when using the AsyncLocalStorage API.

In order to retain ABI compatibility with previous versions, passing NULL for async_resource does not result in an error. However, this is not recommended as this will result poor results with async_hooks init hooks and async_hooks.executionAsyncResource() as the resource is now required by the underlying async_hooks implementation in order to provide the linkage between async callbacks.

napi_async_destroy#

Added in: v8.6.0 N-API version: 1 
napi_status napi_async_destroy(napi_env env,
                               napi_async_context async_context);
  • [in] env: The environment that the API is invoked under.
  • [in] async_context: The async context to be destroyed.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_make_callback#

History
VersionChanges
v8.6.0

Added async_context parameter.

v8.0.0

Added in: v8.0.0

N-API version: 1 
NAPI_EXTERN napi_status napi_make_callback(napi_env env,
                                           napi_async_context async_context,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] async_context: Context for the async operation that is invoking the callback. This should normally be a value previously obtained from napi_async_init. In order to retain ABI compatibility with previous versions, passing NULL for async_context does not result in an error. However, this results in incorrect operation of async hooks. Potential issues include loss of async context when using the AsyncLocalStorage API.
  • [in] recv: The this value passed to the called function.
  • [in] func: napi_value representing the JavaScript function to be invoked.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of JavaScript values as napi_value representing the arguments to the function. If argc is zero this parameter may be omitted by passing in NULL.
  • [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This API is similar to napi_call_function. However, it is used to call from native code back into JavaScript after returning from an async operation (when there is no other script on the stack). It is a fairly simple wrapper around node::MakeCallback.

Note it is not necessary to use napi_make_callback from within a napi_async_complete_callback; in that situation the callback's async context has already been set up, so a direct call to napi_call_function is sufficient and appropriate. Use of the napi_make_callback function may be required when implementing custom async behavior that does not use napi_create_async_work.

Any process.nextTicks or Promises scheduled on the microtask queue by JavaScript during the callback are ran before returning back to C/C++.

napi_open_callback_scope#

Added in: v9.6.0 N-API version: 3 
NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env,
                                                 napi_value resource_object,
                                                 napi_async_context context,
                                                 napi_callback_scope* result)
  • [in] env: The environment that the API is invoked under.
  • [in] resource_object: An object associated with the async work that will be passed to possible async_hooks init hooks. This parameter has been deprecated and is ignored at runtime. Use the async_resource parameter in napi_async_init instead.
  • [in] context: Context for the async operation that is invoking the callback. This should be a value previously obtained from napi_async_init.
  • [out] result: The newly created scope.

There are cases (for example, resolving promises) where it is necessary to have the equivalent of the scope associated with a callback in place when making certain Node-API calls. If there is no other script on the stack the napi_open_callback_scope and napi_close_callback_scope functions can be used to open/close the required scope.

napi_close_callback_scope#

Added in: v9.6.0 N-API version: 3 
NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env,
                                                  napi_callback_scope scope)
  • [in] env: The environment that the API is invoked under.
  • [in] scope: The scope to be closed.

This API can be called even if there is a pending JavaScript exception.

Version management#

napi_get_node_version#

Added in: v8.4.0 N-API version: 1 
typedef struct {
  uint32_t major;
  uint32_t minor;
  uint32_t patch;
  const char* release;
} napi_node_version;

napi_status napi_get_node_version(napi_env env,
                                  const napi_node_version** version);
  • [in] env: The environment that the API is invoked under.
  • [out] version: A pointer to version information for Node.js itself.

Returns napi_ok if the API succeeded.

This function fills the version struct with the major, minor, and patch version of Node.js that is currently running, and the release field with the value of process.release.name.

The returned buffer is statically allocated and does not need to be freed.

napi_get_version#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_version(napi_env env,
                             uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The highest version of Node-API supported.

Returns napi_ok if the API succeeded.

This API returns the highest Node-API version supported by the Node.js runtime. Node-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:

  • Call napi_get_version() to determine if the API is available.
  • If available, dynamically load a pointer to the function using uv_dlsym().
  • Use the dynamically loaded pointer to invoke the function.
  • If the function is not available, provide an alternate implementation that does not use the function.

Memory management#

napi_adjust_external_memory#

Added in: v8.5.0 N-API version: 1 
NAPI_EXTERN napi_status napi_adjust_external_memory(napi_env env,
                                                    int64_t change_in_bytes,
                                                    int64_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] change_in_bytes: The change in externally allocated memory that is kept alive by JavaScript objects.
  • [out] result: The adjusted value

Returns napi_ok if the API succeeded.

This function gives V8 an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native addon). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise.

Promises#

Node-API provides facilities for creating Promise objects as described in Section 25.4 of the ECMA specification. It implements promises as a pair of objects. When a promise is created by napi_create_promise(), a "deferred" object is created and returned alongside the Promise. The deferred object is bound to the created Promise and is the only means to resolve or reject the Promise using napi_resolve_deferred() or napi_reject_deferred(). The deferred object that is created by napi_create_promise() is freed by napi_resolve_deferred() or napi_reject_deferred(). The Promise object may be returned to JavaScript where it can be used in the usual fashion.

For example, to create a promise and pass it to an asynchronous worker:

napi_deferred deferred;
napi_value promise;
napi_status status;

// Create the promise.
status = napi_create_promise(env, &deferred, &promise);
if (status != napi_ok) return NULL;

// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred);

// Return the promise to JS
return promise;

The above function do_something_asynchronous() would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred:

napi_deferred deferred;
napi_value undefined;
napi_status status;

// Create a value with which to conclude the deferred.
status = napi_get_undefined(env, &undefined);
if (status != napi_ok) return NULL;

// Resolve or reject the promise associated with the deferred depending on
// whether the asynchronous action succeeded.
if (asynchronous_action_succeeded) {
  status = napi_resolve_deferred(env, deferred, undefined);
} else {
  status = napi_reject_deferred(env, deferred, undefined);
}
if (status != napi_ok) return NULL;

// At this point the deferred has been freed, so we should assign NULL to it.
deferred = NULL;

napi_create_promise#

Added in: v8.5.0 N-API version: 1 
napi_status napi_create_promise(napi_env env,
                                napi_deferred* deferred,
                                napi_value* promise);
  • [in] env: The environment that the API is invoked under.
  • [out] deferred: A newly created deferred object which can later be passed to napi_resolve_deferred() or napi_reject_deferred() to resolve resp. reject the associated promise.
  • [out] promise: The JavaScript promise associated with the deferred object.

Returns napi_ok if the API succeeded.

This API creates a deferred object and a JavaScript promise.

napi_resolve_deferred#

Added in: v8.5.0 N-API version: 1 
napi_status napi_resolve_deferred(napi_env env,
                                  napi_deferred deferred,
                                  napi_value resolution);
  • [in] env: The environment that the API is invoked under.
  • [in] deferred: The deferred object whose associated promise to resolve.
  • [in] resolution: The value with which to resolve the promise.

This API resolves a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to resolve JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_reject_deferred#

Added in: v8.5.0 N-API version: 1 
napi_status napi_reject_deferred(napi_env env,
                                 napi_deferred deferred,
                                 napi_value rejection);
  • [in] env: The environment that the API is invoked under.
  • [in] deferred: The deferred object whose associated promise to resolve.
  • [in] rejection: The value with which to reject the promise.

This API rejects a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to reject JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_is_promise#

Added in: v8.5.0 N-API version: 1 
napi_status napi_is_promise(napi_env env,
                            napi_value value,
                            bool* is_promise);
  • [in] env: The environment that the API is invoked under.
  • [in] value: The value to examine
  • [out] is_promise: Flag indicating whether promise is a native promise object (that is, a promise object created by the underlying engine).

Script execution#

Node-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.

napi_run_script#

Added in: v8.5.0 N-API version: 1 
NAPI_EXTERN napi_status napi_run_script(napi_env env,
                                        napi_value script,
                                        napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] script: A JavaScript string containing the script to execute.
  • [out] result: The value resulting from having executed the script.

This function executes a string of JavaScript code and returns its result with the following caveats:

  • Unlike eval, this function does not allow the script to access the current lexical scope, and therefore also does not allow to access the module scope, meaning that pseudo-globals such as require will not be available.
  • The script can access the global scope. Function and var declarations in the script will be added to the global object. Variable declarations made using let and const will be visible globally, but will not be added to the global object.
  • The value of this is global within the script.

libuv event loop#

Node-API provides a function for getting the current event loop associated with a specific napi_env.

napi_get_uv_event_loop#

Added in: v9.3.0, v8.10.0 N-API version: 2 
NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               struct uv_loop_s** loop);
  • [in] env: The environment that the API is invoked under.
  • [out] loop: The current libuv loop instance.

Asynchronous thread-safe function calls#

JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then Node-API functions that require a napi_env, napi_value, or napi_ref must not be called from those threads.

When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.

These APIs provide the type napi_threadsafe_function as well as APIs to create, destroy, and call objects of this type. napi_create_threadsafe_function() creates a persistent reference to a napi_value that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function.

Upon creation of a napi_threadsafe_function a napi_finalize callback can be provided. This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction, and provides an opportunity for cleaning up after the threads e.g. by calling uv_thread_join(). Aside from the main loop thread, no threads should be using the thread-safe function after the finalize callback completes.

The context given during the call to napi_create_threadsafe_function() can be retrieved from any thread with a call to napi_get_threadsafe_function_context().

Calling a thread-safe function#

napi_call_threadsafe_function() can be used for initiating a call into JavaScript. napi_call_threadsafe_function() accepts a parameter which controls whether the API behaves blockingly. If set to napi_tsfn_nonblocking, the API behaves non-blockingly, returning napi_queue_full if the queue was full, preventing data from being successfully added to the queue. If set to napi_tsfn_blocking, the API blocks until space becomes available in the queue. napi_call_threadsafe_function() never blocks if the thread-safe function was created with a maximum queue size of 0.

napi_call_threadsafe_function() should not be called with napi_tsfn_blocking from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

The actual call into JavaScript is controlled by the callback given via the call_js_cb parameter. call_js_cb is invoked on the main thread once for each value that was placed into the queue by a successful call to napi_call_threadsafe_function(). If such a callback is not given, a default callback will be used, and the resulting JavaScript call will have no arguments. The call_js_cb callback receives the JavaScript function to call as a napi_value in its parameters, as well as the void* context pointer used when creating the napi_threadsafe_function, and the next data pointer that was created by one of the secondary threads. The callback can then use an API such as napi_call_function() to call into JavaScript.

The callback may also be invoked with env and call_js_cb both set to NULL to indicate that calls into JavaScript are no longer possible, while items remain in the queue that may need to be freed. This normally occurs when the Node.js process exits while there is a thread-safe function still active.

It is not necessary to call into JavaScript via napi_make_callback() because Node-API runs call_js_cb in a context appropriate for callbacks.

Reference counting of thread-safe functions#

Threads can be added to and removed from a napi_threadsafe_function object during its existence. Thus, in addition to specifying an initial number of threads upon creation, napi_acquire_threadsafe_function can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, napi_release_threadsafe_function can be called to indicate that an existing thread will stop making use of the thread-safe function.

napi_threadsafe_function objects are destroyed when every thread which uses the object has called napi_release_threadsafe_function() or has received a return status of napi_closing in response to a call to napi_call_threadsafe_function. The queue is emptied before the napi_threadsafe_function is destroyed. napi_release_threadsafe_function() should be the last API call made in conjunction with a given napi_threadsafe_function, because after the call completes, there is no guarantee that the napi_threadsafe_function is still allocated. For the same reason, do not use a thread-safe function after receiving a return value of napi_closing in response to a call to napi_call_threadsafe_function. Data associated with the napi_threadsafe_function can be freed in its napi_finalize callback which was passed to napi_create_threadsafe_function(). The parameter initial_thread_count of napi_create_threadsafe_function marks the initial number of acquisitions of the thread-safe functions, instead of calling napi_acquire_threadsafe_function multiple times at creation.

Once the number of threads making use of a napi_threadsafe_function reaches zero, no further threads can start making use of it by calling napi_acquire_threadsafe_function(). In fact, all subsequent API calls associated with it, except napi_release_threadsafe_function(), will return an error value of napi_closing.

The thread-safe function can be "aborted" by giving a value of napi_tsfn_abort to napi_release_threadsafe_function(). This will cause all subsequent APIs associated with the thread-safe function except napi_release_threadsafe_function() to return napi_closing even before its reference count reaches zero. In particular, napi_call_threadsafe_function() will return napi_closing, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of napi_closing from napi_call_threadsafe_function() a thread must not use the thread-safe function anymore because it is no longer guaranteed to be allocated.

Deciding whether to keep the process running#

Similarly to libuv handles, thread-safe functions can be "referenced" and "unreferenced". A "referenced" thread-safe function will cause the event loop on the thread on which it is created to remain alive until the thread-safe function is destroyed. In contrast, an "unreferenced" thread-safe function will not prevent the event loop from exiting. The APIs napi_ref_threadsafe_function and napi_unref_threadsafe_function exist for this purpose.

Neither does napi_unref_threadsafe_function mark the thread-safe functions as able to be destroyed nor does napi_ref_threadsafe_function prevent it from being destroyed.

napi_create_threadsafe_function#

History
VersionChanges
v12.6.0, v10.17.0

Made func parameter optional with custom call_js_cb.

v10.6.0

Added in: v10.6.0

N-API version: 4 
NAPI_EXTERN napi_status
napi_create_threadsafe_function(napi_env env,
                                napi_value func,
                                napi_value async_resource,
                                napi_value async_resource_name,
                                size_t max_queue_size,
                                size_t initial_thread_count,
                                void* thread_finalize_data,
                                napi_finalize thread_finalize_cb,
                                void* context,
                                napi_threadsafe_function_call_js call_js_cb,
                                napi_threadsafe_function* result);
  • [in] env: The environment that the API is invoked under.
  • [in] func: An optional JavaScript function to call from another thread. It must be provided if NULL is passed to call_js_cb.
  • [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
  • [in] async_resource_name: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [in] max_queue_size: Maximum size of the queue. 0 for no limit.
  • [in] initial_thread_count: The initial number of acquisitions, i.e. the initial number of threads, including the main thread, which will be making use of this function.
  • [in] thread_finalize_data: Optional data to be passed to thread_finalize_cb.
  • [in] thread_finalize_cb: Optional function to call when the napi_threadsafe_function is being destroyed.
  • [in] context: Optional data to attach to the resulting napi_threadsafe_function.
  • [in] call_js_cb: Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and with undefined as its this value. napi_threadsafe_function_call_js provides more details.
  • [out] result: The asynchronous thread-safe JavaScript function.

napi_get_threadsafe_function_context#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_get_threadsafe_function_context(napi_threadsafe_function func,
                                     void** result);
  • [in] func: The thread-safe function for which to retrieve the context.
  • [out] result: The location where to store the context.

This API may be called from any thread which makes use of func.

napi_call_threadsafe_function#

History
VersionChanges
v14.5.0

Support for napi_would_deadlock has been reverted.

v14.1.0

Return napi_would_deadlock when called with napi_tsfn_blocking from the main thread or a worker thread and the queue is full.

v10.6.0

Added in: v10.6.0

N-API version: 4 
NAPI_EXTERN napi_status
napi_call_threadsafe_function(napi_threadsafe_function func,
                              void* data,
                              napi_threadsafe_function_call_mode is_blocking);
  • [in] func: The asynchronous thread-safe JavaScript function to invoke.
  • [in] data: Data to send into JavaScript via the callback call_js_cb provided during the creation of the thread-safe JavaScript function.
  • [in] is_blocking: Flag whose value can be either napi_tsfn_blocking to indicate that the call should block if the queue is full or napi_tsfn_nonblocking to indicate that the call should return immediately with a status of napi_queue_full whenever the queue is full.

This API should not be called with napi_tsfn_blocking from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

This API will return napi_closing if napi_release_threadsafe_function() was called with abort set to napi_tsfn_abort from any thread. The value is only added to the queue if the API returns napi_ok.

This API may be called from any thread which makes use of func.

napi_acquire_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_acquire_threadsafe_function(napi_threadsafe_function func);
  • [in] func: The asynchronous thread-safe JavaScript function to start making use of.

A thread should call this API before passing func to any other thread-safe function APIs to indicate that it will be making use of func. This prevents func from being destroyed when all other threads have stopped making use of it.

This API may be called from any thread which will start making use of func.

napi_release_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_release_threadsafe_function(napi_threadsafe_function func,
                                 napi_threadsafe_function_release_mode mode);
  • [in] func: The asynchronous thread-safe JavaScript function whose reference count to decrement.
  • [in] mode: Flag whose value can be either napi_tsfn_release to indicate that the current thread will make no further calls to the thread-safe function, or napi_tsfn_abort to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set to napi_tsfn_abort, further calls to napi_call_threadsafe_function() will return napi_closing, and no further values will be placed in the queue.

A thread should call this API when it stops making use of func. Passing func to any thread-safe APIs after having called this API has undefined results, as func may have been destroyed.

This API may be called from any thread which will stop making use of func.

napi_ref_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func);
  • [in] env: The environment that the API is invoked under.
  • [in] func: The thread-safe function to reference.

This API is used to indicate that the event loop running on the main thread should not exit until func has been destroyed. Similar to uv_ref it is also idempotent.

Neither does napi_unref_threadsafe_function mark the thread-safe functions as able to be destroyed nor does napi_ref_threadsafe_function prevent it from being destroyed. napi_acquire_threadsafe_function and napi_release_threadsafe_function are available for that purpose.

This API may only be called from the main thread.

napi_unref_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_unref_threadsafe_function(napi_env env, napi_threadsafe_function func);
  • [in] env: The environment that the API is invoked under.
  • [in] func: The thread-safe function to unreference.

This API is used to indicate that the event loop running on the main thread may exit before func is destroyed. Similar to uv_unref it is also idempotent.

This API may only be called from the main thread.

Miscellaneous utilities#

node_api_get_module_file_name#

Added in: v15.9.0, v14.18.0, v12.22.0

Stability: 1 - Experimental

NAPI_EXTERN napi_status
node_api_get_module_file_name(napi_env env, const char** result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: A URL containing the absolute path of the location from which the add-on was loaded. For a file on the local file system it will start with file://. The string is null-terminated and owned by env and must thus not be modified or freed.

result may be an empty string if the add-on loading process fails to establish the add-on's file name during loading.

C++ embedder API#

Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.

The documentation for these APIs can be found in src/node.h in the Node.js source tree. In addition to the APIs exposed by Node.js, some required concepts are provided by the V8 embedder API.

Because using Node.js as an embedded library is different from writing code that is executed by Node.js, breaking changes do not follow typical Node.js deprecation policy and may occur on each semver-major release without prior warning.

Example embedding application#

The following sections will provide an overview over how to use these APIs to create an application from scratch that will perform the equivalent of node -e <code>, i.e. that will take a piece of JavaScript and run it in a Node.js-specific environment.

The full code can be found in the Node.js source tree.

Setting up per-process state#

Node.js requires some per-process state management in order to run:

  • Arguments parsing for Node.js CLI options,
  • V8 per-process requirements, such as a v8::Platform instance.

The following example shows how these can be set up. Some class names are from the node and v8 C++ namespaces, respectively.

int main(int argc, char** argv) {
  argv = uv_setup_args(argc, argv);
  std::vector<std::string> args(argv, argv + argc);
  // Parse Node.js CLI options, and print any errors that have occurred while
  // trying to parse them.
  std::unique_ptr<node::InitializationResult> result =
      node::InitializeOncePerProcess(args, {
        node::ProcessInitializationFlags::kNoInitializeV8,
        node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
      });

  for (const std::string& error : result->errors())
    fprintf(stderr, "%s: %s\n", args[0].c_str(), error.c_str());
  if (result->early_return() != 0) {
    return result->exit_code();
  }

  // Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way
  // to create a v8::Platform instance that Node.js can use when creating
  // Worker threads. When no `MultiIsolatePlatform` instance is present,
  // Worker threads are disabled.
  std::unique_ptr<MultiIsolatePlatform> platform =
      MultiIsolatePlatform::Create(4);
  V8::InitializePlatform(platform.get());
  V8::Initialize();

  // See below for the contents of this function.
  int ret = RunNodeInstance(
      platform.get(), result->args(), result->exec_args());

  V8::Dispose();
  V8::DisposePlatform();

  node::TearDownOncePerProcess();
  return ret;
}

Per-instance state#

History
VersionChanges
v15.0.0

The CommonEnvironmentSetup and SpinEventLoop utilities were added.

Node.js has a concept of a “Node.js instance”, that is commonly being referred to as node::Environment. Each node::Environment is associated with:

  • Exactly one v8::Isolate, i.e. one JS Engine instance,
  • Exactly one uv_loop_t, i.e. one event loop, and
  • A number of v8::Contexts, but exactly one main v8::Context.
  • One node::IsolateData instance that contains information that could be shared by multiple node::Environments that use the same v8::Isolate. Currently, no testing if performed for this scenario.

In order to set up a v8::Isolate, an v8::ArrayBuffer::Allocator needs to be provided. One possible choice is the default Node.js allocator, which can be created through node::ArrayBufferAllocator::Create(). Using the Node.js allocator allows minor performance optimizations when addons use the Node.js C++ Buffer API, and is required in order to track ArrayBuffer memory in process.memoryUsage().

Additionally, each v8::Isolate that is used for a Node.js instance needs to be registered and unregistered with the MultiIsolatePlatform instance, if one is being used, in order for the platform to know which event loop to use for tasks scheduled by the v8::Isolate.

The node::NewIsolate() helper function creates a v8::Isolate, sets it up with some Node.js-specific hooks (e.g. the Node.js error handler), and registers it with the platform automatically.

int RunNodeInstance(MultiIsolatePlatform* platform,
                    const std::vector<std::string>& args,
                    const std::vector<std::string>& exec_args) {
  int exit_code = 0;

  // Setup up a libuv event loop, v8::Isolate, and Node.js Environment.
  std::vector<std::string> errors;
  std::unique_ptr<CommonEnvironmentSetup> setup =
      CommonEnvironmentSetup::Create(platform, &errors, args, exec_args);
  if (!setup) {
    for (const std::string& err : errors)
      fprintf(stderr, "%s: %s\n", args[0].c_str(), err.c_str());
    return 1;
  }

  Isolate* isolate = setup->isolate();
  Environment* env = setup->env();

  {
    Locker locker(isolate);
    Isolate::Scope isolate_scope(isolate);
    HandleScope handle_scope(isolate);
    // The v8::Context needs to be entered when node::CreateEnvironment() and
    // node::LoadEnvironment() are being called.
    Context::Scope context_scope(setup->context());

    // Set up the Node.js instance for execution, and run code inside of it.
    // There is also a variant that takes a callback and provides it with
    // the `require` and `process` objects, so that it can manually compile
    // and run scripts as needed.
    // The `require` function inside this script does *not* access the file
    // system, and can only load built-in Node.js modules.
    // `module.createRequire()` is being used to create one that is able to
    // load files from the disk, and uses the standard CommonJS file loader
    // instead of the internal-only `require` function.
    MaybeLocal<Value> loadenv_ret = node::LoadEnvironment(
        env,
        "const publicRequire ="
        "  require('node:module').createRequire(process.cwd() + '/');"
        "globalThis.require = publicRequire;"
        "require('node:vm').runInThisContext(process.argv[1]);");

    if (loadenv_ret.IsEmpty())  // There has been a JS exception.
      return 1;

    exit_code = node::SpinEventLoop(env).FromMaybe(1);

    // node::Stop() can be used to explicitly stop the event loop and keep
    // further JavaScript from running. It can be called from any thread,
    // and will act like worker.terminate() if called from another thread.
    node::Stop(env);
  }

  return exit_code;
}

Child process#

Stability: 2 - Stable

Source Code: lib/child_process.js

The node:child_process module provides the ability to spawn subprocesses in a manner that is similar, but not identical, to popen(3). This capability is primarily provided by the child_process.spawn() function:

const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

By default, pipes for stdin, stdout, and stderr are established between the parent Node.js process and the spawned subprocess. These pipes have limited (and platform-specific) capacity. If the subprocess writes to stdout in excess of that limit without the output being captured, the subprocess blocks waiting for the pipe buffer to accept more data. This is identical to the behavior of pipes in the shell. Use the { stdio: 'ignore' } option if the output will not be consumed.

The command lookup is performed using the options.env.PATH environment variable if env is in the options object. Otherwise, process.env.PATH is used. If options.env is set without PATH, lookup on Unix is performed on a default search path search of /usr/bin:/bin (see your operating system's manual for execvpe/execvp), on Windows the current processes environment variable PATH is used.

On Windows, environment variables are case-insensitive. Node.js lexicographically sorts the env keys and uses the first one that case-insensitively matches. Only first (in lexicographic order) entry will be passed to the subprocess. This might lead to issues on Windows when passing objects to the env option that have multiple variants of the same key, such as PATH and Path.

The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the node:child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync().

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

Asynchronous process creation#

The child_process.spawn(), child_process.fork(), child_process.exec(), and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates.

Spawning .bat and .cmd files on Windows#

The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix-type operating systems (Unix, Linux, macOS) child_process.execFile() can be more efficient because it does not spawn a shell by default. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.

// On Windows Only...
const { spawn } = require('node:child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);

bat.stdout.on('data', (data) => {
  console.log(data.toString());
});

bat.stderr.on('data', (data) => {
  console.error(data.toString());
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});
// OR...
const { exec, spawn } = require('node:child_process');
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});

child_process.exec(command[, options][, callback])#

History
VersionChanges
v15.4.0

AbortSignal support was added.

v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v8.8.0

The windowsHide option is supported now.

v0.1.90

Added in: v0.1.90

Spawns a shell then executes the command within that shell, buffering any generated output. The command string passed to the exec function is processed directly by the shell and special characters (vary based on shell) need to be dealt with accordingly:

const { exec } = require('node:child_process');

exec('"/path/to/test file/test.sh" arg1 arg2');
// Double quotes are used so that the space in the path is not interpreted as
// a delimiter of multiple arguments.

exec('echo "The \\$HOME variable is $HOME"');
// The $HOME variable is escaped in the first instance, but not in the second.

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

If a callback function is provided, it is called with the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error. The error.code property will be the exit code of the process. By convention, any exit code other than 0 indicates an error. error.signal will be the signal that terminated the process.

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

const { exec } = require('node:child_process');
exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.error(`stderr: ${stderr}`);
});

If timeout is greater than 0, the parent will send the signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr.

const util = require('node:util');
const exec = util.promisify(require('node:child_process').exec);

async function lsExample() {
  const { stdout, stderr } = await exec('ls');
  console.log('stdout:', stdout);
  console.error('stderr:', stderr);
}
lsExample();

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError:

const { exec } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = exec('grep ssh', { signal }, (error) => {
  console.error(error); // an AbortError
});
controller.abort();

child_process.execFile(file[, args][, options][, callback])#

History
VersionChanges
v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v15.4.0, v14.17.0

AbortSignal support was added.

v8.8.0

The windowsHide option is supported now.

v0.1.91

Added in: v0.1.91

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell by default. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec().

The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

const { execFile } = require('node:child_process');
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
  if (error) {
    throw error;
  }
  console.log(stdout);
});

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr.

const util = require('node:util');
const execFile = util.promisify(require('node:child_process').execFile);
async function getVersion() {
  const { stdout } = await execFile('node', ['--version']);
  console.log(stdout);
}
getVersion();

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError:

const { execFile } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = execFile('node', ['--version'], { signal }, (error) => {
  console.error(error); // an AbortError
});
controller.abort();

child_process.fork(modulePath[, args][, options])#

History
VersionChanges
v17.4.0, v16.14.0

The modulePath parameter can be a WHATWG URL object using file: protocol.

v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v15.13.0, v14.18.0

timeout was added.

v15.11.0, v14.18.0

killSignal for AbortSignal was added.

v15.6.0, v14.17.0

AbortSignal support was added.

v13.2.0, v12.16.0

The serialization option is supported now.

v8.0.0

The stdio option can now be a string.

v6.4.0

The stdio option is supported now.

v0.5.0

Added in: v0.5.0

  • modulePath <string> | <URL> The module to run in the child.
  • args <string[]> List of string arguments.
  • options <Object>
    • cwd <string> | <URL> Current working directory of the child process.
    • detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached).
    • env <Object> Environment key-value pairs. Default: process.env.
    • execPath <string> Executable used to create the child process.
    • execArgv <string[]> List of string arguments passed to the executable. Default: process.execArgv.
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for more details. Default: 'json'.
    • signal <AbortSignal> Allows closing the child process using an AbortSignal.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: 'SIGTERM'.
    • silent <boolean> If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the 'pipe' and 'inherit' options for child_process.spawn()'s stdio for more details. Default: false.
    • stdio <Array> | <string> See child_process.spawn()'s stdio. When this option is provided, it overrides silent. If the array variant is used, it must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false.
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
  • Returns: <ChildProcess>

The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn(), a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See subprocess.send() for details.

Keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used.

Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment variable NODE_CHANNEL_FD on the child process.

Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

The shell option available in child_process.spawn() is not supported by child_process.fork() and will be ignored if set.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError:

if (process.argv[2] === 'child') {
  setTimeout(() => {
    console.log(`Hello from ${process.argv[2]}!`);
  }, 1_000);
} else {
  const { fork } = require('node:child_process');
  const controller = new AbortController();
  const { signal } = controller;
  const child = fork(__filename, ['child'], { signal });
  child.on('error', (err) => {
    // This will be called with err being an AbortError if the controller aborts
  });
  controller.abort(); // Stops the child process
}

child_process.spawn(command[, args][, options])#

History
VersionChanges
v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v15.13.0, v14.18.0

timeout was added.

v15.11.0, v14.18.0

killSignal for AbortSignal was added.

v15.5.0, v14.17.0

AbortSignal support was added.

v13.2.0, v12.16.0

The serialization option is supported now.

v8.8.0

The windowsHide option is supported now.

v6.4.0

The argv0 option is supported now.

v5.7.0

The shell option is supported now.

v0.1.90

Added in: v0.1.90

  • command <string> The command to run.
  • args <string[]> List of string arguments.
  • options <Object>
    • cwd <string> | <URL> Current working directory of the child process.
    • env <Object> Environment key-value pairs. Default: process.env.
    • argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <Array> | <string> Child's stdio configuration (see options.stdio).
    • detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached).
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for more details. Default: 'json'.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: false (no shell).
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
    • signal <AbortSignal> allows aborting the child process using an AbortSignal.
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: 'SIGTERM'.
  • Returns: <ChildProcess>

The child_process.spawn() method spawns a new process using the given command, with command-line arguments in args. If omitted, args defaults to an empty array.

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

A third argument may be used to specify additional options, with these defaults:

const defaults = {
  cwd: undefined,
  env: process.env,
};

Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory. If given, but the path does not exist, the child process emits an ENOENT error and exits immediately. ENOENT is also emitted when the command does not exist.

Use env to specify environment variables that will be visible to the new process, the default is process.env.

undefined values in env will be ignored.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Example: A very elaborate way to run ps ax | grep ssh

const { spawn } = require('node:child_process');
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);

ps.stdout.on('data', (data) => {
  grep.stdin.write(data);
});

ps.stderr.on('data', (data) => {
  console.error(`ps stderr: ${data}`);
});

ps.on('close', (code) => {
  if (code !== 0) {
    console.log(`ps process exited with code ${code}`);
  }
  grep.stdin.end();
});

grep.stdout.on('data', (data) => {
  console.log(data.toString());
});

grep.stderr.on('data', (data) => {
  console.error(`grep stderr: ${data}`);
});

grep.on('close', (code) => {
  if (code !== 0) {
    console.log(`grep process exited with code ${code}`);
  }
});

Example of checking for failed spawn:

const { spawn } = require('node:child_process');
const subprocess = spawn('bad_command');

subprocess.on('error', (err) => {
  console.error('Failed to start subprocess.');
});

Certain platforms (macOS, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command.

Node.js overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent. Retrieve it with the process.argv0 property instead.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError:

const { spawn } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const grep = spawn('grep', ['ssh'], { signal });
grep.on('error', (err) => {
  // This will be called with err being an AbortError if the controller aborts
});
controller.abort(); // Stops the child process
options.detached#
Added in: v0.7.10

On Windows, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if options.detached is set to true, the child process will be made the leader of a new process group and session. Child processes may continue running after the parent exits regardless of whether they are detached or not. See setsid(2) for more information.

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent.

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination:

const { spawn } = require('node:child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();

Alternatively one can redirect the child process' output into files:

const fs = require('node:fs');
const { spawn } = require('node:child_process');
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');

const subprocess = spawn('prg', [], {
  detached: true,
  stdio: [ 'ignore', out, err ],
});

subprocess.unref();
options.stdio#
History
VersionChanges
v15.6.0, v14.18.0

Added the overlapped stdio flag.

v3.3.1

The value 0 is now accepted as a file descriptor.

v0.7.10

Added in: v0.7.10

The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding subprocess.stdin, subprocess.stdout, and subprocess.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'].

For convenience, options.stdio may be one of the following strings:

  • 'pipe': equivalent to ['pipe', 'pipe', 'pipe'] (the default)
  • 'overlapped': equivalent to ['overlapped', 'overlapped', 'overlapped']
  • 'ignore': equivalent to ['ignore', 'ignore', 'ignore']
  • 'inherit': equivalent to ['inherit', 'inherit', 'inherit'] or [0, 1, 2]

Otherwise, the value of options.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following:

  1. 'pipe': Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as subprocess.stdio[fd]. Pipes created for fds 0, 1, and 2 are also available as subprocess.stdin, subprocess.stdout and subprocess.stderr, respectively. These are not actual Unix pipes and therefore the child process can not use them by their descriptor files, e.g. /dev/fd/2 or /dev/stdout.

  2. 'overlapped': Same as 'pipe' except that the FILE_FLAG_OVERLAPPED flag is set on the handle. This is necessary for overlapped I/O on the child process's stdio handles. See the docs for more details. This is exactly the same as 'pipe' on non-Windows systems.

  3. 'ipc': Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the subprocess.send() method. If the child is a Node.js process, the presence of an IPC channel will enable process.send() and process.disconnect() methods, as well as 'disconnect' and 'message' events within the child.

    Accessing the IPC channel fd in any way other than process.send() or using the IPC channel with a child process that is not a Node.js instance is not supported.

  4. 'ignore': Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0, 1, and 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to open /dev/null and attach it to the child's fd.

  5. 'inherit': Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent to process.stdin, process.stdout, and process.stderr, respectively. In any other position, equivalent to 'ignore'.

  6. <Stream> object: Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. The stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).

  7. Positive integer: The integer value is interpreted as a file descriptor that is open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared. Passing sockets is not supported on Windows.

  8. null, undefined: Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

const { spawn } = require('node:child_process');

// Child will use parent's stdios.
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr.
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref()) until the child registers an event handler for the 'disconnect' event or the 'message' event. This allows the child to exit normally without the process being held open by the open IPC channel.

On Unix-like operating systems, the child_process.spawn() method performs memory operations synchronously before decoupling the event loop from the child. Applications with a large memory footprint may find frequent child_process.spawn() calls to be a bottleneck. For more information, see V8 issue 7381.

See also: child_process.exec() and child_process.fork().

Synchronous process creation#

The child_process.spawnSync(), child_process.execSync(), and child_process.execFileSync() methods are synchronous and will block the Node.js event loop, pausing execution of any additional code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

child_process.execFileSync(file[, args][, options])#

History
VersionChanges
v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v6.2.1, v4.5.0

The encoding option can now explicitly be set to buffer.

v0.11.12

Added in: v0.11.12

  • file <string> The name or path of the executable file to run.
  • args <string[]> List of string arguments.
  • options <Object>
    • cwd <string> | <URL> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe'.
    • env <Object> Environment key-value pairs. Default: process.env.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: false (no shell).
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited.

If the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw an Error that will include the full result of the underlying child_process.spawnSync().

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.execSync(command[, options])#

History
VersionChanges
v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v0.11.12

Added in: v0.11.12

  • command <string> The command to run.
  • options <Object>
    • cwd <string> | <URL> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe'.
    • env <Object> Environment key-value pairs. Default: process.env.
    • shell <string> Shell to execute the command with. See Shell requirements and Default Windows shell. Default: '/bin/sh' on Unix, process.env.ComSpec on Windows.
    • uid <number> Sets the user identity of the process. (See setuid(2)).
    • gid <number> Sets the group identity of the process. (See setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. If the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync().

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.spawnSync(command[, args][, options])#

History
VersionChanges
v16.4.0, v14.18.0

The cwd option can be a WHATWG URL object using file: protocol.

v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v5.7.0

The shell option is supported now.

v6.2.1, v4.5.0

The encoding option can now explicitly be set to buffer.

v0.11.12

Added in: v0.11.12

  • command <string> The command to run.
  • args <string[]> List of string arguments.
  • options <Object>
    • cwd <string> | <URL> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <string> | <Array> Child's stdio configuration.
    • env <Object> Environment key-value pairs. Default: process.env.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: false (no shell).
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • Returns: <Object>
    • pid <number> Pid of the child process.
    • output <Array> Array of results from stdio output.
    • stdout <Buffer> | <string> The contents of output[1].
    • stderr <Buffer> | <string> The contents of output[2].
    • status <number> | <null> The exit code of the subprocess, or null if the subprocess terminated due to a signal.
    • signal <string> | <null> The signal used to kill the subprocess, or null if the subprocess did not terminate due to a signal.
    • error <Error> The error object if the child process failed or timed out.

The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. If the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

Class: ChildProcess#

Added in: v2.2.0

Instances of the ChildProcess represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

Event: 'close'#

Added in: v0.7.7
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'close' event is emitted after a process has ended and the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams. The 'close' event will always emit after 'exit' was already emitted, or 'error' if the child failed to spawn.

const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process close all stdio with code ${code}`);
});

ls.on('exit', (code) => {
  console.log(`child process exited with code ${code}`);
});

Event: 'disconnect'#

Added in: v0.7.2

The 'disconnect' event is emitted after calling the subprocess.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the subprocess.connected property is false.

Event: 'error'#

The 'error' event is emitted whenever:

  • The process could not be spawned.
  • The process could not be killed.
  • Sending a message to the child process failed.
  • The child process was aborted via the signal option.

The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, guard against accidentally invoking handler functions multiple times.

See also subprocess.kill() and subprocess.send().

Event: 'exit'#

Added in: v0.1.90
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

When the 'exit' event is triggered, child process stdio streams might still be open.

Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

Event: 'message'#

Added in: v0.5.9

The 'message' event is triggered when a child process uses process.send() to send messages.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the serialization option was set to 'advanced' used when spawning the child process, the message argument can contain data that JSON is not able to represent. See Advanced serialization for more details.

Event: 'spawn'#

Added in: v15.1.0, v14.17.0

The 'spawn' event is emitted once the child process has spawned successfully. If the child process does not spawn successfully, the 'spawn' event is not emitted and the 'error' event is emitted instead.

If emitted, the 'spawn' event comes before all other events and before any data is received via stdout or stderr.

The 'spawn' event will fire regardless of whether an error occurs within the spawned process. For example, if bash some-command spawns successfully, the 'spawn' event will fire, though bash may fail to spawn some-command. This caveat also applies when using { shell: true }.

subprocess.channel#

History
VersionChanges
v14.0.0

The object no longer accidentally exposes native C++ bindings.

v7.1.0

Added in: v7.1.0

  • <Object> A pipe representing the IPC channel to the child process.

The subprocess.channel property is a reference to the child's IPC channel. If no IPC channel exists, this property is undefined.

subprocess.channel.ref()#
Added in: v7.1.0

This method makes the IPC channel keep the event loop of the parent process running if .unref() has been called before.

subprocess.channel.unref()#
Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.

subprocess.connected#

Added in: v0.7.2
  • <boolean> Set to false after subprocess.disconnect() is called.

The subprocess.connected property indicates whether it is still possible to send and receive messages from a child process. When subprocess.connected is false, it is no longer possible to send or receive messages.

subprocess.disconnect()#

Added in: v0.7.2

Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect().

When the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

subprocess.exitCode#

The subprocess.exitCode property indicates the exit code of the child process. If the child process is still running, the field will be null.

subprocess.kill([signal])#

Added in: v0.1.90

The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals. This function returns true if kill(2) succeeds, and false otherwise.

const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process.
grep.kill('SIGHUP');

The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

While the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

On Windows, where POSIX signals do not exist, the signal argument will be ignored, and the process will be killed forcefully and abruptly (similar to 'SIGKILL'). See Signal Events for more details.

On Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with the use of the shell option of ChildProcess:

'use strict';
const { spawn } = require('node:child_process');

const subprocess = spawn(
  'sh',
  [
    '-c',
    `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`,
  ], {
    stdio: ['inherit', 'inherit', 'inherit'],
  },
);

setTimeout(() => {
  subprocess.kill(); // Does not terminate the Node.js process in the shell.
}, 2000);

subprocess.killed#

Added in: v0.5.10
  • <boolean> Set to true after subprocess.kill() is used to successfully send a signal to the child process.

The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill(). The killed property does not indicate that the child process has been terminated.

subprocess.pid#

Added in: v0.1.90

Returns the process identifier (PID) of the child process. If the child process fails to spawn due to errors, then the value is undefined and error is emitted.

const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

subprocess.ref()#

Added in: v0.7.10

Calling subprocess.ref() after making a call to subprocess.unref() will restore the removed reference count for the child process, forcing the parent to wait for the child to exit before exiting itself.

const { spawn } = require('node:child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();
subprocess.ref();

subprocess.send(message[, sendHandle[, options]][, callback])#

History
VersionChanges
v5.8.0

The options parameter, and the keepOpen option in particular, is supported now.

v5.0.0

This method returns a boolean for flow control now.

v4.0.0

The callback parameter is supported now.

v0.5.9

Added in: v0.5.9

  • message <Object>
  • sendHandle <Handle>
  • options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:
    • keepOpen <boolean> A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
  • callback <Function>
  • Returns: <boolean>

When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork()), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the 'message' event.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

For example, in the parent script:

const cp = require('node:child_process');
const n = cp.fork(`${__dirname}/sub.js`);

n.on('message', (m) => {
  console.log('PARENT got message:', m);
});

// Causes the child to print: CHILD got message: { hello: 'world' }
n.send({ hello: 'world' });

And then the child script, 'sub.js' might look like this:

process.on('message', (m) => {
  console.log('CHILD got message:', m);
});

// Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
process.send({ foo: 'bar', baz: NaN });

Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent.

There is a special case when sending a {cmd: 'NODE_foo'} message. Messages containing a NODE_ prefix in the cmd property are reserved for use within Node.js core and will not be emitted in the child's 'message' event. Rather, such messages are emitted using the 'internalMessage' event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to subprocess.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the 'message' event. Any data that is received and buffered in the socket will not be sent to the child.

The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Example: sending a server object#

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

const subprocess = require('node:child_process').fork('subprocess.js');

// Open up the server object and send the handle.
const server = require('node:net').createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  subprocess.send('server', server);
});

The child would then receive the server object as:

process.on('message', (m, server) => {
  if (m === 'server') {
    server.on('connection', (socket) => {
      socket.end('handled by child');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the node:net module, node:dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen(). This is, however, only supported on Unix platforms.

Example: sending a socket object#

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

const { fork } = require('node:child_process');
const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);

// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = require('node:net').createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {

  // If this is special priority...
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority.
  normal.send('socket', socket);
});
server.listen(1337);

The subprocess.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    if (socket) {
      // Check that the client socket exists.
      // It is possible for the socket to be closed between the time it is
      // sent and the time it is received in the child process.
      socket.end(`Request handled with ${process.argv[2]} priority`);
    }
  }
});

Do not use .maxConnections on a socket that has been passed to a subprocess. The parent cannot track when the socket is destroyed.

Any 'message' handlers in the subprocess should verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

subprocess.signalCode#

The subprocess.signalCode property indicates the signal received by the child process if any, else null.

subprocess.spawnargs#

The subprocess.spawnargs property represents the full list of command-line arguments the child process was launched with.

subprocess.spawnfile#

The subprocess.spawnfile property indicates the executable file name of the child process that is launched.

For child_process.fork(), its value will be equal to process.execPath. For child_process.spawn(), its value will be the name of the executable file. For child_process.exec(), its value will be the name of the shell in which the child process is launched.

subprocess.stderr#

Added in: v0.1.90

A Readable Stream that represents the child process's stderr.

If the child was spawned with stdio[2] set to anything other than 'pipe', then this will be null.

subprocess.stderr is an alias for subprocess.stdio[2]. Both properties will refer to the same value.

The subprocess.stderr property can be null or undefined if the child process could not be successfully spawned.

subprocess.stdin#

Added in: v0.1.90

A Writable Stream that represents the child process's stdin.

If a child process waits to read all of its input, the child will not continue until this stream has been closed via end().

If the child was spawned with stdio[0] set to anything other than 'pipe', then this will be null.

subprocess.stdin is an alias for subprocess.stdio[0]. Both properties will refer to the same value.

The subprocess.stdin property can be null or undefined if the child process could not be successfully spawned.

subprocess.stdio#

Added in: v0.7.10

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. subprocess.stdio[0], subprocess.stdio[1], and subprocess.stdio[2] are also available as subprocess.stdin, subprocess.stdout, and subprocess.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's subprocess.stdio[1] is a stream, all other values in the array are null.

const assert = require('node:assert');
const fs = require('node:fs');
const child_process = require('node:child_process');

const subprocess = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child.
    'pipe', // Pipe child's stdout to parent.
    fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
  ],
});

assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);

The subprocess.stdio property can be undefined if the child process could not be successfully spawned.

subprocess.stdout#

Added in: v0.1.90

A Readable Stream that represents the child process's stdout.

If the child was spawned with stdio[1] set to anything other than 'pipe', then this will be null.

subprocess.stdout is an alias for subprocess.stdio[1]. Both properties will refer to the same value.

const { spawn } = require('node:child_process');

const subprocess = spawn('ls');

subprocess.stdout.on('data', (data) => {
  console.log(`Received chunk ${data}`);
});

The subprocess.stdout property can be null or undefined if the child process could not be successfully spawned.

subprocess.unref()#

Added in: v0.7.10

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent.

const { spawn } = require('node:child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();

maxBuffer and Unicode#

The maxBuffer option specifies the largest number of bytes allowed on stdout or stderr. If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, console.log('中文测试') will send 13 UTF-8 encoded bytes to stdout although there are only 4 characters.

Shell requirements#

The shell should understand the -c switch. If the shell is 'cmd.exe', it should understand the /d /s /c switches and command-line parsing should be compatible.

Default Windows shell#

Although Microsoft specifies %COMSPEC% must contain the path to 'cmd.exe' in the root environment, child processes are not always subject to the same requirement. Thus, in child_process functions where a shell can be spawned, 'cmd.exe' is used as a fallback if process.env.ComSpec is unavailable.

Advanced serialization#

Added in: v13.2.0, v12.16.0

Child processes support a serialization mechanism for IPC that is based on the serialization API of the node:v8 module, based on the HTML structured clone algorithm. This is generally more powerful and supports more built-in JavaScript object types, such as BigInt, Map and Set, ArrayBuffer and TypedArray, Buffer, Error, RegExp etc.

However, this format is not a full superset of JSON, and e.g. properties set on objects of such built-in types will not be passed on through the serialization step. Additionally, performance may not be equivalent to that of JSON, depending on the structure of the passed data. Therefore, this feature requires opting in by setting the serialization option to 'advanced' when calling child_process.spawn() or child_process.fork().

Cluster#

Stability: 2 - Stable

Source Code: lib/cluster.js

Clusters of Node.js processes can be used to run multiple instances of Node.js that can distribute workloads among their application threads. When process isolation is not needed, use the worker_threads module instead, which allows running multiple application threads within a single Node.js instance.

The cluster module allows easy creation of child processes that all share server ports.

import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';

const numCPUs = availableParallelism();

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}const cluster = require('node:cluster');
const http = require('node:http');
const numCPUs = require('node:os').availableParallelism();
const process = require('node:process');

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Running Node.js will now share port 8000 between the workers:

$ node server.js
Primary 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

On Windows, it is not yet possible to set up a named pipe server in a worker.

How it works#

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows) is the round-robin approach, where the primary process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the primary process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the primary process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

  1. server.listen({fd: 7}) Because the message is passed to the primary, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
  2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the primary process.
  3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.

Node.js does not provide routing logic. It is therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.

Although a primary use case for the node:cluster module is networking, it can also be used for other use cases requiring worker processes.

Class: Worker#

Added in: v0.7.0

A Worker object contains all public information and method about a worker. In the primary it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

Event: 'disconnect'#

Added in: v0.7.7

Similar to the cluster.on('disconnect') event, but specific to this worker.

cluster.fork().on('disconnect', () => {
  // Worker has disconnected
});

Event: 'error'#

Added in: v0.7.3

This event is the same as the one provided by child_process.fork().

Within a worker, process.on('error') may also be used.

Event: 'exit'#

Added in: v0.11.2
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

Similar to the cluster.on('exit') event, but specific to this worker.

import cluster from 'node:cluster';

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.on('exit', (code, signal) => {
    if (signal) {
      console.log(`worker was killed by signal: ${signal}`);
    } else if (code !== 0) {
      console.log(`worker exited with error code: ${code}`);
    } else {
      console.log('worker success!');
    }
  });
}const cluster = require('node:cluster');

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.on('exit', (code, signal) => {
    if (signal) {
      console.log(`worker was killed by signal: ${signal}`);
    } else if (code !== 0) {
      console.log(`worker exited with error code: ${code}`);
    } else {
      console.log('worker success!');
    }
  });
}

Event: 'listening'#

Added in: v0.7.0

Similar to the cluster.on('listening') event, but specific to this worker.

cluster.fork().on('listening', (address) => {
  // Worker is listening
});cluster.fork().on('listening', (address) => {
  // Worker is listening
});

It is not emitted in the worker.

Event: 'message'#

Added in: v0.7.0

Similar to the 'message' event of cluster, but specific to this worker.

Within a worker, process.on('message') may also be used.

See process event: 'message'.

Here is an example using the message system. It keeps a count in the primary process of the number of HTTP requests received by the workers:

import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';

if (cluster.isPrimary) {

  // Keep track of http requests
  let numReqs = 0;
  setInterval(() => {
    console.log(`numReqs = ${numReqs}`);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd === 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = availableParallelism();
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  for (const id in cluster.workers) {
    cluster.workers[id].on('message', messageHandler);
  }

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // Notify primary about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}const cluster = require('node:cluster');
const http = require('node:http');
const process = require('node:process');

if (cluster.isPrimary) {

  // Keep track of http requests
  let numReqs = 0;
  setInterval(() => {
    console.log(`numReqs = ${numReqs}`);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd === 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('node:os').availableParallelism();
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  for (const id in cluster.workers) {
    cluster.workers[id].on('message', messageHandler);
  }

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // Notify primary about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

Event: 'online'#

Added in: v0.7.0

Similar to the cluster.on('online') event, but specific to this worker.

cluster.fork().on('online', () => {
  // Worker is online
});

It is not emitted in the worker.

worker.disconnect()#

History
VersionChanges
v7.3.0

This method now returns a reference to worker.

v0.7.7

Added in: v0.7.7

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the primary, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

After a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

In a worker, process.disconnect exists, but it is not this function; it is disconnect().

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

if (cluster.isPrimary) {
  const worker = cluster.fork();
  let timeout;

  worker.on('listening', (address) => {
    worker.send('shutdown');
    worker.disconnect();
    timeout = setTimeout(() => {
      worker.kill();
    }, 2000);
  });

  worker.on('disconnect', () => {
    clearTimeout(timeout);
  });

} else if (cluster.isWorker) {
  const net = require('node:net');
  const server = net.createServer((socket) => {
    // Connections never end
  });

  server.listen(8000);

  process.on('message', (msg) => {
    if (msg === 'shutdown') {
      // Initiate graceful close of any connections to server
    }
  });
}

worker.exitedAfterDisconnect#

Added in: v6.0.0

This property is true if the worker exited due to .disconnect(). If the worker exited any other way, it is false. If the worker has not exited, it is undefined.

The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the primary may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.exitedAfterDisconnect === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

worker.id#

Added in: v0.8.0

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers.

worker.isConnected()#

Added in: v0.11.14

This function returns true if the worker is connected to its primary via its IPC channel, false otherwise. A worker is connected to its primary after it has been created. It is disconnected after the 'disconnect' event is emitted.

worker.isDead()#

Added in: v0.11.14

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

import cluster from 'node:cluster';
import http from 'node:http';
import { availableParallelism } from 'node:os';
import process from 'node:process';

const numCPUs = availableParallelism();

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('fork', (worker) => {
    console.log('worker is dead:', worker.isDead());
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log('worker is dead:', worker.isDead());
  });
} else {
  // Workers can share any TCP connection. In this case, it is an HTTP server.
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Current process\n ${process.pid}`);
    process.kill(process.pid);
  }).listen(8000);
}const cluster = require('node:cluster');
const http = require('node:http');
const numCPUs = require('node:os').availableParallelism();
const process = require('node:process');

if (cluster.isPrimary) {
  console.log(`Primary ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('fork', (worker) => {
    console.log('worker is dead:', worker.isDead());
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log('worker is dead:', worker.isDead());
  });
} else {
  // Workers can share any TCP connection. In this case, it is an HTTP server.
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Current process\n ${process.pid}`);
    process.kill(process.pid);
  }).listen(8000);
}

worker.kill([signal])#

Added in: v0.9.12
  • signal <string> Name of the kill signal to send to the worker process. Default: 'SIGTERM'

This function will kill the worker. In the primary worker, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by killing the process with signal.

The kill() function kills the worker process without waiting for a graceful disconnect, it has the same behavior as worker.process.kill().

This method is aliased as worker.destroy() for backwards compatibility.

In a worker, process.kill() exists, but it is not this function; it is kill().

worker.process#

Added in: v0.7.0

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module.

Workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.

worker.send(message[, sendHandle[, options]][, callback])#

History
VersionChanges
v4.0.0

The callback parameter is supported now.

v0.7.0

Added in: v0.7.0

  • message <Object>
  • sendHandle <Handle>
  • options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:
    • keepOpen <boolean> A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
  • callback <Function>
  • Returns: <boolean>

Send a message to a worker or primary, optionally with a handle.

In the primary, this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker, this sends a message to the primary. It is identical to process.send().

This example will echo back all messages from the primary:

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.send('hi there');

} else if (cluster.isWorker) {
  process.on('message', (msg) => {
    process.send(msg);
  });
}

Event: 'disconnect'#

Added in: v0.7.9

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', (worker) => {
  console.log(`The worker #${worker.id} has disconnected`);
});

Event: 'exit'#

Added in: v0.7.9
  • worker <cluster.Worker>
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

cluster.on('exit', (worker, code, signal) => {
  console.log('worker %d died (%s). restarting...',
              worker.process.pid, signal || code);
  cluster.fork();
});

See child_process event: 'exit'.

Event: 'fork'#

Added in: v0.7.0

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create a custom timeout.

const timeouts = [];
function errorMsg() {
  console.error('Something must be wrong with the connection ...');
}

cluster.on('fork', (worker) => {
  timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
  clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
  clearTimeout(timeouts[worker.id]);
  errorMsg();
});

Event: 'listening'#

Added in: v0.7.0

After calling listen() from a worker, when the 'listening' event is emitted on the server, a 'listening' event will also be emitted on cluster in the primary.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port, and addressType. This is very useful if the worker is listening on more than one address.

cluster.on('listening', (worker, address) => {
  console.log(
    `A worker is now connected to ${address.address}:${address.port}`);
});

The addressType is one of:

  • 4 (TCPv4)
  • 6 (TCPv6)
  • -1 (Unix domain socket)
  • 'udp4' or 'udp6' (UDPv4 or UDPv6)

Event: 'message'#

History
VersionChanges
v6.0.0

The worker parameter is passed now; see below for details.

v2.5.0

Added in: v2.5.0

Emitted when the cluster primary receives a message from any worker.

See child_process event: 'message'.

Event: 'online'#

Added in: v0.7.0

After forking a new worker, the worker should respond with an online message. When the primary receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the primary forks a worker, and 'online' is emitted when the worker is running.

cluster.on('online', (worker) => {
  console.log('Yay, the worker responded after it was forked');
});

Event: 'setup'#

Added in: v0.7.1

Emitted every time .setupPrimary() is called.

The settings object is the cluster.settings object at the time .setupPrimary() was called and is advisory only, since multiple calls to .setupPrimary() can be made in a single tick.

If accuracy is important, use cluster.settings.

cluster.disconnect([callback])#

Added in: v0.7.7
  • callback <Function> Called when all workers are disconnected and handles are closed.

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the primary process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the primary process.

cluster.fork([env])#

Added in: v0.6.0

Spawn a new worker process.

This can only be called from the primary process.

cluster.isMaster#

Added in: v0.8.1Deprecated since: v16.0.0

Deprecated alias for cluster.isPrimary.

cluster.isPrimary#

Added in: v16.0.0

True if the process is a primary. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isPrimary is true.

cluster.isWorker#

Added in: v0.6.0

True if the process is not a primary (it is the negation of cluster.isPrimary).

cluster.schedulingPolicy#

Added in: v0.11.2

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or .setupPrimary() is called, whichever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none'.

cluster.settings#

History
VersionChanges
v13.2.0, v12.16.0

The serialization option is supported now.

v9.5.0

The cwd option is supported now.

v9.4.0

The windowsHide option is supported now.

v8.2.0

The inspectPort option is supported now.

v6.4.0

The stdio option is supported now.

v0.7.1

Added in: v0.7.1

  • <Object>
    • execArgv <string[]> List of string arguments passed to the Node.js executable. Default: process.execArgv.
    • exec <string> File path to worker file. Default: process.argv[1].
    • args <string[]> String arguments passed to worker. Default: process.argv.slice(2).
    • cwd <string> Current working directory of the worker process. Default: undefined (inherits from parent process).
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for child_process for more details. Default: false.
    • silent <boolean> Whether or not to send output to parent's stdio. Default: false.
    • stdio <Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent.
    • uid <number> Sets the user identity of the process. (See setuid(2).)
    • gid <number> Sets the group identity of the process. (See setgid(2).)
    • inspectPort <number> | <Function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary's process.debugPort.
    • windowsHide <boolean> Hide the forked processes console window that would normally be created on Windows systems. Default: false.

After calling .setupPrimary() (or .fork()) this settings object will contain the settings, including the default values.

This object is not intended to be changed or set manually.

cluster.setupMaster([settings])#

History
VersionChanges
v16.0.0

Deprecated since: v16.0.0

v6.4.0

The stdio option is supported now.

v0.7.1

Added in: v0.7.1

Deprecated alias for .setupPrimary().

cluster.setupPrimary([settings])#

Added in: v16.0.0

setupPrimary is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Any settings changes only affect future calls to .fork() and have no effect on workers that are already running.

The only attribute of a worker that cannot be set via .setupPrimary() is the env passed to .fork().

The defaults above apply to the first call only; the defaults for later calls are the current values at the time of cluster.setupPrimary() is called.

import cluster from 'node:cluster';

cluster.setupPrimary({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true,
});
cluster.fork(); // https worker
cluster.setupPrimary({
  exec: 'worker.js',
  args: ['--use', 'http'],
});
cluster.fork(); // http workerconst cluster = require('node:cluster');

cluster.setupPrimary({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true,
});
cluster.fork(); // https worker
cluster.setupPrimary({
  exec: 'worker.js',
  args: ['--use', 'http'],
});
cluster.fork(); // http worker

This can only be called from the primary process.

cluster.worker#

Added in: v0.7.0

A reference to the current worker object. Not available in the primary process.

import cluster from 'node:cluster';

if (cluster.isPrimary) {
  console.log('I am primary');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}const cluster = require('node:cluster');

if (cluster.isPrimary) {
  console.log('I am primary');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}

cluster.workers#

Added in: v0.7.0

A hash that stores the active worker objects, keyed by id field. This makes it easy to loop through all the workers. It is only available in the primary process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before the last 'disconnect' or 'exit' event is emitted.

import cluster from 'node:cluster';

for (const worker of Object.values(cluster.workers)) {
  worker.send('big announcement to all workers');
}const cluster = require('node:cluster');

for (const worker of Object.values(cluster.workers)) {
  worker.send('big announcement to all workers');
}

Command-line API#

Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in a terminal, run man node.

Synopsis#

node [options] [V8 options] [<program-entry-point> | -e "script" | -] [--] [arguments]

node inspect [<program-entry-point> | -e "script" | <host>:<port>] …

node --v8-options

Execute without arguments to start the REPL.

For more info about node inspect, see the debugger documentation.

Program entry point#

The program entry point is a specifier-like string. If the string is not an absolute path, it's resolved as a relative path from the current working directory. That path is then resolved by CommonJS module loader. If no corresponding file is found, an error is thrown.

If a file is found, its path will be passed to the ECMAScript module loader under any of the following conditions:

  • The program was started with a command-line flag that forces the entry point to be loaded with ECMAScript module loader.
  • The file has an .mjs extension.
  • The file does not have a .cjs extension, and the nearest parent package.json file contains a top-level "type" field with a value of "module".

Otherwise, the file is loaded using the CommonJS module loader. See Modules loaders for more details.

ECMAScript modules loader entry point caveat#

When loading ECMAScript module loader loads the program entry point, the node command will only accept as input only files with .js, .mjs, or .cjs extensions; and with .wasm extensions when --experimental-wasm-modules is enabled.

Options#

History
VersionChanges
v10.12.0

Underscores instead of dashes are now allowed for Node.js options as well, in addition to V8 options.

All options, including V8 options, allow words to be separated by both dashes (-) or underscores (_). For example, --pending-deprecation is equivalent to --pending_deprecation.

If an option that takes a single value (such as --max-http-header-size) is passed more than once, then the last passed value is used. Options from the command line take precedence over options passed through the NODE_OPTIONS environment variable.

-#

Added in: v8.0.0

Alias for stdin. Analogous to the use of - in other command-line utilities, meaning that the script is read from stdin, and the rest of the options are passed to that script.

--#

Added in: v6.11.0

Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument is used as a script filename.

--abort-on-uncaught-exception#

Added in: v0.10.8

Aborting instead of exiting causes a core file to be generated for post-mortem analysis using a debugger (such as lldb, gdb, and mdb).

If this flag is passed, the behavior can still be set to not abort through process.setUncaughtExceptionCaptureCallback() (and through usage of the node:domain module that uses it).

--build-snapshot#

Added in: v18.8.0

Stability: 1 - Experimental

Generates a snapshot blob when the process exits and writes it to disk, which can be loaded later with --snapshot-blob.

When building the snapshot, if --snapshot-blob is not specified, the generated blob will be written, by default, to snapshot.blob in the current working directory. Otherwise it will be written to the path specified by --snapshot-blob.

$ echo "globalThis.foo = 'I am from the snapshot'" > snapshot.js

# Run snapshot.js to intialize the application and snapshot the
# state of it into snapshot.blob.
$ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js

$ echo "console.log(globalThis.foo)" > index.js

# Load the generated snapshot and start the application from index.js.
$ node --snapshot-blob snapshot.blob index.js
I am from the snapshot

The v8.startupSnapshot API can be used to specify an entry point at snapshot building time, thus avoiding the need of an additional entry script at deserialization time:

$ echo "require('v8').startupSnapshot.setDeserializeMainFunction(() => console.log('I am from the snapshot'))" > snapshot.js
$ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js
$ node --snapshot-blob snapshot.blob
I am from the snapshot

For more information, check out the v8.startupSnapshot API documentation.

Currently the support for run-time snapshot is experimental in that:

  1. User-land modules are not yet supported in the snapshot, so only one single file can be snapshotted. Users can bundle their applications into a single script with their bundler of choice before building a snapshot, however.
  2. Only a subset of the built-in modules work in the snapshot, though the Node.js core test suite checks that a few fairly complex applications can be snapshotted. Support for more modules are being added. If any crashes or buggy behaviors occur when building a snapshot, please file a report in the Node.js issue tracker and link to it in the tracking issue for user-land snapshots.

--completion-bash#

Added in: v10.12.0

Print source-able bash completion script for Node.js.

$ node --completion-bash > node_bash_completion
$ source node_bash_completion

-C=condition, --conditions=condition#

Added in: v14.9.0, v12.19.0

Stability: 1 - Experimental

Enable experimental support for custom conditional exports resolution conditions.

Any number of custom string condition names are permitted.

The default Node.js conditions of "node", "default", "import", and "require" will always apply as defined.

For example, to run a module with "development" resolutions:

$ node -C=development app.js

--cpu-prof#

Added in: v12.0.0

Stability: 1 - Experimental

Starts the V8 CPU profiler on start up, and writes the CPU profile to disk before exit.

If --cpu-prof-dir is not specified, the generated profile is placed in the current working directory.

If --cpu-prof-name is not specified, the generated profile is named CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile.

$ node --cpu-prof index.js
$ ls *.cpuprofile
CPU.20190409.202950.15293.0.0.cpuprofile

--cpu-prof-dir#

Added in: v12.0.0

Stability: 1 - Experimental

Specify the directory where the CPU profiles generated by --cpu-prof will be placed.

The default value is controlled by the --diagnostic-dir command-line option.

--cpu-prof-interval#

Added in: v12.2.0

Stability: 1 - Experimental

Specify the sampling interval in microseconds for the CPU profiles generated by --cpu-prof. The default is 1000 microseconds.

--cpu-prof-name#

Added in: v12.0.0

Stability: 1 - Experimental

Specify the file name of the CPU profile generated by --cpu-prof.

--diagnostic-dir=directory#

Set the directory to which all diagnostic output files are written. Defaults to current working directory.

Affects the default output directory of:

--disable-proto=mode#

Added in: v13.12.0, v12.17.0

Disable the Object.prototype.__proto__ property. If mode is delete, the property is removed entirely. If mode is throw, accesses to the property throw an exception with the code ERR_PROTO_ACCESS.

--disallow-code-generation-from-strings#

Added in: v9.8.0

Make built-in language features like eval and new Function that generate code from strings throw an exception instead. This does not affect the Node.js node:vm module.

--dns-result-order=order#

History
VersionChanges
v17.0.0

Changed default value to verbatim.

v16.4.0, v14.18.0

Added in: v16.4.0, v14.18.0

Set the default value of verbatim in dns.lookup() and dnsPromises.lookup(). The value could be:

  • ipv4first: sets default verbatim false.
  • verbatim: sets default verbatim true.

The default is verbatim and dns.setDefaultResultOrder() have higher priority than --dns-result-order.

--enable-fips#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built against FIPS-compatible OpenSSL.)

--enable-network-family-autoselection#

Added in: v19.4.0

Enables the family autoselection algorithm unless connection options explicitly disables it.

--enable-source-maps#

History
VersionChanges
v15.11.0, v14.18.0

This API is no longer experimental.

v12.12.0

Added in: v12.12.0

Enable Source Map v3 support for stack traces.

When using a transpiler, such as TypeScript, stack traces thrown by an application reference the transpiled code, not the original source position. --enable-source-maps enables caching of Source Maps and makes a best effort to report stack traces relative to the original source file.

Overriding Error.prepareStackTrace prevents --enable-source-maps from modifying the stack trace.

Note, enabling source maps can introduce latency to your application when Error.stack is accessed. If you access Error.stack frequently in your application, take into account the performance implications of --enable-source-maps.

--experimental-import-meta-resolve#

Added in: v13.9.0, v12.16.2

Enable experimental import.meta.resolve() support.

--experimental-loader=module#

History
VersionChanges
v12.11.1

This flag was renamed from --loader to --experimental-loader.

v8.8.0

Added in: v8.8.0

Specify the module of a custom experimental ECMAScript module loader. module may be any string accepted as an import specifier.

--experimental-network-imports#

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental

Enable experimental support for the https: protocol in import specifiers.

--experimental-policy#

Added in: v11.8.0

Use the specified file as a security policy.

--no-experimental-fetch#

Added in: v18.0.0

Disable experimental support for the Fetch API.

--no-experimental-global-webcrypto#

Added in: v19.0.0

Disable exposition of Web Crypto API on the global scope.

--no-experimental-global-customevent#

Added in: v19.0.0

Disable exposition of CustomEvent Web API on the global scope.

--no-experimental-repl-await#

Added in: v16.6.0

Use this flag to disable top-level await in REPL.

--experimental-shadow-realm#

Added in: v19.0.0

Use this flag to enable ShadowRealm support.

--experimental-vm-modules#

Added in: v9.6.0

Enable experimental ES Module support in the node:vm module.

--experimental-wasi-unstable-preview1#

History
VersionChanges
v13.6.0

changed from --experimental-wasi-unstable-preview0 to --experimental-wasi-unstable-preview1.

v13.3.0, v12.16.0

Added in: v13.3.0, v12.16.0

Enable experimental WebAssembly System Interface (WASI) support.

--experimental-wasm-modules#

Added in: v12.3.0

Enable experimental WebAssembly module support.

--force-context-aware#

Added in: v12.12.0

Disable loading native addons that are not context-aware.

--force-fips#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips.)

--frozen-intrinsics#

Added in: v11.12.0

Stability: 1 - Experimental

Enable experimental frozen intrinsics like Array and Object.

Only the root context is supported. There is no guarantee that globalThis.Array is indeed the default intrinsic reference. Code may break under this flag.

To allow polyfills to be added, --require and --import both run before freezing intrinsics.

--force-node-api-uncaught-exceptions-policy#

Added in: v18.3.0, v16.17.0

Enforces uncaughtException event on Node-API asynchronous callbacks.

To prevent from an existing add-on from crashing the process, this flag is not enabled by default. In the future, this flag will be enabled by default to enforce the correct behavior.

--heapsnapshot-near-heap-limit=max_count#

Added in: v15.1.0, v14.18.0

Stability: 1 - Experimental

Writes a V8 heap snapshot to disk when the V8 heap usage is approaching the heap limit. count should be a non-negative integer (in which case Node.js will write no more than max_count snapshots to disk).

When generating snapshots, garbage collection may be triggered and bring the heap usage down. Therefore multiple snapshots may be written to disk before the Node.js instance finally runs out of memory. These heap snapshots can be compared to determine what objects are being allocated during the time consecutive snapshots are taken. It's not guaranteed that Node.js will write exactly max_count snapshots to disk, but it will try its best to generate at least one and up to max_count snapshots before the Node.js instance runs out of memory when max_count is greater than 0.

Generating V8 snapshots takes time and memory (both memory managed by the V8 heap and native memory outside the V8 heap). The bigger the heap is, the more resources it needs. Node.js will adjust the V8 heap to accommodate the additional V8 heap memory overhead, and try its best to avoid using up all the memory available to the process. When the process uses more memory than the system deems appropriate, the process may be terminated abruptly by the system, depending on the system configuration.

$ node --max-old-space-size=100 --heapsnapshot-near-heap-limit=3 index.js
Wrote snapshot to Heap.20200430.100036.49580.0.001.heapsnapshot
Wrote snapshot to Heap.20200430.100037.49580.0.002.heapsnapshot
Wrote snapshot to Heap.20200430.100038.49580.0.003.heapsnapshot

<--- Last few GCs --->

[49580:0x110000000]     4826 ms: Mark-sweep 130.6 (147.8) -> 130.5 (147.8) MB, 27.4 / 0.0 ms  (average mu = 0.126, current mu = 0.034) allocation failure scavenge might not succeed
[49580:0x110000000]     4845 ms: Mark-sweep 130.6 (147.8) -> 130.6 (147.8) MB, 18.8 / 0.0 ms  (average mu = 0.088, current mu = 0.031) allocation failure scavenge might not succeed


<--- JS stacktrace --->

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
....

--heapsnapshot-signal=signal#

Added in: v12.0.0

Enables a signal handler that causes the Node.js process to write a heap dump when the specified signal is received. signal must be a valid signal name. Disabled by default.

$ node --heapsnapshot-signal=SIGUSR2 index.js &
$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
node         1  5.5  6.1 787252 247004 ?       Ssl  16:43   0:02 node --heapsnapshot-signal=SIGUSR2 index.js
$ kill -USR2 1
$ ls
Heap.20190718.133405.15554.0.001.heapsnapshot

--heap-prof#

Added in: v12.4.0

Stability: 1 - Experimental

Starts the V8 heap profiler on start up, and writes the heap profile to disk before exit.

If --heap-prof-dir is not specified, the generated profile is placed in the current working directory.

If --heap-prof-name is not specified, the generated profile is named Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile.

$ node --heap-prof index.js
$ ls *.heapprofile
Heap.20190409.202950.15293.0.001.heapprofile

--heap-prof-dir#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the directory where the heap profiles generated by --heap-prof will be placed.

The default value is controlled by the --diagnostic-dir command-line option.

--heap-prof-interval#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the average sampling interval in bytes for the heap profiles generated by --heap-prof. The default is 512 * 1024 bytes.

--heap-prof-name#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the file name of the heap profile generated by --heap-prof.

--icu-data-dir=file#

Added in: v0.11.15

Specify ICU data load path. (Overrides NODE_ICU_DATA.)

--import=module#

Added in: v19.0.0

Stability: 1 - Experimental

Preload the specified module at startup.

Follows ECMAScript module resolution rules. Use --require to load a CommonJS module. Modules preloaded with --require will run before modules preloaded with --import.

--input-type=type#

Added in: v12.0.0

This configures Node.js to interpret string input as CommonJS or as an ES module. String input is input via --eval, --print, or STDIN.

Valid values are "commonjs" and "module". The default is "commonjs".

The REPL does not support this option.

--inspect-brk[=[host:]port]#

Added in: v7.6.0

Activate inspector on host:port and break at start of user script. Default host:port is 127.0.0.1:9229.

--inspect-port=[host:]port#

Added in: v7.6.0

Set the host:port to be used when the inspector is activated. Useful when activating the inspector by sending the SIGUSR1 signal.

Default host is 127.0.0.1.

See the security warning below regarding the host parameter usage.

--inspect[=[host:]port]#

Added in: v6.3.0

Activate inspector on host:port. Default is 127.0.0.1:9229.

V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome DevTools Protocol.

Warning: binding inspector to a public IP:port combination is insecure#

Binding the inspector to a public IP (including 0.0.0.0) with an open port is insecure, as it allows external hosts to connect to the inspector and perform a remote code execution attack.

If specifying a host, make sure that either:

  • The host is not accessible from public networks.
  • A firewall disallows unwanted connections on the port.

More specifically, --inspect=0.0.0.0 is insecure if the port (9229 by default) is not firewall-protected.

See the debugging security implications section for more information.

--inspect-publish-uid=stderr,http#

Specify ways of the inspector web socket url exposure.

By default inspector websocket url is available in stderr and under /json/list endpoint on http://host:port/json/list.

--insecure-http-parser#

Added in: v13.4.0, v12.15.0, v10.19.0

Use an insecure HTTP parser that accepts invalid HTTP headers. This may allow interoperability with non-conformant HTTP implementations. It may also allow request smuggling and other HTTP attacks that rely on invalid headers being accepted. Avoid using this option.

--jitless#

Added in: v12.0.0

Disable runtime allocation of executable memory. This may be required on some platforms for security reasons. It can also reduce attack surface on other platforms, but the performance impact may be severe.

This flag is inherited from V8 and is subject to change upstream. It may disappear in a non-semver-major release.

--max-http-header-size=size#

History
VersionChanges
v13.13.0

Change maximum default size of HTTP headers from 8 KiB to 16 KiB.

v11.6.0, v10.15.0

Added in: v11.6.0, v10.15.0

Specify the maximum size, in bytes, of HTTP headers. Defaults to 16 KiB.

--napi-modules#

Added in: v7.10.0

This option is a no-op. It is kept for compatibility.

--no-addons#

Added in: v16.10.0, v14.19.0

Disable the node-addons exports condition as well as disable loading native addons. When --no-addons is specified, calling process.dlopen or requiring a native C++ addon will fail and throw an exception.

--no-deprecation#

Added in: v0.8.0

Silence deprecation warnings.

--no-extra-info-on-fatal-exception#

Added in: v17.0.0

Hide extra information on fatal exception that causes exit.

--no-force-async-hooks-checks#

Added in: v9.0.0

Disables runtime checks for async_hooks. These will still be enabled dynamically when async_hooks is enabled.

--no-global-search-paths#

Added in: v16.10.0

Do not search modules from global paths like $HOME/.node_modules and $NODE_PATH.

--no-warnings#

Added in: v6.0.0

Silence all process warnings (including deprecations).

--node-memory-debug#

Added in: v15.0.0, v14.18.0

Enable extra debug checks for memory leaks in Node.js internals. This is usually only useful for developers debugging Node.js itself.

--openssl-config=file#

Added in: v6.9.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built against FIPS-enabled OpenSSL.

--openssl-shared-config#

Added in: v18.5.0, v16.17.0

Enable OpenSSL default configuration section, openssl_conf to be read from the OpenSSL configuration file. The default configuration file is named openssl.cnf but this can be changed using the environment variable OPENSSL_CONF, or by using the command line option --openssl-config. The location of the default OpenSSL configuration file depends on how OpenSSL is being linked to Node.js. Sharing the OpenSSL configuration may have unwanted implications and it is recommended to use a configuration section specific to Node.js which is nodejs_conf and is default when this option is not used.

--openssl-legacy-provider#

Added in: v17.0.0, v16.17.0

Enable OpenSSL 3.0 legacy provider. For more information please see OSSL_PROVIDER-legacy.

--pending-deprecation#

Added in: v8.0.0

Emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command-line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

--policy-integrity=sri#

Added in: v12.7.0

Stability: 1 - Experimental

Instructs Node.js to error prior to running any code if the policy does not have the specified integrity. It expects a Subresource Integrity string as a parameter.

--preserve-symlinks#

Added in: v6.3.0

Instructs the module loader to preserve symbolic links when resolving and caching modules.

By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if moduleA attempts to require moduleB as a peer dependency:

{appDir}
 ├── app
 │   ├── index.js
 │   └── node_modules
 │       ├── moduleA -> {appDir}/moduleA
 │       └── moduleB
 │           ├── index.js
 │           └── package.json
 └── moduleA
     ├── index.js
     └── package.json

The --preserve-symlinks command-line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found.

Note, however, that using --preserve-symlinks can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown).

The --preserve-symlinks flag does not apply to the main module, which allows node --preserve-symlinks node_module/.bin/<foo> to work. To apply the same behavior for the main module, also use --preserve-symlinks-main.

--preserve-symlinks-main#

Added in: v10.2.0

Instructs the module loader to preserve symbolic links when resolving and caching the main module (require.main).

This flag exists so that the main module can be opted-in to the same behavior that --preserve-symlinks gives to all other imports; they are separate flags, however, for backward compatibility with older Node.js versions.

--preserve-symlinks-main does not imply --preserve-symlinks; use --preserve-symlinks-main in addition to --preserve-symlinks when it is not desirable to follow symlinks before resolving relative paths.

See --preserve-symlinks for more information.

--prof#

Added in: v2.0.0

Generate V8 profiler output.

--prof-process#

Added in: v5.2.0

Process V8 profiler output generated using the V8 option --prof.

--redirect-warnings=file#

Added in: v8.0.0

Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.

The file name may be an absolute path. If it is not, the default directory it will be written to is controlled by the --diagnostic-dir command-line option.

--report-compact#

Added in: v13.12.0, v12.17.0

Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

--report-dir=directory, report-directory=directory#

History
VersionChanges
v13.12.0, v12.17.0

This option is no longer experimental.

v12.0.0

Changed from --diagnostic-report-directory to --report-directory.

v11.8.0

Added in: v11.8.0

Location at which the report will be generated.

--report-filename=filename#

History
VersionChanges
v13.12.0, v12.17.0

This option is no longer experimental.

v12.0.0

changed from --diagnostic-report-filename to --report-filename.

v11.8.0

Added in: v11.8.0

Name of the file to which the report will be written.

If the filename is set to 'stdout' or 'stderr', the report is written to the stdout or stderr of the process respectively.

--report-on-fatalerror#

History
VersionChanges
v14.0.0, v13.14.0, v12.17.0

This option is no longer experimental.

v12.0.0

changed from --diagnostic-report-on-fatalerror to --report-on-fatalerror.

v11.8.0

Added in: v11.8.0

Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime such as out of memory) that lead to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.

--report-on-signal#

History
VersionChanges
v13.12.0, v12.17.0

This option is no longer experimental.

v12.0.0

changed from --diagnostic-report-on-signal to --report-on-signal.

v11.8.0

Added in: v11.8.0

Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. The signal to trigger the report is specified through --report-signal.

--report-signal=signal#

History
VersionChanges
v13.12.0, v12.17.0

This option is no longer experimental.

v12.0.0

changed from --diagnostic-report-signal to --report-signal.

v11.8.0

Added in: v11.8.0

Sets or resets the signal for report generation (not supported on Windows). Default signal is SIGUSR2.

--report-uncaught-exception#

History
VersionChanges
v18.8.0, v16.18.0

Report is not generated if the uncaught exception is handled.

v13.12.0, v12.17.0

This option is no longer experimental.

v12.0.0

changed from --diagnostic-report-uncaught-exception to --report-uncaught-exception.

v11.8.0

Added in: v11.8.0

Enables report to be generated when the process exits due to an uncaught exception. Useful when inspecting the JavaScript stack in conjunction with native stack and other runtime environment data.

--secure-heap=n#

Added in: v15.6.0

Initializes an OpenSSL secure heap of n bytes. When initialized, the secure heap is used for selected types of allocations within OpenSSL during key generation and other operations. This is useful, for instance, to prevent sensitive information from leaking due to pointer overruns or underruns.

The secure heap is a fixed size and cannot be resized at runtime so, if used, it is important to select a large enough heap to cover all application uses.

The heap size given must be a power of two. Any value less than 2 will disable the secure heap.

The secure heap is disabled by default.

The secure heap is not available on Windows.

See CRYPTO_secure_malloc_init for more details.

--secure-heap-min=n#

Added in: v15.6.0

When using --secure-heap, the --secure-heap-min flag specifies the minimum allocation from the secure heap. The minimum value is 2. The maximum value is the lesser of --secure-heap or 2147483647. The value given must be a power of two.

--snapshot-blob=path#

Added in: v18.8.0

Stability: 1 - Experimental

When used with --build-snapshot, --snapshot-blob specifies the path where the generated snapshot blob is written to. If not specified, the generated blob is written to snapshot.blob in the current working directory.

When used without --build-snapshot, --snapshot-blob specifies the path to the blob that is used to restore the application state.

When loading a snapshot, Node.js checks that:

  1. The version, architecture and platform of the running Node.js binary are exactly the same as that of the binary that generates the snapshot.
  2. The V8 flags and CPU features are compatible with that of the binary that generates the snapshot.

If they don't match, Node.js refuses to load the snapshot and exits with status code 1.

--test#

History
VersionChanges
v19.2.0

Test runner now supports running in watch mode.

v18.1.0, v16.17.0

Added in: v18.1.0, v16.17.0

Starts the Node.js command line test runner. This flag cannot be combined with --watch-path, --check, --eval, --interactive, or the inspector. See the documentation on running tests from the command line for more details.

--test-name-pattern#

Added in: v18.11.0

A regular expression that configures the test runner to only execute tests whose name matches the provided pattern. See the documentation on filtering tests by name for more details.

--test-only#

Added in: v18.0.0, v16.17.0

Configures the test runner to only execute top level tests that have the only option set.

--throw-deprecation#

Added in: v0.11.14

Throw errors for deprecations.

--title=title#

Added in: v10.7.0

Set process.title on startup.

--tls-cipher-list=list#

Added in: v4.0.0

Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default).

--tls-keylog=file#

Added in: v13.2.0, v12.16.0

Log TLS key material to a file. The key material is in NSS SSLKEYLOGFILE format and can be used by software (such as Wireshark) to decrypt the TLS traffic.

--tls-max-v1.2#

Added in: v12.0.0, v10.20.0

Set tls.DEFAULT_MAX_VERSION to 'TLSv1.2'. Use to disable support for TLSv1.3.

--tls-max-v1.3#

Added in: v12.0.0

Set default tls.DEFAULT_MAX_VERSION to 'TLSv1.3'. Use to enable support for TLSv1.3.

--tls-min-v1.0#

Added in: v12.0.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1'. Use for compatibility with old TLS clients or servers.

--tls-min-v1.1#

Added in: v12.0.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.1'. Use for compatibility with old TLS clients or servers.

--tls-min-v1.2#

Added in: v12.2.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.2'. This is the default for 12.x and later, but the option is supported for compatibility with older Node.js versions.

--tls-min-v1.3#

Added in: v12.0.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.3'. Use to disable support for TLSv1.2, which is not as secure as TLSv1.3.

--trace-atomics-wait#

Added in: v14.3.0Deprecated since: v18.8.0, v16.18.0

Stability: 0 - Deprecated

Print short summaries of calls to Atomics.wait() to stderr. The output could look like this:

(node:15701) [Thread 0] Atomics.wait(&lt;address> + 0, 1, inf) started
(node:15701) [Thread 0] Atomics.wait(&lt;address> + 0, 1, inf) did not wait because the values mismatched
(node:15701) [Thread 0] Atomics.wait(&lt;address> + 0, 0, 10) started
(node:15701) [Thread 0] Atomics.wait(&lt;address> + 0, 0, 10) timed out
(node:15701) [Thread 0] Atomics.wait(&lt;address> + 4, 0, inf) started
(node:15701) [Thread 1] Atomics.wait(&lt;address> + 4, -1, inf) started
(node:15701) [Thread 0] Atomics.wait(&lt;address> + 4, 0, inf) was woken up by another thread
(node:15701) [Thread 1] Atomics.wait(&lt;address> + 4, -1, inf) was woken up by another thread

The fields here correspond to:

  • The thread id as given by worker_threads.threadId
  • The base address of the SharedArrayBuffer in question, as well as the byte offset corresponding to the index passed to Atomics.wait()
  • The expected value that was passed to Atomics.wait()
  • The timeout passed to Atomics.wait

--trace-deprecation#

Added in: v0.8.0

Print stack traces for deprecations.

--trace-event-categories#

Added in: v7.7.0

A comma separated list of categories that should be traced when trace event tracing is enabled using --trace-events-enabled.

--trace-event-file-pattern#

Added in: v9.8.0

Template string specifying the filepath for the trace event data, it supports ${rotation} and ${pid}.

--trace-events-enabled#

Added in: v7.7.0

Enables the collection of trace event tracing information.

--trace-exit#

Added in: v13.5.0, v12.16.0

Prints a stack trace whenever an environment is exited proactively, i.e. invoking process.exit().

--trace-sigint#

Added in: v13.9.0, v12.17.0

Prints a stack trace on SIGINT.

--trace-sync-io#

Added in: v2.1.0

Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.

--trace-tls#

Added in: v12.2.0

Prints TLS packet trace information to stderr. This can be used to debug TLS connection problems.

--trace-uncaught#

Added in: v13.1.0

Print stack traces for uncaught exceptions; usually, the stack trace associated with the creation of an Error is printed, whereas this makes Node.js also print the stack trace associated with throwing the value (which does not need to be an Error instance).

Enabling this option may affect garbage collection behavior negatively.

--trace-warnings#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

--track-heap-objects#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

--unhandled-rejections=mode#

History
VersionChanges
v15.0.0

Changed default mode to throw. Previously, a warning was emitted.

v12.0.0, v10.17.0

Added in: v12.0.0, v10.17.0

Using this flag allows to change what should happen when an unhandled rejection occurs. One of the following modes can be chosen:

  • throw: Emit unhandledRejection. If this hook is not set, raise the unhandled rejection as an uncaught exception. This is the default.
  • strict: Raise the unhandled rejection as an uncaught exception. If the exception is handled, unhandledRejection is emitted.
  • warn: Always trigger a warning, no matter if the unhandledRejection hook is set or not but do not print the deprecation warning.
  • warn-with-error-code: Emit unhandledRejection. If this hook is not set, trigger a warning, and set the process exit code to 1.
  • none: Silence all warnings.

If a rejection happens during the command line entry point's ES module static loading phase, it will always raise it as an uncaught exception.

--use-bundled-ca, --use-openssl-ca#

Added in: v6.11.0

Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store. The default store is selectable at build-time.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables.

See SSL_CERT_DIR and SSL_CERT_FILE.

--use-largepages=mode#

Added in: v13.6.0, v12.17.0

Re-map the Node.js static code to large memory pages at startup. If supported on the target system, this will cause the Node.js static code to be moved onto 2 MiB pages instead of 4 KiB pages.

The following values are valid for mode:

  • off: No mapping will be attempted. This is the default.
  • on: If supported by the OS, mapping will be attempted. Failure to map will be ignored and a message will be printed to standard error.
  • silent: If supported by the OS, mapping will be attempted. Failure to map will be ignored and will not be reported.

--v8-options#

Added in: v0.1.3

Print V8 command-line options.

--v8-pool-size=num#

Added in: v5.10.0

Set V8's thread pool size which will be used to allocate background jobs.

If set to 0 then Node.js will choose an appropriate size of the thread pool based on an estimate of the amount of parallelism.

The amount of parallelism refers to the number of computations that can be carried out simultaneously in a given machine. In general, it's the same as the amount of CPUs, but it may diverge in environments such as VMs or containers.

--watch#

History
VersionChanges
v19.2.0

Test runner now supports running in watch mode.

v18.11.0

Added in: v18.11.0

Stability: 1 - Experimental

Starts Node.js in watch mode. When in watch mode, changes in the watched files cause the Node.js process to restart. By default, watch mode will watch the entry point and any required or imported module. Use --watch-path to specify what paths to watch.

This flag cannot be combined with --check, --eval, --interactive, or the REPL.

$ node --watch index.js

--watch-path#

Added in: v18.11.0

Stability: 1 - Experimental

Starts Node.js in watch mode and specifies what paths to watch. When in watch mode, changes in the watched paths cause the Node.js process to restart. This will turn off watching of required or imported modules, even when used in combination with --watch.

This flag cannot be combined with --check, --eval, --interactive, --test, or the REPL.

$ node --watch-path=./src --watch-path=./tests index.js

This option is only supported on macOS and Windows. An ERR_FEATURE_UNAVAILABLE_ON_PLATFORM exception will be thrown when the option is used on a platform that does not support it.

--watch-preserve-output#

Disable the clearing of the console when watch mode restarts the process.

$ node --watch --watch-preserve-output test.js

--zero-fill-buffers#

Added in: v6.0.0

Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.

-c, --check#

History
VersionChanges
v10.0.0

The --require option is now supported when checking a file.

v5.0.0, v4.2.0

Added in: v5.0.0, v4.2.0

Syntax check the script without executing.

-e, --eval "script"#

History
VersionChanges
v5.11.0

Built-in libraries are now available as predefined variables.

v0.5.2

Added in: v0.5.2

Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script.

On Windows, using cmd.exe a single quote will not work correctly because it only recognizes double " for quoting. In Powershell or Git bash, both ' and " are usable.

-h, --help#

Added in: v0.1.3

Print node command-line options. The output of this option is less detailed than this document.

-i, --interactive#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

-p, --print "script"#

History
VersionChanges
v5.11.0

Built-in libraries are now available as predefined variables.

v0.6.4

Added in: v0.6.4

Identical to -e but prints the result.

-r, --require module#

Added in: v1.6.0

Preload the specified module at startup.

Follows require()'s module resolution rules. module may be either a path to a file, or a node module name.

Only CommonJS modules are supported. Use --import to preload an ECMAScript module. Modules preloaded with --require will run before modules preloaded with --import.

-v, --version#

Added in: v0.1.3

Print node's version.

Environment variables#

FORCE_COLOR=[1, 2, 3]#

The FORCE_COLOR environment variable is used to enable ANSI colorized output. The value may be:

  • 1, true, or the empty string '' indicate 16-color support,
  • 2 to indicate 256-color support, or
  • 3 to indicate 16 million-color support.

When FORCE_COLOR is used and set to a supported value, both the NO_COLOR, and NODE_DISABLE_COLORS environment variables are ignored.

Any other value will result in colorized output being disabled.

NODE_DEBUG=module[,…]#

Added in: v0.1.32

','-separated list of core modules that should print debug information.

NODE_DEBUG_NATIVE=module[,…]#

','-separated list of core C++ modules that should print debug information.

NODE_DISABLE_COLORS=1#

Added in: v0.3.0

When set, colors will not be used in the REPL.

NODE_EXTRA_CA_CERTS=file#

Added in: v7.3.0

When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in file. The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with process.emitWarning() if the file is missing or malformed, but any errors are otherwise ignored.

Neither the well known nor extra certificates are used when the ca options property is explicitly specified for a TLS or HTTPS client or server.

This environment variable is ignored when node runs as setuid root or has Linux file capabilities set.

The NODE_EXTRA_CA_CERTS environment variable is only read when the Node.js process is first launched. Changing the value at runtime using process.env.NODE_EXTRA_CA_CERTS has no effect on the current process.

NODE_ICU_DATA=file#

Added in: v0.11.15

Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu support.

NODE_NO_WARNINGS=1#

Added in: v6.11.0

When set to 1, process warnings are silenced.

NODE_OPTIONS=options...#

Added in: v8.0.0

A space-separated list of command-line options. options... are interpreted before command-line options, so command-line options will override or compound after anything in options.... Node.js will exit with an error if an option that is not allowed in the environment is used, such as -p or a script file.

If an option value contains a space, it can be escaped using double quotes:

NODE_OPTIONS='--require "./my path/file.js"'

A singleton flag passed as a command-line option will override the same flag passed into NODE_OPTIONS:

# The inspector will be available on port 5555
NODE_OPTIONS='--inspect=localhost:4444' node --inspect=localhost:5555

A flag that can be passed multiple times will be treated as if its NODE_OPTIONS instances were passed first, and then its command-line instances afterwards:

NODE_OPTIONS='--require "./a.js"' node --require "./b.js"
# is equivalent to:
node --require "./a.js" --require "./b.js"

Node.js options that are allowed are:

  • --conditions, -C
  • --diagnostic-dir
  • --disable-proto
  • --dns-result-order
  • --enable-fips
  • --enable-network-family-autoselection
  • --enable-source-maps
  • --experimental-abortcontroller
  • --experimental-import-meta-resolve
  • --experimental-json-modules
  • --experimental-loader
  • --experimental-modules
  • --experimental-network-imports
  • --experimental-policy
  • --experimental-shadow-realm
  • --experimental-specifier-resolution
  • --experimental-top-level-await
  • --experimental-vm-modules
  • --experimental-wasi-unstable-preview1
  • --experimental-wasm-modules
  • --force-context-aware
  • --force-fips
  • --force-node-api-uncaught-exceptions-policy
  • --frozen-intrinsics
  • --heapsnapshot-near-heap-limit
  • --heapsnapshot-signal
  • --http-parser
  • --icu-data-dir
  • --import
  • --input-type
  • --insecure-http-parser
  • --inspect-brk
  • --inspect-port, --debug-port
  • --inspect-publish-uid
  • --inspect
  • --max-http-header-size
  • --napi-modules
  • --no-addons
  • --no-deprecation
  • --no-experimental-fetch
  • --no-experimental-global-customevent
  • --no-experimental-global-webcrypto
  • --no-experimental-repl-await
  • --no-extra-info-on-fatal-exception
  • --no-force-async-hooks-checks
  • --no-global-search-paths
  • --no-warnings
  • --node-memory-debug
  • --openssl-config
  • --openssl-legacy-provider
  • --openssl-shared-config
  • --pending-deprecation
  • --policy-integrity
  • --preserve-symlinks-main
  • --preserve-symlinks
  • --prof-process
  • --redirect-warnings
  • --report-compact
  • --report-dir, --report-directory
  • --report-filename
  • --report-on-fatalerror
  • --report-on-signal
  • --report-signal
  • --report-uncaught-exception
  • --require, -r
  • --secure-heap-min
  • --secure-heap
  • --snapshot-blob
  • --test-only
  • --throw-deprecation
  • --title
  • --tls-cipher-list
  • --tls-keylog
  • --tls-max-v1.2
  • --tls-max-v1.3
  • --tls-min-v1.0
  • --tls-min-v1.1
  • --tls-min-v1.2
  • --tls-min-v1.3
  • --trace-atomics-wait
  • --trace-deprecation
  • --trace-event-categories
  • --trace-event-file-pattern
  • --trace-events-enabled
  • --trace-exit
  • --trace-sigint
  • --trace-sync-io
  • --trace-tls
  • --trace-uncaught
  • --trace-warnings
  • --track-heap-objects
  • --unhandled-rejections
  • --use-bundled-ca
  • --use-largepages
  • --use-openssl-ca
  • --v8-pool-size
  • --watch-path
  • --watch-preserve-output
  • --watch
  • --zero-fill-buffers

V8 options that are allowed are:

  • --abort-on-uncaught-exception
  • --disallow-code-generation-from-strings
  • --huge-max-old-generation-size
  • --interpreted-frames-native-stack
  • --jitless
  • --max-old-space-size
  • --max-semi-space-size
  • --perf-basic-prof-only-functions
  • --perf-basic-prof
  • --perf-prof-unwinding-info
  • --perf-prof
  • --stack-trace-limit

--perf-basic-prof-only-functions, --perf-basic-prof, --perf-prof-unwinding-info, and --perf-prof are only available on Linux.

NODE_PATH=path[:…]#

Added in: v0.1.32

':'-separated list of directories prefixed to the module search path.

On Windows, this is a ';'-separated list instead.

NODE_PENDING_DEPRECATION=1#

Added in: v8.0.0

When set to 1, emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command-line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

NODE_PENDING_PIPE_INSTANCES=instances#

Set the number of pending pipe instance handles when the pipe server is waiting for connections. This setting applies to Windows only.

NODE_PRESERVE_SYMLINKS=1#

Added in: v7.1.0

When set to 1, instructs the module loader to preserve symbolic links when resolving and caching modules.

NODE_REDIRECT_WARNINGS=file#

Added in: v8.0.0

When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is equivalent to using the --redirect-warnings=file command-line flag.

NODE_REPL_HISTORY=file#

Added in: v3.0.0

Path to the file used to store the persistent REPL history. The default path is ~/.node_repl_history, which is overridden by this variable. Setting the value to an empty string ('' or ' ') disables persistent REPL history.

NODE_REPL_EXTERNAL_MODULE=file#

Added in: v13.0.0, v12.16.0

Path to a Node.js module which will be loaded in place of the built-in REPL. Overriding this value to an empty string ('') will use the built-in REPL.

NODE_SKIP_PLATFORM_CHECK=value#

Added in: v14.5.0

If value equals '1', the check for a supported platform is skipped during Node.js startup. Node.js might not execute correctly. Any issues encountered on unsupported platforms will not be fixed.

NODE_TLS_REJECT_UNAUTHORIZED=value#

If value equals '0', certificate validation is disabled for TLS connections. This makes TLS, and HTTPS by extension, insecure. The use of this environment variable is strongly discouraged.

NODE_V8_COVERAGE=dir#

When set, Node.js will begin outputting V8 JavaScript code coverage and Source Map data to the directory provided as an argument (coverage information is written as JSON to files with a coverage prefix).

NODE_V8_COVERAGE will automatically propagate to subprocesses, making it easier to instrument applications that call the child_process.spawn() family of functions. NODE_V8_COVERAGE can be set to an empty string, to prevent propagation.

Coverage output#

Coverage is output as an array of ScriptCoverage objects on the top-level key result:

{
  "result": [
    {
      "scriptId": "67",
      "url": "internal/tty.js",
      "functions": []
    }
  ]
}
Source map cache#

Stability: 1 - Experimental

If found, source map data is appended to the top-level key source-map-cache on the JSON coverage object.

source-map-cache is an object with keys representing the files source maps were extracted from, and values which include the raw source-map URL (in the key url), the parsed Source Map v3 information (in the key data), and the line lengths of the source file (in the key lineLengths).

{
  "result": [
    {
      "scriptId": "68",
      "url": "file:///absolute/path/to/source.js",
      "functions": []
    }
  ],
  "source-map-cache": {
    "file:///absolute/path/to/source.js": {
      "url": "./path-to-map.json",
      "data": {
        "version": 3,
        "sources": [
          "file:///absolute/path/to/original.js"
        ],
        "names": [
          "Foo",
          "console",
          "info"
        ],
        "mappings": "MAAMA,IACJC,YAAaC",
        "sourceRoot": "./"
      },
      "lineLengths": [
        13,
        62,
        38,
        27
      ]
    }
  }
}

NO_COLOR=<any>#

NO_COLOR is an alias for NODE_DISABLE_COLORS. The value of the environment variable is arbitrary.

OPENSSL_CONF=file#

Added in: v6.11.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

If the --openssl-config command-line option is used, the environment variable is ignored.

SSL_CERT_DIR=dir#

Added in: v7.7.0

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's directory containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

SSL_CERT_FILE=file#

Added in: v7.7.0

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's file containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

TZ#

History
VersionChanges
v16.2.0

Changing the TZ variable using process.env.TZ = changes the timezone on Windows as well.

v13.0.0

Changing the TZ variable using process.env.TZ = changes the timezone on POSIX systems.

v0.0.1

Added in: v0.0.1

The TZ environment variable is used to specify the timezone configuration.

While Node.js does not support all of the various ways that TZ is handled in other environments, it does support basic timezone IDs (such as 'Etc/UTC', 'Europe/Paris', or 'America/New_York'). It may support a few other abbreviations or aliases, but these are strongly discouraged and not guaranteed.

$ TZ=Europe/Dublin node -pe "new Date().toString()"
Wed May 12 2021 20:30:48 GMT+0100 (Irish Standard Time)

UV_THREADPOOL_SIZE=size#

Set the number of threads used in libuv's threadpool to size threads.

Asynchronous system APIs are used by Node.js whenever possible, but where they do not exist, libuv's threadpool is used to create asynchronous node APIs based on synchronous system APIs. Node.js APIs that use the threadpool are:

  • all fs APIs, other than the file watcher APIs and those that are explicitly synchronous
  • asynchronous crypto APIs such as crypto.pbkdf2(), crypto.scrypt(), crypto.randomBytes(), crypto.randomFill(), crypto.generateKeyPair()
  • dns.lookup()
  • all zlib APIs, other than those that are explicitly synchronous

Because libuv's threadpool has a fixed size, it means that if for whatever reason any of these APIs takes a long time, other (seemingly unrelated) APIs that run in libuv's threadpool will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than 4 (its current default value). For more information, see the libuv threadpool documentation.

Useful V8 options#

V8 has its own set of CLI options. Any V8 CLI option that is provided to node will be passed on to V8 to handle. V8's options have no stability guarantee. The V8 team themselves don't consider them to be part of their formal API, and reserve the right to change them at any time. Likewise, they are not covered by the Node.js stability guarantees. Many of the V8 options are of interest only to V8 developers. Despite this, there is a small set of V8 options that are widely applicable to Node.js, and they are documented here:

--max-old-space-size=SIZE (in megabytes)#

Sets the max memory size of V8's old memory section. As memory consumption approaches the limit, V8 will spend more time on garbage collection in an effort to free unused memory.

On a machine with 2 GiB of memory, consider setting this to 1536 (1.5 GiB) to leave some memory for other uses and avoid swapping.

$ node --max-old-space-size=1536 index.js

--max-semi-space-size=SIZE (in megabytes)#

Sets the maximum semi-space size for V8's scavenge garbage collector in MiB (megabytes). Increasing the max size of a semi-space may improve throughput for Node.js at the cost of more memory consumption.

Since the young generation size of the V8 heap is three times (see YoungGenerationSizeFromSemiSpaceSize in V8) the size of the semi-space, an increase of 1 MiB to semi-space applies to each of the three individual semi-spaces and causes the heap size to increase by 3 MiB. The throughput improvement depends on your workload (see #42511).

The default value is 16 MiB for 64-bit systems and 8 MiB for 32-bit systems. To get the best configuration for your application, you should try different max-semi-space-size values when running benchmarks for your application.

For example, benchmark on a 64-bit systems:

for MiB in 16 32 64 128; do
    node --max-semi-space-size=$MiB index.js
done

Console#

Stability: 2 - Stable

Source Code: lib/console.js

The node:console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

  • A Console class with methods such as console.log(), console.error(), and console.warn() that can be used to write to any Node.js stream.
  • A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('node:console').

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

Class: Console#

History
VersionChanges
v8.0.0

Errors that occur while writing to the underlying streams will now be ignored by default.

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('node:console').Console or console.Console (or their destructured counterparts):

const { Console } = require('node:console');
const { Console } = console;

new Console(stdout[, stderr][, ignoreErrors])#

new Console(options)#

History
VersionChanges
v14.2.0, v12.17.0

The groupIndentation option was introduced.

v11.7.0

The inspectOptions option is introduced.

v10.0.0

The Console constructor now supports an options argument, and the colorMode option was introduced.

v8.0.0

The ignoreErrors option was introduced.

  • options <Object>
    • stdout <stream.Writable>
    • stderr <stream.Writable>
    • ignoreErrors <boolean> Ignore errors when writing to the underlying streams. Default: true.
    • colorMode <boolean> | <string> Set color support for this Console instance. Setting to true enables coloring while inspecting values. Setting to false disables coloring while inspecting values. Setting to 'auto' makes color support depend on the value of the isTTY property and the value returned by getColorDepth() on the respective stream. This option can not be used, if inspectOptions.colors is set as well. Default: 'auto'.
    • inspectOptions <Object> Specifies options that are passed along to util.inspect().
    • groupIndentation <number> Set group indentation. Default: 2.

Creates a new Console with one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not provided, stdout is used for stderr.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// Custom simple logger
const logger = new Console({ stdout: output, stderr: errorOutput });
// use it like console
const count = 5;
logger.log('count: %d', count);
// In stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console({ stdout: process.stdout, stderr: process.stderr });

console.assert(value[, ...message])#

History
VersionChanges
v10.0.0

The implementation is now spec compliant and does not throw anymore.

v0.1.101

Added in: v0.1.101

  • value <any> The value tested for being truthy.
  • ...message <any> All arguments besides value are used as error message.

console.assert() writes a message if value is falsy or omitted. It only writes a message and does not otherwise affect execution. The output always starts with "Assertion failed". If provided, message is formatted using util.format().

If value is truthy, nothing happens.

console.assert(true, 'does nothing');

console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work

console.assert();
// Assertion failed

console.clear()#

Added in: v8.3.0

When stdout is a TTY, calling console.clear() will attempt to clear the TTY. When stdout is not a TTY, this method does nothing.

The specific operation of console.clear() can vary across operating systems and terminal types. For most Linux operating systems, console.clear() operates similarly to the clear shell command. On Windows, console.clear() will clear only the output in the current terminal viewport for the Node.js binary.

console.count([label])#

Added in: v8.3.0
  • label <string> The display label for the counter. Default: 'default'.

Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label.

> console.count()
default: 1
undefined
> console.count('default')
default: 2
undefined
> console.count('abc')
abc: 1
undefined
> console.count('xyz')
xyz: 1
undefined
> console.count('abc')
abc: 2
undefined
> console.count()
default: 3
undefined
>

console.countReset([label])#

Added in: v8.3.0
  • label <string> The display label for the counter. Default: 'default'.

Resets the internal counter specific to label.

> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>

console.debug(data[, ...args])#

History
VersionChanges
v8.10.0

console.debug is now an alias for console.log.

v8.0.0

Added in: v8.0.0

The console.debug() function is an alias for console.log().

console.dir(obj[, options])#

Added in: v0.1.101
  • obj <any>
  • options <Object>
    • showHidden <boolean> If true then the object's non-enumerable and symbol properties will be shown too. Default: false.
    • depth <number> Tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse indefinitely, pass null. Default: 2.
    • colors <boolean> If true, then the output will be styled with ANSI color codes. Colors are customizable; see customizing util.inspect() colors. Default: false.

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj.

console.dirxml(...data)#

History
VersionChanges
v9.3.0

console.dirxml now calls console.log for its arguments.

v8.0.0

Added in: v8.0.0

This method calls console.log() passing it the arguments received. This method does not produce any XML formatting.

console.error([data][, ...args])#

Added in: v0.1.100

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.group([...label])#

Added in: v8.5.0

Increases indentation of subsequent lines by spaces for groupIndentation length.

If one or more labels are provided, those are printed first without the additional indentation.

console.groupCollapsed()#

Added in: v8.5.0

An alias for console.group().

console.groupEnd()#

Added in: v8.5.0

Decreases indentation of subsequent lines by spaces for groupIndentation length.

console.info([data][, ...args])#

Added in: v0.1.100

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

console.table(tabularData[, properties])#

Added in: v10.0.0
  • tabularData <any>
  • properties <string[]> Alternate properties for constructing the table.

Try to construct a table with the columns of the properties of tabularData (or use properties) and rows of tabularData and log it. Falls back to just logging the argument if it can't be parsed as tabular.

// These can't be parsed as tabular data
console.table(Symbol());
// Symbol()

console.table(undefined);
// undefined

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]);
// ┌─────────┬─────┬─────┐
// │ (index) │  a  │  b  │
// ├─────────┼─────┼─────┤
// │    0    │  1  │ 'Y' │
// │    1    │ 'Z' │  2  │
// └─────────┴─────┴─────┘

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']);
// ┌─────────┬─────┐
// │ (index) │  a  │
// ├─────────┼─────┤
// │    0    │  1  │
// │    1    │ 'Z' │
// └─────────┴─────┘

console.time([label])#

Added in: v0.1.104

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when calling console.timeEnd() to stop the timer and output the elapsed time in suitable time units to stdout. For example, if the elapsed time is 3869ms, console.timeEnd() displays "3.869s".

console.timeEnd([label])#

History
VersionChanges
v13.0.0

The elapsed time is displayed with a suitable time unit.

v6.0.0

This method no longer supports multiple calls that don't map to individual console.time() calls; see below for details.

v0.1.104

Added in: v0.1.104

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('bunch-of-stuff');
// Do a bunch of stuff.
console.timeEnd('bunch-of-stuff');
// Prints: bunch-of-stuff: 225.438ms

console.timeLog([label][, ...data])#

Added in: v10.7.0

For a timer that was previously started by calling console.time(), prints the elapsed time and other data arguments to stdout:

console.time('process');
const value = expensiveProcess1(); // Returns 42
console.timeLog('process', value);
// Prints "process: 365.227ms 42".
doExpensiveProcess2(value);
console.timeEnd('process');

console.trace([message][, ...args])#

Added in: v0.1.104

Prints to stderr the string 'Trace: ', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
//  Trace: Show me
//    at repl:2:9
//    at REPLServer.defaultEval (repl.js:248:27)
//    at bound (domain.js:287:14)
//    at REPLServer.runBound [as eval] (domain.js:300:12)
//    at REPLServer.<anonymous> (repl.js:412:12)
//    at emitOne (events.js:82:20)
//    at REPLServer.emit (events.js:169:7)
//    at REPLServer.Interface._onLine (readline.js:210:10)
//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)

console.warn([data][, ...args])#

Added in: v0.1.100

The console.warn() function is an alias for console.error().

Inspector only methods#

The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector (--inspect flag).

console.profile([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector.

console.profile('MyLabel');
// Some code
console.profileEnd('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.

console.profileEnd([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. See console.profile() for an example.

If this method is called without a label, the most recently started profile is stopped.

console.timeStamp([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.timeStamp() method adds an event with the label 'label' to the Timeline panel of the inspector.

Corepack#

Added in: v16.9.0, v14.19.0

Stability: 1 - Experimental

Corepack is an experimental tool to help with managing versions of your package managers. It exposes binary proxies for each supported package manager that, when called, will identify whatever package manager is configured for the current project, transparently install it if needed, and finally run it without requiring explicit user interactions.

This feature simplifies two core workflows:

  • It eases new contributor onboarding, since they won't have to follow system-specific installation processes anymore just to have the package manager you want them to.

  • It allows you to ensure that everyone in your team will use exactly the package manager version you intend them to, without them having to manually synchronize it each time you need to make an update.

Workflows#

Enabling the feature#

Due to its experimental status, Corepack currently needs to be explicitly enabled to have any effect. To do that, run corepack enable, which will set up the symlinks in your environment next to the node binary (and overwrite the existing symlinks if necessary).

From this point forward, any call to the supported binaries will work without further setup. Should you experience a problem, run corepack disable to remove the proxies from your system (and consider opening an issue on the Corepack repository to let us know).

Configuring a package#

The Corepack proxies will find the closest package.json file in your current directory hierarchy to extract its "packageManager" property.

If the value corresponds to a supported package manager, Corepack will make sure that all calls to the relevant binaries are run against the requested version, downloading it on demand if needed, and aborting if it cannot be successfully retrieved.

Upgrading the global versions#

When running outside of an existing project (for example when running yarn init), Corepack will by default use predefined versions roughly corresponding to the latest stable releases from each tool. Those versions can be overridden by running the corepack prepare command along with the package manager version you wish to set:

corepack prepare yarn@x.y.z --activate

Alternately, a tag or range may be used:

corepack prepare pnpm@latest --activate
corepack prepare yarn@stable --activate

Offline workflow#

Many production environments don't have network access. Since Corepack usually downloads the package manager releases straight from their registries, it can conflict with such environments. To avoid that happening, call the corepack prepare command while you still have network access (typically at the same time you're preparing your deploy image). This will ensure that the required package managers are available even without network access.

The prepare command has various flags. Consult the detailed Corepack documentation for more information.

Supported package managers#

The following binaries are provided through Corepack:

Package managerBinary names
Yarnyarn, yarnpkg
pnpmpnpm, pnpx

Common questions#

How does Corepack interact with npm?#

While Corepack could support npm like any other package manager, its shims aren't enabled by default. This has a few consequences:

  • It's always possible to run a npm command within a project configured to be used with another package manager, since Corepack cannot intercept it.

  • While npm is a valid option in the "packageManager" property, the lack of shim will cause the global npm to be used.

Running npm install -g yarn doesn't work#

npm prevents accidentally overriding the Corepack binaries when doing a global install. To avoid this problem, consider one of the following options:

  • Don't run this command; Corepack will provide the package manager binaries anyway and will ensure that the requested versions are always available, so installing the package managers explicitly isn't needed.

  • Add the --force flag to npm install; this will tell npm that it's fine to override binaries, but you'll erase the Corepack ones in the process. (Run corepack enable to add them back.)

Crypto#

Stability: 2 - Stable

Source Code: lib/crypto.js

The node:crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions.

const { createHmac } = await import('node:crypto');

const secret = 'abcdefg';
const hash = createHmac('sha256', secret)
               .update('I love cupcakes')
               .digest('hex');
console.log(hash);
// Prints:
//   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658econst crypto = require('node:crypto');

const secret = 'abcdefg';
const hash = crypto.createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
console.log(hash);
// Prints:
//   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the node:crypto module. In such cases, attempting to import from crypto or calling require('node:crypto') will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

let crypto;
try {
  crypto = require('node:crypto');
} catch (err) {
  console.error('crypto support is disabled!');
}

When using the lexical ESM import keyword, the error can only be caught if a handler for process.on('uncaughtException') is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the import() function instead of the lexical import keyword:

let crypto;
try {
  crypto = await import('node:crypto');
} catch (err) {
  console.error('crypto support is disabled!');
}

Class: Certificate#

Added in: v0.11.8

SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and was specified formally as part of HTML5's keygen element.

<keygen> is deprecated since HTML 5.2 and new projects should not use this element anymore.

The node:crypto module provides the Certificate class for working with SPKAC data. The most common usage is handling output generated by the HTML5 <keygen> element. Node.js uses OpenSSL's SPKAC implementation internally.

Static method: Certificate.exportChallenge(spkac[, encoding])#

History
VersionChanges
v15.0.0

The spkac argument can be an ArrayBuffer. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

v9.0.0

Added in: v9.0.0

const { Certificate } = await import('node:crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 stringconst { Certificate } = require('node:crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

Static method: Certificate.exportPublicKey(spkac[, encoding])#

History
VersionChanges
v15.0.0

The spkac argument can be an ArrayBuffer. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

v9.0.0

Added in: v9.0.0

const { Certificate } = await import('node:crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>const { Certificate } = require('node:crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

Static method: Certificate.verifySpkac(spkac[, encoding])#

History
VersionChanges
v15.0.0

The spkac argument can be an ArrayBuffer. Added encoding. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

v9.0.0

Added in: v9.0.0

import { Buffer } from 'node:buffer';
const { Certificate } = await import('node:crypto');

const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or falseconst { Certificate } = require('node:crypto');
const { Buffer } = require('node:buffer');

const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Legacy API#

Stability: 0 - Deprecated

As a legacy interface, it is possible to create new instances of the crypto.Certificate class as illustrated in the examples below.

new crypto.Certificate()#

Instances of the Certificate class can be created using the new keyword or by calling crypto.Certificate() as a function:

const { Certificate } = await import('node:crypto');

const cert1 = new Certificate();
const cert2 = Certificate();const { Certificate } = require('node:crypto');

const cert1 = new Certificate();
const cert2 = Certificate();
certificate.exportChallenge(spkac[, encoding])#
Added in: v0.11.8 
const { Certificate } = await import('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 stringconst { Certificate } = require('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
certificate.exportPublicKey(spkac[, encoding])#
Added in: v0.11.8 
const { Certificate } = await import('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>const { Certificate } = require('node:crypto');
const cert = Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>
certificate.verifySpkac(spkac[, encoding])#
Added in: v0.11.8 
import { Buffer } from 'node:buffer';
const { Certificate } = await import('node:crypto');

const cert = Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or falseconst { Certificate } = require('node:crypto');
const { Buffer } = require('node:buffer');

const cert = Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Class: Cipher#

Added in: v0.1.94

Instances of the Cipher class are used to encrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
  • Using the cipher.update() and cipher.final() methods to produce the encrypted data.

The crypto.createCipher() or crypto.createCipheriv() methods are used to create Cipher instances. Cipher objects are not to be created directly using the new keyword.

Example: Using Cipher objects as streams:

const {
  scrypt,
  randomFill,
  createCipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    // Once we have the key and iv, we can create and use the cipher...
    const cipher = createCipheriv(algorithm, key, iv);

    let encrypted = '';
    cipher.setEncoding('hex');

    cipher.on('data', (chunk) => encrypted += chunk);
    cipher.on('end', () => console.log(encrypted));

    cipher.write('some clear text data');
    cipher.end();
  });
});const {
  scrypt,
  randomFill,
  createCipheriv,
} = require('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    // Once we have the key and iv, we can create and use the cipher...
    const cipher = createCipheriv(algorithm, key, iv);

    let encrypted = '';
    cipher.setEncoding('hex');

    cipher.on('data', (chunk) => encrypted += chunk);
    cipher.on('end', () => console.log(encrypted));

    cipher.write('some clear text data');
    cipher.end();
  });
});

Example: Using Cipher and piped streams:

import {
  createReadStream,
  createWriteStream,
} from 'node:fs';

import {
  pipeline,
} from 'node:stream';

const {
  scrypt,
  randomFill,
  createCipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    const cipher = createCipheriv(algorithm, key, iv);

    const input = createReadStream('test.js');
    const output = createWriteStream('test.enc');

    pipeline(input, cipher, output, (err) => {
      if (err) throw err;
    });
  });
});const {
  createReadStream,
  createWriteStream,
} = require('node:fs');

const {
  pipeline,
} = require('node:stream');

const {
  scrypt,
  randomFill,
  createCipheriv,
} = require('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    const cipher = createCipheriv(algorithm, key, iv);

    const input = createReadStream('test.js');
    const output = createWriteStream('test.enc');

    pipeline(input, cipher, output, (err) => {
      if (err) throw err;
    });
  });
});

Example: Using the cipher.update() and cipher.final() methods:

const {
  scrypt,
  randomFill,
  createCipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    const cipher = createCipheriv(algorithm, key, iv);

    let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
    encrypted += cipher.final('hex');
    console.log(encrypted);
  });
});const {
  scrypt,
  randomFill,
  createCipheriv,
} = require('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';

// First, we'll generate the key. The key length is dependent on the algorithm.
// In this case for aes192, it is 24 bytes (192 bits).
scrypt(password, 'salt', 24, (err, key) => {
  if (err) throw err;
  // Then, we'll generate a random initialization vector
  randomFill(new Uint8Array(16), (err, iv) => {
    if (err) throw err;

    const cipher = createCipheriv(algorithm, key, iv);

    let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
    encrypted += cipher.final('hex');
    console.log(encrypted);
  });
});

cipher.final([outputEncoding])#

Added in: v0.1.94
  • outputEncoding <string> The encoding of the return value.
  • Returns: <Buffer> | <string> Any remaining enciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned.

Once the cipher.final() method has been called, the Cipher object can no longer be used to encrypt data. Attempts to call cipher.final() more than once will result in an error being thrown.

cipher.getAuthTag()#

Added in: v1.0.0
  • Returns: <Buffer> When using an authenticated encryption mode (GCM, CCM, OCB, and chacha20-poly1305 are currently supported), the cipher.getAuthTag() method returns a Buffer containing the authentication tag that has been computed from the given data.

The cipher.getAuthTag() method should only be called after encryption has been completed using the cipher.final() method.

If the authTagLength option was set during the cipher instance's creation, this function will return exactly authTagLength bytes.

cipher.setAAD(buffer[, options])#

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM, OCB, and chacha20-poly1305 are currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The plaintextLength option is optional for GCM and OCB. When using CCM, the plaintextLength option must be specified and its value must match the length of the plaintext in bytes. See CCM mode.

The cipher.setAAD() method must be called before cipher.update().

cipher.setAutoPadding([autoPadding])#

Added in: v0.7.1

When using block encryption algorithms, the Cipher class will automatically add padding to the input data to the appropriate block size. To disable the default padding call cipher.setAutoPadding(false).

When autoPadding is false, the length of the entire input data must be a multiple of the cipher's block size or cipher.final() will throw an error. Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding.

The cipher.setAutoPadding() method must be called before cipher.final().

cipher.update(data[, inputEncoding][, outputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the cipher with data. If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer, TypedArray, or DataView. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned.

The cipher.update() method can be called multiple times with new data until cipher.final() is called. Calling cipher.update() after cipher.final() will result in an error being thrown.

Class: Decipher#

Added in: v0.1.94

Instances of the Decipher class are used to decrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
  • Using the decipher.update() and decipher.final() methods to produce the unencrypted data.

The crypto.createDecipher() or crypto.createDecipheriv() methods are used to create Decipher instances. Decipher objects are not to be created directly using the new keyword.

Example: Using Decipher objects as streams:

import { Buffer } from 'node:buffer';
const {
  scryptSync,
  createDecipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

let decrypted = '';
decipher.on('readable', () => {
  let chunk;
  while (null !== (chunk = decipher.read())) {
    decrypted += chunk.toString('utf8');
  }
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

// Encrypted with same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
decipher.write(encrypted, 'hex');
decipher.end();const {
  scryptSync,
  createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

let decrypted = '';
decipher.on('readable', () => {
  let chunk;
  while (null !== (chunk = decipher.read())) {
    decrypted += chunk.toString('utf8');
  }
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

// Encrypted with same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
decipher.write(encrypted, 'hex');
decipher.end();

Example: Using Decipher and piped streams:

import {
  createReadStream,
  createWriteStream,
} from 'node:fs';
import { Buffer } from 'node:buffer';
const {
  scryptSync,
  createDecipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

const input = createReadStream('test.enc');
const output = createWriteStream('test.js');

input.pipe(decipher).pipe(output);const {
  createReadStream,
  createWriteStream,
} = require('node:fs');
const {
  scryptSync,
  createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

const input = createReadStream('test.enc');
const output = createWriteStream('test.js');

input.pipe(decipher).pipe(output);

Example: Using the decipher.update() and decipher.final() methods:

import { Buffer } from 'node:buffer';
const {
  scryptSync,
  createDecipheriv,
} = await import('node:crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

// Encrypted using same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text dataconst {
  scryptSync,
  createDecipheriv,
} = require('node:crypto');
const { Buffer } = require('node:buffer');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = createDecipheriv(algorithm, key, iv);

// Encrypted using same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data

decipher.final([outputEncoding])#

Added in: v0.1.94
  • outputEncoding <string> The encoding of the return value.
  • Returns: <Buffer> | <string> Any remaining deciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned.

Once the decipher.final() method has been called, the Decipher object can no longer be used to decrypt data. Attempts to call decipher.final() more than once will result in an error being thrown.

decipher.setAAD(buffer[, options])#

History
VersionChanges
v15.0.0

The buffer argument can be a string or ArrayBuffer and is limited to no more than 2 ** 31 - 1 bytes.

v7.2.0

This method now returns a reference to decipher.

v1.0.0

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM, OCB, and chacha20-poly1305 are currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The options argument is optional for GCM. When using CCM, the plaintextLength option must be specified and its value must match the length of the ciphertext in bytes. See CCM mode.

The decipher.setAAD() method must be called before decipher.update().

When passing a string as the buffer, please consider caveats when using strings as inputs to cryptographic APIs.

decipher.setAuthTag(buffer[, encoding])#

History
VersionChanges
v15.0.0

The buffer argument can be a string or ArrayBuffer and is limited to no more than 2 ** 31 - 1 bytes.

v11.0.0

This method now throws if the GCM tag length is invalid.

v7.2.0

This method now returns a reference to decipher.

v1.0.0

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM, OCB, and chacha20-poly1305 are currently supported), the decipher.setAuthTag() method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, decipher.final() will throw, indicating that the cipher text should be discarded due to failed authentication. If the tag length is invalid according to NIST SP 800-38D or does not match the value of the authTagLength option, decipher.setAuthTag() will throw an error.

The decipher.setAuthTag() method must be called before decipher.update() for CCM mode or before decipher.final() for GCM and OCB modes and chacha20-poly1305. decipher.setAuthTag() can only be called once.

When passing a string as the authentication tag, please consider caveats when using strings as inputs to cryptographic APIs.

decipher.setAutoPadding([autoPadding])#

Added in: v0.7.1

When data has been encrypted without standard block padding, calling decipher.setAutoPadding(false) will disable automatic padding to prevent decipher.final() from checking for and removing padding.

Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.

The decipher.setAutoPadding() method must be called before decipher.final().

decipher.update(data[, inputEncoding][, outputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the decipher with data. If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer. If data is a Buffer then inputEncoding is ignored.

The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned.

The decipher.update() method can be called multiple times with new data until decipher.final() is called. Calling decipher.update() after decipher.final() will result in an error being thrown.

Class: DiffieHellman#

Added in: v0.5.0

The DiffieHellman class is a utility for creating Diffie-Hellman key exchanges.

Instances of the DiffieHellman class can be created using the crypto.createDiffieHellman() function.

import assert from 'node:assert';

const {
  createDiffieHellman,
} = await import('node:crypto');

// Generate Alice's keys...
const alice = createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));const assert = require('node:assert');

const {
  createDiffieHellman,
} = require('node:crypto');

// Generate Alice's keys...
const alice = createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));

diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#

Added in: v0.5.0

Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified inputEncoding, and secret is encoded using specified outputEncoding. If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer, TypedArray, or DataView.

If outputEncoding is given a string is returned; otherwise, a Buffer is returned.

diffieHellman.generateKeys([encoding])#

Added in: v0.5.0

Generates private and public Diffie-Hellman key values, and returns the public key in the specified encoding. This key should be transferred to the other party. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getGenerator([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman generator in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrime([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman prime in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrivateKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman private key in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPublicKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman public key in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.setPrivateKey(privateKey[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman private key. If the encoding argument is provided, privateKey is expected to be a string. If no encoding is provided, privateKey is expected to be a Buffer, TypedArray, or DataView.

diffieHellman.setPublicKey(publicKey[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman public key. If the encoding argument is provided, publicKey is expected to be a string. If no encoding is provided, publicKey is expected to be a Buffer, TypedArray, or DataView.

diffieHellman.verifyError#

Added in: v0.11.12

A bit field containing any warnings and/or errors resulting from a check performed during initialization of the DiffieHellman object.

The following values are valid for this property (as defined in node:constants module):

  • DH_CHECK_P_NOT_SAFE_PRIME
  • DH_CHECK_P_NOT_PRIME
  • DH_UNABLE_TO_CHECK_GENERATOR
  • DH_NOT_SUITABLE_GENERATOR

Class: DiffieHellmanGroup#

Added in: v0.7.5

The DiffieHellmanGroup class takes a well-known modp group as its argument. It works the same as DiffieHellman, except that it does not allow changing its keys after creation. In other words, it does not implement setPublicKey() or setPrivateKey() methods.

const { createDiffieHellmanGroup } = await import('node:crypto');
const dh = createDiffieHellmanGroup('modp16');const { createDiffieHellmanGroup } = require('node:crypto');
const dh = createDiffieHellmanGroup('modp16');

The following groups are supported:

  • 'modp14' (2048 bits, RFC 3526 Section 3)
  • 'modp15' (3072 bits, RFC 3526 Section 4)
  • 'modp16' (4096 bits, RFC 3526 Section 5)
  • 'modp17' (6144 bits, RFC 3526 Section 6)
  • 'modp18' (8192 bits, RFC 3526 Section 7)

The following groups are still supported but deprecated (see Caveats):

  • 'modp1' (768 bits, RFC 2409 Section 6.1)
  • 'modp2' (1024 bits, RFC 2409 Section 6.2)
  • 'modp5' (1536 bits, RFC 3526 Section 2)

These deprecated groups might be removed in future versions of Node.js.

Class: ECDH#

Added in: v0.11.14

The ECDH class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges.

Instances of the ECDH class can be created using the crypto.createECDH() function.

import assert from 'node:assert';

const {
  createECDH,
} = await import('node:crypto');

// Generate Alice's keys...
const alice = createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = createECDH('secp521r1');
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OKconst assert = require('node:assert');

const {
  createECDH,
} = require('node:crypto');

// Generate Alice's keys...
const alice = createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = createECDH('secp521r1');
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK

Static method: ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])#

Added in: v10.0.0

Converts the EC Diffie-Hellman public key specified by key and curve to the format specified by format. The format argument specifies point encoding and can be 'compressed', 'uncompressed' or 'hybrid'. The supplied key is interpreted using the specified inputEncoding, and the returned key is encoded using the specified outputEncoding.

Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

If format is not specified the point will be returned in 'uncompressed' format.

If the inputEncoding is not provided, key is expected to be a Buffer, TypedArray, or DataView.

Example (uncompressing a key):

const {
  createECDH,
  ECDH,
} = await import('node:crypto');

const ecdh = createECDH('secp256k1');
ecdh.generateKeys();

const compressedKey = ecdh.getPublicKey('hex', 'compressed');

const uncompressedKey = ECDH.convertKey(compressedKey,
                                        'secp256k1',
                                        'hex',
                                        'hex',
                                        'uncompressed');

// The converted key and the uncompressed public key should be the same
console.log(uncompressedKey === ecdh.getPublicKey('hex'));const {
  createECDH,
  ECDH,
} = require('node:crypto');

const ecdh = createECDH('secp256k1');
ecdh.generateKeys();

const compressedKey = ecdh.getPublicKey('hex', 'compressed');

const uncompressedKey = ECDH.convertKey(compressedKey,
                                        'secp256k1',
                                        'hex',
                                        'hex',
                                        'uncompressed');

// The converted key and the uncompressed public key should be the same
console.log(uncompressedKey === ecdh.getPublicKey('hex'));

ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#

History
VersionChanges
v10.0.0

Changed error format to better support invalid public key error.

v6.0.0

The default inputEncoding changed from binary to utf8.

v0.11.14

Added in: v0.11.14

Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified inputEncoding, and the returned secret is encoded using the specified outputEncoding. If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer, TypedArray, or DataView.

If outputEncoding is given a string will be returned; otherwise a Buffer is returned.

ecdh.computeSecret will throw an ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY error when otherPublicKey lies outside of the elliptic curve. Since otherPublicKey is usually supplied from a remote user over an insecure network, be sure to handle this exception accordingly.

ecdh.generateKeys([encoding[, format]])#

Added in: v0.11.14

Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified format and encoding. This key should be transferred to the other party.

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified, the point will be returned in 'uncompressed' format.

If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPrivateKey([encoding])#

Added in: v0.11.14

If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.getPublicKey([encoding][, format])#

Added in: v0.11.14

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified the point will be returned in 'uncompressed' format.

If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.setPrivateKey(privateKey[, encoding])#

Added in: v0.11.14

Sets the EC Diffie-Hellman private key. If encoding is provided, privateKey is expected to be a string; otherwise privateKey is expected to be a Buffer, TypedArray, or DataView.

If privateKey is not valid for the curve specified when the ECDH object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the ECDH object.

ecdh.setPublicKey(publicKey[, encoding])#

Added in: v0.11.14Deprecated since: v5.2.0

Stability: 0 - Deprecated

Sets the EC Diffie-Hellman public key. If encoding is provided publicKey is expected to be a string; otherwise a Buffer, TypedArray, or DataView is expected.

There is not normally a reason to call this method because ECDH only requires a private key and the other party's public key to compute the shared secret. Typically either ecdh.generateKeys() or ecdh.setPrivateKey() will be called. The ecdh.setPrivateKey() method attempts to generate the public point/key associated with the private key being set.

Example (obtaining a shared secret):

const {
  createECDH,
  createHash,
} = await import('node:crypto');

const alice = createECDH('secp256k1');
const bob = createECDH('secp256k1');

// This is a shortcut way of specifying one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  createHash('sha256').update('alice', 'utf8').digest(),
);

// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);const {
  createECDH,
  createHash,
} = require('node:crypto');

const alice = createECDH('secp256k1');
const bob = createECDH('secp256k1');

// This is a shortcut way of specifying one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  createHash('sha256').update('alice', 'utf8').digest(),
);

// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);

Class: Hash#

Added in: v0.1.92

The Hash class is a utility for creating hash digests of data. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
  • Using the hash.update() and hash.digest() methods to produce the computed hash.

The crypto.createHash() method is used to create Hash instances. Hash objects are not to be created directly using the new keyword.

Example: Using Hash objects as streams:

const {
  createHash,
} = await import('node:crypto');

const hash = createHash('sha256');

hash.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hash.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
  }
});

hash.write('some data to hash');
hash.end();const {
  createHash,
} = require('node:crypto');

const hash = createHash('sha256');

hash.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hash.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
  }
});

hash.write('some data to hash');
hash.end();

Example: Using Hash and piped streams:

import { createReadStream } from 'node:fs';
import { stdout } from 'node:process';
const { createHash } = await import('node:crypto');

const hash = createHash('sha256');

const input = createReadStream('test.js');
input.pipe(hash).setEncoding('hex').pipe(stdout);const { createReadStream } = require('node:fs');
const { createHash } = require('node:crypto');
const { stdout } = require('node:process');

const hash = createHash('sha256');

const input = createReadStream('test.js');
input.pipe(hash).setEncoding('hex').pipe(stdout);

Example: Using the hash.update() and hash.digest() methods:

const {
  createHash,
} = await import('node:crypto');

const hash = createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50const {
  createHash,
} = require('node:crypto');

const hash = createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50

hash.copy([options])#

Added in: v13.1.0

Creates a new Hash object that contains a deep copy of the internal state of the current Hash object.

The optional options argument controls stream behavior. For XOF hash functions such as 'shake256', the outputLength option can be used to specify the desired output length in bytes.

An error is thrown when an attempt is made to copy the Hash object after its hash.digest() method has been called.

// Calculate a rolling hash.
const {
  createHash,
} = await import('node:crypto');

const hash = createHash('sha256');

hash.update('one');
console.log(hash.copy().digest('hex'));

hash.update('two');
console.log(hash.copy().digest('hex'));

hash.update('three');
console.log(hash.copy().digest('hex'));

// Etc.// Calculate a rolling hash.
const {
  createHash,
} = require('node:crypto');

const hash = createHash('sha256');

hash.update('one');
console.log(hash.copy().digest('hex'));

hash.update('two');
console.log(hash.copy().digest('hex'));

hash.update('three');
console.log(hash.copy().digest('hex'));

// Etc.

hash.digest([encoding])#

Added in: v0.1.92

Calculates the digest of all of the data passed to be hashed (using the hash.update() method). If encoding is provided a string will be returned; otherwise a Buffer is returned.

The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown.

hash.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the hash content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: Hmac#

Added in: v0.1.94

The Hmac class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
  • Using the hmac.update() and hmac.digest() methods to produce the computed HMAC digest.

The crypto.createHmac() method is used to create Hmac instances. Hmac objects are not to be created directly using the new keyword.

Example: Using Hmac objects as streams:

const {
  createHmac,
} = await import('node:crypto');

const hmac = createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hmac.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
  }
});

hmac.write('some data to hash');
hmac.end();const {
  createHmac,
} = require('node:crypto');

const hmac = createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hmac.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
  }
});

hmac.write('some data to hash');
hmac.end();

Example: Using Hmac and piped streams:

import { createReadStream } from 'node:fs';
import { stdout } from 'node:process';
const {
  createHmac,
} = await import('node:crypto');

const hmac = createHmac('sha256', 'a secret');

const input = createReadStream('test.js');
input.pipe(hmac).pipe(stdout);const {
  createReadStream,
} = require('node:fs');
const {
  createHmac,
} = require('node:crypto');
const { stdout } = require('node:process');

const hmac = createHmac('sha256', 'a secret');

const input = createReadStream('test.js');
input.pipe(hmac).pipe(stdout);

Example: Using the hmac.update() and hmac.digest() methods:

const {
  createHmac,
} = await import('node:crypto');

const hmac = createHmac('sha256', 'a secret');

hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
//   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77econst {
  createHmac,
} = require('node:crypto');

const hmac = createHmac('sha256', 'a secret');

hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
//   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e

hmac.digest([encoding])#

Added in: v0.1.94

Calculates the HMAC digest of all of the data passed using hmac.update(). If encoding is provided a string is returned; otherwise a Buffer is returned;

The Hmac object can not be used again after hmac.digest() has been called. Multiple calls to hmac.digest() will result in an error being thrown.

hmac.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the Hmac content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: KeyObject#

History
VersionChanges
v14.5.0, v12.19.0

Instances of this class can now be passed to worker threads using postMessage.

v11.13.0

This class is now exported.

v11.6.0

Added in: v11.6.0

Node.js uses a KeyObject class to represent a symmetric or asymmetric key, and each kind of key exposes different functions. The crypto.createSecretKey(), crypto.createPublicKey() and crypto.createPrivateKey() methods are used to create KeyObject instances. KeyObject objects are not to be created directly using the new keyword.

Most applications should consider using the new KeyObject API instead of passing keys as strings or Buffers due to improved security features.

KeyObject instances can be passed to other threads via postMessage(). The receiver obtains a cloned KeyObject, and the KeyObject does not need to be listed in the transferList argument.

Static method: KeyObject.from(key)#

Added in: v15.0.0

Example: Converting a CryptoKey instance to a KeyObject:

const { KeyObject } = await import('node:crypto');
const { subtle } = globalThis.crypto;

const key = await subtle.generateKey({
  name: 'HMAC',
  hash: 'SHA-256',
  length: 256,
}, true, ['sign', 'verify']);

const keyObject = KeyObject.from(key);
console.log(keyObject.symmetricKeySize);
// Prints: 32 (symmetric key size in bytes)const { KeyObject } = require('node:crypto');
const { subtle } = globalThis.crypto;

(async function() {
  const key = await subtle.generateKey({
    name: 'HMAC',
    hash: 'SHA-256',
    length: 256,
  }, true, ['sign', 'verify']);

  const keyObject = KeyObject.from(key);
  console.log(keyObject.symmetricKeySize);
  // Prints: 32 (symmetric key size in bytes)
})();

keyObject.asymmetricKeyDetails#

History
VersionChanges
v16.9.0

Expose RSASSA-PSS-params sequence parameters for RSA-PSS keys.

v15.7.0

Added in: v15.7.0

  • <Object>
    • modulusLength: <number> Key size in bits (RSA, DSA).
    • publicExponent: <bigint> Public exponent (RSA).
    • hashAlgorithm: <string> Name of the message digest (RSA-PSS).
    • mgf1HashAlgorithm: <string> Name of the message digest used by MGF1 (RSA-PSS).
    • saltLength: <number> Minimal salt length in bytes (RSA-PSS).
    • divisorLength: <number> Size of q in bits (DSA).
    • namedCurve: <string> Name of the curve (EC).

This property exists only on asymmetric keys. Depending on the type of the key, this object contains information about the key. None of the information obtained through this property can be used to uniquely identify a key or to compromise the security of the key.

For RSA-PSS keys, if the key material contains a RSASSA-PSS-params sequence, the hashAlgorithm, mgf1HashAlgorithm, and saltLength properties will be set.

Other key details might be exposed via this API using additional attributes.

keyObject.asymmetricKeyType#

History
VersionChanges
v13.9.0, v12.17.0

Added support for 'dh'.

v12.0.0

Added support for 'rsa-pss'.

v12.0.0

This property now returns undefined for KeyObject instances of unrecognized type instead of aborting.

v12.0.0

Added support for 'x25519' and 'x448'.

v12.0.0

Added support for 'ed25519' and 'ed448'.

v11.6.0

Added in: v11.6.0

For asymmetric keys, this property represents the type of the key. Supported key types are:

  • 'rsa' (OID 1.2.840.113549.1.1.1)
  • 'rsa-pss' (OID 1.2.840.113549.1.1.10)
  • 'dsa' (OID 1.2.840.10040.4.1)
  • 'ec' (OID 1.2.840.10045.2.1)
  • 'x25519' (OID 1.3.101.110)
  • 'x448' (OID 1.3.101.111)
  • 'ed25519' (OID 1.3.101.112)
  • 'ed448' (OID 1.3.101.113)
  • 'dh' (OID 1.2.840.113549.1.3.1)

This property is undefined for unrecognized KeyObject types and symmetric keys.

keyObject.export([options])#

History
VersionChanges
v15.9.0

Added support for 'jwk' format.

v11.6.0

Added in: v11.6.0

For symmetric keys, the following encoding options can be used:

  • format: <string> Must be 'buffer' (default) or 'jwk'.

For public keys, the following encoding options can be used:

  • type: <string> Must be one of 'pkcs1' (RSA only) or 'spki'.
  • format: <string> Must be 'pem', 'der', or 'jwk'.

For private keys, the following encoding options can be used:

  • type: <string> Must be one of 'pkcs1' (RSA only), 'pkcs8' or 'sec1' (EC only).
  • format: <string> Must be 'pem', 'der', or 'jwk'.
  • cipher: <string> If specified, the private key will be encrypted with the given cipher and passphrase using PKCS#5 v2.0 password based encryption.
  • passphrase: <string> | <Buffer> The passphrase to use for encryption, see cipher.

The result type depends on the selected encoding format, when PEM the result is a string, when DER it will be a buffer containing the data encoded as DER, when JWK it will be an object.

When JWK encoding format was selected, all other encoding options are ignored.

PKCS#1, SEC1, and PKCS#8 type keys can be encrypted by using a combination of the cipher and format options. The PKCS#8 type can be used with any format to encrypt any key algorithm (RSA, EC, or DH) by specifying a cipher. PKCS#1 and SEC1 can only be encrypted by specifying a cipher when the PEM format is used. For maximum compatibility, use PKCS#8 for encrypted private keys. Since PKCS#8 defines its own encryption mechanism, PEM-level encryption is not supported when encrypting a PKCS#8 key. See RFC 5208 for PKCS#8 encryption and RFC 1421 for PKCS#1 and SEC1 encryption.

keyObject.equals(otherKeyObject)#

Added in: v17.7.0, v16.15.0

Returns true or false depending on whether the keys have exactly the same type, value, and parameters. This method is not constant time.

keyObject.symmetricKeySize#

Added in: v11.6.0

For secret keys, this property represents the size of the key in bytes. This property is undefined for asymmetric keys.

keyObject.type#

Added in: v11.6.0

Depending on the type of this KeyObject, this property is either 'secret' for secret (symmetric) keys, 'public' for public (asymmetric) keys or 'private' for private (asymmetric) keys.

Class: Sign#

Added in: v0.1.92

The Sign class is a utility for generating signatures. It can be used in one of two ways:

  • As a writable stream, where data to be signed is written and the sign.sign() method is used to generate and return the signature, or
  • Using the sign.update() and sign.sign() methods to produce the signature.

The crypto.createSign() method is used to create Sign instances. The argument is the string name of the hash function to use. Sign objects are not to be created directly using the new keyword.

Example: Using Sign and Verify objects as streams:

const {
  generateKeyPairSync,
  createSign,
  createVerify,
} = await import('node:crypto');

const { privateKey, publicKey } = generateKeyPairSync('ec', {
  namedCurve: 'sect239k1',
});

const sign = createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');

const verify = createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: trueconst {
  generateKeyPairSync,
  createSign,
  createVerify,
} = require('node:crypto');

const { privateKey, publicKey } = generateKeyPairSync('ec', {
  namedCurve: 'sect239k1',
});

const sign = createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');

const verify = createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: true

Example: Using the sign.update() and verify.update() methods:

const {
  generateKeyPairSync,
  createSign,
  createVerify,
} = await import('node:crypto');

const { privateKey, publicKey } = generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

const sign = createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);

const verify = createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: trueconst {
  generateKeyPairSync,
  createSign,
  createVerify,
} = require('node:crypto');

const { privateKey, publicKey } = generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

const sign = createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);

const verify = createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: true

sign.sign(privateKey[, outputEncoding])#

History
VersionChanges
v15.0.0

The privateKey can also be an ArrayBuffer and CryptoKey.

v13.2.0, v12.16.0

This function now supports IEEE-P1363 DSA and ECDSA signatures.

v12.0.0

This function now supports RSA-PSS keys.

v11.6.0

This function now supports key objects.

v8.0.0

Support for RSASSA-PSS and additional options was added.

v0.1.92

Added in: v0.1.92

Calculates the signature on all the data passed through using either sign.update() or sign.write().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

If outputEncoding is provided a string is returned; otherwise a Buffer is returned.

The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown.

sign.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the Sign content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: Verify#

Added in: v0.1.92

The Verify class is a utility for verifying signatures. It can be used in one of two ways:

The crypto.createVerify() method is used to create Verify instances. Verify objects are not to be created directly using the new keyword.

See Sign for examples.

verify.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the Verify content with the given data, the encoding of which is given in inputEncoding. If inputEncoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

verify.verify(object, signature[, signatureEncoding])#

History
VersionChanges
v15.0.0

The object can also be an ArrayBuffer and CryptoKey.

v13.2.0, v12.16.0

This function now supports IEEE-P1363 DSA and ECDSA signatures.

v12.0.0

This function now supports RSA-PSS keys.

v11.7.0

The key can now be a private key.

v8.0.0

Support for RSASSA-PSS and additional options was added.

v0.1.92

Added in: v0.1.92

Verifies the provided data using the given object and signature.

If object is not a KeyObject, this function behaves as if object had been passed to crypto.createPublicKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_AUTO (default) causes it to be determined automatically.

The signature argument is the previously calculated signature for the data, in the signatureEncoding. If a signatureEncoding is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer, TypedArray, or DataView.

The verify object can not be used again after verify.verify() has been called. Multiple calls to verify.verify() will result in an error being thrown.

Because public keys can be derived from private keys, a private key may be passed instead of a public key.

Class: X509Certificate#

Added in: v15.6.0

Encapsulates an X509 certificate and provides read-only access to its information.

const { X509Certificate } = await import('node:crypto');

const x509 = new X509Certificate('{... pem encoded cert ...}');

console.log(x509.subject);const { X509Certificate } = require('node:crypto');

const x509 = new X509Certificate('{... pem encoded cert ...}');

console.log(x509.subject);

new X509Certificate(buffer)#

Added in: v15.6.0

x509.ca#

Added in: v15.6.0
  • Type: <boolean> Will be true if this is a Certificate Authority (CA) certificate.

x509.checkEmail(email[, options])#

History
VersionChanges
v18.0.0

The subject option now defaults to 'default'.

v17.5.0, v16.15.0

The subject option can now be set to 'default'.

v17.5.0, v16.14.1

The wildcards, partialWildcards, multiLabelWildcards, and singleLabelSubdomains options have been removed since they had no effect.

v15.6.0

Added in: v15.6.0

Checks whether the certificate matches the given email address.

If the 'subject' option is undefined or set to 'default', the certificate subject is only considered if the subject alternative name extension either does not exist or does not contain any email addresses.

If the 'subject' option is set to 'always' and if the subject alternative name extension either does not exist or does not contain a matching email address, the certificate subject is considered.

If the 'subject' option is set to 'never', the certificate subject is never considered, even if the certificate contains no subject alternative names.

x509.checkHost(name[, options])#

History
VersionChanges
v18.0.0

The subject option now defaults to 'default'.

v17.5.0, v16.15.0

The subject option can now be set to 'default'.

v15.6.0

Added in: v15.6.0

Checks whether the certificate matches the given host name.

If the certificate matches the given host name, the matching subject name is returned. The returned name might be an exact match (e.g., foo.example.com) or it might contain wildcards (e.g., *.example.com). Because host name comparisons are case-insensitive, the returned subject name might also differ from the given name in capitalization.

If the 'subject' option is undefined or set to 'default', the certificate subject is only considered if the subject alternative name extension either does not exist or does not contain any DNS names. This behavior is consistent with RFC 2818 ("HTTP Over TLS").

If the 'subject' option is set to 'always' and if the subject alternative name extension either does not exist or does not contain a matching DNS name, the certificate subject is considered.

If the 'subject' option is set to 'never', the certificate subject is never considered, even if the certificate contains no subject alternative names.

x509.checkIP(ip)#

History
VersionChanges
v17.5.0, v16.14.1

The options argument has been removed since it had no effect.

v15.6.0

Added in: v15.6.0

Checks whether the certificate matches the given IP address (IPv4 or IPv6).

Only RFC 5280 iPAddress subject alternative names are considered, and they must match the given ip address exactly. Other subject alternative names as well as the subject field of the certificate are ignored.

x509.checkIssued(otherCert)#

Added in: v15.6.0

Checks whether this certificate was issued by the given otherCert.

x509.checkPrivateKey(privateKey)#

Added in: v15.6.0

Checks whether the public key for this certificate is consistent with the given private key.

x509.fingerprint#

Added in: v15.6.0

The SHA-1 fingerprint of this certificate.

Because SHA-1 is cryptographically broken and because the security of SHA-1 is significantly worse than that of algorithms that are commonly used to sign certificates, consider using x509.fingerprint256 instead.

x509.fingerprint256#

Added in: v15.6.0

The SHA-256 fingerprint of this certificate.

x509.fingerprint512#

Added in: v17.2.0, v16.14.0

The SHA-512 fingerprint of this certificate.

Because computing the SHA-256 fingerprint is usually faster and because it is only half the size of the SHA-512 fingerprint, x509.fingerprint256 may be a better choice. While SHA-512 presumably provides a higher level of security in general, the security of SHA-256 matches that of most algorithms that are commonly used to sign certificates.

x509.infoAccess#

History
VersionChanges
v17.3.1, v16.13.2

Parts of this string may be encoded as JSON string literals in response to CVE-2021-44532.

v15.6.0

Added in: v15.6.0

A textual representation of the certificate's authority information access extension.

This is a line feed separated list of access descriptions. Each line begins with the access method and the kind of the access location, followed by a colon and the value associated with the access location.

After the prefix denoting the access method and the kind of the access location, the remainder of each line might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.

x509.issuer#

Added in: v15.6.0

The issuer identification included in this certificate.

x509.issuerCertificate#

Added in: v15.9.0

The issuer certificate or undefined if the issuer certificate is not available.

x509.keyUsage#

Added in: v15.6.0

An array detailing the key usages for this certificate.

x509.publicKey#

Added in: v15.6.0

The public key <KeyObject> for this certificate.

x509.raw#

Added in: v15.6.0

A Buffer containing the DER encoding of this certificate.

x509.serialNumber#

Added in: v15.6.0

The serial number of this certificate.

Serial numbers are assigned by certificate authorities and do not uniquely identify certificates. Consider using x509.fingerprint256 as a unique identifier instead.

x509.subject#

Added in: v15.6.0

The complete subject of this certificate.

x509.subjectAltName#

History
VersionChanges
v17.3.1, v16.13.2

Parts of this string may be encoded as JSON string literals in response to CVE-2021-44532.

v15.6.0

Added in: v15.6.0

The subject alternative name specified for this certificate.

This is a comma-separated list of subject alternative names. Each entry begins with a string identifying the kind of the subject alternative name followed by a colon and the value associated with the entry.

Earlier versions of Node.js incorrectly assumed that it is safe to split this property at the two-character sequence ', ' (see CVE-2021-44532). However, both malicious and legitimate certificates can contain subject alternative names that include this sequence when represented as a string.

After the prefix denoting the type of the entry, the remainder of each entry might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.

x509.toJSON()#

Added in: v15.6.0

There is no standard JSON encoding for X509 certificates. The toJSON() method returns a string containing the PEM encoded certificate.

x509.toLegacyObject()#

Added in: v15.6.0

Returns information about this certificate using the legacy certificate object encoding.

x509.toString()#

Added in: v15.6.0

Returns the PEM-encoded certificate.

x509.validFrom#

Added in: v15.6.0

The date/time from which this certificate is considered valid.

x509.validTo#

Added in: v15.6.0

The date/time until which this certificate is considered valid.

x509.verify(publicKey)#

Added in: v15.6.0

Verifies that this certificate was signed by the given public key. Does not perform any other validation checks on the certificate.

node:crypto module methods and properties#

crypto.constants#

Added in: v6.3.0

An object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto constants.

crypto.DEFAULT_ENCODING#

Added in: v0.9.3Deprecated since: v10.0.0

Stability: 0 - Deprecated

The default encoding to use for functions that can take either strings or buffers. The default value is 'buffer', which makes methods default to Buffer objects.

The crypto.DEFAULT_ENCODING mechanism is provided for backward compatibility with legacy programs that expect 'latin1' to be the default encoding.

New applications should expect the default to be 'buffer'.

This property is deprecated.

crypto.fips#

Added in: v6.0.0Deprecated since: v10.0.0

Stability: 0 - Deprecated

Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.

This property is deprecated. Please use crypto.setFips() and crypto.getFips() instead.

crypto.checkPrime(candidate[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.8.0

Added in: v15.8.0

  • candidate <ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint> A possible prime encoded as a sequence of big endian octets of arbitrary length.
  • options <Object>
    • checks <number> The number of Miller-Rabin probabilistic primality iterations to perform. When the value is 0 (zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for the BN_is_prime_ex function nchecks options for more details. Default: 0
  • callback <Function>
    • err <Error> Set to an <Error> object if an error occurred during check.
    • result <boolean> true if the candidate is a prime with an error probability less than 0.25 ** options.checks.

Checks the primality of the candidate.

crypto.checkPrimeSync(candidate[, options])#

Added in: v15.8.0
  • candidate <ArrayBuffer> | <SharedArrayBuffer> | <TypedArray> | <Buffer> | <DataView> | <bigint> A possible prime encoded as a sequence of big endian octets of arbitrary length.
  • options <Object>
    • checks <number> The number of Miller-Rabin probabilistic primality iterations to perform. When the value is 0 (zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for the BN_is_prime_ex function nchecks options for more details. Default: 0
  • Returns: <boolean> true if the candidate is a prime with an error probability less than 0.25 ** options.checks.

Checks the primality of the candidate.

crypto.createCipher(algorithm, password[, options])#

History
VersionChanges
v17.9.0, v16.17.0

The authTagLength option is now optional when using the chacha20-poly1305 cipher and defaults to 16 bytes.

v15.0.0

The password argument can be an ArrayBuffer and is limited to a maximum of 2 ** 31 - 1 bytes.

v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to produce shorter authentication tags in GCM mode and defaults to 16 bytes.

v10.0.0

Deprecated since: v10.0.0

v0.1.94

Added in: v0.1.94

Stability: 0 - Deprecated: Use crypto.createCipheriv() instead.

Creates and returns a Cipher object that uses the given algorithm and password.

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. 'aes-128-ccm') is used. In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes. For chacha20-poly1305, the authTagLength option defaults to 16 bytes.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms will display the available cipher algorithms.

The password is used to derive the cipher key and initialization vector (IV). The value must be either a 'latin1' encoded string, a Buffer, a TypedArray, or a DataView.

This function is semantically insecure for all supported ciphers and fatally flawed for ciphers in counter mode (such as CTR, GCM, or CCM).

The implementation of crypto.createCipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createCipheriv() to create the Cipher object. Users should not use ciphers with counter mode (e.g. CTR, GCM, or CCM) in crypto.createCipher(). A warning is emitted when they are used in order to avoid the risk of IV reuse that causes vulnerabilities. For the case when IV is reused in GCM, see Nonce-Disrespecting Adversaries for details.

crypto.createCipheriv(algorithm, key, iv[, options])#

History
VersionChanges
v17.9.0, v16.17.0

The authTagLength option is now optional when using the chacha20-poly1305 cipher and defaults to 16 bytes.

v15.0.0

The password and iv arguments can be an ArrayBuffer and are each limited to a maximum of 2 ** 31 - 1 bytes.

v11.6.0

The key argument can now be a KeyObject.

v11.2.0, v10.17.0

The cipher chacha20-poly1305 (the IETF variant of ChaCha20-Poly1305) is now supported.

v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to produce shorter authentication tags in GCM mode and defaults to 16 bytes.

v9.9.0

The iv parameter may now be null for ciphers which do not need an initialization vector.

v0.1.94

Added in: v0.1.94

Creates and returns a Cipher object, with the given algorithm, key and initialization vector (iv).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. 'aes-128-ccm') is used. In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes. For chacha20-poly1305, the authTagLength option defaults to 16 bytes.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings, Buffers, TypedArray, or DataViews. The key may optionally be a KeyObject of type secret. If the cipher does not need an initialization vector, iv may be null.

When passing strings for key or iv, please consider caveats when using strings as inputs to cryptographic APIs.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

crypto.createDecipher(algorithm, password[, options])#

History
VersionChanges
v17.9.0, v16.17.0

The authTagLength option is now optional when using the chacha20-poly1305 cipher and defaults to 16 bytes.

v10.10.0

Ciphers in OCB mode are now supported.

v10.0.0

Deprecated since: v10.0.0

v0.1.94

Added in: v0.1.94

Stability: 0 - Deprecated: Use crypto.createDecipheriv() instead.

Creates and returns a Decipher object that uses the given algorithm and password (key).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. 'aes-128-ccm') is used. In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. For chacha20-poly1305, the authTagLength option defaults to 16 bytes.

This function is semantically insecure for all supported ciphers and fatally flawed for ciphers in counter mode (such as CTR, GCM, or CCM).

The implementation of crypto.createDecipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createDecipheriv() to create the Decipher object.

crypto.createDecipheriv(algorithm, key, iv[, options])#

History
VersionChanges
v17.9.0, v16.17.0

The authTagLength option is now optional when using the chacha20-poly1305 cipher and defaults to 16 bytes.

v11.6.0

The key argument can now be a KeyObject.

v11.2.0, v10.17.0

The cipher chacha20-poly1305 (the IETF variant of ChaCha20-Poly1305) is now supported.

v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to restrict accepted GCM authentication tag lengths.

v9.9.0

The iv parameter may now be null for ciphers which do not need an initialization vector.

v0.1.94

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm, key and initialization vector (iv).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. 'aes-128-ccm') is used. In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to restrict accepted authentication tags to those with the specified length. For chacha20-poly1305, the authTagLength option defaults to 16 bytes.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings, Buffers, TypedArray, or DataViews. The key may optionally be a KeyObject of type secret. If the cipher does not need an initialization vector, iv may be null.

When passing strings for key or iv, please consider caveats when using strings as inputs to cryptographic APIs.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])#

History
VersionChanges
v8.0.0

The prime argument can be any TypedArray or DataView now.

v8.0.0

The prime argument can be a Uint8Array now.

v6.0.0

The default for the encoding parameters changed from binary to utf8.

v0.11.12

Added in: v0.11.12

Creates a DiffieHellman key exchange object using the supplied prime and an optional specific generator.

The generator argument can be a number, string, or Buffer. If generator is not specified, the value 2 is used.

If primeEncoding is specified, prime is expected to be a string; otherwise a Buffer, TypedArray, or DataView is expected.

If generatorEncoding is specified, generator is expected to be a string; otherwise a number, Buffer, TypedArray, or DataView is expected.

crypto.createDiffieHellman(primeLength[, generator])#

Added in: v0.5.0

Creates a DiffieHellman key exchange object and generates a prime of primeLength bits using an optional specific numeric generator. If generator is not specified, the value 2 is used.

crypto.createDiffieHellmanGroup(name)#

Added in: v0.9.3

An alias for crypto.getDiffieHellman()

crypto.createECDH(curveName)#

Added in: v0.11.14

Creates an Elliptic Curve Diffie-Hellman (ECDH) key exchange object using a predefined curve specified by the curveName string. Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

crypto.createHash(algorithm[, options])#

History
VersionChanges
v12.8.0

The outputLength option was added for XOF hash functions.

v0.1.92

Added in: v0.1.92

Creates and returns a Hash object that can be used to generate hash digests using the given algorithm. Optional options argument controls stream behavior. For XOF hash functions such as 'shake256', the outputLength option can be used to specify the desired output length in bytes.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list -digest-algorithms will display the available digest algorithms.

Example: generating the sha256 sum of a file

import {
  createReadStream,
} from 'node:fs';
import { argv } from 'node:process';
const {
  createHash,
} = await import('node:crypto');

const filename = argv[2];

const hash = createHash('sha256');

const input = createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});const {
  createReadStream,
} = require('node:fs');
const {
  createHash,
} = require('node:crypto');
const { argv } = require('node:process');

const filename = argv[2];

const hash = createHash('sha256');

const input = createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});

crypto.createHmac(algorithm, key[, options])#

History
VersionChanges
v15.0.0

The key can also be an ArrayBuffer or CryptoKey. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.

v11.6.0

The key argument can now be a KeyObject.

v0.1.94

Added in: v0.1.94

Creates and returns an Hmac object that uses the given algorithm and key. Optional options argument controls stream behavior.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list -digest-algorithms will display the available digest algorithms.

The key is the HMAC key used to generate the cryptographic HMAC hash. If it is a KeyObject, its type must be secret.

Example: generating the sha256 HMAC of a file

import {
  createReadStream,
} from 'node:fs';
import { argv } from 'node:process';
const {
  createHmac,
} = await import('node:crypto');

const filename = argv[2];

const hmac = createHmac('sha256', 'a secret');

const input = createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});const {
  createReadStream,
} = require('node:fs');
const {
  createHmac,
} = require('node:crypto');
const { argv } = require('node:process');

const filename = argv[2];

const hmac = createHmac('sha256', 'a secret');

const input = createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});

crypto.createPrivateKey(key)#

History
VersionChanges
v15.12.0

The key can also be a JWK object.

v15.0.0

The key can also be an ArrayBuffer. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.

v11.6.0

Added in: v11.6.0

Creates and returns a new key object containing a private key. If key is a string or Buffer, format is assumed to be 'pem'; otherwise, key must be an object with the properties described above.

If the private key is encrypted, a passphrase must be specified. The length of the passphrase is limited to 1024 bytes.

crypto.createPublicKey(key)#

History
VersionChanges
v15.12.0

The key can also be a JWK object.

v15.0.0

The key can also be an ArrayBuffer. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.

v11.13.0

The key argument can now be a KeyObject with type private.

v11.7.0

The key argument can now be a private key.

v11.6.0

Added in: v11.6.0

Creates and returns a new key object containing a public key. If key is a string or Buffer, format is assumed to be 'pem'; if key is a KeyObject with type 'private', the public key is derived from the given private key; otherwise, key must be an object with the properties described above.

If the format is 'pem', the 'key' may also be an X.509 certificate.

Because public keys can be derived from private keys, a private key may be passed instead of a public key. In that case, this function behaves as if crypto.createPrivateKey() had been called, except that the type of the returned KeyObject will be 'public' and that the private key cannot be extracted from the returned KeyObject. Similarly, if a KeyObject with type 'private' is given, a new KeyObject with type 'public' will be returned and it will be impossible to extract the private key from the returned object.

crypto.createSecretKey(key[, encoding])#

History
VersionChanges
v18.8.0, v16.18.0

The key can now be zero-length.

v15.0.0

The key can also be an ArrayBuffer or string. The encoding argument was added. The key cannot contain more than 2 ** 32 - 1 bytes.

v11.6.0

Added in: v11.6.0

Creates and returns a new key object containing a secret key for symmetric encryption or Hmac.

crypto.createSign(algorithm[, options])#

Added in: v0.1.92

Creates and returns a Sign object that uses the given algorithm. Use crypto.getHashes() to obtain the names of the available digest algorithms. Optional options argument controls the stream.Writable behavior.

In some cases, a Sign instance can be created using the name of a signature algorithm, such as 'RSA-SHA256', instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256', so it is best to always use digest algorithm names.

crypto.createVerify(algorithm[, options])#

Added in: v0.1.92

Creates and returns a Verify object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms. Optional options argument controls the stream.Writable behavior.

In some cases, a Verify instance can be created using the name of a signature algorithm, such as 'RSA-SHA256', instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256', so it is best to always use digest algorithm names.

crypto.diffieHellman(options)#

Added in: v13.9.0, v12.17.0

Computes the Diffie-Hellman secret based on a privateKey and a publicKey. Both keys must have the same asymmetricKeyType, which must be one of 'dh' (for Diffie-Hellman), 'ec' (for ECDH), 'x448', or 'x25519' (for ECDH-ES).

crypto.generateKey(type, options, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.0.0

Added in: v15.0.0

  • type: <string> The intended use of the generated secret key. Currently accepted values are 'hmac' and 'aes'.
  • options: <Object>
    • length: <number> The bit length of the key to generate. This must be a value greater than 0.
      • If type is 'hmac', the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated to Math.floor(length / 8).
      • If type is 'aes', the length must be one of 128, 192, or 256.
  • callback: <Function>

Asynchronously generates a new random secret key of the given length. The type will determine which validations will be performed on the length.

const {
  generateKey,
} = await import('node:crypto');

generateKey('hmac', { length: 64 }, (err, key) => {
  if (err) throw err;
  console.log(key.export().toString('hex'));  // 46e..........620
});const {
  generateKey,
} = require('node:crypto');

generateKey('hmac', { length: 64 }, (err, key) => {
  if (err) throw err;
  console.log(key.export().toString('hex'));  // 46e..........620
});

crypto.generateKeyPair(type, options, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.10.0

Add ability to define RSASSA-PSS-params sequence parameters for RSA-PSS keys pairs.

v13.9.0, v12.17.0

Add support for Diffie-Hellman.

v12.0.0

Add support for RSA-PSS key pairs.

v12.0.0

Add ability to generate X25519 and X448 key pairs.

v12.0.0

Add ability to generate Ed25519 and Ed448 key pairs.

v11.6.0

The generateKeyPair and generateKeyPairSync functions now produce key objects if no encoding was specified.

v10.12.0

Added in: v10.12.0

Generates a new asymmetric key pair of the given type. RSA, RSA-PSS, DSA, EC, Ed25519, Ed448, X25519, X448, and DH are currently supported.

If a publicKeyEncoding or privateKeyEncoding was specified, this function behaves as if keyObject.export() had been called on its result. Otherwise, the respective part of the key is returned as a KeyObject.

It is recommended to encode public keys as 'spki' and private keys as 'pkcs8' with encryption for long-term storage:

const {
  generateKeyPair,
} = await import('node:crypto');

generateKeyPair('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem',
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret',
  },
}, (err, publicKey, privateKey) => {
  // Handle errors and use the generated key pair.
});const {
  generateKeyPair,
} = require('node:crypto');

generateKeyPair('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem',
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret',
  },
}, (err, publicKey, privateKey) => {
  // Handle errors and use the generated key pair.
});

On completion, callback will be called with err set to undefined and publicKey / privateKey representing the generated key pair.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with publicKey and privateKey properties.

crypto.generateKeyPairSync(type, options)#

History
VersionChanges
v16.10.0

Add ability to define RSASSA-PSS-params sequence parameters for RSA-PSS keys pairs.

v13.9.0, v12.17.0

Add support for Diffie-Hellman.

v12.0.0

Add support for RSA-PSS key pairs.

v12.0.0

Add ability to generate X25519 and X448 key pairs.

v12.0.0

Add ability to generate Ed25519 and Ed448 key pairs.

v11.6.0

The generateKeyPair and generateKeyPairSync functions now produce key objects if no encoding was specified.

v10.12.0

Added in: v10.12.0

Generates a new asymmetric key pair of the given type. RSA, RSA-PSS, DSA, EC, Ed25519, Ed448, X25519, X448, and DH are currently supported.

If a publicKeyEncoding or privateKeyEncoding was specified, this function behaves as if keyObject.export() had been called on its result. Otherwise, the respective part of the key is returned as a KeyObject.

When encoding public keys, it is recommended to use 'spki'. When encoding private keys, it is recommended to use 'pkcs8' with a strong passphrase, and to keep the passphrase confidential.

const {
  generateKeyPairSync,
} = await import('node:crypto');

const {
  publicKey,
  privateKey,
} = generateKeyPairSync('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem',
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret',
  },
});const {
  generateKeyPairSync,
} = require('node:crypto');

const {
  publicKey,
  privateKey,
} = generateKeyPairSync('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem',
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret',
  },
});

The return value { publicKey, privateKey } represents the generated key pair. When PEM encoding was selected, the respective key will be a string, otherwise it will be a buffer containing the data encoded as DER.

crypto.generateKeySync(type, options)#

Added in: v15.0.0
  • type: <string> The intended use of the generated secret key. Currently accepted values are 'hmac' and 'aes'.
  • options: <Object>
    • length: <number> The bit length of the key to generate.
      • If type is 'hmac', the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated to Math.floor(length / 8).
      • If type is 'aes', the length must be one of 128, 192, or 256.
  • Returns: <KeyObject>

Synchronously generates a new random secret key of the given length. The type will determine which validations will be performed on the length.

const {
  generateKeySync,
} = await import('node:crypto');

const key = generateKeySync('hmac', { length: 64 });
console.log(key.export().toString('hex'));  // e89..........41econst {
  generateKeySync,
} = require('node:crypto');

const key = generateKeySync('hmac', { length: 64 });
console.log(key.export().toString('hex'));  // e89..........41e

crypto.generatePrime(size[, options[, callback]])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.8.0

Added in: v15.8.0

Generates a pseudorandom prime of size bits.

If options.safe is true, the prime will be a safe prime -- that is, (prime - 1) / 2 will also be a prime.

The options.add and options.rem parameters can be used to enforce additional requirements, e.g., for Diffie-Hellman:

  • If options.add and options.rem are both set, the prime will satisfy the condition that prime % add = rem.
  • If only options.add is set and options.safe is not true, the prime will satisfy the condition that prime % add = 1.
  • If only options.add is set and options.safe is set to true, the prime will instead satisfy the condition that prime % add = 3. This is necessary because prime % add = 1 for options.add > 2 would contradict the condition enforced by options.safe.
  • options.rem is ignored if options.add is not given.

Both options.add and options.rem must be encoded as big-endian sequences if given as an ArrayBuffer, SharedArrayBuffer, TypedArray, Buffer, or DataView.

By default, the prime is encoded as a big-endian sequence of octets in an <ArrayBuffer>. If the bigint option is true, then a <bigint> is provided.

crypto.generatePrimeSync(size[, options])#

Added in: v15.8.0

Generates a pseudorandom prime of size bits.

If options.safe is true, the prime will be a safe prime -- that is, (prime - 1) / 2 will also be a prime.

The options.add and options.rem parameters can be used to enforce additional requirements, e.g., for Diffie-Hellman:

  • If options.add and options.rem are both set, the prime will satisfy the condition that prime % add = rem.
  • If only options.add is set and options.safe is not true, the prime will satisfy the condition that prime % add = 1.
  • If only options.add is set and options.safe is set to true, the prime will instead satisfy the condition that prime % add = 3. This is necessary because prime % add = 1 for options.add > 2 would contradict the condition enforced by options.safe.
  • options.rem is ignored if options.add is not given.

Both options.add and options.rem must be encoded as big-endian sequences if given as an ArrayBuffer, SharedArrayBuffer, TypedArray, Buffer, or DataView.

By default, the prime is encoded as a big-endian sequence of octets in an <ArrayBuffer>. If the bigint option is true, then a <bigint> is provided.

crypto.getCipherInfo(nameOrNid[, options])#

Added in: v15.0.0
  • nameOrNid: <string> | <number> The name or nid of the cipher to query.
  • options: <Object>
  • Returns: <Object>
    • name <string> The name of the cipher
    • nid <number> The nid of the cipher
    • blockSize <number> The block size of the cipher in bytes. This property is omitted when mode is 'stream'.
    • ivLength <number> The expected or default initialization vector length in bytes. This property is omitted if the cipher does not use an initialization vector.
    • keyLength <number> The expected or default key length in bytes.
    • mode <string> The cipher mode. One of 'cbc', 'ccm', 'cfb', 'ctr', 'ecb', 'gcm', 'ocb', 'ofb', 'stream', 'wrap', 'xts'.

Returns information about a given cipher.

Some ciphers accept variable length keys and initialization vectors. By default, the crypto.getCipherInfo() method will return the default values for these ciphers. To test if a given key length or iv length is acceptable for given cipher, use the keyLength and ivLength options. If the given values are unacceptable, undefined will be returned.

crypto.getCiphers()#

Added in: v0.9.3
  • Returns: <string[]> An array with the names of the supported cipher algorithms.
const {
  getCiphers,
} = await import('node:crypto');

console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]const {
  getCiphers,
} = require('node:crypto');

console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]

crypto.getCurves()#

Added in: v2.3.0
  • Returns: <string[]> An array with the names of the supported elliptic curves.
const {
  getCurves,
} = await import('node:crypto');

console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]const {
  getCurves,
} = require('node:crypto');

console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]

crypto.getDiffieHellman(groupName)#

Added in: v0.7.5

Creates a predefined DiffieHellmanGroup key exchange object. The supported groups are listed in the documentation for DiffieHellmanGroup.

The returned object mimics the interface of objects created by crypto.createDiffieHellman(), but will not allow changing the keys (with diffieHellman.setPublicKey(), for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time.

Example (obtaining a shared secret):

const {
  getDiffieHellman,
} = await import('node:crypto');
const alice = getDiffieHellman('modp14');
const bob = getDiffieHellman('modp14');

alice.generateKeys();
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);const {
  getDiffieHellman,
} = require('node:crypto');

const alice = getDiffieHellman('modp14');
const bob = getDiffieHellman('modp14');

alice.generateKeys();
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);

crypto.getFips()#

Added in: v10.0.0
  • Returns: <number> 1 if and only if a FIPS compliant crypto provider is currently in use, 0 otherwise. A future semver-major release may change the return type of this API to a <boolean>.

crypto.getHashes()#

Added in: v0.9.3
  • Returns: <string[]> An array of the names of the supported hash algorithms, such as 'RSA-SHA256'. Hash algorithms are also called "digest" algorithms.
const {
  getHashes,
} = await import('node:crypto');

console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]const {
  getHashes,
} = require('node:crypto');

console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]

crypto.getRandomValues(typedArray)#

Added in: v17.4.0

A convenient alias for crypto.webcrypto.getRandomValues(). This implementation is not compliant with the Web Crypto spec, to write web-compatible code use crypto.webcrypto.getRandomValues() instead.

crypto.hkdf(digest, ikm, salt, info, keylen, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v18.8.0, v16.18.0

The input keying material can now be zero-length.

v15.0.0

Added in: v15.0.0

HKDF is a simple key derivation function defined in RFC 5869. The given ikm, salt and info are used with the digest to derive a key of keylen bytes.

The supplied callback function is called with two arguments: err and derivedKey. If an errors occurs while deriving the key, err will be set; otherwise err will be null. The successfully generated derivedKey will be passed to the callback as an <ArrayBuffer>. An error will be thrown if any of the input arguments specify invalid values or types.

import { Buffer } from 'node:buffer';
const {
  hkdf,
} = await import('node:crypto');

hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
  if (err) throw err;
  console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
});const {
  hkdf,
} = require('node:crypto');
const { Buffer } = require('node:buffer');

hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
  if (err) throw err;
  console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
});

crypto.hkdfSync(digest, ikm, salt, info, keylen)#

History
VersionChanges
v18.8.0, v16.18.0

The input keying material can now be zero-length.

v15.0.0

Added in: v15.0.0

Provides a synchronous HKDF key derivation function as defined in RFC 5869. The given ikm, salt and info are used with the digest to derive a key of keylen bytes.

The successfully generated derivedKey will be returned as an <ArrayBuffer>.

An error will be thrown if any of the input arguments specify invalid values or types, or if the derived key cannot be generated.

import { Buffer } from 'node:buffer';
const {
  hkdfSync,
} = await import('node:crypto');

const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'const {
  hkdfSync,
} = require('node:crypto');
const { Buffer } = require('node:buffer');

const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'

crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.0.0

The password and salt arguments can also be ArrayBuffer instances.

v14.0.0

The iterations parameter is now restricted to positive values. Earlier releases treated other values as one.

v8.0.0

The digest parameter is always required now.

v6.0.0

Calling this function without passing the digest parameter is deprecated now and will emit a warning.

v6.0.0

The default encoding for password if it is a string changed from binary to utf8.

v0.5.5

Added in: v0.5.5

Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

The supplied callback function is called with two arguments: err and derivedKey. If an error occurs while deriving the key, err will be set; otherwise err will be null. By default, the successfully generated derivedKey will be passed to the callback as a Buffer. An error will be thrown if any of the input arguments specify invalid values or types.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for password or salt, please consider caveats when using strings as inputs to cryptographic APIs.

const {
  pbkdf2,
} = await import('node:crypto');

pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});const {
  pbkdf2,
} = require('node:crypto');

pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});

The crypto.DEFAULT_ENCODING property can be used to change the way the derivedKey is passed to the callback. This property, however, has been deprecated and use should be avoided.

import crypto from 'node:crypto';
crypto.DEFAULT_ENCODING = 'hex';
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey);  // '3745e48...aa39b34'
});const crypto = require('node:crypto');
crypto.DEFAULT_ENCODING = 'hex';
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey);  // '3745e48...aa39b34'
});

An array of supported digest functions can be retrieved using crypto.getHashes().

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)#

History
VersionChanges
v14.0.0

The iterations parameter is now restricted to positive values. Earlier releases treated other values as one.

v6.0.0

Calling this function without passing the digest parameter is deprecated now and will emit a warning.

v6.0.0

The default encoding for password if it is a string changed from binary to utf8.

v0.9.3

Added in: v0.9.3

Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

If an error occurs an Error will be thrown, otherwise the derived key will be returned as a Buffer.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for password or salt, please consider caveats when using strings as inputs to cryptographic APIs.

const {
  pbkdf2Sync,
} = await import('node:crypto');

const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
console.log(key.toString('hex'));  // '3745e48...08d59ae'const {
  pbkdf2Sync,
} = require('node:crypto');

const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
console.log(key.toString('hex'));  // '3745e48...08d59ae'

The crypto.DEFAULT_ENCODING property may be used to change the way the derivedKey is returned. This property, however, is deprecated and use should be avoided.

import crypto from 'node:crypto';
crypto.DEFAULT_ENCODING = 'hex';
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key);  // '3745e48...aa39b34'const crypto = require('node:crypto');
crypto.DEFAULT_ENCODING = 'hex';
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key);  // '3745e48...aa39b34'

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.privateDecrypt(privateKey, buffer)#

History
VersionChanges
v15.0.0

Added string, ArrayBuffer, and CryptoKey as allowable key types. The oaepLabel can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.

v12.11.0

The oaepLabel option was added.

v12.9.0

The oaepHash option was added.

v11.6.0

This function now supports key objects.

v0.11.14

Added in: v0.11.14

Decrypts buffer with privateKey. buffer was previously encrypted using the corresponding public key, for example using crypto.publicEncrypt().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_OAEP_PADDING.

crypto.privateEncrypt(privateKey, buffer)#

History
VersionChanges
v15.0.0

Added string, ArrayBuffer, and CryptoKey as allowable key types. The passphrase can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.

v11.6.0

This function now supports key objects.

v1.1.0

Added in: v1.1.0

Encrypts buffer with privateKey. The returned data can be decrypted using the corresponding public key, for example using crypto.publicDecrypt().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_PADDING.

crypto.publicDecrypt(key, buffer)#

History
VersionChanges
v15.0.0

Added string, ArrayBuffer, and CryptoKey as allowable key types. The passphrase can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.

v11.6.0

This function now supports key objects.

v1.1.0

Added in: v1.1.0

Decrypts buffer with key.buffer was previously encrypted using the corresponding private key, for example using crypto.privateEncrypt().

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_PADDING.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

crypto.publicEncrypt(key, buffer)#

History
VersionChanges
v15.0.0

Added string, ArrayBuffer, and CryptoKey as allowable key types. The oaepLabel and passphrase can be ArrayBuffers. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.

v12.11.0

The oaepLabel option was added.

v12.9.0

The oaepHash option was added.

v11.6.0

This function now supports key objects.

v0.11.14

Added in: v0.11.14

Encrypts the content of buffer with key and returns a new Buffer with encrypted content. The returned data can be decrypted using the corresponding private key, for example using crypto.privateDecrypt().

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_OAEP_PADDING.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

crypto.randomBytes(size[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v9.0.0

Passing null as the callback argument now throws ERR_INVALID_CALLBACK.

v0.5.8

Added in: v0.5.8

Generates cryptographically strong pseudorandom data. The size argument is a number indicating the number of bytes to generate.

If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two arguments: err and buf. If an error occurs, err will be an Error object; otherwise it is null. The buf argument is a Buffer containing the generated bytes.

// Asynchronous
const {
  randomBytes,
} = await import('node:crypto');

randomBytes(256, (err, buf) => {
  if (err) throw err;
  console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});// Asynchronous
const {
  randomBytes,
} = require('node:crypto');

randomBytes(256, (err, buf) => {
  if (err) throw err;
  console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});

If the callback function is not provided, the random bytes are generated synchronously and returned as a Buffer. An error will be thrown if there is a problem generating the bytes.

// Synchronous
const {
  randomBytes,
} = await import('node:crypto');

const buf = randomBytes(256);
console.log(
  `${buf.length} bytes of random data: ${buf.toString('hex')}`);// Synchronous
const {
  randomBytes,
} = require('node:crypto');

const buf = randomBytes(256);
console.log(
  `${buf.length} bytes of random data: ${buf.toString('hex')}`);

The crypto.randomBytes() method will not complete until there is sufficient entropy available. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy.

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

The asynchronous version of crypto.randomBytes() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomBytes requests when doing so as part of fulfilling a client request.

crypto.randomFillSync(buffer[, offset][, size])#

History
VersionChanges
v9.0.0

The buffer argument may be any TypedArray or DataView.

v7.10.0, v6.13.0

Added in: v7.10.0, v6.13.0

Synchronous version of crypto.randomFill().

import { Buffer } from 'node:buffer';
const { randomFillSync } = await import('node:crypto');

const buf = Buffer.alloc(10);
console.log(randomFillSync(buf).toString('hex'));

randomFillSync(buf, 5);
console.log(buf.toString('hex'));

// The above is equivalent to the following:
randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));const { randomFillSync } = require('node:crypto');
const { Buffer } = require('node:buffer');

const buf = Buffer.alloc(10);
console.log(randomFillSync(buf).toString('hex'));

randomFillSync(buf, 5);
console.log(buf.toString('hex'));

// The above is equivalent to the following:
randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));

Any ArrayBuffer, TypedArray or DataView instance may be passed as buffer.

import { Buffer } from 'node:buffer';
const { randomFillSync } = await import('node:crypto');

const a = new Uint32Array(10);
console.log(Buffer.from(randomFillSync(a).buffer,
                        a.byteOffset, a.byteLength).toString('hex'));

const b = new DataView(new ArrayBuffer(10));
console.log(Buffer.from(randomFillSync(b).buffer,
                        b.byteOffset, b.byteLength).toString('hex'));

const c = new ArrayBuffer(10);
console.log(Buffer.from(randomFillSync(c)).toString('hex'));const { randomFillSync } = require('node:crypto');
const { Buffer } = require('node:buffer');

const a = new Uint32Array(10);
console.log(Buffer.from(randomFillSync(a).buffer,
                        a.byteOffset, a.byteLength).toString('hex'));

const b = new DataView(new ArrayBuffer(10));
console.log(Buffer.from(randomFillSync(b).buffer,
                        b.byteOffset, b.byteLength).toString('hex'));

const c = new ArrayBuffer(10);
console.log(Buffer.from(randomFillSync(c)).toString('hex'));

crypto.randomFill(buffer[, offset][, size], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v9.0.0

The buffer argument may be any TypedArray or DataView.

v7.10.0, v6.13.0

Added in: v7.10.0, v6.13.0

This function is similar to crypto.randomBytes() but requires the first argument to be a Buffer that will be filled. It also requires that a callback is passed in.

If the callback function is not provided, an error will be thrown.

import { Buffer } from 'node:buffer';
const { randomFill } = await import('node:crypto');

const buf = Buffer.alloc(10);
randomFill(buf, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

randomFill(buf, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

// The above is equivalent to the following:
randomFill(buf, 5, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});const { randomFill } = require('node:crypto');
const { Buffer } = require('node:buffer');

const buf = Buffer.alloc(10);
randomFill(buf, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

randomFill(buf, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

// The above is equivalent to the following:
randomFill(buf, 5, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

Any ArrayBuffer, TypedArray, or DataView instance may be passed as buffer.

While this includes instances of Float32Array and Float64Array, this function should not be used to generate random floating-point numbers. The result may contain +Infinity, -Infinity, and NaN, and even if the array contains finite numbers only, they are not drawn from a uniform random distribution and have no meaningful lower or upper bounds.

import { Buffer } from 'node:buffer';
const { randomFill } = await import('node:crypto');

const a = new Uint32Array(10);
randomFill(a, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const b = new DataView(new ArrayBuffer(10));
randomFill(b, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const c = new ArrayBuffer(10);
randomFill(c, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf).toString('hex'));
});const { randomFill } = require('node:crypto');
const { Buffer } = require('node:buffer');

const a = new Uint32Array(10);
randomFill(a, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const b = new DataView(new ArrayBuffer(10));
randomFill(b, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const c = new ArrayBuffer(10);
randomFill(c, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf).toString('hex'));
});

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

The asynchronous version of crypto.randomFill() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomFill requests when doing so as part of fulfilling a client request.

crypto.randomInt([min, ]max[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v14.10.0, v12.19.0

Added in: v14.10.0, v12.19.0

  • min <integer> Start of random range (inclusive). Default: 0.
  • max <integer> End of random range (exclusive).
  • callback <Function> function(err, n) {}.

Return a random integer n such that min <= n < max. This implementation avoids modulo bias.

The range (max - min) must be less than 248. min and max must be safe integers.

If the callback function is not provided, the random integer is generated synchronously.

// Asynchronous
const {
  randomInt,
} = await import('node:crypto');

randomInt(3, (err, n) => {
  if (err) throw err;
  console.log(`Random number chosen from (0, 1, 2): ${n}`);
});// Asynchronous
const {
  randomInt,
} = require('node:crypto');

randomInt(3, (err, n) => {
  if (err) throw err;
  console.log(`Random number chosen from (0, 1, 2): ${n}`);
});
// Synchronous
const {
  randomInt,
} = await import('node:crypto');

const n = randomInt(3);
console.log(`Random number chosen from (0, 1, 2): ${n}`);// Synchronous
const {
  randomInt,
} = require('node:crypto');

const n = randomInt(3);
console.log(`Random number chosen from (0, 1, 2): ${n}`);
// With `min` argument
const {
  randomInt,
} = await import('node:crypto');

const n = randomInt(1, 7);
console.log(`The dice rolled: ${n}`);// With `min` argument
const {
  randomInt,
} = require('node:crypto');

const n = randomInt(1, 7);
console.log(`The dice rolled: ${n}`);

crypto.randomUUID([options])#

Added in: v15.6.0, v14.17.0
  • options <Object>
    • disableEntropyCache <boolean> By default, to improve performance, Node.js generates and caches enough random data to generate up to 128 random UUIDs. To generate a UUID without using the cache, set disableEntropyCache to true. Default: false.
  • Returns: <string>

Generates a random RFC 4122 version 4 UUID. The UUID is generated using a cryptographic pseudorandom number generator.

crypto.scrypt(password, salt, keylen[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.0.0

The password and salt arguments can also be ArrayBuffer instances.

v12.8.0, v10.17.0

The maxmem value can now be any safe integer.

v10.9.0

The cost, blockSize and parallelization option names have been added.

v10.5.0

Added in: v10.5.0

Provides an asynchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for password or salt, please consider caveats when using strings as inputs to cryptographic APIs.

The callback function is called with two arguments: err and derivedKey. err is an exception object when key derivation fails, otherwise err is null. derivedKey is passed to the callback as a Buffer.

An exception is thrown when any of the input arguments specify invalid values or types.

const {
  scrypt,
} = await import('node:crypto');

// Using the factory defaults.
scrypt('password', 'salt', 64, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});
// Using a custom N parameter. Must be a power of two.
scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...aa39b34'
});const {
  scrypt,
} = require('node:crypto');

// Using the factory defaults.
scrypt('password', 'salt', 64, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});
// Using a custom N parameter. Must be a power of two.
scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...aa39b34'
});

crypto.scryptSync(password, salt, keylen[, options])#

History
VersionChanges
v12.8.0, v10.17.0

The maxmem value can now be any safe integer.

v10.9.0

The cost, blockSize and parallelization option names have been added.

v10.5.0

Added in: v10.5.0

Provides a synchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for password or salt, please consider caveats when using strings as inputs to cryptographic APIs.

An exception is thrown when key derivation fails, otherwise the derived key is returned as a Buffer.

An exception is thrown when any of the input arguments specify invalid values or types.

const {
  scryptSync,
} = await import('node:crypto');
// Using the factory defaults.

const key1 = scryptSync('password', 'salt', 64);
console.log(key1.toString('hex'));  // '3745e48...08d59ae'
// Using a custom N parameter. Must be a power of two.
const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
console.log(key2.toString('hex'));  // '3745e48...aa39b34'const {
  scryptSync,
} = require('node:crypto');
// Using the factory defaults.

const key1 = scryptSync('password', 'salt', 64);
console.log(key1.toString('hex'));  // '3745e48...08d59ae'
// Using a custom N parameter. Must be a power of two.
const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
console.log(key2.toString('hex'));  // '3745e48...aa39b34'

crypto.secureHeapUsed()#

Added in: v15.6.0
  • Returns: <Object>
    • total <number> The total allocated secure heap size as specified using the --secure-heap=n command-line flag.
    • min <number> The minimum allocation from the secure heap as specified using the --secure-heap-min command-line flag.
    • used <number> The total number of bytes currently allocated from the secure heap.
    • utilization <number> The calculated ratio of used to total allocated bytes.

crypto.setEngine(engine[, flags])#

Added in: v0.11.11

Load and set the engine for some or all OpenSSL functions (selected by flags).

engine could be either an id or a path to the engine's shared library.

The optional flags argument uses ENGINE_METHOD_ALL by default. The flags is a bit field taking one of or a mix of the following flags (defined in crypto.constants):

  • crypto.constants.ENGINE_METHOD_RSA
  • crypto.constants.ENGINE_METHOD_DSA
  • crypto.constants.ENGINE_METHOD_DH
  • crypto.constants.ENGINE_METHOD_RAND
  • crypto.constants.ENGINE_METHOD_EC
  • crypto.constants.ENGINE_METHOD_CIPHERS
  • crypto.constants.ENGINE_METHOD_DIGESTS
  • crypto.constants.ENGINE_METHOD_PKEY_METHS
  • crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
  • crypto.constants.ENGINE_METHOD_ALL
  • crypto.constants.ENGINE_METHOD_NONE

crypto.setFips(bool)#

Added in: v10.0.0

Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available.

crypto.sign(algorithm, data, key[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.12.0

Optional callback argument added.

v13.2.0, v12.16.0

This function now supports IEEE-P1363 DSA and ECDSA signatures.

v12.0.0

Added in: v12.0.0

Calculates and returns the signature for data using the given private key and algorithm. If algorithm is null or undefined, then the algorithm is dependent upon the key type (especially Ed25519 and Ed448).

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPrivateKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

If the callback function is provided this function uses libuv's threadpool.

crypto.subtle#

Added in: v17.4.0

A convenient alias for crypto.webcrypto.subtle.

crypto.timingSafeEqual(a, b)#

History
VersionChanges
v15.0.0

The a and b arguments can also be ArrayBuffer.

v6.6.0

Added in: v6.6.0

This function compares the underlying bytes that represent the given ArrayBuffer, TypedArray, or DataView instances using a constant-time algorithm.

This function does not leak timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.

a and b must both be Buffers, TypedArrays, or DataViews, and they must have the same byte length. An error is thrown if a and b have different byte lengths.

If at least one of a and b is a TypedArray with more than one byte per entry, such as Uint16Array, the result will be computed using the platform byte order.

When both of the inputs are Float32Arrays or Float64Arrays, this function might return unexpected results due to IEEE 754 encoding of floating-point numbers. In particular, neither x === y nor Object.is(x, y) implies that the byte representations of two floating-point numbers x and y are equal.

Use of crypto.timingSafeEqual does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities.

crypto.verify(algorithm, data, key, signature[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.12.0

Optional callback argument added.

v15.0.0

The data, key, and signature arguments can also be ArrayBuffer.

v13.2.0, v12.16.0

This function now supports IEEE-P1363 DSA and ECDSA signatures.

v12.0.0

Added in: v12.0.0

Verifies the given signature for data using the given key and algorithm. If algorithm is null or undefined, then the algorithm is dependent upon the key type (especially Ed25519 and Ed448).

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

The signature argument is the previously calculated signature for the data.

Because public keys can be derived from private keys, a private key or a public key may be passed for key.

If the callback function is provided this function uses libuv's threadpool.

crypto.webcrypto#

Added in: v15.0.0

Type: <Crypto> An implementation of the Web Crypto API standard.

See the Web Crypto API documentation for details.

Notes#

Using strings as inputs to cryptographic APIs#

For historical reasons, many cryptographic APIs provided by Node.js accept strings as inputs where the underlying cryptographic algorithm works on byte sequences. These instances include plaintexts, ciphertexts, symmetric keys, initialization vectors, passphrases, salts, authentication tags, and additional authenticated data.

When passing strings to cryptographic APIs, consider the following factors.

  • Not all byte sequences are valid UTF-8 strings. Therefore, when a byte sequence of length n is derived from a string, its entropy is generally lower than the entropy of a random or pseudorandom n byte sequence. For example, no UTF-8 string will result in the byte sequence c0 af. Secret keys should almost exclusively be random or pseudorandom byte sequences.

  • Similarly, when converting random or pseudorandom byte sequences to UTF-8 strings, subsequences that do not represent valid code points may be replaced by the Unicode replacement character (U+FFFD). The byte representation of the resulting Unicode string may, therefore, not be equal to the byte sequence that the string was created from.

    const original = [0xc0, 0xaf];
const bytesAsString = Buffer.from(original).toString('utf8');
const stringAsBytes = Buffer.from(bytesAsString, 'utf8');
console.log(stringAsBytes);
// Prints '<Buffer ef bf bd ef bf bd>'.

    The outputs of ciphers, hash functions, signature algorithms, and key derivation functions are pseudorandom byte sequences and should not be used as Unicode strings.

  • When strings are obtained from user input, some Unicode characters can be represented in multiple equivalent ways that result in different byte sequences. For example, when passing a user passphrase to a key derivation function, such as PBKDF2 or scrypt, the result of the key derivation function depends on whether the string uses composed or decomposed characters. Node.js does not normalize character representations. Developers should consider using String.prototype.normalize() on user inputs before passing them to cryptographic APIs.

Legacy streams API (prior to Node.js 0.10)#

The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were Buffer objects for handling binary data. As such, the many of the crypto defined classes have methods not typically found on other Node.js classes that implement the streams API (e.g. update(), final(), or digest()). Also, many methods accepted and returned 'latin1' encoded strings by default rather than Buffers. This default was changed after Node.js v0.8 to use Buffer objects by default instead.

Support for weak or compromised algorithms#

The node:crypto module still supports some algorithms which are already compromised and are not currently recommended for use. The API also allows the use of ciphers and hashes with a small key size that are too weak for safe use.

Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.

Based on the recommendations of NIST SP 800-131A:

  • MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
  • The key used with RSA, DSA, and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
  • The DH groups of modp1, modp2 and modp5 have a key size smaller than 2048 bits and are not recommended.

See the reference for other recommendations and details.

Some algorithms that have known weaknesses and are of little relevance in practice are only available through the legacy provider, which is not enabled by default.

CCM mode#

CCM is one of the supported AEAD algorithms. Applications which use this mode must adhere to certain restrictions when using the cipher API:

  • The authentication tag length must be specified during cipher creation by setting the authTagLength option and must be one of 4, 6, 8, 10, 12, 14 or 16 bytes.
  • The length of the initialization vector (nonce) N must be between 7 and 13 bytes (7 ≤ N ≤ 13).
  • The length of the plaintext is limited to 2 ** (8 * (15 - N)) bytes.
  • When decrypting, the authentication tag must be set via setAuthTag() before calling update(). Otherwise, decryption will fail and final() will throw an error in compliance with section 2.6 of RFC 3610.
  • Using stream methods such as write(data), end(data) or pipe() in CCM mode might fail as CCM cannot handle more than one chunk of data per instance.
  • When passing additional authenticated data (AAD), the length of the actual message in bytes must be passed to setAAD() via the plaintextLength option. Many crypto libraries include the authentication tag in the ciphertext, which means that they produce ciphertexts of the length plaintextLength + authTagLength. Node.js does not include the authentication tag, so the ciphertext length is always plaintextLength. This is not necessary if no AAD is used.
  • As CCM processes the whole message at once, update() must be called exactly once.
  • Even though calling update() is sufficient to encrypt/decrypt the message, applications must call final() to compute or verify the authentication tag.
import { Buffer } from 'node:buffer';
const {
  createCipheriv,
  createDecipheriv,
  randomBytes,
} = await import('node:crypto');

const key = 'keykeykeykeykeykeykeykey';
const nonce = randomBytes(12);

const aad = Buffer.from('0123456789', 'hex');

const cipher = createCipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16,
});
const plaintext = 'Hello world';
cipher.setAAD(aad, {
  plaintextLength: Buffer.byteLength(plaintext),
});
const ciphertext = cipher.update(plaintext, 'utf8');
cipher.final();
const tag = cipher.getAuthTag();

// Now transmit { ciphertext, nonce, tag }.

const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16,
});
decipher.setAuthTag(tag);
decipher.setAAD(aad, {
  plaintextLength: ciphertext.length,
});
const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');

try {
  decipher.final();
} catch (err) {
  throw new Error('Authentication failed!', { cause: err });
}

console.log(receivedPlaintext);const { Buffer } = require('node:buffer');
const {
  createCipheriv,
  createDecipheriv,
  randomBytes,
} = require('node:crypto');

const key = 'keykeykeykeykeykeykeykey';
const nonce = randomBytes(12);

const aad = Buffer.from('0123456789', 'hex');

const cipher = createCipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16,
});
const plaintext = 'Hello world';
cipher.setAAD(aad, {
  plaintextLength: Buffer.byteLength(plaintext),
});
const ciphertext = cipher.update(plaintext, 'utf8');
cipher.final();
const tag = cipher.getAuthTag();

// Now transmit { ciphertext, nonce, tag }.

const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16,
});
decipher.setAuthTag(tag);
decipher.setAAD(aad, {
  plaintextLength: ciphertext.length,
});
const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');

try {
  decipher.final();
} catch (err) {
  throw new Error('Authentication failed!', { cause: err });
}

console.log(receivedPlaintext);

Crypto constants#

The following constants exported by crypto.constants apply to various uses of the node:crypto, node:tls, and node:https modules and are generally specific to OpenSSL.

OpenSSL options#

See the list of SSL OP Flags for details.

Constant Description
SSL_OP_ALL Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail.
SSL_OP_ALLOW_NO_DHE_KEX Instructs OpenSSL to allow a non-[EC]DHE-based key exchange mode for TLS v1.3
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CIPHER_SERVER_PREFERENCE Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CISCO_ANYCONNECT Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER.
SSL_OP_COOKIE_EXCHANGE Instructs OpenSSL to turn on cookie exchange.
SSL_OP_CRYPTOPRO_TLSEXT_BUG Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft.
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d.
SSL_OP_EPHEMERAL_RSA Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations.
SSL_OP_LEGACY_SERVER_CONNECT Allows initial connection to servers that do not support RI.
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol-version vulnerability in the SSL 2.0 server implementation.
SSL_OP_NETSCAPE_CA_DN_BUG
SSL_OP_NETSCAPE_CHALLENGE_BUG
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
SSL_OP_NO_COMPRESSION Instructs OpenSSL to disable support for SSL/TLS compression.
SSL_OP_NO_ENCRYPT_THEN_MAC Instructs OpenSSL to disable encrypt-then-MAC.
SSL_OP_NO_QUERY_MTU
SSL_OP_NO_RENEGOTIATION Instructs OpenSSL to disable renegotiation.
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION Instructs OpenSSL to always start a new session when performing renegotiation.
SSL_OP_NO_SSLv2 Instructs OpenSSL to turn off SSL v2
SSL_OP_NO_SSLv3 Instructs OpenSSL to turn off SSL v3
SSL_OP_NO_TICKET Instructs OpenSSL to disable use of RFC4507bis tickets.
SSL_OP_NO_TLSv1 Instructs OpenSSL to turn off TLS v1
SSL_OP_NO_TLSv1_1 Instructs OpenSSL to turn off TLS v1.1
SSL_OP_NO_TLSv1_2 Instructs OpenSSL to turn off TLS v1.2
SSL_OP_NO_TLSv1_3 Instructs OpenSSL to turn off TLS v1.3
SSL_OP_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
SSL_OP_PRIORITIZE_CHACHA Instructs OpenSSL server to prioritize ChaCha20-Poly1305 when the client does. This option has no effect if SSL_OP_CIPHER_SERVER_PREFERENCE is not enabled.
SSL_OP_SINGLE_DH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters.
SSL_OP_SINGLE_ECDH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters.
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
SSL_OP_TLS_BLOCK_PADDING_BUG
SSL_OP_TLS_D5_BUG
SSL_OP_TLS_ROLLBACK_BUG Instructs OpenSSL to disable version rollback attack detection.

OpenSSL engine constants#

Constant Description
ENGINE_METHOD_RSA Limit engine usage to RSA
ENGINE_METHOD_DSA Limit engine usage to DSA
ENGINE_METHOD_DH Limit engine usage to DH
ENGINE_METHOD_RAND Limit engine usage to RAND
ENGINE_METHOD_EC Limit engine usage to EC
ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS
ENGINE_METHOD_PKEY_METHS Limit engine usage to PKEY_METHDS
ENGINE_METHOD_PKEY_ASN1_METHS Limit engine usage to PKEY_ASN1_METHS
ENGINE_METHOD_ALL
ENGINE_METHOD_NONE

Other OpenSSL constants#

Constant Description
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
ALPN_ENABLED
RSA_PKCS1_PADDING
RSA_SSLV23_PADDING
RSA_NO_PADDING
RSA_PKCS1_OAEP_PADDING
RSA_X931_PADDING
RSA_PKCS1_PSS_PADDING
RSA_PSS_SALTLEN_DIGEST Sets the salt length for RSA_PKCS1_PSS_PADDING to the digest size when signing or verifying.
RSA_PSS_SALTLEN_MAX_SIGN Sets the salt length for RSA_PKCS1_PSS_PADDING to the maximum permissible value when signing data.
RSA_PSS_SALTLEN_AUTO Causes the salt length for RSA_PKCS1_PSS_PADDING to be determined automatically when verifying a signature.
POINT_CONVERSION_COMPRESSED
POINT_CONVERSION_UNCOMPRESSED
POINT_CONVERSION_HYBRID

Node.js crypto constants#

Constant Description
defaultCoreCipherList Specifies the built-in default cipher list used by Node.js.
defaultCipherList Specifies the active default cipher list used by the current Node.js process.

Debugger#

Stability: 2 - Stable

Node.js includes a command-line debugging utility. The Node.js debugger client is not a full-featured debugger, but simple stepping and inspection are possible.

To use it, start Node.js with the inspect argument followed by the path to the script to debug.

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
 ok
Break on start in myscript.js:2
  1 // myscript.js
> 2 global.x = 5;
  3 setTimeout(() => {
  4   debugger;
debug>

The debugger automatically breaks on the first executable line. To instead run until the first breakpoint (specified by a debugger statement), set the NODE_INSPECT_RESUME_ON_START environment variable to 1.

$ cat myscript.js
// myscript.js
global.x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');
$ NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
< hello
<
break in myscript.js:4
  2 global.x = 5;
  3 setTimeout(() => {
> 4   debugger;
  5   console.log('world');
  6 }, 1000);
debug> next
break in myscript.js:5
  3 setTimeout(() => {
  4   debugger;
> 5   console.log('world');
  6 }, 1000);
  7 console.log('hello');
debug> repl
Press Ctrl+C to leave debug repl
> x
5
> 2 + 2
4
debug> next
< world
<
break in myscript.js:6
  4   debugger;
  5   console.log('world');
> 6 }, 1000);
  7 console.log('hello');
  8
debug> .exit
$

The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available.

Pressing enter without typing a command will repeat the previous debugger command.

Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type watch('my_expression'). The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression').

Command reference#

Stepping#

  • cont, c: Continue execution
  • next, n: Step next
  • step, s: Step in
  • out, o: Step out
  • pause: Pause running code (like pause button in Developer Tools)

Breakpoints#

  • setBreakpoint(), sb(): Set breakpoint on current line
  • setBreakpoint(line), sb(line): Set breakpoint on specific line
  • setBreakpoint('fn()'), sb(...): Set breakpoint on a first statement in function's body
  • setBreakpoint('script.js', 1), sb(...): Set breakpoint on first line of script.js
  • setBreakpoint('script.js', 1, 'num < 4'), sb(...): Set conditional breakpoint on first line of script.js that only breaks when num < 4 evaluates to true
  • clearBreakpoint('script.js', 1), cb(...): Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
<
Break on start in main.js:1
> 1 const mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 22)
Warning: script 'mod.js' was not loaded yet.
debug> c
break in mod.js:22
 20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 21
>22 exports.hello = function() {
 23   return 'hello from module';
 24 };
debug>

It is also possible to set a conditional breakpoint that only breaks when a given expression evaluates to true:

$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35
< For help, see: https://nodejs.org/en/docs/inspector
<
connecting to 127.0.0.1:9229 ... ok
< Debugger attached.
Break on start in main.js:7
  5 }
  6
> 7 addOne(10);
  8 addOne(-1);
  9
debug> setBreakpoint('main.js', 4, 'num < 0')
  1 'use strict';
  2
  3 function addOne(num) {
> 4   return num + 1;
  5 }
  6
  7 addOne(10);
  8 addOne(-1);
  9
debug> cont
break in main.js:4
  2
  3 function addOne(num) {
> 4   return num + 1;
  5 }
  6
debug> exec('num')
-1
debug>

Information#

  • backtrace, bt: Print backtrace of current execution frame
  • list(5): List scripts source code with 5 line context (5 lines before and after)
  • watch(expr): Add expression to watch list
  • unwatch(expr): Remove expression from watch list
  • watchers: List all watchers and their values (automatically listed on each breakpoint)
  • repl: Open debugger's repl for evaluation in debugging script's context
  • exec expr, p expr: Execute an expression in debugging script's context and print its value

Execution control#

  • run: Run script (automatically runs on debugger's start)
  • restart: Restart script
  • kill: Kill script

Various#

  • scripts: List all loaded scripts
  • version: Display V8's version

Advanced usage#

V8 inspector integration for Node.js#

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol.

V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

To break on the first line of the application code, pass the --inspect-brk flag instead of --inspect.

$ node --inspect index.js
Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
For help, see: https://nodejs.org/en/docs/inspector

(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)

If the Chrome browser is older than 66.0.3345.0, use inspector.html instead of js_app.html in the above URL.

Chrome DevTools doesn't support debugging worker threads yet. ndb can be used to debug them.

Deprecated APIs#

Node.js APIs might be deprecated for any of the following reasons:

  • Use of the API is unsafe.
  • An improved alternative API is available.
  • Breaking changes to the API are expected in a future major release.

Node.js uses three kinds of Deprecations:

  • Documentation-only
  • Runtime
  • End-of-Life

A Documentation-only deprecation is one that is expressed only within the Node.js API docs. These generate no side-effects while running Node.js. Some Documentation-only deprecations trigger a runtime warning when launched with --pending-deprecation flag (or its alternative, NODE_PENDING_DEPRECATION=1 environment variable), similarly to Runtime deprecations below. Documentation-only deprecations that support that flag are explicitly labeled as such in the list of Deprecated APIs.

A Runtime deprecation will, by default, generate a process warning that will be printed to stderr the first time the deprecated API is used. When the --throw-deprecation command-line flag is used, a Runtime deprecation will cause an error to be thrown.

An End-of-Life deprecation is used when functionality is or will soon be removed from Node.js.

Revoking deprecations#

Occasionally, the deprecation of an API might be reversed. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified.

List of deprecated APIs#

DEP0001: http.OutgoingMessage.prototype.flush#

History
VersionChanges
v14.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v1.6.0

Runtime deprecation.

Type: End-of-Life

OutgoingMessage.prototype.flush() has been removed. Use OutgoingMessage.prototype.flushHeaders() instead.

DEP0002: require('_linklist')#

History
VersionChanges
v8.0.0

End-of-Life.

v6.12.0

A deprecation code has been assigned.

v5.0.0

Runtime deprecation.

Type: End-of-Life

The _linklist module is deprecated. Please use a userland alternative.

DEP0003: _writableState.buffer#

History
VersionChanges
v14.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.15

Runtime deprecation.

Type: End-of-Life

The _writableState.buffer has been removed. Use _writableState.getBuffer() instead.

DEP0004: CryptoStream.prototype.readyState#

History
VersionChanges
v10.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.4.0

Documentation-only deprecation.

Type: End-of-Life

The CryptoStream.prototype.readyState property was removed.

DEP0005: Buffer() constructor#

History
VersionChanges
v10.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Runtime (supports --pending-deprecation)

The Buffer() function and new Buffer() constructor are deprecated due to API usability issues that can lead to accidental security issues.

As an alternative, use one of the following methods of constructing Buffer objects:

Without --pending-deprecation, runtime warnings occur only for code not in node_modules. This means there will not be deprecation warnings for Buffer() usage in dependencies. With --pending-deprecation, a runtime warning results no matter where the Buffer() usage occurs.

DEP0006: child_process options.customFds#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.14

Runtime deprecation.

v0.5.10

Documentation-only deprecation.

Type: End-of-Life

Within the child_process module's spawn(), fork(), and exec() methods, the options.customFds option is deprecated. The options.stdio option should be used instead.

DEP0007: Replace cluster worker.suicide with worker.exitedAfterDisconnect#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: End-of-Life

In an earlier version of the Node.js cluster, a boolean property with the name suicide was added to the Worker object. The intent of this property was to provide an indication of how and why the Worker instance exited. In Node.js 6.0.0, the old property was deprecated and replaced with a new worker.exitedAfterDisconnect property. The old property name did not precisely describe the actual semantics and was unnecessarily emotion-laden.

DEP0008: require('node:constants')#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.3.0

Documentation-only deprecation.

Type: Documentation-only

The node:constants module is deprecated. When requiring access to constants relevant to specific Node.js builtin modules, developers should instead refer to the constants property exposed by the relevant module. For instance, require('node:fs').constants and require('node:os').constants.

DEP0009: crypto.pbkdf2 without digest#

History
VersionChanges
v14.0.0

End-of-Life (for digest === null).

v11.0.0

Runtime deprecation (for digest === null).

v8.0.0

End-of-Life (for digest === undefined).

v6.12.0

A deprecation code has been assigned.

v6.0.0

Runtime deprecation (for digest === undefined).

Type: End-of-Life

Use of the crypto.pbkdf2() API without specifying a digest was deprecated in Node.js 6.0 because the method defaulted to using the non-recommended 'SHA1' digest. Previously, a deprecation warning was printed. Starting in Node.js 8.0.0, calling crypto.pbkdf2() or crypto.pbkdf2Sync() with digest set to undefined will throw a TypeError.

Beginning in Node.js v11.0.0, calling these functions with digest set to null would print a deprecation warning to align with the behavior when digest is undefined.

Now, however, passing either undefined or null will throw a TypeError.

DEP0010: crypto.createCredentials#

History
VersionChanges
v11.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.13

Runtime deprecation.

Type: End-of-Life

The crypto.createCredentials() API was removed. Please use tls.createSecureContext() instead.

DEP0011: crypto.Credentials#

History
VersionChanges
v11.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.13

Runtime deprecation.

Type: End-of-Life

The crypto.Credentials class was removed. Please use tls.SecureContext instead.

DEP0012: Domain.dispose#

History
VersionChanges
v9.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.7

Runtime deprecation.

Type: End-of-Life

Domain.dispose() has been removed. Recover from failed I/O actions explicitly via error event handlers set on the domain instead.

DEP0013: fs asynchronous function without callback#

History
VersionChanges
v10.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

Calling an asynchronous function without a callback throws a TypeError in Node.js 10.0.0 onwards. See https://github.com/nodejs/node/pull/12562.

DEP0014: fs.read legacy String interface#

History
VersionChanges
v8.0.0

End-of-Life.

v6.0.0

Runtime deprecation.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.1.96

Documentation-only deprecation.

Type: End-of-Life

The fs.read() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead.

DEP0015: fs.readSync legacy String interface#

History
VersionChanges
v8.0.0

End-of-Life.

v6.0.0

Runtime deprecation.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.1.96

Documentation-only deprecation.

Type: End-of-Life

The fs.readSync() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead.

DEP0016: GLOBAL/root#

History
VersionChanges
v14.0.0

End-of-Life.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Runtime deprecation.

Type: End-of-Life

The GLOBAL and root aliases for the global property were deprecated in Node.js 6.0.0 and have since been removed.

DEP0017: Intl.v8BreakIterator#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

Intl.v8BreakIterator was a non-standard extension and has been removed. See Intl.Segmenter.

DEP0018: Unhandled promise rejections#

History
VersionChanges
v15.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

Unhandled promise rejections are deprecated. By default, promise rejections that are not handled terminate the Node.js process with a non-zero exit code. To change the way Node.js treats unhandled rejections, use the --unhandled-rejections command-line option.

DEP0019: require('.') resolved outside directory#

History
VersionChanges
v12.0.0

Removed functionality.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v1.8.1

Runtime deprecation.

Type: End-of-Life

In certain cases, require('.') could resolve outside the package directory. This behavior has been removed.

DEP0020: Server.connections#

History
VersionChanges
v15.0.0

Server.connections has been removed.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.9.7

Runtime deprecation.

Type: End-of-Life

The Server.connections property was deprecated in Node.js v0.9.7 and has been removed. Please use the Server.getConnections() method instead.

DEP0021: Server.listenFD#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.7.12

Runtime deprecation.

Type: End-of-Life

The Server.listenFD() method was deprecated and removed. Please use Server.listen({fd: <number>}) instead.

DEP0022: os.tmpDir()#

History
VersionChanges
v14.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

The os.tmpDir() API was deprecated in Node.js 7.0.0 and has since been removed. Please use os.tmpdir() instead.

DEP0023: os.getNetworkInterfaces()#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.6.0

Runtime deprecation.

Type: End-of-Life

The os.getNetworkInterfaces() method is deprecated. Please use the os.networkInterfaces() method instead.

DEP0024: REPLServer.prototype.convertToContext()#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

The REPLServer.prototype.convertToContext() API has been removed.

DEP0025: require('node:sys')#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v1.0.0

Runtime deprecation.

Type: Runtime

The node:sys module is deprecated. Please use the util module instead.

DEP0026: util.print()#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.print() has been removed. Please use console.log() instead.

DEP0027: util.puts()#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.puts() has been removed. Please use console.log() instead.

DEP0028: util.debug()#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.debug() has been removed. Please use console.error() instead.

DEP0029: util.error()#

History
VersionChanges
v12.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.error() has been removed. Please use console.error() instead.

DEP0030: SlowBuffer#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The SlowBuffer class is deprecated. Please use Buffer.allocUnsafeSlow(size) instead.

DEP0031: ecdh.setPublicKey()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v5.2.0

Documentation-only deprecation.

Type: Documentation-only

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful.

DEP0032: node:domain module#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v1.4.2

Documentation-only deprecation.

Type: Documentation-only

The domain module is deprecated and should not be used.

DEP0033: EventEmitter.listenerCount()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v3.2.0

Documentation-only deprecation.

Type: Documentation-only

The events.listenerCount(emitter, eventName) API is deprecated. Please use emitter.listenerCount(eventName) instead.

DEP0034: fs.exists(path, callback)#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v1.0.0

Documentation-only deprecation.

Type: Documentation-only

The fs.exists(path, callback) API is deprecated. Please use fs.stat() or fs.access() instead.

DEP0035: fs.lchmod(path, mode, callback)#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Documentation-only

The fs.lchmod(path, mode, callback) API is deprecated.

DEP0036: fs.lchmodSync(path, mode)#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Documentation-only

The fs.lchmodSync(path, mode) API is deprecated.

DEP0037: fs.lchown(path, uid, gid, callback)#

History
VersionChanges
v10.6.0

Deprecation revoked.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Deprecation revoked

The fs.lchown(path, uid, gid, callback) API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

DEP0038: fs.lchownSync(path, uid, gid)#

History
VersionChanges
v10.6.0

Deprecation revoked.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Deprecation revoked

The fs.lchownSync(path, uid, gid) API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

DEP0039: require.extensions#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.10.6

Documentation-only deprecation.

Type: Documentation-only

The require.extensions property is deprecated.

DEP0040: node:punycode module#

History
VersionChanges
v16.6.0

Added support for --pending-deprecation.

v7.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

The punycode module is deprecated. Please use a userland alternative instead.

DEP0041: NODE_REPL_HISTORY_FILE environment variable#

History
VersionChanges
v10.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v3.0.0

Documentation-only deprecation.

Type: End-of-Life

The NODE_REPL_HISTORY_FILE environment variable was removed. Please use NODE_REPL_HISTORY instead.

DEP0042: tls.CryptoStream#

History
VersionChanges
v10.0.0

End-of-Life.

v6.12.0, v4.8.6

A deprecation code has been assigned.

v0.11.3

Documentation-only deprecation.

Type: End-of-Life

The tls.CryptoStream class was removed. Please use tls.TLSSocket instead.

DEP0043: tls.SecurePair#

History
VersionChanges
v8.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

v0.11.15

Deprecation revoked.

v0.11.3

Runtime deprecation.

Type: Documentation-only

The tls.SecurePair class is deprecated. Please use tls.TLSSocket instead.

DEP0044: util.isArray()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isArray() API is deprecated. Please use Array.isArray() instead.

DEP0045: util.isBoolean()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isBoolean() API is deprecated.

DEP0046: util.isBuffer()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isBuffer() API is deprecated. Please use Buffer.isBuffer() instead.

DEP0047: util.isDate()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isDate() API is deprecated.

DEP0048: util.isError()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isError() API is deprecated.

DEP0049: util.isFunction()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isFunction() API is deprecated.

DEP0050: util.isNull()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isNull() API is deprecated.

DEP0051: util.isNullOrUndefined()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isNullOrUndefined() API is deprecated.

DEP0052: util.isNumber()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isNumber() API is deprecated.

DEP0053: util.isObject()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isObject() API is deprecated.

DEP0054: util.isPrimitive()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isPrimitive() API is deprecated.

DEP0055: util.isRegExp()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isRegExp() API is deprecated.

DEP0056: util.isString()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isString() API is deprecated.

DEP0057: util.isSymbol()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isSymbol() API is deprecated.

DEP0058: util.isUndefined()#

History
VersionChanges
v6.12.0, v4.8.6

A deprecation code has been assigned.

v4.0.0, v3.3.1

Documentation-only deprecation.

Type: Documentation-only

The util.isUndefined() API is deprecated.

DEP0059: util.log()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.log() API is deprecated.

DEP0060: util._extend()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The util._extend() API is deprecated.

DEP0061: fs.SyncWriteStream#

History
VersionChanges
v11.0.0

End-of-Life.

v8.0.0

Runtime deprecation.

v7.0.0

Documentation-only deprecation.

Type: End-of-Life

The fs.SyncWriteStream class was never intended to be a publicly accessible API and has been removed. No alternative API is available. Please use a userland alternative.

DEP0062: node --debug#

History
VersionChanges
v12.0.0

End-of-Life.

v8.0.0

Runtime deprecation.

Type: End-of-Life

--debug activates the legacy V8 debugger interface, which was removed as of V8 5.8. It is replaced by Inspector which is activated with --inspect instead.

DEP0063: ServerResponse.prototype.writeHeader()#

History
VersionChanges
v8.0.0

Documentation-only deprecation.

Type: Documentation-only

The node:http module ServerResponse.prototype.writeHeader() API is deprecated. Please use ServerResponse.prototype.writeHead() instead.

The ServerResponse.prototype.writeHeader() method was never documented as an officially supported API.

DEP0064: tls.createSecurePair()#

History
VersionChanges
v8.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

v0.11.15

Deprecation revoked.

v0.11.3

Runtime deprecation.

Type: Runtime

The tls.createSecurePair() API was deprecated in documentation in Node.js 0.11.3. Users should use tls.Socket instead.

DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic#

History
VersionChanges
v10.0.0

End-of-Life.

v8.0.0

Documentation-only deprecation.

Type: End-of-Life

The node:repl module's REPL_MODE_MAGIC constant, used for replMode option, has been removed. Its behavior has been functionally identical to that of REPL_MODE_SLOPPY since Node.js 6.0.0, when V8 5.0 was imported. Please use REPL_MODE_SLOPPY instead.

The NODE_REPL_MODE environment variable is used to set the underlying replMode of an interactive node session. Its value, magic, is also removed. Please use sloppy instead.

DEP0066: OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames#

History
VersionChanges
v12.0.0

Runtime deprecation.

v8.0.0

Documentation-only deprecation.

Type: Runtime

The node:http module OutgoingMessage.prototype._headers and OutgoingMessage.prototype._headerNames properties are deprecated. Use one of the public methods (e.g. OutgoingMessage.prototype.getHeader(), OutgoingMessage.prototype.getHeaders(), OutgoingMessage.prototype.getHeaderNames(), OutgoingMessage.prototype.getRawHeaderNames(), OutgoingMessage.prototype.hasHeader(), OutgoingMessage.prototype.removeHeader(), OutgoingMessage.prototype.setHeader()) for working with outgoing headers.

The OutgoingMessage.prototype._headers and OutgoingMessage.prototype._headerNames properties were never documented as officially supported properties.

DEP0067: OutgoingMessage.prototype._renderHeaders#

History
VersionChanges
v8.0.0

Documentation-only deprecation.

Type: Documentation-only

The node:http module OutgoingMessage.prototype._renderHeaders() API is deprecated.

The OutgoingMessage.prototype._renderHeaders property was never documented as an officially supported API.

DEP0068: node debug#

History
VersionChanges
v15.0.0

The legacy node debug command was removed.

v8.0.0

Runtime deprecation.

Type: End-of-Life

node debug corresponds to the legacy CLI debugger which has been replaced with a V8-inspector based CLI debugger available through node inspect.

DEP0069: vm.runInDebugContext(string)#

History
VersionChanges
v10.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

v8.0.0

Documentation-only deprecation.

Type: End-of-Life

DebugContext has been removed in V8 and is not available in Node.js 10+.

DebugContext was an experimental API.

DEP0070: async_hooks.currentId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.currentId() was renamed to async_hooks.executionAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0071: async_hooks.triggerId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.triggerId() was renamed to async_hooks.triggerAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0072: async_hooks.AsyncResource.triggerId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.AsyncResource.triggerId() was renamed to async_hooks.AsyncResource.triggerAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0073: Several internal properties of net.Server#

History
VersionChanges
v10.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

Accessing several internal, undocumented properties of net.Server instances with inappropriate names is deprecated.

As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided.

DEP0074: REPLServer.bufferedCommand#

History
VersionChanges
v15.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

The REPLServer.bufferedCommand property was deprecated in favor of REPLServer.clearBufferedCommand().

DEP0075: REPLServer.parseREPLKeyword()#

History
VersionChanges
v15.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

REPLServer.parseREPLKeyword() was removed from userland visibility.

DEP0076: tls.parseCertString()#

History
VersionChanges
v18.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

v8.6.0

Documentation-only deprecation.

Type: End-of-Life

tls.parseCertString() was a trivial parsing helper that was made public by mistake. While it was supposed to parse certificate subject and issuer strings, it never handled multi-value Relative Distinguished Names correctly.

Earlier versions of this document suggested using querystring.parse() as an alternative to tls.parseCertString(). However, querystring.parse() also does not handle all certificate subjects correctly and should not be used.

DEP0077: Module._debug()#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

Module._debug() is deprecated.

The Module._debug() function was never documented as an officially supported API.

DEP0078: REPLServer.turnOffEditorMode()#

History
VersionChanges
v15.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

REPLServer.turnOffEditorMode() was removed from userland visibility.

DEP0079: Custom inspection function on objects via .inspect()#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

v8.7.0

Documentation-only deprecation.

Type: End-of-Life

Using a property named inspect on an object to specify a custom inspection function for util.inspect() is deprecated. Use util.inspect.custom instead. For backward compatibility with Node.js prior to version 6.4.0, both can be specified.

DEP0080: path._makeLong()#

History
VersionChanges
v9.0.0

Documentation-only deprecation.

Type: Documentation-only

The internal path._makeLong() was not intended for public use. However, userland modules have found it useful. The internal API is deprecated and replaced with an identical, public path.toNamespacedPath() method.

DEP0081: fs.truncate() using a file descriptor#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

fs.truncate() fs.truncateSync() usage with a file descriptor is deprecated. Please use fs.ftruncate() or fs.ftruncateSync() to work with file descriptors.

DEP0082: REPLServer.prototype.memory()#

History
VersionChanges
v15.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

REPLServer.prototype.memory() is only necessary for the internal mechanics of the REPLServer itself. Do not use this function.

DEP0083: Disabling ECDH by setting ecdhCurve to false#

History
VersionChanges
v10.0.0

End-of-Life.

v9.2.0

Runtime deprecation.

Type: End-of-Life.

The ecdhCurve option to tls.createSecureContext() and tls.TLSSocket could be set to false to disable ECDH entirely on the server only. This mode was deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with the client and is now unsupported. Use the ciphers parameter instead.

DEP0084: requiring bundled internal dependencies#

History
VersionChanges
v12.0.0

This functionality has been removed.

v10.0.0

Runtime deprecation.

Type: End-of-Life

Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for internal usage were mistakenly exposed to user code through require(). These modules were:

  • v8/tools/codemap
  • v8/tools/consarray
  • v8/tools/csvparser
  • v8/tools/logreader
  • v8/tools/profile_view
  • v8/tools/profile
  • v8/tools/SourceMap
  • v8/tools/splaytree
  • v8/tools/tickprocessor-driver
  • v8/tools/tickprocessor
  • node-inspect/lib/_inspect (from 7.6.0)
  • node-inspect/lib/internal/inspect_client (from 7.6.0)
  • node-inspect/lib/internal/inspect_repl (from 7.6.0)

The v8/* modules do not have any exports, and if not imported in a specific order would in fact throw errors. As such there are virtually no legitimate use cases for importing them through require().

On the other hand, node-inspect can be installed locally through a package manager, as it is published on the npm registry under the same name. No source code modification is necessary if that is done.

DEP0085: AsyncHooks sensitive API#

History
VersionChanges
v10.0.0

End-of-Life.

v9.4.0, v8.10.0

Runtime deprecation.

Type: End-of-Life

The AsyncHooks sensitive API was never documented and had various minor issues. Use the AsyncResource API instead. See https://github.com/nodejs/node/issues/15572.

DEP0086: Remove runInAsyncIdScope#

History
VersionChanges
v10.0.0

End-of-Life.

v9.4.0, v8.10.0

Runtime deprecation.

Type: End-of-Life

runInAsyncIdScope doesn't emit the 'before' or 'after' event and can thus cause a lot of issues. See https://github.com/nodejs/node/issues/14328.

DEP0089: require('node:assert')#

History
VersionChanges
v12.8.0

Deprecation revoked.

v9.9.0, v8.13.0

Documentation-only deprecation.

Type: Deprecation revoked

Importing assert directly was not recommended as the exposed functions use loose equality checks. The deprecation was revoked because use of the node:assert module is not discouraged, and the deprecation caused developer confusion.

DEP0090: Invalid GCM authentication tag lengths#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

Node.js used to support all GCM authentication tag lengths which are accepted by OpenSSL when calling decipher.setAuthTag(). Beginning with Node.js v11.0.0, only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32 bits are allowed. Authentication tags of other lengths are invalid per NIST SP 800-38D.

DEP0091: crypto.DEFAULT_ENCODING#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

The crypto.DEFAULT_ENCODING property is deprecated.

DEP0092: Top-level this bound to module.exports#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only

Assigning properties to the top-level this as an alternative to module.exports is deprecated. Developers should use exports or module.exports instead.

DEP0093: crypto.fips is deprecated and replaced#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only

The crypto.fips property is deprecated. Please use crypto.setFips() and crypto.getFips() instead.

DEP0094: Using assert.fail() with more than one argument#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

Using assert.fail() with more than one argument is deprecated. Use assert.fail() with only one argument or use a different node:assert module method.

DEP0095: timers.enroll()#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

timers.enroll() is deprecated. Please use the publicly documented setTimeout() or setInterval() instead.

DEP0096: timers.unenroll()#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

timers.unenroll() is deprecated. Please use the publicly documented clearTimeout() or clearInterval() instead.

DEP0097: MakeCallback with domain property#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

Users of MakeCallback that add the domain property to carry context, should start using the async_context variant of MakeCallback or CallbackScope, or the high-level AsyncResource class.

DEP0098: AsyncHooks embedder AsyncResource.emitBefore and AsyncResource.emitAfter APIs#

History
VersionChanges
v12.0.0

End-of-Life.

v10.0.0, v9.6.0, v8.12.0

Runtime deprecation.

Type: End-of-Life

The embedded API provided by AsyncHooks exposes .emitBefore() and .emitAfter() methods which are very easy to use incorrectly which can lead to unrecoverable errors.

Use asyncResource.runInAsyncScope() API instead which provides a much safer, and more convenient, alternative. See https://github.com/nodejs/node/pull/18513.

DEP0099: Async context-unaware node::MakeCallback C++ APIs#

History
VersionChanges
v10.0.0

Compile-time deprecation.

Type: Compile-time

Certain versions of node::MakeCallback APIs available to native addons are deprecated. Please use the versions of the API that accept an async_context parameter.

DEP0100: process.assert()#

History
VersionChanges
v10.0.0

Runtime deprecation.

v0.3.7

Documentation-only deprecation.

Type: Runtime

process.assert() is deprecated. Please use the assert module instead.

This was never a documented feature.

DEP0101: --with-lttng#

History
VersionChanges
v10.0.0

End-of-Life.

Type: End-of-Life

The --with-lttng compile-time option has been removed.

DEP0102: Using noAssert in Buffer#(read|write) operations#

History
VersionChanges
v10.0.0

End-of-Life.

Type: End-of-Life

Using the noAssert argument has no functionality anymore. All input is verified regardless of the value of noAssert. Skipping the verification could lead to hard-to-find errors and crashes.

DEP0103: process.binding('util').is[...] typechecks#

History
VersionChanges
v10.9.0

Superseded by DEP0111.

v10.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

Using process.binding() in general should be avoided. The type checking methods in particular can be replaced by using util.types.

This deprecation has been superseded by the deprecation of the process.binding() API (DEP0111).

DEP0104: process.env string coercion#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

When assigning a non-string property to process.env, the assigned value is implicitly converted to a string. This behavior is deprecated if the assigned value is not a string, boolean, or number. In the future, such assignment might result in a thrown error. Please convert the property to a string before assigning it to process.env.

DEP0105: decipher.finaltol#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

decipher.finaltol() has never been documented and was an alias for decipher.final(). This API has been removed, and it is recommended to use decipher.final() instead.

DEP0106: crypto.createCipher and crypto.createDecipher#

History
VersionChanges
v11.0.0

Runtime deprecation.

v10.0.0

Documentation-only deprecation.

Type: Runtime

Using crypto.createCipher() and crypto.createDecipher() must be avoided as they use a weak key derivation function (MD5 with no salt) and static initialization vectors. It is recommended to derive a key using crypto.pbkdf2() or crypto.scrypt() with random salts and to use crypto.createCipheriv() and crypto.createDecipheriv() to obtain the Cipher and Decipher objects respectively.

DEP0107: tls.convertNPNProtocols()#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

This was an undocumented helper function not intended for use outside Node.js core and obsoleted by the removal of NPN (Next Protocol Negotiation) support.

DEP0108: zlib.bytesRead#

History
VersionChanges
v11.0.0

Runtime deprecation.

v10.0.0

Documentation-only deprecation.

Type: Runtime

Deprecated alias for zlib.bytesWritten. This original name was chosen because it also made sense to interpret the value as the number of bytes read by the engine, but is inconsistent with other streams in Node.js that expose values under these names.

DEP0109: http, https, and tls support for invalid URLs#

History
VersionChanges
v16.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Some previously supported (but strictly invalid) URLs were accepted through the http.request(), http.get(), https.request(), https.get(), and tls.checkServerIdentity() APIs because those were accepted by the legacy url.parse() API. The mentioned APIs now use the WHATWG URL parser that requires strictly valid URLs. Passing an invalid URL is deprecated and support will be removed in the future.

DEP0110: vm.Script cached data#

History
VersionChanges
v10.6.0

Documentation-only deprecation.

Type: Documentation-only

The produceCachedData option is deprecated. Use script.createCachedData() instead.

DEP0111: process.binding()#

History
VersionChanges
v11.12.0

Added support for --pending-deprecation.

v10.9.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

process.binding() is for use by Node.js internal code only.

DEP0112: dgram private APIs#

History
VersionChanges
v11.0.0

Runtime deprecation.

Type: Runtime

The node:dgram module previously contained several APIs that were never meant to accessed outside of Node.js core: Socket.prototype._handle, Socket.prototype._receiving, Socket.prototype._bindState, Socket.prototype._queue, Socket.prototype._reuseAddr, Socket.prototype._healthCheck(), Socket.prototype._stopReceiving(), and dgram._createSocketHandle().

DEP0113: Cipher.setAuthTag(), Decipher.getAuthTag()#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Cipher.setAuthTag() and Decipher.getAuthTag() are no longer available. They were never documented and would throw when called.

DEP0114: crypto._toBuf()#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

The crypto._toBuf() function was not designed to be used by modules outside of Node.js core and was removed.

DEP0115: crypto.prng(), crypto.pseudoRandomBytes(), crypto.rng()#

History
VersionChanges
v11.0.0

Added documentation-only deprecation with --pending-deprecation support.

Type: Documentation-only (supports --pending-deprecation)

In recent versions of Node.js, there is no difference between crypto.randomBytes() and crypto.pseudoRandomBytes(). The latter is deprecated along with the undocumented aliases crypto.prng() and crypto.rng() in favor of crypto.randomBytes() and might be removed in a future release.

DEP0116: Legacy URL API#

History
VersionChanges
v19.0.0

`url.parse()` is deprecated again in DEP0169.

v15.13.0, v14.17.0

Deprecation revoked. Status changed to "Legacy".

v11.0.0

Documentation-only deprecation.

Type: Deprecation revoked

The legacy URL API is deprecated. This includes url.format(), url.parse(), url.resolve(), and the legacy urlObject. Please use the WHATWG URL API instead.

DEP0117: Native crypto handles#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Previous versions of Node.js exposed handles to internal native objects through the _handle property of the Cipher, Decipher, DiffieHellman, DiffieHellmanGroup, ECDH, Hash, Hmac, Sign, and Verify classes. The _handle property has been removed because improper use of the native object can lead to crashing the application.

DEP0118: dns.lookup() support for a falsy host name#

History
VersionChanges
v11.0.0

Runtime deprecation.

Type: Runtime

Previous versions of Node.js supported dns.lookup() with a falsy host name like dns.lookup(false) due to backward compatibility. This behavior is undocumented and is thought to be unused in real world apps. It will become an error in future versions of Node.js.

DEP0119: process.binding('uv').errname() private API#

History
VersionChanges
v11.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

process.binding('uv').errname() is deprecated. Please use util.getSystemErrorName() instead.

DEP0120: Windows Performance Counter support#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Windows Performance Counter support has been removed from Node.js. The undocumented COUNTER_NET_SERVER_CONNECTION(), COUNTER_NET_SERVER_CONNECTION_CLOSE(), COUNTER_HTTP_SERVER_REQUEST(), COUNTER_HTTP_SERVER_RESPONSE(), COUNTER_HTTP_CLIENT_REQUEST(), and COUNTER_HTTP_CLIENT_RESPONSE() functions have been deprecated.

DEP0121: net._setSimultaneousAccepts()#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

The undocumented net._setSimultaneousAccepts() function was originally intended for debugging and performance tuning when using the node:child_process and node:cluster modules on Windows. The function is not generally useful and is being removed. See discussion here: https://github.com/nodejs/node/issues/18391

DEP0122: tls Server.prototype.setOptions()#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

Please use Server.prototype.setSecureContext() instead.

DEP0123: setting the TLS ServerName to an IP address#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

Setting the TLS ServerName to an IP address is not permitted by RFC 6066. This will be ignored in a future version.

DEP0124: using REPLServer.rli#

History
VersionChanges
v15.0.0

End-of-Life.

v12.0.0

Runtime deprecation.

Type: End-of-Life

This property is a reference to the instance itself.

DEP0125: require('node:_stream_wrap')#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

The node:_stream_wrap module is deprecated.

DEP0126: timers.active()#

History
VersionChanges
v11.14.0

Runtime deprecation.

Type: Runtime

The previously undocumented timers.active() is deprecated. Please use the publicly documented timeout.refresh() instead. If re-referencing the timeout is necessary, timeout.ref() can be used with no performance impact since Node.js 10.

DEP0127: timers._unrefActive()#

History
VersionChanges
v11.14.0

Runtime deprecation.

Type: Runtime

The previously undocumented and "private" timers._unrefActive() is deprecated. Please use the publicly documented timeout.refresh() instead. If unreferencing the timeout is necessary, timeout.unref() can be used with no performance impact since Node.js 10.

DEP0128: modules with an invalid main entry and an index.js file#

History
VersionChanges
v16.0.0

Runtime deprecation.

v12.0.0

Documentation-only.

Type: Runtime

Modules that have an invalid main entry (e.g., ./does-not-exist.js) and also have an index.js file in the top level directory will resolve the index.js file. That is deprecated and is going to throw an error in future Node.js versions.

DEP0129: ChildProcess._channel#

History
VersionChanges
v13.0.0

Runtime deprecation.

v11.14.0

Documentation-only.

Type: Runtime

The _channel property of child process objects returned by spawn() and similar functions is not intended for public use. Use ChildProcess.channel instead.

DEP0130: Module.createRequireFromPath()#

History
VersionChanges
v16.0.0

End-of-life.

v13.0.0

Runtime deprecation.

v12.2.0

Documentation-only.

Type: End-of-Life

Use module.createRequire() instead.

DEP0131: Legacy HTTP parser#

History
VersionChanges
v13.0.0

This feature has been removed.

v12.22.0

Runtime deprecation.

v12.3.0

Documentation-only.

Type: End-of-Life

The legacy HTTP parser, used by default in versions of Node.js prior to 12.0.0, is deprecated and has been removed in v13.0.0. Prior to v13.0.0, the --http-parser=legacy command-line flag could be used to revert to using the legacy parser.

DEP0132: worker.terminate() with callback#

History
VersionChanges
v12.5.0

Runtime deprecation.

Type: Runtime

Passing a callback to worker.terminate() is deprecated. Use the returned Promise instead, or a listener to the worker's 'exit' event.

DEP0133: http connection#

History
VersionChanges
v12.12.0

Documentation-only deprecation.

Type: Documentation-only

Prefer response.socket over response.connection and request.socket over request.connection.

DEP0134: process._tickCallback#

History
VersionChanges
v12.12.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

The process._tickCallback property was never documented as an officially supported API.

DEP0135: WriteStream.open() and ReadStream.open() are internal#

History
VersionChanges
v13.0.0

Runtime deprecation.

Type: Runtime

WriteStream.open() and ReadStream.open() are undocumented internal APIs that do not make sense to use in userland. File streams should always be opened through their corresponding factory methods fs.createWriteStream() and fs.createReadStream()) or by passing a file descriptor in options.

DEP0136: http finished#

History
VersionChanges
v13.4.0, v12.16.0

Documentation-only deprecation.

Type: Documentation-only

response.finished indicates whether response.end() has been called, not whether 'finish' has been emitted and the underlying data is flushed.

Use response.writableFinished or response.writableEnded accordingly instead to avoid the ambiguity.

To maintain existing behavior response.finished should be replaced with response.writableEnded.

DEP0137: Closing fs.FileHandle on garbage collection#

History
VersionChanges
v14.0.0

Runtime deprecation.

Type: Runtime

Allowing a fs.FileHandle object to be closed on garbage collection is deprecated. In the future, doing so might result in a thrown error that will terminate the process.

Please ensure that all fs.FileHandle objects are explicitly closed using FileHandle.prototype.close() when the fs.FileHandle is no longer needed:

const fsPromises = require('node:fs').promises;
async function openAndClose() {
  let filehandle;
  try {
    filehandle = await fsPromises.open('thefile.txt', 'r');
  } finally {
    if (filehandle !== undefined)
      await filehandle.close();
  }
}

DEP0138: process.mainModule#

History
VersionChanges
v14.0.0

Documentation-only deprecation.

Type: Documentation-only

process.mainModule is a CommonJS-only feature while process global object is shared with non-CommonJS environment. Its use within ECMAScript modules is unsupported.

It is deprecated in favor of require.main, because it serves the same purpose and is only available on CommonJS environment.

DEP0139: process.umask() with no arguments#

History
VersionChanges
v14.0.0, v12.19.0

Documentation-only deprecation.

Type: Documentation-only

Calling process.umask() with no argument causes the process-wide umask to be written twice. This introduces a race condition between threads, and is a potential security vulnerability. There is no safe, cross-platform alternative API.

DEP0140: Use request.destroy() instead of request.abort()#

History
VersionChanges
v14.1.0, v13.14.0

Documentation-only deprecation.

Type: Documentation-only

Use request.destroy() instead of request.abort().

DEP0141: repl.inputStream and repl.outputStream#

History
VersionChanges
v14.3.0

Documentation-only (supports [--pending-deprecation][]).

Type: Documentation-only (supports --pending-deprecation)

The node:repl module exported the input and output stream twice. Use .input instead of .inputStream and .output instead of .outputStream.

DEP0142: repl._builtinLibs#

History
VersionChanges
v14.3.0

Documentation-only (supports [--pending-deprecation][]).

Type: Documentation-only

The node:repl module exports a _builtinLibs property that contains an array of built-in modules. It was incomplete so far and instead it's better to rely upon require('node:module').builtinModules.

DEP0143: Transform._transformState#

History
VersionChanges
v14.5.0

Runtime deprecation.

Type: Runtime Transform._transformState will be removed in future versions where it is no longer required due to simplification of the implementation.

DEP0144: module.parent#

History
VersionChanges
v14.6.0, v12.19.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

A CommonJS module can access the first module that required it using module.parent. This feature is deprecated because it does not work consistently in the presence of ECMAScript modules and because it gives an inaccurate representation of the CommonJS module graph.

Some modules use it to check if they are the entry point of the current process. Instead, it is recommended to compare require.main and module:

if (require.main === module) {
  // Code section that will run only if current file is the entry point.
}

When looking for the CommonJS modules that have required the current one, require.cache and module.children can be used:

const moduleParents = Object.values(require.cache)
  .filter((m) => m.children.includes(module));

DEP0145: socket.bufferSize#

History
VersionChanges
v14.6.0

Documentation-only deprecation.

Type: Documentation-only

socket.bufferSize is just an alias for writable.writableLength.

DEP0146: new crypto.Certificate()#

History
VersionChanges
v14.9.0

Documentation-only deprecation.

Type: Documentation-only

The crypto.Certificate() constructor is deprecated. Use static methods of crypto.Certificate() instead.

DEP0147: fs.rmdir(path, { recursive: true })#

History
VersionChanges
v16.0.0

Runtime deprecation.

v15.0.0

Runtime deprecation for permissive behavior.

v14.14.0

Documentation-only deprecation.

Type: Runtime

In future versions of Node.js, recursive option will be ignored for fs.rmdir, fs.rmdirSync, and fs.promises.rmdir.

Use fs.rm(path, { recursive: true, force: true }), fs.rmSync(path, { recursive: true, force: true }) or fs.promises.rm(path, { recursive: true, force: true }) instead.

DEP0148: Folder mappings in "exports" (trailing "/")#

History
VersionChanges
v17.0.0

End-of-Life.

v16.0.0

Runtime deprecation.

v15.1.0

Runtime deprecation for self-referencing imports.

v14.13.0

Documentation-only deprecation.

Type: Runtime

Using a trailing "/" to define subpath folder mappings in the subpath exports or subpath imports fields is deprecated. Use subpath patterns instead.

DEP0149: http.IncomingMessage#connection#

History
VersionChanges
v16.0.0

Documentation-only deprecation.

Type: Documentation-only.

Prefer message.socket over message.connection.

DEP0150: Changing the value of process.config#

History
VersionChanges
v19.0.0

End-of-Life.

v16.0.0

Runtime deprecation.

Type: End-of-Life

The process.config property provides access to Node.js compile-time settings. However, the property is mutable and therefore subject to tampering. The ability to change the value will be removed in a future version of Node.js.

DEP0151: Main index lookup and extension searching#

History
VersionChanges
v16.0.0

Runtime deprecation.

v15.8.0, v14.18.0

Documentation-only deprecation with --pending-deprecation support.

Type: Runtime

Previously, index.js and extension searching lookups would apply to import 'pkg' main entry point resolution, even when resolving ES modules.

With this deprecation, all ES module main entry point resolutions require an explicit "exports" or "main" entry with the exact file extension.

DEP0152: Extension PerformanceEntry properties#

History
VersionChanges
v16.0.0

Runtime deprecation.

Type: Runtime

The 'gc', 'http2', and 'http' <PerformanceEntry> object types have additional properties assigned to them that provide additional information. These properties are now available within the standard detail property of the PerformanceEntry object. The existing accessors have been deprecated and should no longer be used.

DEP0153: dns.lookup and dnsPromises.lookup options type coercion#

History
VersionChanges
v18.0.0

End-of-Life.

v17.0.0

Runtime deprecation.

v16.8.0

Documentation-only deprecation.

Type: End-of-Life

Using a non-nullish non-integer value for family option, a non-nullish non-number value for hints option, a non-nullish non-boolean value for all option, or a non-nullish non-boolean value for verbatim option in dns.lookup() and dnsPromises.lookup() throws an ERR_INVALID_ARG_TYPE error.

DEP0154: RSA-PSS generate key pair options#

History
VersionChanges
v16.10.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

The 'hash' and 'mgf1Hash' options are replaced with 'hashAlgorithm' and 'mgf1HashAlgorithm'.

DEP0155: Trailing slashes in pattern specifier resolutions#

History
VersionChanges
v17.0.0

Runtime deprecation.

v16.10.0

Documentation-only deprecation with --pending-deprecation support.

Type: Runtime

The remapping of specifiers ending in "/" like import 'pkg/x/' is deprecated for package "exports" and "imports" pattern resolutions.

DEP0156: .aborted property and 'abort', 'aborted' event in http#

History
VersionChanges
v17.0.0, v16.12.0

Documentation-only deprecation.

Type: Documentation-only

Move to <Stream> API instead, as the http.ClientRequest, http.ServerResponse, and http.IncomingMessage are all stream-based. Check stream.destroyed instead of the .aborted property, and listen for 'close' instead of 'abort', 'aborted' event.

The .aborted property and 'abort' event are only useful for detecting .abort() calls. For closing a request early, use the Stream .destroy([error]) then check the .destroyed property and 'close' event should have the same effect. The receiving end should also check the readable.readableEnded value on http.IncomingMessage to get whether it was an aborted or graceful destroy.

DEP0157: Thenable support in streams#

History
VersionChanges
v18.0.0

End-of-life.

v17.2.0, v16.14.0

Documentation-only deprecation.

Type: End-of-Life

An undocumented feature of Node.js streams was to support thenables in implementation methods. This is now deprecated, use callbacks instead and avoid use of async function for streams implementation methods.

This feature caused users to encounter unexpected problems where the user implements the function in callback style but uses e.g. an async method which would cause an error since mixing promise and callback semantics is not valid.

const w = new Writable({
  async final(callback) {
    await someOp();
    callback();
  },
});

DEP0158: buffer.slice(start, end)#

History
VersionChanges
v17.5.0, v16.15.0

Documentation-only deprecation.

Type: Documentation-only

This method was deprecated because it is not compatible with Uint8Array.prototype.slice(), which is a superclass of Buffer.

Use buffer.subarray which does the same thing instead.

DEP0159: ERR_INVALID_CALLBACK#

History
VersionChanges
v18.0.0

End-of-Life.

Type: End-of-Life

This error code was removed due to adding more confusion to the errors used for value type validation.

DEP0160: process.on('multipleResolves', handler)#

History
VersionChanges
v18.0.0

Runtime deprecation.

v17.6.0, v16.15.0

Documentation-only deprecation.

Type: Runtime.

This event was deprecated because it did not work with V8 promise combinators which diminished its usefulness.

DEP0161: process._getActiveRequests() and process._getActiveHandles()#

History
VersionChanges
v17.6.0, v16.15.0

Documentation-only deprecation.

Type: Documentation-only

The process._getActiveHandles() and process._getActiveRequests() functions are not intended for public use and can be removed in future releases.

Use process.getActiveResourcesInfo() to get a list of types of active resources and not the actual references.

DEP0162: fs.write(), fs.writeFileSync() coercion to string#

History
VersionChanges
v19.0.0

End-of-Life.

v18.0.0

Runtime deprecation.

v17.8.0, v16.15.0

Documentation-only deprecation.

Type: End-of-Life

Implicit coercion of objects with own toString property, passed as second parameter in fs.write(), fs.writeFile(), fs.appendFile(), fs.writeFileSync(), and fs.appendFileSync() is deprecated. Convert them to primitive strings.

DEP0163: channel.subscribe(onMessage), channel.unsubscribe(onMessage)#

History
VersionChanges
v18.7.0, v16.17.0

Documentation-only deprecation.

Type: Documentation-only

These methods were deprecated because they can be used in a way which does not hold the channel reference alive long enough to receive the events.

Use diagnostics_channel.subscribe(name, onMessage) or diagnostics_channel.unsubscribe(name, onMessage) which does the same thing instead.

DEP0164: process.exit(code), process.exitCode coercion to integer#

History
VersionChanges
v19.0.0

Runtime deprecation.

v18.10.0, v16.18.0

Documentation-only deprecation of process.exitCode integer coercion.

v18.7.0, v16.17.0

Documentation-only deprecation of process.exit(code) integer coercion.

Type: Runtime

Values other than undefined, null, integer numbers, and integer strings (e.g., '1') are deprecated as value for the code parameter in process.exit() and as value to assign to process.exitCode.

DEP0165: --trace-atomics-wait#

History
VersionChanges
v18.8.0, v16.18.0

Documentation-only deprecation.

Type: Documentation-only

The --trace-atomics-wait flag is deprecated.

DEP0166: Double slashes in imports and exports targets#

History
VersionChanges
v19.0.0

Runtime deprecation.

v18.10.0

Documentation-only deprecation with --pending-deprecation support.

Type: Runtime

Package imports and exports targets mapping into paths including a double slash (of "/" or "\") are deprecated and will fail with a resolution validation error in a future release. This same deprecation also applies to pattern matches starting or ending in a slash.

DEP0167: Weak DiffieHellmanGroup instances (modp1, modp2, modp5)#

History
VersionChanges
v18.10.0, v16.18.0

Documentation-only deprecation.

Type: Documentation-only

The well-known MODP groups modp1, modp2, and modp5 are deprecated because they are not secure against practical attacks. See RFC 8247 Section 2.4 for details.

These groups might be removed in future versions of Node.js. Applications that rely on these groups should evaluate using stronger MODP groups instead.

DEP0168: Unhandled exception in Node-API callbacks#

History
VersionChanges
v18.3.0, v16.17.0

Runtime deprecation.

Type: Runtime

The implicit suppression of uncaught exceptions in Node-API callbacks is now deprecated.

Set the flag --force-node-api-uncaught-exceptions-policy to force Node.js to emit an 'uncaughtException' event if the exception is not handled in Node-API callbacks.

DEP0169: Insecure url.parse()#

History
VersionChanges
v19.0.0

Documentation-only deprecation.

Type: Documentation-only

url.parse() behavior is not standardized and prone to errors that have security implications. Use the WHATWG URL API instead. CVEs are not issued for url.parse() vulnerabilities.

DEP0170: Invalid port when using url.parse()#

History
VersionChanges
v19.2.0

Documentation-only deprecation.

Type: Documentation-only

url.parse() accepts URLs with ports that are not numbers. This behavior might result in host name spoofing with unexpected input. These URLs will throw an error in future versions of Node.js, as the WHATWG URL API does already.

DEP0171: Setters for http.IncomingMessage headers and trailers#

History
VersionChanges
v19.3.0

Documentation-only deprecation.

Type: Documentation-only

In a future version of Node.js, message.headers, message.headersDistinct, message.trailers, and message.trailersDistinct will be read-only.

Diagnostics Channel#

History
VersionChanges
v19.2.0

diagnostics_channel is now Stable.

v15.1.0

Added in: v15.1.0

Stability: 2 - Stable

Source Code: lib/diagnostics_channel.js

The node:diagnostics_channel module provides an API to create named channels to report arbitrary message data for diagnostics purposes.

It can be accessed using:

import diagnostics_channel from 'node:diagnostics_channel';const diagnostics_channel = require('node:diagnostics_channel');

It is intended that a module writer wanting to report diagnostics messages will create one or many top-level channels to report messages through. Channels may also be acquired at runtime but it is not encouraged due to the additional overhead of doing so. Channels may be exported for convenience, but as long as the name is known it can be acquired anywhere.

If you intend for your module to produce diagnostics data for others to consume it is recommended that you include documentation of what named channels are used along with the shape of the message data. Channel names should generally include the module name to avoid collisions with data from other modules.

Public API#

Overview#

Following is a simple overview of the public API.

import diagnostics_channel from 'node:diagnostics_channel';

// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);

// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
  // Publish data to the channel
  channel.publish({
    some: 'data',
  });
}

// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);const diagnostics_channel = require('node:diagnostics_channel');

// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);

// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
  // Publish data to the channel
  channel.publish({
    some: 'data',
  });
}

// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);
diagnostics_channel.hasSubscribers(name)#
Added in: v15.1.0, v14.17.0

Check if there are active subscribers to the named channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

import diagnostics_channel from 'node:diagnostics_channel';

if (diagnostics_channel.hasSubscribers('my-channel')) {
  // There are subscribers, prepare and publish message
}const diagnostics_channel = require('node:diagnostics_channel');

if (diagnostics_channel.hasSubscribers('my-channel')) {
  // There are subscribers, prepare and publish message
}
diagnostics_channel.channel(name)#
Added in: v15.1.0, v14.17.0

This is the primary entry-point for anyone wanting to publish to a named channel. It produces a channel object which is optimized to reduce overhead at publish time as much as possible.

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');const diagnostics_channel = require('node:diagnostics_channel');

const channel = diagnostics_channel.channel('my-channel');
diagnostics_channel.subscribe(name, onMessage)#
Added in: v18.7.0, v16.17.0

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an 'uncaughtException'.

import diagnostics_channel from 'node:diagnostics_channel';

diagnostics_channel.subscribe('my-channel', (message, name) => {
  // Received data
});const diagnostics_channel = require('node:diagnostics_channel');

diagnostics_channel.subscribe('my-channel', (message, name) => {
  // Received data
});
diagnostics_channel.unsubscribe(name, onMessage)#
Added in: v18.7.0, v16.17.0

Remove a message handler previously registered to this channel with diagnostics_channel.subscribe(name, onMessage).

import diagnostics_channel from 'node:diagnostics_channel';

function onMessage(message, name) {
  // Received data
}

diagnostics_channel.subscribe('my-channel', onMessage);

diagnostics_channel.unsubscribe('my-channel', onMessage);const diagnostics_channel = require('node:diagnostics_channel');

function onMessage(message, name) {
  // Received data
}

diagnostics_channel.subscribe('my-channel', onMessage);

diagnostics_channel.unsubscribe('my-channel', onMessage);

Class: Channel#

Added in: v15.1.0, v14.17.0

The class Channel represents an individual named channel within the data pipeline. It is used to track subscribers and to publish messages when there are subscribers present. It exists as a separate object to avoid channel lookups at publish time, enabling very fast publish speeds and allowing for heavy use while incurring very minimal cost. Channels are created with diagnostics_channel.channel(name), constructing a channel directly with new Channel(name) is not supported.

channel.hasSubscribers#
Added in: v15.1.0, v14.17.0
  • Returns: <boolean> If there are active subscribers

Check if there are active subscribers to this channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

if (channel.hasSubscribers) {
  // There are subscribers, prepare and publish message
}const diagnostics_channel = require('node:diagnostics_channel');

const channel = diagnostics_channel.channel('my-channel');

if (channel.hasSubscribers) {
  // There are subscribers, prepare and publish message
}
channel.publish(message)#
Added in: v15.1.0, v14.17.0
  • message <any> The message to send to the channel subscribers

Publish a message to any subscribers to the channel. This will trigger message handlers synchronously so they will execute within the same context.

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

channel.publish({
  some: 'message',
});const diagnostics_channel = require('node:diagnostics_channel');

const channel = diagnostics_channel.channel('my-channel');

channel.publish({
  some: 'message',
});
channel.subscribe(onMessage)#
Added in: v15.1.0, v14.17.0Deprecated since: v18.7.0, v16.17.0

Stability: 0 - Deprecated: Use diagnostics_channel.subscribe(name, onMessage)

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an 'uncaughtException'.

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

channel.subscribe((message, name) => {
  // Received data
});const diagnostics_channel = require('node:diagnostics_channel');

const channel = diagnostics_channel.channel('my-channel');

channel.subscribe((message, name) => {
  // Received data
});
channel.unsubscribe(onMessage)#
History
VersionChanges
v18.7.0, v16.17.0

Deprecated since: v18.7.0, v16.17.0

v17.1.0, v16.14.0, v14.19.0

Added return value. Added to channels without subscribers.

v15.1.0, v14.17.0

Added in: v15.1.0, v14.17.0

Stability: 0 - Deprecated: Use diagnostics_channel.unsubscribe(name, onMessage)

  • onMessage <Function> The previous subscribed handler to remove
  • Returns: <boolean> true if the handler was found, false otherwise.

Remove a message handler previously registered to this channel with channel.subscribe(onMessage).

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

channel.subscribe(onMessage);

channel.unsubscribe(onMessage);const diagnostics_channel = require('node:diagnostics_channel');

const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

channel.subscribe(onMessage);

channel.unsubscribe(onMessage);

Built-in Channels#

Stability: 1 - Experimental

While the diagnostics_channel API is now considered stable, the built-in channels currently available are not. Each channel must be declared stable independently.

HTTP#

http.client.request.start

Emitted when client starts a request.

http.client.response.finish

Emitted when client receives a response.

http.server.request.start

Emitted when server receives a request.

http.server.response.finish

Emitted when server sends a response.

NET#

net.client.socket

Emitted when a new TCP or pipe client socket is created.

net.server.socket

Emitted when a new TCP or pipe connection is received.

UDP#

udp.socket

Emitted when a new UDP socket is created.

Process#
Added in: v16.18.0

child_process

Emitted when a new process is created.

Worker Thread#
Added in: v16.18.0

worker_threads

Emitted when a new thread is created.

DNS#

Stability: 2 - Stable

Source Code: lib/dns.js

The node:dns module enables name resolution. For example, use it to look up IP addresses of host names.

Although named for the Domain Name System (DNS), it does not always use the DNS protocol for lookups. dns.lookup() uses the operating system facilities to perform name resolution. It may not need to perform any network communication. To perform name resolution the way other applications on the same system do, use dns.lookup().

const dns = require('node:dns');

dns.lookup('example.org', (err, address, family) => {
  console.log('address: %j family: IPv%s', address, family);
});
// address: "93.184.216.34" family: IPv4

All other functions in the node:dns module connect to an actual DNS server to perform name resolution. They will always use the network to perform DNS queries. These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts). Use these functions to always perform DNS queries, bypassing other name-resolution facilities.

const dns = require('node:dns');

dns.resolve4('archive.org', (err, addresses) => {
  if (err) throw err;

  console.log(`addresses: ${JSON.stringify(addresses)}`);

  addresses.forEach((a) => {
    dns.reverse(a, (err, hostnames) => {
      if (err) {
        throw err;
      }
      console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
    });
  });
});

See the Implementation considerations section for more information.

Class: dns.Resolver#

Added in: v8.3.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers:

const { Resolver } = require('node:dns');
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org', (err, addresses) => {
  // ...
});

The following methods from the node:dns module are available:

Resolver([options])#

History
VersionChanges
v16.7.0, v14.18.0

The options object now accepts a tries option.

v12.18.3

The constructor now accepts an options object. The single supported option is timeout.

v8.3.0

Added in: v8.3.0

Create a new resolver.

  • options <Object>
    • timeout <integer> Query timeout in milliseconds, or -1 to use the default timeout.
    • tries <integer> The number of tries the resolver will try contacting each name server before giving up. Default: 4

resolver.cancel()#

Added in: v8.3.0

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code ECANCELLED.

resolver.setLocalAddress([ipv4][, ipv6])#

Added in: v15.1.0, v14.17.0
  • ipv4 <string> A string representation of an IPv4 address. Default: '0.0.0.0'
  • ipv6 <string> A string representation of an IPv6 address. Default: '::0'

The resolver instance will send its requests from the specified IP address. This allows programs to specify outbound interfaces when used on multi-homed systems.

If a v4 or v6 address is not specified, it is set to the default and the operating system will choose a local address automatically.

The resolver will use the v4 local address when making requests to IPv4 DNS servers, and the v6 local address when making requests to IPv6 DNS servers. The rrtype of resolution requests has no impact on the local address used.

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

[
  '4.4.4.4',
  '2001:4860:4860::8888',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053',
]

dns.lookup(hostname[, options], callback)#

History
VersionChanges
v18.4.0

For compatibility with node:net, when passing an option object the family option can be the string 'IPv4' or the string 'IPv6'.

v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v17.0.0

The verbatim options defaults to true now.

v8.5.0

The verbatim option is supported now.

v1.2.0

The all option is supported now.

v0.1.90

Added in: v0.1.90

  • hostname <string>
  • options <integer> | <Object>
    • family <integer> | <string> The record family. Must be 4, 6, or 0. For backward compatibility reasons,'IPv4' and 'IPv6' are interpreted as 4 and 6 respectively. The value 0 indicates that IPv4 and IPv6 addresses are both returned. Default: 0.
    • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
    • all <boolean> When true, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: false.
    • verbatim <boolean> When true, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false, IPv4 addresses are placed before IPv6 addresses. Default: true (addresses are not reordered). Default value is configurable using dns.setDefaultResultOrder() or --dns-result-order.
  • callback <Function>
    • err <Error>
    • address <string> A string representation of an IPv4 or IPv6 address.
    • family <integer> 4 or 6, denoting the family of address, or 0 if the address is not an IPv4 or IPv6 address. 0 is a likely indicator of a bug in the name resolution service used by the operating system.

Resolves a host name (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is 0 or not provided, then IPv4 and IPv6 addresses are both returned if found.

With the all option set to true, the arguments for callback change to (err, addresses), with addresses being an array of objects with the properties address and family.

On error, err is an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOTFOUND' not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

dns.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dns.lookup().

Example usage:

const dns = require('node:dns');
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
  console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6

// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
  console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]

If this method is invoked as its util.promisify()ed version, and all is not set to true, it returns a Promise for an Object with address and family properties.

Supported getaddrinfo flags#

History
VersionChanges
v13.13.0, v12.17.0

Added support for the dns.ALL flag.

The following flags can be passed as hints to dns.lookup().

  • dns.ADDRCONFIG: Limits returned address types to the types of non-loopback addresses configured on the system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured.
  • dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. It is not supported on some operating systems (e.g. FreeBSD 10.1).
  • dns.ALL: If dns.V4MAPPED is specified, return resolved IPv6 addresses as well as IPv4 mapped IPv6 addresses.

dns.lookupService(address, port, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.11.14

Added in: v0.11.14

Resolves the given address and port into a host name and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

On an error, err is an Error object, where err.code is the error code.

const dns = require('node:dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
  console.log(hostname, service);
  // Prints: localhost ssh
});

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with hostname and service properties.

dns.resolve(hostname[, rrtype], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.27

Added in: v0.1.27

Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org') into an array of the resource records. The callback function has arguments (err, records). When successful, records will be an array of resource records. The type and structure of individual results varies based on rrtype:

rrtyperecords containsResult typeShorthand method
'A'IPv4 addresses (default)<string>dns.resolve4()
'AAAA'IPv6 addresses<string>dns.resolve6()
'ANY'any records<Object>dns.resolveAny()
'CAA'CA authorization records<Object>dns.resolveCaa()
'CNAME'canonical name records<string>dns.resolveCname()
'MX'mail exchange records<Object>dns.resolveMx()
'NAPTR'name authority pointer records<Object>dns.resolveNaptr()
'NS'name server records<string>dns.resolveNs()
'PTR'pointer records<string>dns.resolvePtr()
'SOA'start of authority records<Object>dns.resolveSoa()
'SRV'service records<Object>dns.resolveSrv()
'TXT'text records<string[]>dns.resolveTxt()

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Host name to resolve.
  • options <Object>
    • ttl <boolean> Retrieves the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.
  • callback <Function>

Uses the DNS protocol to resolve a IPv4 addresses (A records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dns.resolve6(hostname[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Host name to resolve.
  • options <Object>
    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.
  • callback <Function>

Uses the DNS protocol to resolve IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

dns.resolveAny(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

Uses the DNS protocol to resolve all records (also known as ANY or * query). The ret argument passed to the callback function will be an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type, additional properties will be present on the object:

TypeProperties
'A'address/ttl
'AAAA'address/ttl
'CNAME'value
'MX'Refer to dns.resolveMx()
'NAPTR'Refer to dns.resolveNaptr()
'NS'value
'PTR'value
'SOA'Refer to dns.resolveSoa()
'SRV'Refer to dns.resolveSrv()
'TXT'This type of record contains an array property called entries which refers to dns.resolveTxt(), e.g. { entries: ['...'], type: 'TXT' }

Here is an example of the ret object passed to the callback:

[ { type: 'A', address: '127.0.0.1', ttl: 299 },
  { type: 'CNAME', value: 'example.com' },
  { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
  { type: 'NS', value: 'ns1.example.com' },
  { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
  { type: 'SOA',
    nsname: 'ns1.example.com',
    hostmaster: 'admin.example.com',
    serial: 156696742,
    refresh: 900,
    retry: 900,
    expire: 1800,
    minttl: 60 } ]

DNS server operators may choose not to respond to ANY queries. It may be better to call individual methods like dns.resolve4(), dns.resolveMx(), and so on. For more details, see RFC 8482.

dns.resolveCname(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.3.2

Added in: v0.3.2

Uses the DNS protocol to resolve CNAME records for the hostname. The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dns.resolveCaa(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.0.0, v14.17.0

Added in: v15.0.0, v14.17.0

Uses the DNS protocol to resolve CAA records for the hostname. The addresses argument passed to the callback function will contain an array of certification authority authorization records available for the hostname (e.g. [{critical: 0, iodef: 'mailto:pki@example.com'}, {critical: 128, issue: 'pki.example.com'}]).

dns.resolveMx(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.27

Added in: v0.1.27

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dns.resolveNaptr(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.9.12

Added in: v0.9.12

Uses the DNS protocol to resolve regular expression-based records (NAPTR records) for the hostname. The addresses argument passed to the callback function will contain an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference
{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.90

Added in: v0.1.90

Uses the DNS protocol to resolve name server records (NS records) for the hostname. The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dns.resolvePtr(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v6.0.0

Added in: v6.0.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. The addresses argument passed to the callback function will be an array of strings containing the reply records.

dns.resolveSoa(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.11.10

Added in: v0.11.10

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The address argument passed to the callback function will be an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dns.resolveSrv(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.27

Added in: v0.1.27

Uses the DNS protocol to resolve service records (SRV records) for the hostname. The addresses argument passed to the callback function will be an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dns.resolveTxt(hostname, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.27

Added in: v0.1.27

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The records argument passed to the callback function is a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dns.reverse(ip, callback)#

Added in: v0.1.16

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.setDefaultResultOrder(order)#

History
VersionChanges
v17.0.0

Changed default value to verbatim.

v16.4.0, v14.18.0

Added in: v16.4.0, v14.18.0

  • order <string> must be 'ipv4first' or 'verbatim'.

Set the default value of verbatim in dns.lookup() and dnsPromises.lookup(). The value could be:

  • ipv4first: sets default verbatim false.
  • verbatim: sets default verbatim true.

The default is verbatim and dns.setDefaultResultOrder() have higher priority than --dns-result-order. When using worker threads, dns.setDefaultResultOrder() from the main thread won't affect the default dns orders in workers.

dns.setServers(servers)#

Added in: v0.11.3

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

dns.setServers([
  '4.4.4.4',
  '[2001:4860:4860::8888]',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053',
]);

An error will be thrown if an invalid address is provided.

The dns.setServers() method must not be called while a DNS query is in progress.

The dns.setServers() method affects only dns.resolve(), dns.resolve*() and dns.reverse() (and specifically not dns.lookup()).

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

DNS promises API#

History
VersionChanges
v15.0.0

Exposed as require('dns/promises').

v11.14.0, v10.17.0

This API is no longer experimental.

v10.6.0

Added in: v10.6.0

The dns.promises API provides an alternative set of asynchronous DNS methods that return Promise objects rather than using callbacks. The API is accessible via require('node:dns').promises or require('node:dns/promises').

Class: dnsPromises.Resolver#

Added in: v10.6.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers:

const { Resolver } = require('node:dns').promises;
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org').then((addresses) => {
  // ...
});

// Alternatively, the same code can be written using async-await style.
(async function() {
  const addresses = await resolver.resolve4('example.org');
})();

The following methods from the dnsPromises API are available:

resolver.cancel()#

Added in: v15.3.0, v14.17.0

Cancel all outstanding DNS queries made by this resolver. The corresponding promises will be rejected with an error with the code ECANCELLED.

dnsPromises.getServers()#

Added in: v10.6.0

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

[
  '4.4.4.4',
  '2001:4860:4860::8888',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053',
]

dnsPromises.lookup(hostname[, options])#

Added in: v10.6.0
  • hostname <string>
  • options <integer> | <Object>
    • family <integer> The record family. Must be 4, 6, or 0. The value 0 indicates that IPv4 and IPv6 addresses are both returned. Default: 0.
    • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
    • all <boolean> When true, the Promise is resolved with all addresses in an array. Otherwise, returns a single address. Default: false.
    • verbatim <boolean> When true, the Promise is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false, IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. Default value is configurable using dns.setDefaultResultOrder() or --dns-result-order. New code should use { verbatim: true }.

Resolves a host name (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found.

With the all option set to true, the Promise is resolved with addresses being an array of objects with the properties address and family.

On error, the Promise is rejected with an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOTFOUND' not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

dnsPromises.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dnsPromises.lookup().

Example usage:

const dns = require('node:dns');
const dnsPromises = dns.promises;
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};

dnsPromises.lookup('example.com', options).then((result) => {
  console.log('address: %j family: IPv%s', result.address, result.family);
  // address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6
});

// When options.all is true, the result will be an Array.
options.all = true;
dnsPromises.lookup('example.com', options).then((result) => {
  console.log('addresses: %j', result);
  // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
});

dnsPromises.lookupService(address, port)#

Added in: v10.6.0

Resolves the given address and port into a host name and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

On error, the Promise is rejected with an Error object, where err.code is the error code.

const dnsPromises = require('node:dns').promises;
dnsPromises.lookupService('127.0.0.1', 22).then((result) => {
  console.log(result.hostname, result.service);
  // Prints: localhost ssh
});

dnsPromises.resolve(hostname[, rrtype])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.
  • rrtype <string> Resource record type. Default: 'A'.

Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org') into an array of the resource records. When successful, the Promise is resolved with an array of resource records. The type and structure of individual results vary based on rrtype:

rrtyperecords containsResult typeShorthand method
'A'IPv4 addresses (default)<string>dnsPromises.resolve4()
'AAAA'IPv6 addresses<string>dnsPromises.resolve6()
'ANY'any records<Object>dnsPromises.resolveAny()
'CAA'CA authorization records<Object>dnsPromises.resolveCaa()
'CNAME'canonical name records<string>dnsPromises.resolveCname()
'MX'mail exchange records<Object>dnsPromises.resolveMx()
'NAPTR'name authority pointer records<Object>dnsPromises.resolveNaptr()
'NS'name server records<string>dnsPromises.resolveNs()
'PTR'pointer records<string>dnsPromises.resolvePtr()
'SOA'start of authority records<Object>dnsPromises.resolveSoa()
'SRV'service records<Object>dnsPromises.resolveSrv()
'TXT'text records<string[]>dnsPromises.resolveTxt()

On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes.

dnsPromises.resolve4(hostname[, options])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.
  • options <Object>
    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the Promise is resolved with an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv4 addresses (A records) for the hostname. On success, the Promise is resolved with an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dnsPromises.resolve6(hostname[, options])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.
  • options <Object>
    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the Promise is resolved with an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv6 addresses (AAAA records) for the hostname. On success, the Promise is resolved with an array of IPv6 addresses.

dnsPromises.resolveAny(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve all records (also known as ANY or * query). On success, the Promise is resolved with an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type, additional properties will be present on the object:

TypeProperties
'A'address/ttl
'AAAA'address/ttl
'CNAME'value
'MX'Refer to dnsPromises.resolveMx()
'NAPTR'Refer to dnsPromises.resolveNaptr()
'NS'value
'PTR'value
'SOA'Refer to dnsPromises.resolveSoa()
'SRV'Refer to dnsPromises.resolveSrv()
'TXT'This type of record contains an array property called entries which refers to dnsPromises.resolveTxt(), e.g. { entries: ['...'], type: 'TXT' }

Here is an example of the result object:

[ { type: 'A', address: '127.0.0.1', ttl: 299 },
  { type: 'CNAME', value: 'example.com' },
  { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
  { type: 'NS', value: 'ns1.example.com' },
  { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
  { type: 'SOA',
    nsname: 'ns1.example.com',
    hostmaster: 'admin.example.com',
    serial: 156696742,
    refresh: 900,
    retry: 900,
    expire: 1800,
    minttl: 60 } ]

dnsPromises.resolveCaa(hostname)#

Added in: v15.0.0, v14.17.0

Uses the DNS protocol to resolve CAA records for the hostname. On success, the Promise is resolved with an array of objects containing available certification authority authorization records available for the hostname (e.g. [{critical: 0, iodef: 'mailto:pki@example.com'},{critical: 128, issue: 'pki.example.com'}]).

dnsPromises.resolveCname(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve CNAME records for the hostname. On success, the Promise is resolved with an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dnsPromises.resolveMx(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. On success, the Promise is resolved with an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dnsPromises.resolveNaptr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve regular expression-based records (NAPTR records) for the hostname. On success, the Promise is resolved with an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference
{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dnsPromises.resolveNs(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve name server records (NS records) for the hostname. On success, the Promise is resolved with an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dnsPromises.resolvePtr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. On success, the Promise is resolved with an array of strings containing the reply records.

dnsPromises.resolveSoa(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. On success, the Promise is resolved with an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dnsPromises.resolveSrv(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve service records (SRV records) for the hostname. On success, the Promise is resolved with an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dnsPromises.resolveTxt(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. On success, the Promise is resolved with a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dnsPromises.reverse(ip)#

Added in: v10.6.0

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes.

dnsPromises.setDefaultResultOrder(order)#

History
VersionChanges
v17.0.0

Changed default value to verbatim.

v16.4.0, v14.18.0

Added in: v16.4.0, v14.18.0

  • order <string> must be 'ipv4first' or 'verbatim'.

Set the default value of verbatim in dns.lookup() and dnsPromises.lookup(). The value could be:

  • ipv4first: sets default verbatim false.
  • verbatim: sets default verbatim true.

The default is verbatim and dnsPromises.setDefaultResultOrder() have higher priority than --dns-result-order. When using worker threads, dnsPromises.setDefaultResultOrder() from the main thread won't affect the default dns orders in workers.

dnsPromises.setServers(servers)#

Added in: v10.6.0

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

dnsPromises.setServers([
  '4.4.4.4',
  '[2001:4860:4860::8888]',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053',
]);

An error will be thrown if an invalid address is provided.

The dnsPromises.setServers() method must not be called while a DNS query is in progress.

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

Error codes#

Each DNS query can return one of the following error codes:

  • dns.NODATA: DNS server returned an answer with no data.
  • dns.FORMERR: DNS server claims query was misformatted.
  • dns.SERVFAIL: DNS server returned general failure.
  • dns.NOTFOUND: Domain name not found.
  • dns.NOTIMP: DNS server does not implement the requested operation.
  • dns.REFUSED: DNS server refused query.
  • dns.BADQUERY: Misformatted DNS query.
  • dns.BADNAME: Misformatted host name.
  • dns.BADFAMILY: Unsupported address family.
  • dns.BADRESP: Misformatted DNS reply.
  • dns.CONNREFUSED: Could not contact DNS servers.
  • dns.TIMEOUT: Timeout while contacting DNS servers.
  • dns.EOF: End of file.
  • dns.FILE: Error reading file.
  • dns.NOMEM: Out of memory.
  • dns.DESTRUCTION: Channel is being destroyed.
  • dns.BADSTR: Misformatted string.
  • dns.BADFLAGS: Illegal flags specified.
  • dns.NONAME: Given host name is not numeric.
  • dns.BADHINTS: Illegal hints flags specified.
  • dns.NOTINITIALIZED: c-ares library initialization not yet performed.
  • dns.LOADIPHLPAPI: Error loading iphlpapi.dll.
  • dns.ADDRGETNETWORKPARAMS: Could not find GetNetworkParams function.
  • dns.CANCELLED: DNS query cancelled.

The dnsPromises API also exports the above error codes, e.g., dnsPromises.NODATA.

Implementation considerations#

Although dns.lookup() and the various dns.resolve*()/dns.reverse() functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

dns.lookup()#

Under the hood, dns.lookup() uses the same operating system facilities as most other programs. For instance, dns.lookup() will almost always resolve a given name the same way as the ping command. On most POSIX-like operating systems, the behavior of the dns.lookup() function can be modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5), but changing these files will change the behavior of all other programs running on the same operating system.

Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. This can have surprising negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information.

Various networking APIs will call dns.lookup() internally to resolve host names. If that is an issue, consider resolving the host name to an address using dns.resolve() and using the address instead of a host name. Also, some networking APIs (such as socket.connect() and dgram.createSocket()) allow the default resolver, dns.lookup(), to be replaced.

dns.resolve(), dns.resolve*(), and dns.reverse()#

These functions are implemented quite differently than dns.lookup(). They do not use getaddrinfo(3) and they always perform a DNS query on the network. This network communication is always done asynchronously and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that dns.lookup() can have.

They do not use the same set of configuration files that dns.lookup() uses. For instance, they do not use the configuration from /etc/hosts.

Domain#

History
VersionChanges
v8.8.0

Any Promises created in VM contexts no longer have a .domain property. Their handlers are still executed in the proper domain, however, and Promises created in the main context still possess a .domain property.

v8.0.0

Handlers for Promises are now invoked in the domain in which the first promise of a chain was created.

v1.4.2

Deprecated since: v1.4.2

Stability: 0 - Deprecated

Source Code: lib/domain.js

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most developers should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an 'error' event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code.

Warning: Don't ignore errors!#

Domain error handlers are not a substitute for closing down a process when an error occurs.

By the very nature of how throw works in JavaScript, there is almost never any way to safely "pick up where it left off", without leaking references, or creating some other sort of undefined brittle state.

The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.

The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

In this way, domain usage goes hand-in-hand with the cluster module, since the primary process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly.

For example, this is not a good idea:

// XXX WARNING! BAD IDEA!

const d = require('node:domain').create();
d.on('error', (er) => {
  // The error won't crash the process, but what it does is worse!
  // Though we've prevented abrupt process restarting, we are leaking
  // a lot of resources if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
  require('node:http').createServer((req, res) => {
    handleRequest(req, res);
  }).listen(PORT);
});

By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.

// Much better!

const cluster = require('node:cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isPrimary) {
  // A more realistic scenario would have more than 2 workers,
  // and perhaps not put the primary and worker in the same file.
  //
  // It is also possible to get a bit fancier about logging, and
  // implement whatever custom logic is needed to prevent DoS
  // attacks and other bad behavior.
  //
  // See the options in the cluster documentation.
  //
  // The important thing is that the primary does very little,
  // increasing our resilience to unexpected errors.

  cluster.fork();
  cluster.fork();

  cluster.on('disconnect', (worker) => {
    console.error('disconnect!');
    cluster.fork();
  });

} else {
  // the worker
  //
  // This is where we put our bugs!

  const domain = require('node:domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests. How it works, caveats, etc.

  const server = require('node:http').createServer((req, res) => {
    const d = domain.create();
    d.on('error', (er) => {
      console.error(`error ${er.stack}`);

      // We're in dangerous territory!
      // By definition, something unexpected occurred,
      // which we probably didn't want.
      // Anything can happen now! Be very careful!

      try {
        // Make sure we close down within 30 seconds
        const killtimer = setTimeout(() => {
          process.exit(1);
        }, 30000);
        // But don't keep the process open just for that!
        killtimer.unref();

        // Stop taking new requests.
        server.close();

        // Let the primary know we're dead. This will trigger a
        // 'disconnect' in the cluster primary, and then it will fork
        // a new worker.
        cluster.worker.disconnect();

        // Try to send an error to the request that triggered the problem
        res.statusCode = 500;
        res.setHeader('content-type', 'text/plain');
        res.end('Oops, there was a problem!\n');
      } catch (er2) {
        // Oh well, not much we can do at this point.
        console.error(`Error sending 500! ${er2.stack}`);
      }
    });

    // Because req and res were created before this domain existed,
    // we need to explicitly add them.
    // See the explanation of implicit vs explicit binding below.
    d.add(req);
    d.add(res);

    // Now run the handler function in the domain.
    d.run(() => {
      handleRequest(req, res);
    });
  });
  server.listen(PORT);
}

// This part is not important. Just an example routing thing.
// Put fancy application logic here.
function handleRequest(req, res) {
  switch (req.url) {
    case '/error':
      // We do some async stuff, and then...
      setTimeout(() => {
        // Whoops!
        flerb.bark();
      }, timeout);
      break;
    default:
      res.end('ok');
  }
}

Additions to Error objects#

Any time an Error object is routed through a domain, a few extra fields are added to it.

  • error.domain The domain that first handled the error.
  • error.domainEmitter The event emitter that emitted an 'error' event with the error object.
  • error.domainBound The callback function which was bound to the domain, and passed an error as its first argument.
  • error.domainThrown A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.

Implicit binding#

If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.

Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open(), or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.

In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.

To nest Domain objects as children of a parent Domain they must be explicitly added.

Implicit binding routes thrown errors and 'error' events to the Domain's 'error' event, but does not register the EventEmitter on the Domain. Implicit binding only takes care of thrown errors and 'error' events.

Explicit binding#

Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.

For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.

That is possible via explicit binding.

// Create a top-level domain for the server
const domain = require('node:domain');
const http = require('node:http');
const serverDomain = domain.create();

serverDomain.run(() => {
  // Server is created in the scope of serverDomain
  http.createServer((req, res) => {
    // Req and res are also created in the scope of serverDomain
    // however, we'd prefer to have a separate domain for each request.
    // create it first thing, and add req and res to it.
    const reqd = domain.create();
    reqd.add(req);
    reqd.add(res);
    reqd.on('error', (er) => {
      console.error('Error', er, req.url);
      try {
        res.writeHead(500);
        res.end('Error occurred, sorry.');
      } catch (er2) {
        console.error('Error sending 500', er2, req.url);
      }
    });
  }).listen(1337);
});

domain.create()#

Class: Domain#

The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.

To handle the errors that it catches, listen to its 'error' event.

domain.members#

An array of timers and event emitters that have been explicitly added to the domain.

domain.add(emitter)#

Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an 'error' event, it will be routed to the domain's 'error' event, just like with implicit binding.

This also works with timers that are returned from setInterval() and setTimeout(). If their callback function throws, it will be caught by the domain 'error' handler.

If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead.

domain.bind(callback)#

The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's 'error' event.

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.bind((er, data) => {
    // If this throws, it will also be passed to the domain.
    return cb(er, data ? JSON.parse(data) : null);
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.enter()#

The enter() method is plumbing used by the run(), bind(), and intercept() methods to set the active domain. It sets domain.active and process.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see domain.exit() for details on the domain stack). The call to enter() delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling enter() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain.

domain.exit()#

The exit() method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to exit() delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain.

If there are multiple, nested domains bound to the current execution context, exit() will exit any domains nested within this domain.

Calling exit() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain.

domain.intercept(callback)#

This method is almost identical to domain.bind(callback). However, in addition to catching thrown errors, it will also intercept Error objects sent as the first argument to the function.

In this way, the common if (err) return callback(err); pattern can be replaced with a single error handler in a single place.

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.intercept((data) => {
    // Note, the first argument is never passed to the
    // callback since it is assumed to be the 'Error' argument
    // and thus intercepted by the domain.

    // If this throws, it will also be passed to the domain
    // so the error-handling logic can be moved to the 'error'
    // event on the domain instead of being repeated throughout
    // the program.
    return cb(null, JSON.parse(data));
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.remove(emitter)#

The opposite of domain.add(emitter). Removes domain handling from the specified emitter.

domain.run(fn[, ...args])#

Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.

This is the most basic way to use a domain.

const domain = require('node:domain');
const fs = require('node:fs');
const d = domain.create();
d.on('error', (er) => {
  console.error('Caught error!', er);
});
d.run(() => {
  process.nextTick(() => {
    setTimeout(() => { // Simulating some various async stuff
      fs.open('non-existent file', 'r', (er, fd) => {
        if (er) throw er;
        // proceed...
      });
    }, 100);
  });
});

In this example, the d.on('error') handler will be triggered, rather than crashing the program.

Domains and promises#

As of Node.js 8.0.0, the handlers of promises are run inside the domain in which the call to .then() or .catch() itself was made:

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then((v) => {
    // running in d2
  });
});

A callback may be bound to a specific domain using domain.bind(callback):

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then(p.domain.bind((v) => {
    // running in d1
  }));
});

Domains will not interfere with the error handling mechanisms for promises. In other words, no 'error' event will be emitted for unhandled Promise rejections.

Errors#

Applications running in Node.js will generally experience four categories of errors:

  • Standard JavaScript errors such as <EvalError>, <SyntaxError>, <RangeError>, <ReferenceError>, <TypeError>, and <URIError>.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket.
  • User-specified errors triggered by application code.
  • AssertionErrors are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by the node:assert module.

All JavaScript and system errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.

Error propagation and interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try…catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is not defined.
try {
  const m = 1;
  const n = m + z;
} catch (err) {
  // Handle the error here.
}

Any use of the JavaScript throw mechanism will raise an exception that must be handled using try…catch or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not accept a callback function, such as fs.readFileSync), will use throw to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

  • Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error, then an error occurred that should be handled.

    const fs = require('node:fs');
fs.readFile('a file that does not exist', (err, data) => {
  if (err) {
    console.error('There was an error reading the file!', err);
    return;
  }
  // Otherwise handle the data
});

  • When an asynchronous method is called on an object that is an EventEmitter, errors can be routed to that object's 'error' event.

    const net = require('node:net');
const connection = net.connect('localhost');

// Adding an 'error' event handler to a stream:
connection.on('error', (err) => {
  // If the connection is reset by the server, or if it can't
  // connect at all, or on any sort of error encountered by
  // the connection, the error will be sent here.
  console.error(err);
});

connection.pipe(process.stdout);

  • A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try…catch. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an uncaught exception and crash unless either: The domain module is used appropriately or a handler has been registered for the 'uncaughtException' event.

const EventEmitter = require('node:events');
const ee = new EventEmitter();

setImmediate(() => {
  // This will crash the process because no 'error' event
  // handler has been added.
  ee.emit('error', new Error('This will crash'));
});

Errors generated in this way cannot be intercepted using try…catch as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

Error-first callbacks#

Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as an error-first callback. With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null.

const fs = require('node:fs');

function errorFirstCallback(err, data) {
  if (err) {
    console.error('There was an error', err);
    return;
  }
  console.log(data);
}

fs.readFile('/some/file/that/does-not-exist', errorFirstCallback);
fs.readFile('/some/file/that/does-exist', errorFirstCallback);

The JavaScript try…catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside an error-first callback:

// THIS WILL NOT WORK:
const fs = require('node:fs');

try {
  fs.readFile('/some/file/that/does-not-exist', (err, data) => {
    // Mistaken assumption: throwing here...
    if (err) {
      throw err;
    }
  });
} catch (err) {
  // This will not catch the throw!
  console.error(err);
}

This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code, including the try…catch block, will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException'), such errors can be intercepted.

Class: Error#

A generic JavaScript <Error> object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all system and JavaScript errors, will either be instances of, or inherit from, the Error class.

new Error(message[, options])#

Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message, the text message is generated by calling String(message). If the cause option is provided, it is assigned to the error.cause property. The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit, whichever is smaller.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // Similar to `new Error().stack`

The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message}.

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from the user. For instance:

function MyError() {
  Error.captureStackTrace(this, MyError);
}

// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;

Error.stackTraceLimit#

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

error.cause#

Added in: v16.9.0

If present, the error.cause property is the underlying cause of the Error. It is used when catching an error and throwing a new one with a different message or code in order to still have access to the original error.

The error.cause property is typically set by calling new Error(message, { cause }). It is not set by the constructor if the cause option is not provided.

This property allows errors to be chained. When serializing Error objects, util.inspect() recursively serializes error.cause if it is set.

const cause = new Error('The remote HTTP server responded with a 500 status');
const symptom = new Error('The message failed to send', { cause });

console.log(symptom);
// Prints:
//   Error: The message failed to send
//       at REPL2:1:17
//       at Script.runInThisContext (node:vm:130:12)
//       ... 7 lines matching cause stack trace ...
//       at [_line] [as _line] (node:internal/readline/interface:886:18) {
//     [cause]: Error: The remote HTTP server responded with a 500 status
//         at REPL1:1:15
//         at Script.runInThisContext (node:vm:130:12)
//         at REPLServer.defaultEval (node:repl:574:29)
//         at bound (node:domain:426:15)
//         at REPLServer.runBound [as eval] (node:domain:437:12)
//         at REPLServer.onLine (node:repl:902:10)
//         at REPLServer.emit (node:events:549:35)
//         at REPLServer.emit (node:domain:482:12)
//         at [_onLine] [as _onLine] (node:internal/readline/interface:425:12)
//         at [_line] [as _line] (node:internal/readline/interface:886:18)

error.code#

The error.code property is a string label that identifies the kind of error. error.code is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, error.message strings may change between any versions of Node.js. See Node.js error codes for details about specific codes.

error.message#

The error.message property is the string description of the error as set by calling new Error(message). The message passed to the constructor will also appear in the first line of the stack trace of the Error, however changing this property after the Error object is created may not change the first line of the stack trace (for example, when error.stack is read before this property is changed).

const err = new Error('The message');
console.error(err.message);
// Prints: The message

error.stack#

The error.stack property is a string describing the point in the code at which the Error was instantiated.

Error: Things keep happening!
   at /home/gbusey/file.js:525:2
   at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
   at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
   at increaseSynergy (/home/gbusey/actors.js:701:6)

The first line is formatted as <error class name>: <error message>, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces:

const cheetahify = require('./native-binding.node');

function makeFaster() {
  // `cheetahify()` *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster();
// will throw:
//   /home/gbusey/file.js:6
//       throw new Error('oh no!');
//           ^
//   Error: oh no!
//       at speedy (/home/gbusey/file.js:6:11)
//       at makeFaster (/home/gbusey/file.js:5:3)
//       at Object.<anonymous> (/home/gbusey/file.js:10:1)
//       at Module._compile (module.js:456:26)
//       at Object.Module._extensions..js (module.js:474:10)
//       at Module.load (module.js:356:32)
//       at Function.Module._load (module.js:312:12)
//       at Function.Module.runMain (module.js:497:10)
//       at startup (node.js:119:16)
//       at node.js:906:3

The location information will be one of:

  • native, if the frame represents a call internal to V8 (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Node.js.
  • /absolute/path/to/file.js:line:column, if the frame represents a call in a user program (using CommonJS module system), or its dependencies.
  • <transport-protocol>:///url/to/module/file.mjs:line:column, if the frame represents a call in a user program (using ES module system), or its dependencies.

The string representing the stack trace is lazily generated when the error.stack property is accessed.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

Class: AssertionError#

Indicates the failure of an assertion. For details, see Class: assert.AssertionError.

Class: RangeError#

Indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

require('node:net').connect(-1);
// Throws "RangeError: "port" option should be >= 0 and < 65536: -1"

Node.js will generate and throw RangeError instances immediately as a form of argument validation.

Class: ReferenceError#

Indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

doesNotExist;
// Throws ReferenceError, doesNotExist is not a variable in this program.

Unless an application is dynamically generating and running code, ReferenceError instances indicate a bug in the code or its dependencies.

Class: SyntaxError#

Indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require('node:vm').runInThisContext('binary ! isNotOk');
} catch (err) {
  // 'err' will be a SyntaxError.
}

SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts.

Class: SystemError#

Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist.

  • address <string> If present, the address to which a network connection failed
  • code <string> The string error code
  • dest <string> If present, the file path destination when reporting a file system error
  • errno <number> The system-provided error number
  • info <Object> If present, extra details about the error condition
  • message <string> A system-provided human-readable description of the error
  • path <string> If present, the file path when reporting a file system error
  • port <number> If present, the network connection port that is not available
  • syscall <string> The name of the system call that triggered the error

error.address#

If present, error.address is a string describing the address to which a network connection failed.

error.code#

The error.code property is a string representing the error code.

error.dest#

If present, error.dest is the file path destination when reporting a file system error.

error.errno#

The error.errno property is a negative number which corresponds to the error code defined in libuv Error handling.

On Windows the error number provided by the system will be normalized by libuv.

To get the string representation of the error code, use util.getSystemErrorName(error.errno).

error.info#

If present, error.info is an object with details about the error condition.

error.message#

error.message is a system-provided human-readable description of the error.

error.path#

If present, error.path is a string containing a relevant invalid pathname.

error.port#

If present, error.port is the network connection port that is not available.

error.syscall#

The error.syscall property is a string describing the syscall that failed.

Common system errors#

This is a list of system errors commonly-encountered when writing a Node.js program. For a comprehensive list, see the errno(3) man page.

  • EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

  • EADDRINUSE (Address already in use): An attempt to bind a server (net, http, or https) to a local address failed due to another server on the local system already occupying that address.

  • ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

  • ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules.

  • EEXIST (File exists): An existing file was the target of an operation that required that the target not exist.

  • EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory.

  • EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process.

  • ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist. No entity (file or directory) could be found by the given path.

  • ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.

  • ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usually fs.unlink.

  • ENOTFOUND (DNS lookup failed): Indicates a DNS failure of either EAI_NODATA or EAI_NONAME. This is not a standard POSIX error.

  • EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

  • EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed.

  • ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net. Often a sign that a socket.end() was not properly called.

Class: TypeError#

Indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be a TypeError.

require('node:url').parse(() => { });
// Throws TypeError, since it expected a string.

Node.js will generate and throw TypeError instances immediately as a form of argument validation.

Exceptions vs. errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error, all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer.

OpenSSL errors#

Errors originating in crypto or tls are of class Error, and in addition to the standard .code and .message properties, may have some additional OpenSSL-specific properties.

error.opensslErrorStack#

An array of errors that can give context to where in the OpenSSL library an error originates from.

error.function#

The OpenSSL function the error originates in.

error.library#

The OpenSSL library the error originates in.

error.reason#

A human-readable string describing the reason for the error.

Node.js error codes#

ABORT_ERR#

Added in: v15.0.0

Used when an operation has been aborted (typically using an AbortController).

APIs not using AbortSignals typically do not raise an error with this code.

This code does not use the regular ERR_* convention Node.js errors use in order to be compatible with the web platform's AbortError.

ERR_AMBIGUOUS_ARGUMENT#

A function argument is being used in a way that suggests that the function signature may be misunderstood. This is thrown by the node:assert module when the message parameter in assert.throws(block, message) matches the error message thrown by block because that usage suggests that the user believes message is the expected message rather than the message the AssertionError will display if block does not throw.

ERR_ARG_NOT_ITERABLE#

An iterable argument (i.e. a value that works with for...of loops) was required, but not provided to a Node.js API.

ERR_ASSERTION#

A special type of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the node:assert module.

ERR_ASYNC_CALLBACK#

An attempt was made to register something that is not a function as an AsyncHooks callback.

ERR_ASYNC_TYPE#

The type of an asynchronous resource was invalid. Users are also able to define their own types if using the public embedder API.

ERR_BROTLI_COMPRESSION_FAILED#

Data passed to a Brotli stream was not successfully compressed.

ERR_BROTLI_INVALID_PARAM#

An invalid parameter key was passed during construction of a Brotli stream.

ERR_BUFFER_CONTEXT_NOT_AVAILABLE#

An attempt was made to create a Node.js Buffer instance from addon or embedder code, while in a JS engine Context that is not associated with a Node.js instance. The data passed to the Buffer method will have been released by the time the method returns.

When encountering this error, a possible alternative to creating a Buffer instance is to create a normal Uint8Array, which only differs in the prototype of the resulting object. Uint8Arrays are generally accepted in all Node.js core APIs where Buffers are; they are available in all Contexts.

ERR_BUFFER_OUT_OF_BOUNDS#

An operation outside the bounds of a Buffer was attempted.

ERR_BUFFER_TOO_LARGE#

An attempt has been made to create a Buffer larger than the maximum allowed size.

ERR_CANNOT_WATCH_SIGINT#

Node.js was unable to watch for the SIGINT signal.

ERR_CHILD_CLOSED_BEFORE_REPLY#

A child process was closed before the parent received a reply.

ERR_CHILD_PROCESS_IPC_REQUIRED#

Used when a child process is being forked without specifying an IPC channel.

ERR_CHILD_PROCESS_STDIO_MAXBUFFER#

Used when the main process is trying to read data from the child process's STDERR/STDOUT, and the data's length is longer than the maxBuffer option.

ERR_CLOSED_MESSAGE_PORT#

There was an attempt to use a MessagePort instance in a closed state, usually after .close() has been called.

ERR_CONSOLE_WRITABLE_STREAM#

Console was instantiated without stdout stream, or Console has a non-writable stdout or stderr stream.

ERR_CONSTRUCT_CALL_INVALID#

A class constructor was called that is not callable.

ERR_CONSTRUCT_CALL_REQUIRED#

A constructor for a class was called without new.

ERR_CONTEXT_NOT_INITIALIZED#

The vm context passed into the API is not yet initialized. This could happen when an error occurs (and is caught) during the creation of the context, for example, when the allocation fails or the maximum call stack size is reached when the context is created.

ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED#

A client certificate engine was requested that is not supported by the version of OpenSSL being used.

ERR_CRYPTO_ECDH_INVALID_FORMAT#

An invalid value for the format argument was passed to the crypto.ECDH() class getPublicKey() method.

ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY#

An invalid value for the key argument has been passed to the crypto.ECDH() class computeSecret() method. It means that the public key lies outside of the elliptic curve.

ERR_CRYPTO_ENGINE_UNKNOWN#

An invalid crypto engine identifier was passed to require('node:crypto').setEngine().

ERR_CRYPTO_FIPS_FORCED#

The --force-fips command-line argument was used but there was an attempt to enable or disable FIPS mode in the node:crypto module.

ERR_CRYPTO_FIPS_UNAVAILABLE#

An attempt was made to enable or disable FIPS mode, but FIPS mode was not available.

ERR_CRYPTO_HASH_FINALIZED#

hash.digest() was called multiple times. The hash.digest() method must be called no more than one time per instance of a Hash object.

ERR_CRYPTO_HASH_UPDATE_FAILED#

hash.update() failed for any reason. This should rarely, if ever, happen.

ERR_CRYPTO_INCOMPATIBLE_KEY#

The given crypto keys are incompatible with the attempted operation.

ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS#

The selected public or private key encoding is incompatible with other options.

ERR_CRYPTO_INITIALIZATION_FAILED#

Added in: v15.0.0

Initialization of the crypto subsystem failed.

ERR_CRYPTO_INVALID_AUTH_TAG#

Added in: v15.0.0

An invalid authentication tag was provided.

ERR_CRYPTO_INVALID_COUNTER#

Added in: v15.0.0

An invalid counter was provided for a counter-mode cipher.

ERR_CRYPTO_INVALID_CURVE#

Added in: v15.0.0

An invalid elliptic-curve was provided.

ERR_CRYPTO_INVALID_DIGEST#

An invalid crypto digest algorithm was specified.

ERR_CRYPTO_INVALID_IV#

Added in: v15.0.0

An invalid initialization vector was provided.

ERR_CRYPTO_INVALID_JWK#

Added in: v15.0.0

An invalid JSON Web Key was provided.

ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE#

The given crypto key object's type is invalid for the attempted operation.

ERR_CRYPTO_INVALID_KEYLEN#

Added in: v15.0.0

An invalid key length was provided.

ERR_CRYPTO_INVALID_KEYPAIR#

Added in: v15.0.0

An invalid key pair was provided.

ERR_CRYPTO_INVALID_KEYTYPE#

Added in: v15.0.0

An invalid key type was provided.

ERR_CRYPTO_INVALID_MESSAGELEN#

Added in: v15.0.0

An invalid message length was provided.

ERR_CRYPTO_INVALID_SCRYPT_PARAMS#

Added in: v15.0.0

Invalid scrypt algorithm parameters were provided.

ERR_CRYPTO_INVALID_STATE#

A crypto method was used on an object that was in an invalid state. For instance, calling cipher.getAuthTag() before calling cipher.final().

ERR_CRYPTO_INVALID_TAG_LENGTH#

Added in: v15.0.0

An invalid authentication tag length was provided.

ERR_CRYPTO_JOB_INIT_FAILED#

Added in: v15.0.0

Initialization of an asynchronous crypto operation failed.

ERR_CRYPTO_JWK_UNSUPPORTED_CURVE#

Key's Elliptic Curve is not registered for use in the JSON Web Key Elliptic Curve Registry.

ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE#

Key's Asymmetric Key Type is not registered for use in the JSON Web Key Types Registry.

ERR_CRYPTO_OPERATION_FAILED#

Added in: v15.0.0

A crypto operation failed for an otherwise unspecified reason.

ERR_CRYPTO_PBKDF2_ERROR#

The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js.

ERR_CRYPTO_SCRYPT_INVALID_PARAMETER#

One or more crypto.scrypt() or crypto.scryptSync() parameters are outside their legal range.

ERR_CRYPTO_SCRYPT_NOT_SUPPORTED#

Node.js was compiled without scrypt support. Not possible with the official release binaries but can happen with custom builds, including distro builds.

ERR_CRYPTO_SIGN_KEY_REQUIRED#

A signing key was not provided to the sign.sign() method.

ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH#

crypto.timingSafeEqual() was called with Buffer, TypedArray, or DataView arguments of different lengths.

ERR_CRYPTO_UNKNOWN_CIPHER#

An unknown cipher was specified.

ERR_CRYPTO_UNKNOWN_DH_GROUP#

An unknown Diffie-Hellman group name was given. See crypto.getDiffieHellman() for a list of valid group names.

ERR_CRYPTO_UNSUPPORTED_OPERATION#

Added in: v15.0.0, v14.18.0

An attempt to invoke an unsupported crypto operation was made.

ERR_DEBUGGER_ERROR#

Added in: v16.4.0, v14.17.4

An error occurred with the debugger.

ERR_DEBUGGER_STARTUP_ERROR#

Added in: v16.4.0, v14.17.4

The debugger timed out waiting for the required host/port to be free.

ERR_DLOPEN_DISABLED#

Added in: v16.10.0, v14.19.0

Loading native addons has been disabled using --no-addons.

ERR_DLOPEN_FAILED#

Added in: v15.0.0

A call to process.dlopen() failed.

ERR_DIR_CLOSED#

The fs.Dir was previously closed.

ERR_DIR_CONCURRENT_OPERATION#

Added in: v14.3.0

A synchronous read or close call was attempted on an fs.Dir which has ongoing asynchronous operations.

ERR_DNS_SET_SERVERS_FAILED#

c-ares failed to set the DNS server.

ERR_DOMAIN_CALLBACK_NOT_AVAILABLE#

The node:domain module was not usable since it could not establish the required error handling hooks, because process.setUncaughtExceptionCaptureCallback() had been called at an earlier point in time.

ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE#

process.setUncaughtExceptionCaptureCallback() could not be called because the node:domain module has been loaded at an earlier point in time.

The stack trace is extended to include the point in time at which the node:domain module had been loaded.

ERR_DUPLICATE_STARTUP_SNAPSHOT_MAIN_FUNCTION#

v8.startupSnapshot.setDeserializeMainFunction() could not be called because it had already been called before.

ERR_ENCODING_INVALID_ENCODED_DATA#

Data provided to TextDecoder() API was invalid according to the encoding provided.

ERR_ENCODING_NOT_SUPPORTED#

Encoding provided to TextDecoder() API was not one of the WHATWG Supported Encodings.

ERR_EVAL_ESM_CANNOT_PRINT#

--print cannot be used with ESM input.

ERR_EVENT_RECURSION#

Thrown when an attempt is made to recursively dispatch an event on EventTarget.

ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE#

The JS execution context is not associated with a Node.js environment. This may occur when Node.js is used as an embedded library and some hooks for the JS engine are not set up properly.

ERR_FALSY_VALUE_REJECTION#

A Promise that was callbackified via util.callbackify() was rejected with a falsy value.

ERR_FEATURE_UNAVAILABLE_ON_PLATFORM#

Added in: v14.0.0

Used when a feature that is not available to the current platform which is running Node.js is used.

ERR_FS_CP_DIR_TO_NON_DIR#

An attempt was made to copy a directory to a non-directory (file, symlink, etc.) using fs.cp().

ERR_FS_CP_EEXIST#

An attempt was made to copy over a file that already existed with fs.cp(), with the force and errorOnExist set to true.

ERR_FS_CP_EINVAL#

When using fs.cp(), src or dest pointed to an invalid path.

ERR_HTTP_CONTENT_LENGTH_MISMATCH#

Response body size doesn't match with the specified content-length header value.

ERR_FS_CP_FIFO_PIPE#

An attempt was made to copy a named pipe with fs.cp().

ERR_FS_CP_NON_DIR_TO_DIR#

An attempt was made to copy a non-directory (file, symlink, etc.) to a directory using fs.cp().

ERR_FS_CP_SOCKET#

An attempt was made to copy to a socket with fs.cp().

ERR_FS_CP_SYMLINK_TO_SUBDIRECTORY#

When using fs.cp(), a symlink in dest pointed to a subdirectory of src.

ERR_FS_CP_UNKNOWN#

An attempt was made to copy to an unknown file type with fs.cp().

ERR_FS_EISDIR#

Path is a directory.

ERR_FS_FILE_TOO_LARGE#

An attempt has been made to read a file whose size is larger than the maximum allowed size for a Buffer.

ERR_FS_INVALID_SYMLINK_TYPE#

An invalid symlink type was passed to the fs.symlink() or fs.symlinkSync() methods.

ERR_HTTP_HEADERS_SENT#

An attempt was made to add more headers after the headers had already been sent.

ERR_HTTP_INVALID_HEADER_VALUE#

An invalid HTTP header value was specified.

ERR_HTTP_INVALID_STATUS_CODE#

Status code was outside the regular status code range (100-999).

ERR_HTTP_REQUEST_TIMEOUT#

The client has not sent the entire request within the allowed time.

ERR_HTTP_SOCKET_ENCODING#

Changing the socket encoding is not allowed per RFC 7230 Section 3.

ERR_HTTP_TRAILER_INVALID#

The Trailer header was set even though the transfer encoding does not support that.

ERR_HTTP2_ALTSVC_INVALID_ORIGIN#

HTTP/2 ALTSVC frames require a valid origin.

ERR_HTTP2_ALTSVC_LENGTH#

HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes.

ERR_HTTP2_CONNECT_AUTHORITY#

For HTTP/2 requests using the CONNECT method, the :authority pseudo-header is required.

ERR_HTTP2_CONNECT_PATH#

For HTTP/2 requests using the CONNECT method, the :path pseudo-header is forbidden.

ERR_HTTP2_CONNECT_SCHEME#

For HTTP/2 requests using the CONNECT method, the :scheme pseudo-header is forbidden.

ERR_HTTP2_ERROR#

A non-specific HTTP/2 error has occurred.

ERR_HTTP2_GOAWAY_SESSION#

New HTTP/2 Streams may not be opened after the Http2Session has received a GOAWAY frame from the connected peer.

ERR_HTTP2_HEADER_SINGLE_VALUE#

Multiple values were provided for an HTTP/2 header field that was required to have only a single value.

ERR_HTTP2_HEADERS_AFTER_RESPOND#

An additional headers was specified after an HTTP/2 response was initiated.

ERR_HTTP2_HEADERS_SENT#

An attempt was made to send multiple response headers.

ERR_HTTP2_INFO_STATUS_NOT_ALLOWED#

Informational HTTP status codes (1xx) may not be set as the response status code on HTTP/2 responses.

ERR_HTTP2_INVALID_CONNECTION_HEADERS#

HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses.

ERR_HTTP2_INVALID_HEADER_VALUE#

An invalid HTTP/2 header value was specified.

ERR_HTTP2_INVALID_INFO_STATUS#

An invalid HTTP informational status code has been specified. Informational status codes must be an integer between 100 and 199 (inclusive).

ERR_HTTP2_INVALID_ORIGIN#

HTTP/2 ORIGIN frames require a valid origin.

ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH#

Input Buffer and Uint8Array instances passed to the http2.getUnpackedSettings() API must have a length that is a multiple of six.

ERR_HTTP2_INVALID_PSEUDOHEADER#

Only valid HTTP/2 pseudoheaders (:status, :path, :authority, :scheme, and :method) may be used.

ERR_HTTP2_INVALID_SESSION#

An action was performed on an Http2Session object that had already been destroyed.

ERR_HTTP2_INVALID_SETTING_VALUE#

An invalid value has been specified for an HTTP/2 setting.

ERR_HTTP2_INVALID_STREAM#

An operation was performed on a stream that had already been destroyed.

ERR_HTTP2_MAX_PENDING_SETTINGS_ACK#

Whenever an HTTP/2 SETTINGS frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new SETTINGS. By default, a maximum number of unacknowledged SETTINGS frames may be sent at any given time. This error code is used when that limit has been reached.

ERR_HTTP2_NESTED_PUSH#

An attempt was made to initiate a new push stream from within a push stream. Nested push streams are not permitted.

ERR_HTTP2_NO_MEM#

Out of memory when using the http2session.setLocalWindowSize(windowSize) API.

ERR_HTTP2_NO_SOCKET_MANIPULATION#

An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an Http2Session.

ERR_HTTP2_ORIGIN_LENGTH#

HTTP/2 ORIGIN frames are limited to a length of 16382 bytes.

ERR_HTTP2_OUT_OF_STREAMS#

The number of streams created on a single HTTP/2 session reached the maximum limit.

ERR_HTTP2_PAYLOAD_FORBIDDEN#

A message payload was specified for an HTTP response code for which a payload is forbidden.

ERR_HTTP2_PING_CANCEL#

An HTTP/2 ping was canceled.

ERR_HTTP2_PING_LENGTH#

HTTP/2 ping payloads must be exactly 8 bytes in length.

ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED#

An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header key names that begin with the : prefix.

ERR_HTTP2_PUSH_DISABLED#

An attempt was made to create a push stream, which had been disabled by the client.

ERR_HTTP2_SEND_FILE#

An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send a directory.

ERR_HTTP2_SEND_FILE_NOSEEK#

An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send something other than a regular file, but offset or length options were provided.

ERR_HTTP2_SESSION_ERROR#

The Http2Session closed with a non-zero error code.

ERR_HTTP2_SETTINGS_CANCEL#

The Http2Session settings canceled.

ERR_HTTP2_SOCKET_BOUND#

An attempt was made to connect a Http2Session object to a net.Socket or tls.TLSSocket that had already been bound to another Http2Session object.

ERR_HTTP2_SOCKET_UNBOUND#

An attempt was made to use the socket property of an Http2Session that has already been closed.

ERR_HTTP2_STATUS_101#

Use of the 101 Informational status code is forbidden in HTTP/2.

ERR_HTTP2_STATUS_INVALID#

An invalid HTTP status code has been specified. Status codes must be an integer between 100 and 599 (inclusive).

ERR_HTTP2_STREAM_CANCEL#

An Http2Stream was destroyed before any data was transmitted to the connected peer.

ERR_HTTP2_STREAM_ERROR#

A non-zero error code was been specified in an RST_STREAM frame.

ERR_HTTP2_STREAM_SELF_DEPENDENCY#

When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself.

ERR_HTTP2_TOO_MANY_INVALID_FRAMES#

The limit of acceptable invalid HTTP/2 protocol frames sent by the peer, as specified through the maxSessionInvalidFrames option, has been exceeded.

ERR_HTTP2_TRAILERS_ALREADY_SENT#

Trailing headers have already been sent on the Http2Stream.

ERR_HTTP2_TRAILERS_NOT_READY#

The http2stream.sendTrailers() method cannot be called until after the 'wantTrailers' event is emitted on an Http2Stream object. The 'wantTrailers' event will only be emitted if the waitForTrailers option is set for the Http2Stream.

ERR_HTTP2_UNSUPPORTED_PROTOCOL#

http2.connect() was passed a URL that uses any protocol other than http: or https:.

ERR_ILLEGAL_CONSTRUCTOR#

An attempt was made to construct an object using a non-public constructor.

ERR_IMPORT_ASSERTION_TYPE_FAILED#

Added in: v17.1.0, v16.14.0

An import assertion has failed, preventing the specified module to be imported.

ERR_IMPORT_ASSERTION_TYPE_MISSING#

Added in: v17.1.0, v16.14.0

An import assertion is missing, preventing the specified module to be imported.

ERR_IMPORT_ASSERTION_TYPE_UNSUPPORTED#

Added in: v17.1.0, v16.14.0

An import assertion is not supported by this version of Node.js.

ERR_INCOMPATIBLE_OPTION_PAIR#

An option pair is incompatible with each other and cannot be used at the same time.

ERR_INPUT_TYPE_NOT_ALLOWED#

Stability: 1 - Experimental

The --input-type flag was used to attempt to execute a file. This flag can only be used with input via --eval, --print, or STDIN.

ERR_INSPECTOR_ALREADY_ACTIVATED#

While using the node:inspector module, an attempt was made to activate the inspector when it already started to listen on a port. Use inspector.close() before activating it on a different address.

ERR_INSPECTOR_ALREADY_CONNECTED#

While using the node:inspector module, an attempt was made to connect when the inspector was already connected.

ERR_INSPECTOR_CLOSED#

While using the node:inspector module, an attempt was made to use the inspector after the session had already closed.

ERR_INSPECTOR_COMMAND#

An error occurred while issuing a command via the node:inspector module.

ERR_INSPECTOR_NOT_ACTIVE#

The inspector is not active when inspector.waitForDebugger() is called.

ERR_INSPECTOR_NOT_AVAILABLE#

The node:inspector module is not available for use.

ERR_INSPECTOR_NOT_CONNECTED#

While using the node:inspector module, an attempt was made to use the inspector before it was connected.

ERR_INSPECTOR_NOT_WORKER#

An API was called on the main thread that can only be used from the worker thread.

ERR_INTERNAL_ASSERTION#

There was a bug in Node.js or incorrect usage of Node.js internals. To fix the error, open an issue at https://github.com/nodejs/node/issues.

ERR_INVALID_ADDRESS_FAMILY#

The provided address family is not understood by the Node.js API.

ERR_INVALID_ARG_TYPE#

An argument of the wrong type was passed to a Node.js API.

ERR_INVALID_ARG_VALUE#

An invalid or unsupported value was passed for a given argument.

ERR_INVALID_ASYNC_ID#

An invalid asyncId or triggerAsyncId was passed using AsyncHooks. An id less than -1 should never happen.

ERR_INVALID_BUFFER_SIZE#

A swap was performed on a Buffer but its size was not compatible with the operation.

ERR_INVALID_CHAR#

Invalid characters were detected in headers.

ERR_INVALID_CURSOR_POS#

A cursor on a given stream cannot be moved to a specified row without a specified column.

ERR_INVALID_FD#

A file descriptor ('fd') was not valid (e.g. it was a negative value).

ERR_INVALID_FD_TYPE#

A file descriptor ('fd') type was not valid.

ERR_INVALID_FILE_URL_HOST#

A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible host. This situation can only occur on Unix-like systems where only localhost or an empty host is supported.

ERR_INVALID_FILE_URL_PATH#

A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible path. The exact semantics for determining whether a path can be used is platform-dependent.

ERR_INVALID_HANDLE_TYPE#

An attempt was made to send an unsupported "handle" over an IPC communication channel to a child process. See subprocess.send() and process.send() for more information.

ERR_INVALID_HTTP_TOKEN#

An invalid HTTP token was supplied.

ERR_INVALID_IP_ADDRESS#

An IP address is not valid.

ERR_INVALID_MIME_SYNTAX#

The syntax of a MIME is not valid.

ERR_INVALID_MODULE#

Added in: v15.0.0, v14.18.0

An attempt was made to load a module that does not exist or was otherwise not valid.

ERR_INVALID_MODULE_SPECIFIER#

The imported module string is an invalid URL, package name, or package subpath specifier.

ERR_INVALID_OBJECT_DEFINE_PROPERTY#

An error occurred while setting an invalid attribute on the property of an object.

ERR_INVALID_PACKAGE_CONFIG#

An invalid package.json file failed parsing.

ERR_INVALID_PACKAGE_TARGET#

The package.json "exports" field contains an invalid target mapping value for the attempted module resolution.

ERR_INVALID_PERFORMANCE_MARK#

While using the Performance Timing API (perf_hooks), a performance mark is invalid.

ERR_INVALID_PROTOCOL#

An invalid options.protocol was passed to http.request().

ERR_INVALID_REPL_EVAL_CONFIG#

Both breakEvalOnSigint and eval options were set in the REPL config, which is not supported.

ERR_INVALID_REPL_INPUT#

The input may not be used in the REPL. The conditions under which this error is used are described in the REPL documentation.

ERR_INVALID_RETURN_PROPERTY#

Thrown in case a function option does not provide a valid value for one of its returned object properties on execution.

ERR_INVALID_RETURN_PROPERTY_VALUE#

Thrown in case a function option does not provide an expected value type for one of its returned object properties on execution.

ERR_INVALID_RETURN_VALUE#

Thrown in case a function option does not return an expected value type on execution, such as when a function is expected to return a promise.

ERR_INVALID_STATE#

Added in: v15.0.0

Indicates that an operation cannot be completed due to an invalid state. For instance, an object may have already been destroyed, or may be performing another operation.

ERR_INVALID_SYNC_FORK_INPUT#

A Buffer, TypedArray, DataView, or string was provided as stdio input to an asynchronous fork. See the documentation for the child_process module for more information.

ERR_INVALID_THIS#

A Node.js API function was called with an incompatible this value.

const urlSearchParams = new URLSearchParams('foo=bar&baz=new');

const buf = Buffer.alloc(1);
urlSearchParams.has.call(buf, 'foo');
// Throws a TypeError with code 'ERR_INVALID_THIS'

ERR_INVALID_TRANSFER_OBJECT#

An invalid transfer object was passed to postMessage().

ERR_INVALID_TUPLE#

An element in the iterable provided to the WHATWG URLSearchParams constructor did not represent a [name, value] tuple – that is, if an element is not iterable, or does not consist of exactly two elements.

ERR_INVALID_URI#

An invalid URI was passed.

ERR_INVALID_URL#

An invalid URL was passed to the WHATWG URL constructor or the legacy url.parse() to be parsed. The thrown error object typically has an additional property 'input' that contains the URL that failed to parse.

ERR_INVALID_URL_SCHEME#

An attempt was made to use a URL of an incompatible scheme (protocol) for a specific purpose. It is only used in the WHATWG URL API support in the fs module (which only accepts URLs with 'file' scheme), but may be used in other Node.js APIs as well in the future.

ERR_IPC_CHANNEL_CLOSED#

An attempt was made to use an IPC communication channel that was already closed.

ERR_IPC_DISCONNECTED#

An attempt was made to disconnect an IPC communication channel that was already disconnected. See the documentation for the child_process module for more information.

ERR_IPC_ONE_PIPE#

An attempt was made to create a child Node.js process using more than one IPC communication channel. See the documentation for the child_process module for more information.

ERR_IPC_SYNC_FORK#

An attempt was made to open an IPC communication channel with a synchronously forked Node.js process. See the documentation for the child_process module for more information.

ERR_LOADER_CHAIN_INCOMPLETE#

Added in: v18.6.0, v16.17.0

An ESM loader hook returned without calling next() and without explicitly signaling a short circuit.

ERR_MANIFEST_ASSERT_INTEGRITY#

An attempt was made to load a resource, but the resource did not match the integrity defined by the policy manifest. See the documentation for policy manifests for more information.

ERR_MANIFEST_DEPENDENCY_MISSING#

An attempt was made to load a resource, but the resource was not listed as a dependency from the location that attempted to load it. See the documentation for policy manifests for more information.

ERR_MANIFEST_INTEGRITY_MISMATCH#

An attempt was made to load a policy manifest, but the manifest had multiple entries for a resource which did not match each other. Update the manifest entries to match in order to resolve this error. See the documentation for policy manifests for more information.

ERR_MANIFEST_INVALID_RESOURCE_FIELD#

A policy manifest resource had an invalid value for one of its fields. Update the manifest entry to match in order to resolve this error. See the documentation for policy manifests for more information.

ERR_MANIFEST_INVALID_SPECIFIER#

A policy manifest resource had an invalid value for one of its dependency mappings. Update the manifest entry to match to resolve this error. See the documentation for policy manifests for more information.

ERR_MANIFEST_PARSE_POLICY#

An attempt was made to load a policy manifest, but the manifest was unable to be parsed. See the documentation for policy manifests for more information.

ERR_MANIFEST_TDZ#

An attempt was made to read from a policy manifest, but the manifest initialization has not yet taken place. This is likely a bug in Node.js.

ERR_MANIFEST_UNKNOWN_ONERROR#

A policy manifest was loaded, but had an unknown value for its "onerror" behavior. See the documentation for policy manifests for more information.

ERR_MEMORY_ALLOCATION_FAILED#

An attempt was made to allocate memory (usually in the C++ layer) but it failed.

ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE#

Added in: v14.5.0, v12.19.0

A message posted to a MessagePort could not be deserialized in the target vm Context. Not all Node.js objects can be successfully instantiated in any context at this time, and attempting to transfer them using postMessage() can fail on the receiving side in that case.

ERR_METHOD_NOT_IMPLEMENTED#

A method is required but not implemented.

ERR_MISSING_ARGS#

A required argument of a Node.js API was not passed. This is only used for strict compliance with the API specification (which in some cases may accept func(undefined) but not func()). In most native Node.js APIs, func(undefined) and func() are treated identically, and the ERR_INVALID_ARG_TYPE error code may be used instead.

ERR_MISSING_OPTION#

For APIs that accept options objects, some options might be mandatory. This code is thrown if a required option is missing.

ERR_MISSING_PASSPHRASE#

An attempt was made to read an encrypted key without specifying a passphrase.

ERR_MISSING_PLATFORM_FOR_WORKER#

The V8 platform used by this instance of Node.js does not support creating Workers. This is caused by lack of embedder support for Workers. In particular, this error will not occur with standard builds of Node.js.

ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST#

Added in: v15.0.0

An object that needs to be explicitly listed in the transferList argument is in the object passed to a postMessage() call, but is not provided in the transferList for that call. Usually, this is a MessagePort.

In Node.js versions prior to v15.0.0, the error code being used here was ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST. However, the set of transferable object types has been expanded to cover more types than MessagePort.

ERR_MODULE_NOT_FOUND#

A module file could not be resolved by the ECMAScript modules loader while attempting an import operation or when loading the program entry point.

ERR_MULTIPLE_CALLBACK#

A callback was called more than once.

A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once.

ERR_NAPI_CONS_FUNCTION#

While using Node-API, a constructor passed was not a function.

ERR_NAPI_INVALID_DATAVIEW_ARGS#

While calling napi_create_dataview(), a given offset was outside the bounds of the dataview or offset + length was larger than a length of given buffer.

ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT#

While calling napi_create_typedarray(), the provided offset was not a multiple of the element size.

ERR_NAPI_INVALID_TYPEDARRAY_LENGTH#

While calling napi_create_typedarray(), (length * size_of_element) + byte_offset was larger than the length of given buffer.

ERR_NAPI_TSFN_CALL_JS#

An error occurred while invoking the JavaScript portion of the thread-safe function.

ERR_NAPI_TSFN_GET_UNDEFINED#

An error occurred while attempting to retrieve the JavaScript undefined value.

ERR_NAPI_TSFN_START_IDLE_LOOP#

On the main thread, values are removed from the queue associated with the thread-safe function in an idle loop. This error indicates that an error has occurred when attempting to start the loop.

ERR_NAPI_TSFN_STOP_IDLE_LOOP#

Once no more items are left in the queue, the idle loop must be suspended. This error indicates that the idle loop has failed to stop.

ERR_NOT_BUILDING_SNAPSHOT#

An attempt was made to use operations that can only be used when building V8 startup snapshot even though Node.js isn't building one.

ERR_NO_CRYPTO#

An attempt was made to use crypto features while Node.js was not compiled with OpenSSL crypto support.

ERR_NO_ICU#

An attempt was made to use features that require ICU, but Node.js was not compiled with ICU support.

ERR_NON_CONTEXT_AWARE_DISABLED#

A non-context-aware native addon was loaded in a process that disallows them.

ERR_OUT_OF_RANGE#

A given value is out of the accepted range.

ERR_PACKAGE_IMPORT_NOT_DEFINED#

The package.json "imports" field does not define the given internal package specifier mapping.

ERR_PACKAGE_PATH_NOT_EXPORTED#

The package.json "exports" field does not export the requested subpath. Because exports are encapsulated, private internal modules that are not exported cannot be imported through the package resolution, unless using an absolute URL.

ERR_PARSE_ARGS_INVALID_OPTION_VALUE#

Added in: v18.3.0, v16.17.0

When strict set to true, thrown by util.parseArgs() if a <boolean> value is provided for an option of type <string>, or if a <string> value is provided for an option of type <boolean>.

ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL#

Added in: v18.3.0, v16.17.0

Thrown by util.parseArgs(), when a positional argument is provided and allowPositionals is set to false.

ERR_PARSE_ARGS_UNKNOWN_OPTION#

Added in: v18.3.0, v16.17.0

When strict set to true, thrown by util.parseArgs() if an argument is not configured in options.

ERR_PERFORMANCE_INVALID_TIMESTAMP#

An invalid timestamp value was provided for a performance mark or measure.

ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS#

Invalid options were provided for a performance measure.

ERR_PROTO_ACCESS#

Accessing Object.prototype.__proto__ has been forbidden using --disable-proto=throw. Object.getPrototypeOf and Object.setPrototypeOf should be used to get and set the prototype of an object.

ERR_REQUIRE_ESM#

Stability: 1 - Experimental

An attempt was made to require() an ES Module.

ERR_SCRIPT_EXECUTION_INTERRUPTED#

Script execution was interrupted by SIGINT (For example, Ctrl+C was pressed.)

ERR_SCRIPT_EXECUTION_TIMEOUT#

Script execution timed out, possibly due to bugs in the script being executed.

ERR_SERVER_ALREADY_LISTEN#

The server.listen() method was called while a net.Server was already listening. This applies to all instances of net.Server, including HTTP, HTTPS, and HTTP/2 Server instances.

ERR_SERVER_NOT_RUNNING#

The server.close() method was called when a net.Server was not running. This applies to all instances of net.Server, including HTTP, HTTPS, and HTTP/2 Server instances.

ERR_SOCKET_ALREADY_BOUND#

An attempt was made to bind a socket that has already been bound.

ERR_SOCKET_BAD_BUFFER_SIZE#

An invalid (negative) size was passed for either the recvBufferSize or sendBufferSize options in dgram.createSocket().

ERR_SOCKET_BAD_PORT#

An API function expecting a port >= 0 and < 65536 received an invalid value.

ERR_SOCKET_BAD_TYPE#

An API function expecting a socket type (udp4 or udp6) received an invalid value.

ERR_SOCKET_BUFFER_SIZE#

While using dgram.createSocket(), the size of the receive or send Buffer could not be determined.

ERR_SOCKET_CLOSED#

An attempt was made to operate on an already closed socket.

ERR_SOCKET_CLOSED_BEFORE_CONNECTION#

When calling net.Socket.write() on a connecting socket and the socket was closed before the connection was established.

ERR_SOCKET_DGRAM_IS_CONNECTED#

A dgram.connect() call was made on an already connected socket.

ERR_SOCKET_DGRAM_NOT_CONNECTED#

A dgram.disconnect() or dgram.remoteAddress() call was made on a disconnected socket.

ERR_SOCKET_DGRAM_NOT_RUNNING#

A call was made and the UDP subsystem was not running.

ERR_SRI_PARSE#

A string was provided for a Subresource Integrity check, but was unable to be parsed. Check the format of integrity attributes by looking at the Subresource Integrity specification.

ERR_STREAM_ALREADY_FINISHED#

A stream method was called that cannot complete because the stream was finished.

ERR_STREAM_CANNOT_PIPE#

An attempt was made to call stream.pipe() on a Writable stream.

ERR_STREAM_DESTROYED#

A stream method was called that cannot complete because the stream was destroyed using stream.destroy().

ERR_STREAM_NULL_VALUES#

An attempt was made to call stream.write() with a null chunk.

ERR_STREAM_PREMATURE_CLOSE#

An error returned by stream.finished() and stream.pipeline(), when a stream or a pipeline ends non gracefully with no explicit error.

ERR_STREAM_PUSH_AFTER_EOF#

An attempt was made to call stream.push() after a null(EOF) had been pushed to the stream.

ERR_STREAM_UNSHIFT_AFTER_END_EVENT#

An attempt was made to call stream.unshift() after the 'end' event was emitted.

ERR_STREAM_WRAP#

Prevents an abort if a string decoder was set on the Socket or if the decoder is in objectMode.

const Socket = require('node:net').Socket;
const instance = new Socket();

instance.setEncoding('utf8');

ERR_STREAM_WRITE_AFTER_END#

An attempt was made to call stream.write() after stream.end() has been called.

ERR_STRING_TOO_LONG#

An attempt has been made to create a string longer than the maximum allowed length.

ERR_SYNTHETIC#

An artificial error object used to capture the call stack for diagnostic reports.

ERR_SYSTEM_ERROR#

An unspecified or non-specific system error has occurred within the Node.js process. The error object will have an err.info object property with additional details.

ERR_TAP_LEXER_ERROR#

An error representing a failing lexer state.

ERR_TAP_PARSER_ERROR#

An error representing a failing parser state. Additional information about the token causing the error is available via the cause property.

ERR_TAP_VALIDATION_ERROR#

This error represents a failed TAP validation.

ERR_TEST_FAILURE#

This error represents a failed test. Additional information about the failure is available via the cause property. The failureType property specifies what the test was doing when the failure occurred.

ERR_TLS_CERT_ALTNAME_FORMAT#

This error is thrown by checkServerIdentity if a user-supplied subjectaltname property violates encoding rules. Certificate objects produced by Node.js itself always comply with encoding rules and will never cause this error.

ERR_TLS_CERT_ALTNAME_INVALID#

While using TLS, the host name/IP of the peer did not match any of the subjectAltNames in its certificate.

ERR_TLS_DH_PARAM_SIZE#

While using TLS, the parameter offered for the Diffie-Hellman (DH) key-agreement protocol is too small. By default, the key length must be greater than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly recommended to use 2048 bits or larger for stronger security.

ERR_TLS_HANDSHAKE_TIMEOUT#

A TLS/SSL handshake timed out. In this case, the server must also abort the connection.

ERR_TLS_INVALID_CONTEXT#

Added in: v13.3.0

The context must be a SecureContext.

ERR_TLS_INVALID_PROTOCOL_METHOD#

The specified secureProtocol method is invalid. It is either unknown, or disabled because it is insecure.

ERR_TLS_INVALID_PROTOCOL_VERSION#

Valid TLS protocol versions are 'TLSv1', 'TLSv1.1', or 'TLSv1.2'.

ERR_TLS_INVALID_STATE#

Added in: v13.10.0, v12.17.0

The TLS socket must be connected and securely established. Ensure the 'secure' event is emitted before continuing.

ERR_TLS_PROTOCOL_VERSION_CONFLICT#

Attempting to set a TLS protocol minVersion or maxVersion conflicts with an attempt to set the secureProtocol explicitly. Use one mechanism or the other.

ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED#

Failed to set PSK identity hint. Hint may be too long.

ERR_TLS_RENEGOTIATION_DISABLED#

An attempt was made to renegotiate TLS on a socket instance with renegotiation disabled.

ERR_TLS_REQUIRED_SERVER_NAME#

While using TLS, the server.addContext() method was called without providing a host name in the first parameter.

ERR_TLS_SESSION_ATTACK#

An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks.

ERR_TLS_SNI_FROM_SERVER#

An attempt was made to issue Server Name Indication from a TLS server-side socket, which is only valid from a client.

ERR_TRACE_EVENTS_CATEGORY_REQUIRED#

The trace_events.createTracing() method requires at least one trace event category.

ERR_TRACE_EVENTS_UNAVAILABLE#

The node:trace_events module could not be loaded because Node.js was compiled with the --without-v8-platform flag.

ERR_TRANSFORM_ALREADY_TRANSFORMING#

A Transform stream finished while it was still transforming.

ERR_TRANSFORM_WITH_LENGTH_0#

A Transform stream finished with data still in the write buffer.

ERR_TTY_INIT_FAILED#

The initialization of a TTY failed due to a system error.

ERR_UNAVAILABLE_DURING_EXIT#

Function was called within a process.on('exit') handler that shouldn't be called within process.on('exit') handler.

ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET#

process.setUncaughtExceptionCaptureCallback() was called twice, without first resetting the callback to null.

This error is designed to prevent accidentally overwriting a callback registered from another module.

ERR_UNESCAPED_CHARACTERS#

A string that contained unescaped characters was received.

ERR_UNHANDLED_ERROR#

An unhandled error occurred (for instance, when an 'error' event is emitted by an EventEmitter but an 'error' handler is not registered).

ERR_UNKNOWN_BUILTIN_MODULE#

Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.

ERR_UNKNOWN_CREDENTIAL#

A Unix group or user identifier that does not exist was passed.

ERR_UNKNOWN_ENCODING#

An invalid or unknown encoding option was passed to an API.

ERR_UNKNOWN_FILE_EXTENSION#

Stability: 1 - Experimental

An attempt was made to load a module with an unknown or unsupported file extension.

ERR_UNKNOWN_MODULE_FORMAT#

Stability: 1 - Experimental

An attempt was made to load a module with an unknown or unsupported format.

ERR_UNKNOWN_SIGNAL#

An invalid or unknown process signal was passed to an API expecting a valid signal (such as subprocess.kill()).

ERR_UNSUPPORTED_DIR_IMPORT#

import a directory URL is unsupported. Instead, self-reference a package using its name and define a custom subpath in the "exports" field of the package.json file.

import './'; // unsupported
import './index.js'; // supported
import 'package-name'; // supported

ERR_UNSUPPORTED_ESM_URL_SCHEME#

import with URL schemes other than file and data is unsupported.

ERR_USE_AFTER_CLOSE#

Stability: 1 - Experimental

An attempt was made to use something that was already closed.

ERR_VALID_PERFORMANCE_ENTRY_TYPE#

While using the Performance Timing API (perf_hooks), no valid performance entry types are found.

ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING#

A dynamic import callback was not specified.

ERR_VM_MODULE_ALREADY_LINKED#

The module attempted to be linked is not eligible for linking, because of one of the following reasons:

  • It has already been linked (linkingStatus is 'linked')
  • It is being linked (linkingStatus is 'linking')
  • Linking has failed for this module (linkingStatus is 'errored')

ERR_VM_MODULE_CACHED_DATA_REJECTED#

The cachedData option passed to a module constructor is invalid.

ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA#

Cached data cannot be created for modules which have already been evaluated.

ERR_VM_MODULE_DIFFERENT_CONTEXT#

The module being returned from the linker function is from a different context than the parent module. Linked modules must share the same context.

ERR_VM_MODULE_LINK_FAILURE#

The module was unable to be linked due to a failure.

ERR_VM_MODULE_NOT_MODULE#

The fulfilled value of a linking promise is not a vm.Module object.

ERR_VM_MODULE_STATUS#

The current module's status does not allow for this operation. The specific meaning of the error depends on the specific function.

ERR_WASI_ALREADY_STARTED#

The WASI instance has already started.

ERR_WASI_NOT_STARTED#

The WASI instance has not been started.

ERR_WEBASSEMBLY_RESPONSE#

Added in: v18.1.0

The Response that has been passed to WebAssembly.compileStreaming or to WebAssembly.instantiateStreaming is not a valid WebAssembly response.

ERR_WORKER_INIT_FAILED#

The Worker initialization failed.

ERR_WORKER_INVALID_EXEC_ARGV#

The execArgv option passed to the Worker constructor contains invalid flags.

ERR_WORKER_NOT_RUNNING#

An operation failed because the Worker instance is not currently running.

ERR_WORKER_OUT_OF_MEMORY#

The Worker instance terminated because it reached its memory limit.

ERR_WORKER_PATH#

The path for the main script of a worker is neither an absolute path nor a relative path starting with ./ or ../.

ERR_WORKER_UNSERIALIZABLE_ERROR#

All attempts at serializing an uncaught exception from a worker thread failed.

ERR_WORKER_UNSUPPORTED_OPERATION#

The requested functionality is not supported in worker threads.

ERR_ZLIB_INITIALIZATION_FAILED#

Creation of a zlib object failed due to incorrect configuration.

HPE_HEADER_OVERFLOW#

History
VersionChanges
v11.4.0, v10.15.0

Max header size in http_parser was set to 8 KiB.

Too much HTTP header data was received. In order to protect against malicious or malconfigured clients, if more than 8 KiB of HTTP header data is received then HTTP parsing will abort without a request or response object being created, and an Error with this code will be emitted.

HPE_UNEXPECTED_CONTENT_LENGTH#

Server is sending both a Content-Length header and Transfer-Encoding: chunked.

Transfer-Encoding: chunked allows the server to maintain an HTTP persistent connection for dynamically generated content. In this case, the Content-Length HTTP header cannot be used.

Use Content-Length or Transfer-Encoding: chunked.

MODULE_NOT_FOUND#

History
VersionChanges
v12.0.0

Added requireStack property.

A module file could not be resolved by the CommonJS modules loader while attempting a require() operation or when loading the program entry point.

Legacy Node.js error codes#

Stability: 0 - Deprecated. These error codes are either inconsistent, or have been removed.

ERR_CANNOT_TRANSFER_OBJECT#

The value passed to postMessage() contained an object that is not supported for transferring.

ERR_CRYPTO_HASH_DIGEST_NO_UTF16#

Added in: v9.0.0Removed in: v12.12.0

The UTF-16 encoding was used with hash.digest(). While the hash.digest() method does allow an encoding argument to be passed in, causing the method to return a string rather than a Buffer, the UTF-16 encoding (e.g. ucs or utf16le) is not supported.

ERR_HTTP2_FRAME_ERROR#

Added in: v9.0.0Removed in: v10.0.0

Used when a failure occurs sending an individual frame on the HTTP/2 session.

ERR_HTTP2_HEADERS_OBJECT#

Added in: v9.0.0Removed in: v10.0.0

Used when an HTTP/2 Headers Object is expected.

ERR_HTTP2_HEADER_REQUIRED#

Added in: v9.0.0Removed in: v10.0.0

Used when a required header is missing in an HTTP/2 message.

ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND#

Added in: v9.0.0Removed in: v10.0.0

HTTP/2 informational headers must only be sent prior to calling the Http2Stream.prototype.respond() method.

ERR_HTTP2_STREAM_CLOSED#

Added in: v9.0.0Removed in: v10.0.0

Used when an action has been performed on an HTTP/2 Stream that has already been closed.

ERR_HTTP_INVALID_CHAR#

Added in: v9.0.0Removed in: v10.0.0

Used when an invalid character is found in an HTTP response status message (reason phrase).

ERR_INDEX_OUT_OF_RANGE#

Added in: v10.0.0Removed in: v11.0.0

A given index was out of the accepted range (e.g. negative offsets).

ERR_INVALID_OPT_VALUE#

Added in: v8.0.0Removed in: v15.0.0

An invalid or unexpected value was passed in an options object.

ERR_INVALID_OPT_VALUE_ENCODING#

Added in: v9.0.0Removed in: v15.0.0

An invalid or unknown file encoding was passed.

ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST#

Removed in: v15.0.0

This error code was replaced by ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST in Node.js v15.0.0, because it is no longer accurate as other types of transferable objects also exist now.

ERR_NAPI_CONS_PROTOTYPE_OBJECT#

Added in: v9.0.0Removed in: v10.0.0

Used by the Node-API when Constructor.prototype is not an object.

ERR_NETWORK_IMPORT_BAD_RESPONSE#

Stability: 1 - Experimental

Response was received but was invalid when importing a module over the network.

ERR_NETWORK_IMPORT_DISALLOWED#

Stability: 1 - Experimental

A network module attempted to load another module that it is not allowed to load. Likely this restriction is for security reasons.

ERR_NO_LONGER_SUPPORTED#

A Node.js API was called in an unsupported manner, such as Buffer.write(string, encoding, offset[, length]).

ERR_OPERATION_FAILED#

Added in: v15.0.0

An operation failed. This is typically used to signal the general failure of an asynchronous operation.

ERR_OUTOFMEMORY#

Added in: v9.0.0Removed in: v10.0.0

Used generically to identify that an operation caused an out of memory condition.

ERR_PARSE_HISTORY_DATA#

Added in: v9.0.0Removed in: v10.0.0

The node:repl module was unable to parse data from the REPL history file.

ERR_SOCKET_CANNOT_SEND#

Added in: v9.0.0Removed in: v14.0.0

Data could not be sent on a socket.

ERR_STDERR_CLOSE#

History
VersionChanges
v10.12.0

Rather than emitting an error, process.stderr.end() now only closes the stream side but not the underlying resource, making this error obsolete.

v10.12.0

Removed in: v10.12.0

An attempt was made to close the process.stderr stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.

ERR_STDOUT_CLOSE#

History
VersionChanges
v10.12.0

Rather than emitting an error, process.stderr.end() now only closes the stream side but not the underlying resource, making this error obsolete.

v10.12.0

Removed in: v10.12.0

An attempt was made to close the process.stdout stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.

ERR_STREAM_READ_NOT_IMPLEMENTED#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a readable stream that has not implemented readable._read().

ERR_TLS_RENEGOTIATION_FAILED#

Added in: v9.0.0Removed in: v10.0.0

Used when a TLS renegotiation request has failed in a non-specific way.

ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER#

Added in: v10.5.0Removed in: v14.0.0

A SharedArrayBuffer whose memory is not managed by the JavaScript engine or by Node.js was encountered during serialization. Such a SharedArrayBuffer cannot be serialized.

This can only happen when native addons create SharedArrayBuffers in "externalized" mode, or put existing SharedArrayBuffer into externalized mode.

ERR_UNKNOWN_STDIN_TYPE#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown stdin file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

ERR_UNKNOWN_STREAM_TYPE#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown stdout or stderr file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

ERR_V8BREAKITERATOR#

The V8 BreakIterator API was used but the full ICU data set is not installed.

ERR_VALUE_OUT_OF_RANGE#

Added in: v9.0.0Removed in: v10.0.0

Used when a given value is out of the accepted range.

ERR_VM_MODULE_NOT_LINKED#

The module must be successfully linked before instantiation.

ERR_VM_MODULE_LINKING_ERRORED#

Added in: v10.0.0Removed in: v18.1.0, v16.17.0

The linker function returned a module for which linking has failed.

ERR_WORKER_UNSUPPORTED_EXTENSION#

Added in: v11.0.0Removed in: v16.9.0

The pathname used for the main script of a worker has an unknown file extension.

ERR_ZLIB_BINDING_CLOSED#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a zlib object after it has already been closed.

ERR_CPU_USAGE#

Removed in: v15.0.0

The native call from process.cpuUsage could not be processed.

Events#

Stability: 2 - Stable

Source Code: lib/events.js

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") emit named events that cause Function objects ("listeners") to be called.

For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and discarded.

The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event.

import { EventEmitter } from 'node:events';

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');const EventEmitter = require('node:events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');

Passing arguments and this to listeners#

The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');

Asynchronous vs. synchronous#

The EventEmitter calls all listeners synchronously in the order in which they were registered. This ensures the proper sequencing of events and helps avoid race conditions and logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

Handling events only once#

When a listener is registered using the eventEmitter.on() method, that listener is invoked every time the named event is emitted.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2

Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignoredconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored

Error events#

When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.

If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.jsconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js

To guard against crashing the Node.js process the domain module can be used. (Note, however, that the node:domain module is deprecated.)

As a best practice, listeners should always be added for the 'error' events.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an errorconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

It is possible to monitor 'error' events without consuming the emitted error by installing a listener using the symbol events.errorMonitor.

import { EventEmitter, errorMonitor } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.jsconst { EventEmitter, errorMonitor } = require('node:events');

const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js

Capture rejections of promises#

Using async functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});const EventEmitter = require('node:events');
const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});

The captureRejections option in the EventEmitter constructor or the global setting change this behavior, installing a .then(undefined, handler) handler on the Promise. This handler routes the exception asynchronously to the Symbol.for('nodejs.rejection') method if there is one, or to 'error' event handler if there is none.

import { EventEmitter } from 'node:events';
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;const EventEmitter = require('node:events');
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;

Setting events.captureRejections = true will change the default for all new instances of EventEmitter.

import { EventEmitter } from 'node:events';

EventEmitter.captureRejections = true;
const ee1 = new EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);const events = require('node:events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

The 'error' events that are generated by the captureRejections behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use async functions as 'error' event handlers.

Class: EventEmitter#

History
VersionChanges
v13.4.0, v12.16.0

Added captureRejections option.

v0.1.26

Added in: v0.1.26

The EventEmitter class is defined and exposed by the node:events module:

import { EventEmitter } from 'node:events';const EventEmitter = require('node:events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

It supports the following option:

Event: 'newListener'#

Added in: v0.1.26

The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.

Listeners registered for the 'newListener' event are passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback are inserted before the listener that is in the process of being added.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   Aconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A

Event: 'removeListener'#

History
VersionChanges
v6.1.0, v4.7.0

For listeners attached using .once(), the listener argument now yields the original listener function.

v0.9.3

Added in: v0.9.3

The 'removeListener' event is emitted after the listener is removed.

emitter.addListener(eventName, listener)#

Added in: v0.1.26

Alias for emitter.on(eventName, listener).

emitter.emit(eventName[, ...args])#

Added in: v0.1.26

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listenerconst EventEmitter = require('node:events');
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener

emitter.eventNames()#

Added in: v6.0.0

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from 'node:events';

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]const EventEmitter = require('node:events');

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

emitter.getMaxListeners()#

Added in: v1.0.0

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to events.defaultMaxListeners.

emitter.listenerCount(eventName)#

Added in: v3.2.0

Returns the number of listeners listening to the event named eventName.

emitter.listeners(eventName)#

History
VersionChanges
v7.0.0

For listeners attached using .once() this returns the original listeners instead of wrapper functions now.

v0.1.26

Added in: v0.1.26

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

emitter.off(eventName, listener)#

Added in: v10.0.0

Alias for emitter.removeListener().

emitter.on(eventName, listener)#

Added in: v0.1.101

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   aconst EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.once(eventName, listener)#

Added in: v0.3.0

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   aconst EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.prependListener(eventName, listener)#

Added in: v6.0.0

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.prependOnceListener(eventName, listener)#

Added in: v6.0.0

Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeAllListeners([eventName])#

Added in: v0.1.26

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeListener(eventName, listener)#

Added in: v0.1.26

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events behave as expected.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   Aconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');const EventEmitter = require('node:events');
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.setMaxListeners(n)#

Added in: v0.3.5

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.rawListeners(eventName)#

Added in: v9.4.0

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');const EventEmitter = require('node:events');
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])#

History
VersionChanges
v17.4.0, v16.14.0

No longer experimental.

v13.4.0, v12.16.0

Added in: v13.4.0, v12.16.0

The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').

import { EventEmitter, captureRejectionSymbol } from 'node:events';

class MyClass extends EventEmitter {
  constructor() {
    super({ captureRejections: true });
  }

  [captureRejectionSymbol](err, event, ...args) {
    console.log('rejection happened for', event, 'with', err, ...args);
    this.destroy(err);
  }

  destroy(err) {
    // Tear the resource down here.
  }
}const { EventEmitter, captureRejectionSymbol } = require('node:events');

class MyClass extends EventEmitter {
  constructor() {
    super({ captureRejections: true });
  }

  [captureRejectionSymbol](err, event, ...args) {
    console.log('rejection happened for', event, 'with', err, ...args);
    this.destroy(err);
  }

  destroy(err) {
    // Tear the resource down here.
  }
}

events.defaultMaxListeners#

Added in: v0.11.2

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.

This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});const EventEmitter = require('node:events');
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command-line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

events.errorMonitor#

Added in: v13.6.0, v12.17.0

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

events.getEventListeners(emitterOrTarget, eventName)#

Added in: v15.2.0, v14.17.0

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  getEventListeners(ee, 'foo'); // [listener]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  getEventListeners(et, 'foo'); // [listener]
}const { getEventListeners, EventEmitter } = require('node:events');

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  getEventListeners(ee, 'foo'); // [listener]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  getEventListeners(et, 'foo'); // [listener]
}

events.once(emitter, name[, options])#

History
VersionChanges
v15.0.0

The signal option is supported now.

v11.13.0, v10.16.0

Added in: v11.13.0, v10.16.0

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special 'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}const { once, EventEmitter } = require('node:events');

async function run() {
  const ee = new EventEmitter();

  process.nextTick(() => {
    ee.emit('myevent', 42);
  });

  const [value] = await once(ee, 'myevent');
  console.log(value);

  const err = new Error('kaboom');
  process.nextTick(() => {
    ee.emit('error', err);
  });

  try {
    await once(ee, 'myevent');
  } catch (err) {
    console.error('error happened', err);
  }
}

run();

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boomconst { EventEmitter, once } = require('node:events');

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

An <AbortSignal> can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!const { EventEmitter, once } = require('node:events');

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!

Awaiting multiple events emitted on process.nextTick()#

There is an edge case worth noting when using the events.once() function to await multiple events emitted on in the same batch of process.nextTick() operations, or whenever multiple events are emitted synchronously. Specifically, because the process.nextTick() queue is drained before the Promise microtask queue, and because EventEmitter emits all events synchronously, it is possible for events.once() to miss an event.

import { EventEmitter, once } from 'node:events';
import process from 'node:process';

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

To catch both events, create each of the Promises before awaiting either of them, then it becomes possible to use Promise.all(), Promise.race(), or Promise.allSettled():

import { EventEmitter, once } from 'node:events';
import process from 'node:process';

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

events.captureRejections#

History
VersionChanges
v17.4.0, v16.14.0

No longer experimental.

v13.4.0, v12.16.0

Added in: v13.4.0, v12.16.0

Value: <boolean>

Change the default captureRejections option on all new EventEmitter objects.

events.captureRejectionSymbol#

History
VersionChanges
v17.4.0, v16.14.0

No longer experimental.

v13.4.0, v12.16.0

Added in: v13.4.0, v12.16.0

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

events.listenerCount(emitter, eventName)#

Added in: v0.9.12Deprecated since: v3.2.0

Stability: 0 - Deprecated: Use emitter.listenerCount() instead.

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2const { EventEmitter, listenerCount } = require('node:events');

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

events.on(emitter, eventName[, options])#

Added in: v13.6.0, v12.16.0 
import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable hereconst { on, EventEmitter } = require('node:events');

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo')) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An <AbortSignal> can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());const { on, EventEmitter } = require('node:events');

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

events.setMaxListeners(n[, ...eventTargets])#

Added in: v15.4.0 
import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);const {
  setMaxListeners,
  EventEmitter,
} = require('node:events');

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);

Class: events.EventEmitterAsyncResource extends EventEmitter#

Added in: v17.4.0, v16.14.0

Integrates EventEmitter with <AsyncResource> for EventEmitters that require manual async tracking. Specifically, all events emitted by instances of events.EventEmitterAsyncResource will run within its async context.

import { EventEmitterAsyncResource, EventEmitter } from 'node:events';
import { notStrictEqual, strictEqual } from 'node:assert';
import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
  strictEqual(executionAsyncId(), ee1.asyncId);
  strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});

const ee2 = new EventEmitter();

// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
  notStrictEqual(executionAsyncId(), ee2.asyncId);
  notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});

Promise.resolve().then(() => {
  ee1.emit('foo');
  ee2.emit('foo');
});const { EventEmitterAsyncResource, EventEmitter } = require('node:events');
const { notStrictEqual, strictEqual } = require('node:assert');
const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');

// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
  strictEqual(executionAsyncId(), ee1.asyncId);
  strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});

const ee2 = new EventEmitter();

// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
  notStrictEqual(executionAsyncId(), ee2.asyncId);
  notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});

Promise.resolve().then(() => {
  ee1.emit('foo');
  ee2.emit('foo');
});

The EventEmitterAsyncResource class has the same methods and takes the same options as EventEmitter and AsyncResource themselves.

new events.EventEmitterAsyncResource([options])#

  • options <Object>
    • captureRejections <boolean> It enables automatic capturing of promise rejection. Default: false.
    • name <string> The type of async event. Default:: new.target.name.
    • triggerAsyncId <number> The ID of the execution context that created this async event. Default: executionAsyncId().
    • requireManualDestroy <boolean> If set to true, disables emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. When set to false, the emitDestroy call on garbage collection will only take place if there is at least one active destroy hook. Default: false.

eventemitterasyncresource.asyncId#

  • Type: <number> The unique asyncId assigned to the resource.

eventemitterasyncresource.asyncResource#

The returned AsyncResource object has an additional eventEmitter property that provides a reference to this EventEmitterAsyncResource.

eventemitterasyncresource.emitDestroy()#

Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.

eventemitterasyncresource.triggerAsyncId#

  • Type: <number> The same triggerAsyncId that is passed to the AsyncResource constructor.

EventTarget and Event API#

History
VersionChanges
v16.0.0

changed EventTarget error handling.

v15.4.0

No longer experimental.

v15.0.0

The EventTarget and Event classes are now available as globals.

v14.5.0

Added in: v14.5.0

The EventTarget and Event objects are a Node.js-specific implementation of the EventTarget Web API that are exposed by some Node.js core APIs.

const target = new EventTarget();

target.addEventListener('foo', (event) => {
  console.log('foo event happened!');
});

Node.js EventTarget vs. DOM EventTarget#

There are two key differences between the Node.js EventTarget and the EventTarget Web API:

  1. Whereas DOM EventTarget instances may be hierarchical, there is no concept of hierarchy and event propagation in Node.js. That is, an event dispatched to an EventTarget does not propagate through a hierarchy of nested target objects that may each have their own set of handlers for the event.
  2. In the Node.js EventTarget, if an event listener is an async function or returns a Promise, and the returned Promise rejects, the rejection is automatically captured and handled the same way as a listener that throws synchronously (see EventTarget error handling for details).

NodeEventTarget vs. EventEmitter#

The NodeEventTarget object implements a modified subset of the EventEmitter API that allows it to closely emulate an EventEmitter in certain situations. A NodeEventTarget is not an instance of EventEmitter and cannot be used in place of an EventEmitter in most cases.

  1. Unlike EventEmitter, any given listener can be registered at most once per event type. Attempts to register a listener multiple times are ignored.
  2. The NodeEventTarget does not emulate the full EventEmitter API. Specifically the prependListener(), prependOnceListener(), rawListeners(), and errorMonitor APIs are not emulated. The 'newListener' and 'removeListener' events will also not be emitted.
  3. The NodeEventTarget does not implement any special default behavior for events with type 'error'.
  4. The NodeEventTarget supports EventListener objects as well as functions as handlers for all event types.

Event listener#

Event listeners registered for an event type may either be JavaScript functions or objects with a handleEvent property whose value is a function.

In either case, the handler function is invoked with the event argument passed to the eventTarget.dispatchEvent() function.

Async functions may be used as event listeners. If an async handler function rejects, the rejection is captured and handled as described in EventTarget error handling.

An error thrown by one handler function does not prevent the other handlers from being invoked.

The return value of a handler function is ignored.

Handlers are always invoked in the order they were added.

Handler functions may mutate the event object.

function handler1(event) {
  console.log(event.type);  // Prints 'foo'
  event.a = 1;
}

async function handler2(event) {
  console.log(event.type);  // Prints 'foo'
  console.log(event.a);  // Prints 1
}

const handler3 = {
  handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  },
};

const handler4 = {
  async handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  },
};

const target = new EventTarget();

target.addEventListener('foo', handler1);
target.addEventListener('foo', handler2);
target.addEventListener('foo', handler3);
target.addEventListener('foo', handler4, { once: true });

EventTarget error handling#

When a registered event listener throws (or returns a Promise that rejects), by default the error is treated as an uncaught exception on process.nextTick(). This means uncaught exceptions in EventTargets will terminate the Node.js process by default.

Throwing within an event listener will not stop the other registered handlers from being invoked.

The EventTarget does not implement any special default handling for 'error' type events like EventEmitter.

Currently errors are first forwarded to the process.on('error') event before reaching process.on('uncaughtException'). This behavior is deprecated and will change in a future release to align EventTarget with other Node.js APIs. Any code relying on the process.on('error') event should be aligned with the new behavior.

Class: Event#

History
VersionChanges
v15.0.0

The Event class is now available through the global object.

v14.5.0

Added in: v14.5.0

The Event object is an adaptation of the Event Web API. Instances are created internally by Node.js.

event.bubbles#
Added in: v14.5.0

This is not used in Node.js and is provided purely for completeness.

event.cancelBubble#
Added in: v14.5.0

Stability: 3 - Legacy: Use event.stopPropagation() instead.

Alias for event.stopPropagation() if set to true. This is not used in Node.js and is provided purely for completeness.

event.cancelable#
Added in: v14.5.0
  • Type: <boolean> True if the event was created with the cancelable option.
event.composed#
Added in: v14.5.0

This is not used in Node.js and is provided purely for completeness.

event.composedPath()#
Added in: v14.5.0

Returns an array containing the current EventTarget as the only entry or empty if the event is not being dispatched. This is not used in Node.js and is provided purely for completeness.

event.currentTarget#
Added in: v14.5.0

Alias for event.target.

event.defaultPrevented#
Added in: v14.5.0

Is true if cancelable is true and event.preventDefault() has been called.

event.eventPhase#
Added in: v14.5.0
  • Type: <number> Returns 0 while an event is not being dispatched, 2 while it is being dispatched.

This is not used in Node.js and is provided purely for completeness.

event.initEvent(type[, bubbles[, cancelable]])#
Added in: v19.5.0

Stability: 3 - Legacy: The WHATWG spec considers it deprecated and users shouldn't use it at all.

Redundant with event constructors and incapable of setting composed. This is not used in Node.js and is provided purely for completeness.

event.isTrusted#
Added in: v14.5.0

The <AbortSignal> "abort" event is emitted with isTrusted set to true. The value is false in all other cases.

event.preventDefault()#
Added in: v14.5.0

Sets the defaultPrevented property to true if cancelable is true.

event.returnValue#
Added in: v14.5.0

Stability: 3 - Legacy: Use event.defaultPrevented instead.

  • Type: <boolean> True if the event has not been canceled.

The value of event.returnValue is always the opposite of event.defaultPrevented. This is not used in Node.js and is provided purely for completeness.

event.srcElement#
Added in: v14.5.0

Stability: 3 - Legacy: Use event.target instead.

Alias for event.target.

event.stopImmediatePropagation()#
Added in: v14.5.0

Stops the invocation of event listeners after the current one completes.

event.stopPropagation()#
Added in: v14.5.0

This is not used in Node.js and is provided purely for completeness.

event.target#
Added in: v14.5.0
event.timeStamp#
Added in: v14.5.0

The millisecond timestamp when the Event object was created.

event.type#
Added in: v14.5.0

The event type identifier.

Class: EventTarget#

History
VersionChanges
v15.0.0

The EventTarget class is now available through the global object.

v14.5.0

Added in: v14.5.0

eventTarget.addEventListener(type, listener[, options])#
History
VersionChanges
v15.4.0

add support for signal option.

v14.5.0

Added in: v14.5.0

  • type <string>
  • listener <Function> | <EventListener>
  • options <Object>
    • once <boolean> When true, the listener is automatically removed when it is first invoked. Default: false.
    • passive <boolean> When true, serves as a hint that the listener will not call the Event object's preventDefault() method. Default: false.
    • capture <boolean> Not directly used by Node.js. Added for API completeness. Default: false.
    • signal <AbortSignal> The listener will be removed when the given AbortSignal object's abort() method is called.

Adds a new handler for the type event. Any given listener is added only once per type and per capture option value.

If the once option is true, the listener is removed after the next time a type event is dispatched.

The capture option is not used by Node.js in any functional way other than tracking registered event listeners per the EventTarget specification. Specifically, the capture option is used as part of the key when registering a listener. Any individual listener may be added once with capture = false, and once with capture = true.

function handler(event) {}

const target = new EventTarget();
target.addEventListener('foo', handler, { capture: true });  // first
target.addEventListener('foo', handler, { capture: false }); // second

// Removes the second instance of handler
target.removeEventListener('foo', handler);

// Removes the first instance of handler
target.removeEventListener('foo', handler, { capture: true });
eventTarget.dispatchEvent(event)#
Added in: v14.5.0
  • event <Event>
  • Returns: <boolean> true if either event's cancelable attribute value is false or its preventDefault() method was not invoked, otherwise false.

Dispatches the event to the list of handlers for event.type.

The registered event listeners is synchronously invoked in the order they were registered.

eventTarget.removeEventListener(type, listener[, options])#
Added in: v14.5.0

Removes the listener from the list of handlers for event type.

Class: CustomEvent#

Added in: v18.7.0, v16.17.0

Stability: 1 - Experimental.

The CustomEvent object is an adaptation of the CustomEvent Web API. Instances are created internally by Node.js.

event.detail#
Added in: v18.7.0, v16.17.0

Stability: 1 - Experimental.

  • Type: <any> Returns custom data passed when initializing.

Read-only.

Class: NodeEventTarget#

Added in: v14.5.0

The NodeEventTarget is a Node.js-specific extension to EventTarget that emulates a subset of the EventEmitter API.

nodeEventTarget.addListener(type, listener)#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.

nodeEventTarget.eventNames()#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that returns an array of event type names for which event listeners are registered.

nodeEventTarget.listenerCount(type)#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that returns the number of event listeners registered for the type.

nodeEventTarget.setMaxListeners(n)#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that sets the number of max event listeners as n.

nodeEventTarget.getMaxListeners()#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that returns the number of max event listeners.

nodeEventTarget.off(type, listener[, options])#
Added in: v14.5.0

Node.js-specific alias for eventTarget.removeListener().

nodeEventTarget.on(type, listener)#
Added in: v14.5.0

Node.js-specific alias for eventTarget.addListener().

nodeEventTarget.once(type, listener)#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.

nodeEventTarget.removeAllListeners([type])#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class. If type is specified, removes all registered listeners for type, otherwise removes all registered listeners.

nodeEventTarget.removeListener(type, listener[, options])#
Added in: v14.5.0

Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.

File system#

Stability: 2 - Stable

Source Code: lib/fs.js

The node:fs module enables interacting with the file system in a way modeled on standard POSIX functions.

To use the promise-based APIs:

import * as fs from 'node:fs/promises';const fs = require('node:fs/promises');

To use the callback and sync APIs:

import * as fs from 'node:fs';const fs = require('node:fs');

All file system operations have synchronous, callback, and promise-based forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).

Promise example#

Promise-based operations return a promise that is fulfilled when the asynchronous operation is complete.

import { unlink } from 'node:fs/promises';

try {
  await unlink('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (error) {
  console.error('there was an error:', error.message);
}const { unlink } = require('node:fs/promises');

(async function(path) {
  try {
    await unlink(path);
    console.log(`successfully deleted ${path}`);
  } catch (error) {
    console.error('there was an error:', error.message);
  }
})('/tmp/hello');

Callback example#

The callback form takes a completion callback function as its last argument and invokes the operation asynchronously. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation is completed successfully, then the first argument is null or undefined.

import { unlink } from 'node:fs';

unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});const { unlink } = require('node:fs');

unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

The callback-based versions of the node:fs module APIs are preferable over the use of the promise APIs when maximal performance (both in terms of execution time and memory allocation) is required.

Synchronous example#

The synchronous APIs block the Node.js event loop and further JavaScript execution until the operation is complete. Exceptions are thrown immediately and can be handled using try…catch, or can be allowed to bubble up.

import { unlinkSync } from 'node:fs';

try {
  unlinkSync('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (err) {
  // handle the error
}const { unlinkSync } = require('node:fs');

try {
  unlinkSync('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (err) {
  // handle the error
}

Promises API#

History
VersionChanges
v14.0.0

Exposed as require('fs/promises').

v11.14.0, v10.17.0

This API is no longer experimental.

v10.1.0

The API is accessible via require('fs').promises only.

v10.0.0

Added in: v10.0.0

The fs/promises API provides asynchronous file system methods that return promises.

The promise APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

Class: FileHandle#

Added in: v10.0.0

A <FileHandle> object is an object wrapper for a numeric file descriptor.

Instances of the <FileHandle> object are created by the fsPromises.open() method.

All <FileHandle> objects are <EventEmitter>s.

If a <FileHandle> is not closed using the filehandle.close() method, it will try to automatically close the file descriptor and emit a process warning, helping to prevent memory leaks. Please do not rely on this behavior because it can be unreliable and the file may not be closed. Instead, always explicitly close <FileHandle>s. Node.js may change this behavior in the future.

Event: 'close'#
Added in: v15.4.0

The 'close' event is emitted when the <FileHandle> has been closed and can no longer be used.

filehandle.appendFile(data[, options])#
History
VersionChanges
v15.14.0, v14.18.0

The data argument supports AsyncIterable, Iterable, and Stream.

v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Alias of filehandle.writeFile().

When operating on file handles, the mode cannot be changed from what it was set to with fsPromises.open(). Therefore, this is equivalent to filehandle.writeFile().

filehandle.chmod(mode)#
Added in: v10.0.0
  • mode <integer> the file mode bit mask.
  • Returns: <Promise> Fulfills with undefined upon success.

Modifies the permissions on the file. See chmod(2).

filehandle.chown(uid, gid)#
Added in: v10.0.0
  • uid <integer> The file's new owner's user id.
  • gid <integer> The file's new group's group id.
  • Returns: <Promise> Fulfills with undefined upon success.

Changes the ownership of the file. A wrapper for chown(2).

filehandle.close()#
Added in: v10.0.0
  • Returns: <Promise> Fulfills with undefined upon success.

Closes the file handle after waiting for any pending operation on the handle to complete.

import { open } from 'node:fs/promises';

let filehandle;
try {
  filehandle = await open('thefile.txt', 'r');
} finally {
  await filehandle?.close();
}
filehandle.createReadStream([options])#
Added in: v16.11.0

Unlike the 16 KiB default highWaterMark for a <stream.Readable>, the stream returned by this method has a default highWaterMark of 64 KiB.

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. If start is omitted or undefined, filehandle.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by <Buffer>.

If the FileHandle points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will emit a 'close' event after it has been destroyed. Set the emitClose option to false to change this behavior.

import { open } from 'node:fs/promises';

const fd = await open('/dev/input/event0');
// Create a stream from some character device.
const stream = fd.createReadStream();
setTimeout(() => {
  stream.close(); // This may not close the stream.
  // Artificially marking end-of-stream, as if the underlying resource had
  // indicated end-of-file by itself, allows the stream to close.
  // This does not cancel pending read operations, and if there is such an
  // operation, the process may still not be able to exit successfully
  // until it finishes.
  stream.push(null);
  stream.read(0);
}, 100);

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on 'error' or 'end' the file descriptor will be closed automatically.

An example to read the last 10 bytes of a file which is 100 bytes long:

import { open } from 'node:fs/promises';

const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
filehandle.createWriteStream([options])#
Added in: v16.11.0

options may also include a start option to allow writing data at some position past the beginning of the file, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than replacing it may require the flags open option to be set to r+ rather than the default r. The encoding can be any one of those accepted by <Buffer>.

If autoClose is set to true (default behavior) on 'error' or 'finish' the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak.

By default, the stream will emit a 'close' event after it has been destroyed. Set the emitClose option to false to change this behavior.

filehandle.datasync()#
Added in: v10.0.0
  • Returns: <Promise> Fulfills with undefined upon success.

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details.

Unlike filehandle.sync this method does not flush modified metadata.

filehandle.fd#
Added in: v10.0.0
filehandle.read(buffer, offset, length, position)#
Added in: v10.0.0
  • buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.
  • offset <integer> The location in the buffer at which to start filling.
  • length <integer> The number of bytes to read.
  • position <integer> | <null> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged.
  • Returns: <Promise> Fulfills upon success with an object with two properties:

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

filehandle.read([options])#
Added in: v13.11.0, v12.17.0
  • options <Object>
    • buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read. Default: Buffer.alloc(16384)
    • offset <integer> The location in the buffer at which to start filling. Default: 0
    • length <integer> The number of bytes to read. Default: buffer.byteLength - offset
    • position <integer> | <null> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged. Default:: null
  • Returns: <Promise> Fulfills upon success with an object with two properties:

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

filehandle.read(buffer[, options])#
Added in: v18.2.0, v16.17.0
  • buffer <Buffer> | <TypedArray> | <DataView> A buffer that will be filled with the file data read.
  • options <Object>
    • offset <integer> The location in the buffer at which to start filling. Default: 0
    • length <integer> The number of bytes to read. Default: buffer.byteLength - offset
    • position <integer> The location where to begin reading data from the file. If null, data will be read from the current file position, and the position will be updated. If position is an integer, the current file position will remain unchanged. Default:: null
  • Returns: <Promise> Fulfills upon success with an object with two properties:

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

filehandle.readableWebStream()#
Added in: v17.0.0

Stability: 1 - Experimental

Returns a ReadableStream that may be used to read the files data.

An error will be thrown if this method is called more than once or is called after the FileHandle is closed or closing.

import {
  open,
} from 'node:fs/promises';

const file = await open('./some/file/to/read');

for await (const chunk of file.readableWebStream())
  console.log(chunk);

await file.close();const {
  open,
} = require('node:fs/promises');

(async () => {
  const file = await open('./some/file/to/read');

  for await (const chunk of file.readableWebStream())
    console.log(chunk);

  await file.close();
})();

While the ReadableStream will read the file to completion, it will not close the FileHandle automatically. User code must still call the fileHandle.close() method.

filehandle.readFile(options)#
Added in: v10.0.0
  • options <Object> | <string>
  • Returns: <Promise> Fulfills upon a successful read with the contents of the file. If no encoding is specified (using options.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.

Asynchronously reads the entire contents of a file.

If options is a string, then it specifies the encoding.

The <FileHandle> has to support reading.

If one or more filehandle.read() calls are made on a file handle and then a filehandle.readFile() call is made, the data will be read from the current position till the end of the file. It doesn't always read from the beginning of the file.

filehandle.readLines([options])#
Added in: v18.11.0

Convenience method to create a readline interface and stream over the file. See filehandle.createReadStream() for the options.

import { open } from 'node:fs/promises';

const file = await open('./some/file/to/read');

for await (const line of file.readLines()) {
  console.log(line);
}const { open } = require('node:fs/promises');

(async () => {
  const file = await open('./some/file/to/read');

  for await (const line of file.readLines()) {
    console.log(line);
  }
})();
filehandle.readv(buffers[, position])#
Added in: v13.13.0, v12.17.0

Read from a file and write to an array of <ArrayBufferView>s

filehandle.stat([options])#
History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

filehandle.sync()#
Added in: v10.0.0
  • Returns: <Promise> Fulfills with undefined upon success.

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail.

filehandle.truncate(len)#
Added in: v10.0.0

Truncates the file.

If the file was larger than len bytes, only the first len bytes will be retained in the file.

The following example retains only the first four bytes of the file:

import { open } from 'node:fs/promises';

let filehandle = null;
try {
  filehandle = await open('temp.txt', 'r+');
  await filehandle.truncate(4);
} finally {
  await filehandle?.close();
}

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):

If len is negative then 0 will be used.

filehandle.utimes(atime, mtime)#
Added in: v10.0.0

Change the file system timestamps of the object referenced by the <FileHandle> then resolves the promise with no arguments upon success.

filehandle.write(buffer, offset[, length[, position]])#
History
VersionChanges
v14.0.0

The buffer parameter won't coerce unsupported input to buffers anymore.

v10.0.0

Added in: v10.0.0

  • buffer <Buffer> | <TypedArray> | <DataView>
  • offset <integer> The start position from within buffer where the data to write begins.
  • length <integer> The number of bytes from buffer to write. Default: buffer.byteLength - offset
  • position <integer> | <null> The offset from the beginning of the file where the data from buffer should be written. If position is not a number, the data will be written at the current position. See the POSIX pwrite(2) documentation for more detail. Default: null
  • Returns: <Promise>

Write buffer to the file.

The promise is resolved with an object containing two properties:

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be resolved (or rejected). For this scenario, use filehandle.createWriteStream().

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

filehandle.write(buffer[, options])#
Added in: v18.3.0, v16.17.0

Write buffer to the file.

Similar to the above filehandle.write function, this version takes an optional options object. If no options object is specified, it will default with the above values.

filehandle.write(string[, position[, encoding]])#
History
VersionChanges
v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

  • string <string>
  • position <integer> | <null> The offset from the beginning of the file where the data from string should be written. If position is not a number the data will be written at the current position. See the POSIX pwrite(2) documentation for more detail. Default: null
  • encoding <string> The expected string encoding. Default: 'utf8'
  • Returns: <Promise>

Write string to the file. If string is not a string, the promise is rejected with an error.

The promise is resolved with an object containing two properties:

  • bytesWritten <integer> the number of bytes written
  • buffer <string> a reference to the string written.

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the promise to be resolved (or rejected). For this scenario, use filehandle.createWriteStream().

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

filehandle.writeFile(data, options)#
History
VersionChanges
v15.14.0, v14.18.0

The data argument supports AsyncIterable, Iterable, and Stream.

v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object. The promise is resolved with no arguments upon success.

If options is a string, then it specifies the encoding.

The <FileHandle> has to support writing.

It is unsafe to use filehandle.writeFile() multiple times on the same file without waiting for the promise to be resolved (or rejected).

If one or more filehandle.write() calls are made on a file handle and then a filehandle.writeFile() call is made, the data will be written from the current position till the end of the file. It doesn't always write from the beginning of the file.

filehandle.writev(buffers[, position])#
Added in: v12.9.0

Write an array of <ArrayBufferView>s to the file.

The promise is resolved with an object containing a two properties:

It is unsafe to call writev() multiple times on the same file without waiting for the promise to be resolved (or rejected).

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fsPromises.access(path[, mode])#

Added in: v10.0.0

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

If the accessibility check is successful, the promise is resolved with no value. If any of the accessibility checks fail, the promise is rejected with an <Error> object. The following example checks if the file /etc/passwd can be read and written by the current process.

import { access, constants } from 'node:fs/promises';

try {
  await access('/etc/passwd', constants.R_OK | constants.W_OK);
  console.log('can access');
} catch {
  console.error('cannot access');
}

Using fsPromises.access() to check for the accessibility of a file before calling fsPromises.open() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

fsPromises.appendFile(path, data[, options])#

Added in: v10.0.0

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.

If options is a string, then it specifies the encoding.

The mode option only affects the newly created file. See fs.open() for more details.

The path may be specified as a <FileHandle> that has been opened for appending (using fsPromises.open()).

fsPromises.chmod(path, mode)#

Added in: v10.0.0

Changes the permissions of a file.

fsPromises.chown(path, uid, gid)#

Added in: v10.0.0

Changes the ownership of a file.

fsPromises.copyFile(src, dest[, mode])#

History
VersionChanges
v14.0.0

Changed flags argument to mode and imposed stricter type validation.

v10.0.0

Added in: v10.0.0

  • src <string> | <Buffer> | <URL> source filename to copy
  • dest <string> | <Buffer> | <URL> destination filename of the copy operation
  • mode <integer> Optional modifiers that specify the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE) Default: 0.
    • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
    • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
    • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
  • Returns: <Promise> Fulfills with undefined upon success.

Asynchronously copies src to dest. By default, dest is overwritten if it already exists.

No guarantees are made about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, an attempt will be made to remove the destination.

import { copyFile, constants } from 'node:fs/promises';

try {
  await copyFile('source.txt', 'destination.txt');
  console.log('source.txt was copied to destination.txt');
} catch {
  console.error('The file could not be copied');
}

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
try {
  await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
  console.log('source.txt was copied to destination.txt');
} catch {
  console.error('The file could not be copied');
}

fsPromises.cp(src, dest[, options])#

History
VersionChanges
v17.6.0, v16.15.0

Accepts an additional verbatimSymlinks option to specify whether to perform path resolution for symlinks.

v16.7.0

Added in: v16.7.0

Stability: 1 - Experimental

  • src <string> | <URL> source path to copy.
  • dest <string> | <URL> destination path to copy to.
  • options <Object>
    • dereference <boolean> dereference symlinks. Default: false.
    • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
    • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. Can also return a Promise that resolves to true or false Default: undefined.
    • force <boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
    • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
    • recursive <boolean> copy directories recursively Default: false
    • verbatimSymlinks <boolean> When true, path resolution for symlinks will be skipped. Default: false
  • Returns: <Promise> Fulfills with undefined upon success.

Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.

fsPromises.lchmod(path, mode)#

Deprecated since: v10.0.0

Changes the permissions on a symbolic link.

This method is only implemented on macOS.

fsPromises.lchown(path, uid, gid)#

History
VersionChanges
v10.6.0

This API is no longer deprecated.

v10.0.0

Added in: v10.0.0

Changes the ownership on a symbolic link.

fsPromises.lutimes(path, atime, mtime)#

Added in: v14.5.0, v12.19.0

Changes the access and modification times of a file in the same way as fsPromises.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

fsPromises.link(existingPath, newPath)#

Added in: v10.0.0

Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail.

fsPromises.lstat(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

Equivalent to fsPromises.stat() unless path refers to a symbolic link, in which case the link itself is stat-ed, not the file that it refers to. Refer to the POSIX lstat(2) document for more detail.

fsPromises.mkdir(path[, options])#

Added in: v10.0.0

Asynchronously creates a directory.

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fsPromises.mkdir() when path is a directory that exists results in a rejection only when recursive is false.

import { mkdir } from 'node:fs/promises';

try {
  const projectFolder = new URL('./test/project/', import.meta.url);
  const createDir = await mkdir(projectFolder, { recursive: true });

  console.log(`created ${createDir}`);
} catch (err) {
  console.error(err.message);
}const { mkdir } = require('node:fs/promises');
const { resolve, join } = require('node:path');

async function makeDirectory() {
  const projectFolder = join(__dirname, 'test', 'project');
  const dirCreation = await mkdir(projectFolder, { recursive: true });

  console.log(dirCreation);
  return dirCreation;
}

makeDirectory().catch(console.error);

fsPromises.mkdtemp(prefix[, options])#

History
VersionChanges
v16.5.0, v14.18.0

The prefix parameter now accepts an empty string.

v10.0.0

Added in: v10.0.0

Creates a unique temporary directory. A unique directory name is generated by appending six random characters to the end of the provided prefix. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

import { mkdtemp } from 'node:fs/promises';

try {
  await mkdtemp(path.join(os.tmpdir(), 'foo-'));
} catch (err) {
  console.error(err);
}

The fsPromises.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('node:path').sep).

fsPromises.open(path, flags[, mode])#

History
VersionChanges
v11.1.0

The flags argument is now optional and defaults to 'r'.

v10.0.0

Added in: v10.0.0

Opens a <FileHandle>.

Refer to the POSIX open(2) documentation for more detail.

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

fsPromises.opendir(path[, options])#

History
VersionChanges
v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

Asynchronously open a directory for iterative scanning. See the POSIX opendir(3) documentation for more detail.

Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

Example using async iteration:

import { opendir } from 'node:fs/promises';

try {
  const dir = await opendir('./');
  for await (const dirent of dir)
    console.log(dirent.name);
} catch (err) {
  console.error(err);
}

When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.

fsPromises.readdir(path[, options])#

History
VersionChanges
v10.11.0

New option withFileTypes was added.

v10.0.0

Added in: v10.0.0

Reads the contents of a directory.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.

If options.withFileTypes is set to true, the resolved array will contain <fs.Dirent> objects.

import { readdir } from 'node:fs/promises';

try {
  const files = await readdir(path);
  for (const file of files)
    console.log(file);
} catch (err) {
  console.error(err);
}

fsPromises.readFile(path[, options])#

History
VersionChanges
v15.2.0, v14.17.0

The options argument may include an AbortSignal to abort an ongoing readFile request.

v10.0.0

Added in: v10.0.0

Asynchronously reads the entire contents of a file.

If no encoding is specified (using options.encoding), the data is returned as a <Buffer> object. Otherwise, the data will be a string.

If options is a string, then it specifies the encoding.

When the path is a directory, the behavior of fsPromises.readFile() is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned.

An example of reading a package.json file located in the same directory of the running code:

import { readFile } from 'node:fs/promises';
try {
  const filePath = new URL('./package.json', import.meta.url);
  const contents = await readFile(filePath, { encoding: 'utf8' });
  console.log(contents);
} catch (err) {
  console.error(err.message);
}const { readFile } = require('node:fs/promises');
const { resolve } = require('node:path');
async function logFile() {
  try {
    const filePath = resolve('./package.json');
    const contents = await readFile(filePath, { encoding: 'utf8' });
    console.log(contents);
  } catch (err) {
    console.error(err.message);
  }
}
logFile();

It is possible to abort an ongoing readFile using an <AbortSignal>. If a request is aborted the promise returned is rejected with an AbortError:

import { readFile } from 'node:fs/promises';

try {
  const controller = new AbortController();
  const { signal } = controller;
  const promise = readFile(fileName, { signal });

  // Abort the request before the promise settles.
  controller.abort();

  await promise;
} catch (err) {
  // When a request is aborted - err is an AbortError
  console.error(err);
}

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.

Any specified <FileHandle> has to support reading.

fsPromises.readlink(path[, options])#

Added in: v10.0.0

Reads the contents of the symbolic link referred to by path. See the POSIX readlink(2) documentation for more detail. The promise is resolved with the linkString upon success.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.

fsPromises.realpath(path[, options])#

Added in: v10.0.0

Determines the actual location of path using the same semantics as the fs.realpath.native() function.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fsPromises.rename(oldPath, newPath)#

Added in: v10.0.0

Renames oldPath to newPath.

fsPromises.rmdir(path[, options])#

History
VersionChanges
v16.0.0

Using fsPromises.rmdir(path, { recursive: true }) on a path that is a file is no longer permitted and results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

v16.0.0

Using fsPromises.rmdir(path, { recursive: true }) on a path that does not exist is no longer permitted and results in a ENOENT error.

v16.0.0

The recursive option is deprecated, using it triggers a deprecation warning.

v14.14.0

The recursive option is deprecated, use fsPromises.rm instead.

v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v10.0.0

Added in: v10.0.0

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
  • Returns: <Promise> Fulfills with undefined upon success.

Removes the directory identified by path.

Using fsPromises.rmdir() on a file (not a directory) results in the promise being rejected with an ENOENT error on Windows and an ENOTDIR error on POSIX.

To get a behavior similar to the rm -rf Unix command, use fsPromises.rm() with options { recursive: true, force: true }.

fsPromises.rm(path[, options])#

Added in: v14.14.0
  • path <string> | <Buffer> | <URL>
  • options <Object>
    • force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
  • Returns: <Promise> Fulfills with undefined upon success.

Removes files and directories (modeled on the standard POSIX rm utility).

fsPromises.stat(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

fsPromises.symlink(target, path[, type])#

History
VersionChanges
v19.0.0

If the type argument is null or omitted, Node.js will autodetect target type and automatically select dir or file.

v10.0.0

Added in: v10.0.0

Creates a symbolic link.

The type argument is only used on Windows platforms and can be one of 'dir', 'file', or 'junction'. If the type argument is not a string, Node.js will autodetect target type and use 'file' or 'dir'. If the target does not exist, 'file' will be used. Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

fsPromises.truncate(path[, len])#

Added in: v10.0.0

Truncates (shortens or extends the length) of the content at path to len bytes.

fsPromises.unlink(path)#

Added in: v10.0.0

If path refers to a symbolic link, then the link is removed without affecting the file or directory to which that link refers. If the path refers to a file path that is not a symbolic link, the file is deleted. See the POSIX unlink(2) documentation for more detail.

fsPromises.utimes(path, atime, mtime)#

Added in: v10.0.0

Change the file system timestamps of the object referenced by path.

The atime and mtime arguments follow these rules:

  • Values can be either numbers representing Unix epoch time, Dates, or a numeric string like '123456789.0'.
  • If the value can not be converted to a number, or is NaN, Infinity, or -Infinity, an Error will be thrown.

fsPromises.watch(filename[, options])#

Added in: v15.9.0, v14.18.0
  • filename <string> | <Buffer> | <URL>
  • options <string> | <Object>
    • persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true.
    • recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: false.
    • encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.
    • signal <AbortSignal> An <AbortSignal> used to signal when the watcher should stop.
  • Returns: <AsyncIterator> of objects with the properties:

Returns an async iterator that watches for changes on filename, where filename is either a file or a directory.

const { watch } = require('node:fs/promises');

const ac = new AbortController();
const { signal } = ac;
setTimeout(() => ac.abort(), 10000);

(async () => {
  try {
    const watcher = watch(__filename, { signal });
    for await (const event of watcher)
      console.log(event);
  } catch (err) {
    if (err.name === 'AbortError')
      return;
    throw err;
  }
})();

On most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

All the caveats for fs.watch() also apply to fsPromises.watch().

fsPromises.writeFile(file, data[, options])#

History
VersionChanges
v15.14.0, v14.18.0

The data argument supports AsyncIterable, Iterable, and Stream.

v15.2.0, v14.17.0

The options argument may include an AbortSignal to abort an ongoing writeFile request.

v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string, a buffer, an <AsyncIterable>, or an <Iterable> object.

The encoding option is ignored if data is a buffer.

If options is a string, then it specifies the encoding.

The mode option only affects the newly created file. See fs.open() for more details.

Any specified <FileHandle> has to support writing.

It is unsafe to use fsPromises.writeFile() multiple times on the same file without waiting for the promise to be settled.

Similarly to fsPromises.readFile - fsPromises.writeFile is a convenience method that performs multiple write calls internally to write the buffer passed to it. For performance sensitive code consider using fs.createWriteStream() or filehandle.createWriteStream().

It is possible to use an <AbortSignal> to cancel an fsPromises.writeFile(). Cancelation is "best effort", and some amount of data is likely still to be written.

import { writeFile } from 'node:fs/promises';
import { Buffer } from 'node:buffer';

try {
  const controller = new AbortController();
  const { signal } = controller;
  const data = new Uint8Array(Buffer.from('Hello Node.js'));
  const promise = writeFile('message.txt', data, { signal });

  // Abort the request before the promise settles.
  controller.abort();

  await promise;
} catch (err) {
  // When a request is aborted - err is an AbortError
  console.error(err);
}

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.writeFile performs.

fsPromises.constants#

Added in: v18.4.0, v16.17.0

Returns an object containing commonly used constants for file system operations. The object is the same as fs.constants. See FS constants for more details.

Callback API#

The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.

The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

fs.access(path[, mode], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v6.3.0

The constants like fs.R_OK, etc which were present directly on fs were moved into fs.constants as a soft deprecation. Thus for Node.js < v6.3.0 use fs to access those constants, or do something like (fs.constants || fs).R_OK to work with all versions.

v0.11.15

Added in: v0.11.15

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

The final argument, callback, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be an Error object. The following examples check if package.json exists, and if it is readable or writable.

import { access, constants } from 'node:fs';

const file = 'package.json';

// Check if the file exists in the current directory.
access(file, constants.F_OK, (err) => {
  console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});

// Check if the file is readable.
access(file, constants.R_OK, (err) => {
  console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});

// Check if the file is writable.
access(file, constants.W_OK, (err) => {
  console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});

// Check if the file is readable and writable.
access(file, constants.R_OK | constants.W_OK, (err) => {
  console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
});

Do not use fs.access() to check for the accessibility of a file before calling fs.open(), fs.readFile(), or fs.writeFile(). Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

write (NOT RECOMMENDED)

import { access, open, close } from 'node:fs';

access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  open('myfile', 'wx', (err, fd) => {
    if (err) throw err;

    try {
      writeMyData(fd);
    } finally {
      close(fd, (err) => {
        if (err) throw err;
      });
    }
  });
});

write (RECOMMENDED)

import { open, close } from 'node:fs';

open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

read (NOT RECOMMENDED)

import { access, open, close } from 'node:fs';
access('myfile', (err) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  open('myfile', 'r', (err, fd) => {
    if (err) throw err;

    try {
      readMyData(fd);
    } finally {
      close(fd, (err) => {
        if (err) throw err;
      });
    }
  });
});

read (RECOMMENDED)

import { open, close } from 'node:fs';

open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process.

On Windows, access-control policies (ACLs) on a directory may limit access to a file or directory. The fs.access() function, however, does not check the ACL and therefore may report that a path is accessible even if the ACL restricts the user from reading or writing to it.

fs.appendFile(path, data[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v7.0.0

The passed options object will never be modified.

v5.0.0

The file parameter can be a file descriptor now.

v0.6.7

Added in: v0.6.7

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.

The mode option only affects the newly created file. See fs.open() for more details.

import { appendFile } from 'node:fs';

appendFile('message.txt', 'data to append', (err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

If options is a string, then it specifies the encoding:

import { appendFile } from 'node:fs';

appendFile('message.txt', 'data to append', 'utf8', callback);

The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync()). The file descriptor will not be closed automatically.

import { open, close, appendFile } from 'node:fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('message.txt', 'a', (err, fd) => {
  if (err) throw err;

  try {
    appendFile(fd, 'data to append', 'utf8', (err) => {
      closeFd(fd);
      if (err) throw err;
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

fs.chmod(path, mode, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.30

Added in: v0.1.30

Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX chmod(2) documentation for more detail.

import { chmod } from 'node:fs';

chmod('my_file.txt', 0o775, (err) => {
  if (err) throw err;
  console.log('The permissions for file "my_file.txt" have been changed!');
});
File modes#

The mode argument used in both the fs.chmod() and fs.chmodSync() methods is a numeric bitmask created using a logical OR of the following constants:

ConstantOctalDescription
fs.constants.S_IRUSR0o400read by owner
fs.constants.S_IWUSR0o200write by owner
fs.constants.S_IXUSR0o100execute/search by owner
fs.constants.S_IRGRP0o40read by group
fs.constants.S_IWGRP0o20write by group
fs.constants.S_IXGRP0o10execute/search by group
fs.constants.S_IROTH0o4read by others
fs.constants.S_IWOTH0o2write by others
fs.constants.S_IXOTH0o1execute/search by others

An easier method of constructing the mode is to use a sequence of three octal digits (e.g. 765). The left-most digit (7 in the example), specifies the permissions for the file owner. The middle digit (6 in the example), specifies permissions for the group. The right-most digit (5 in the example), specifies the permissions for others.

NumberDescription
7read, write, and execute
6read and write
5read and execute
4read only
3write and execute
2write only
1execute only
0no permission

For example, the octal value 0o765 means:

  • The owner may read, write, and execute the file.
  • The group may read and write the file.
  • Others may read and execute the file.

When using raw numbers where file modes are expected, any value larger than 0o777 may result in platform-specific behaviors that are not supported to work consistently. Therefore constants like S_ISVTX, S_ISGID, or S_ISUID are not exposed in fs.constants.

Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner, or others is not implemented.

fs.chown(path, uid, gid, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.97

Added in: v0.1.97

Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX chown(2) documentation for more detail.

fs.close(fd[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v15.9.0, v14.17.0

A default callback is now used if one is not provided.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.

Calling fs.close() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.

See the POSIX close(2) documentation for more detail.

fs.copyFile(src, dest[, mode], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v14.0.0

Changed flags argument to mode and imposed stricter type validation.

v8.5.0

Added in: v8.5.0

Asynchronously copies src to dest. By default, dest is overwritten if it already exists. No arguments other than a possible exception are given to the callback function. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFile, constants } from 'node:fs';

function callback(err) {
  if (err) throw err;
  console.log('source.txt was copied to destination.txt');
}

// destination.txt will be created or overwritten by default.
copyFile('source.txt', 'destination.txt', callback);

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);

fs.cp(src, dest[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v17.6.0, v16.15.0

Accepts an additional verbatimSymlinks option to specify whether to perform path resolution for symlinks.

v16.7.0

Added in: v16.7.0

Stability: 1 - Experimental

  • src <string> | <URL> source path to copy.
  • dest <string> | <URL> destination path to copy to.
  • options <Object>
    • dereference <boolean> dereference symlinks. Default: false.
    • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
    • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. Can also return a Promise that resolves to true or false Default: undefined.
    • force <boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
    • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
    • recursive <boolean> copy directories recursively Default: false
    • verbatimSymlinks <boolean> When true, path resolution for symlinks will be skipped. Default: false
  • callback <Function>

Asynchronously copies the entire directory structure from src to dest, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.

fs.createReadStream(path[, options])#

History
VersionChanges
v16.10.0

The fs option does not need open method if an fd was provided.

v16.10.0

The fs option does not need close method if autoClose is false.

v15.4.0

The fd option accepts FileHandle arguments.

v14.0.0

Change emitClose default to true.

v13.6.0, v12.17.0

The fs options allow overriding the used fs implementation.

v12.10.0

Enable emitClose option.

v11.0.0

Impose new restrictions on start and end, throwing more appropriate errors in cases when we cannot reasonably handle the input values.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The passed options object will never be modified.

v2.3.0

The passed options object can be a string now.

v0.1.31

Added in: v0.1.31

Unlike the 16 KiB default highWaterMark for a <stream.Readable>, the stream returned by this method has a default highWaterMark of 64 KiB.

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by <Buffer>.

If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fds should be passed to <net.Socket>.

If fd points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will emit a 'close' event after it has been destroyed. Set the emitClose option to false to change this behavior.

By providing the fs option, it is possible to override the corresponding fs implementations for open, read, and close. When providing the fs option, an override for read is required. If no fd is provided, an override for open is also required. If autoClose is true, an override for close is also required.

import { createReadStream } from 'node:fs';

// Create a stream from some character device.
const stream = createReadStream('/dev/input/event0');
setTimeout(() => {
  stream.close(); // This may not close the stream.
  // Artificially marking end-of-stream, as if the underlying resource had
  // indicated end-of-file by itself, allows the stream to close.
  // This does not cancel pending read operations, and if there is such an
  // operation, the process may still not be able to exit successfully
  // until it finishes.
  stream.push(null);
  stream.read(0);
}, 100);

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on 'error' or 'end' the file descriptor will be closed automatically.

mode sets the file mode (permission and sticky bits), but only if the file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

import { createReadStream } from 'node:fs';

createReadStream('sample.txt', { start: 90, end: 99 });

If options is a string, then it specifies the encoding.

fs.createWriteStream(path[, options])#

History
VersionChanges
v16.10.0

The fs option does not need open method if an fd was provided.

v16.10.0

The fs option does not need close method if autoClose is false.

v15.4.0

The fd option accepts FileHandle arguments.

v14.0.0

Change emitClose default to true.

v13.6.0, v12.17.0

The fs options allow overriding the used fs implementation.

v12.10.0

Enable emitClose option.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The passed options object will never be modified.

v5.5.0

The autoClose option is supported now.

v2.3.0

The passed options object can be a string now.

v0.1.31

Added in: v0.1.31

options may also include a start option to allow writing data at some position past the beginning of the file, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than replacing it may require the flags option to be set to r+ rather than the default w. The encoding can be any one of those accepted by <Buffer>.

If autoClose is set to true (default behavior) on 'error' or 'finish' the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak.

By default, the stream will emit a 'close' event after it has been destroyed. Set the emitClose option to false to change this behavior.

By providing the fs option it is possible to override the corresponding fs implementations for open, write, writev, and close. Overriding write() without writev() can reduce performance as some optimizations (_writev()) will be disabled. When providing the fs option, overrides for at least one of write and writev are required. If no fd option is supplied, an override for open is also required. If autoClose is true, an override for close is also required.

Like <fs.ReadStream>, if fd is specified, <fs.WriteStream> will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fds should be passed to <net.Socket>.

If options is a string, then it specifies the encoding.

fs.exists(path, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v1.0.0

Deprecated since: v1.0.0

v0.0.2

Added in: v0.0.2

Stability: 0 - Deprecated: Use fs.stat() or fs.access() instead.

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false:

import { exists } from 'node:fs';

exists('/etc/passwd', (e) => {
  console.log(e ? 'it exists' : 'no passwd!');
});

The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile(), or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

write (NOT RECOMMENDED)

import { exists, open, close } from 'node:fs';

exists('myfile', (e) => {
  if (e) {
    console.error('myfile already exists');
  } else {
    open('myfile', 'wx', (err, fd) => {
      if (err) throw err;

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  }
});

write (RECOMMENDED)

import { open, close } from 'node:fs';
open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

read (NOT RECOMMENDED)

import { open, close, exists } from 'node:fs';

exists('myfile', (e) => {
  if (e) {
    open('myfile', 'r', (err, fd) => {
      if (err) throw err;

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

import { open, close } from 'node:fs';

open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won't be used directly, for example when its existence is a signal from another process.

fs.fchmod(fd, mode, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Added in: v0.4.7

Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX fchmod(2) documentation for more detail.

fs.fchown(fd, uid, gid, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Added in: v0.4.7

Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX fchown(2) documentation for more detail.

fs.fdatasync(fd, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.96

Added in: v0.1.96

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details. No arguments other than a possible exception are given to the completion callback.

fs.fstat(fd[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.95

Added in: v0.1.95

Invokes the callback with the <fs.Stats> for the file descriptor.

See the POSIX fstat(2) documentation for more detail.

fs.fsync(fd, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.96

Added in: v0.1.96

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.

fs.ftruncate(fd[, len], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.8.6

Added in: v0.8.6

Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.

See the POSIX ftruncate(2) documentation for more detail.

If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file:

import { open, close, ftruncate } from 'node:fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('temp.txt', 'r+', (err, fd) => {
  if (err) throw err;

  try {
    ftruncate(fd, 4, (err) => {
      closeFd(fd);
      if (err) throw err;
    });
  } catch (err) {
    closeFd(fd);
    if (err) throw err;
  }
});

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):

If len is negative then 0 will be used.

fs.futimes(fd, atime, mtime, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v4.1.0

Numeric strings, NaN, and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes().

fs.lchmod(path, mode, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.0.0

The error returned may be an AggregateError if more than one error is returned.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Deprecated since: v0.4.7

Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.

This method is only implemented on macOS.

See the POSIX lchmod(2) documentation for more detail.

fs.lchown(path, uid, gid, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.6.0

This API is no longer deprecated.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Documentation-only deprecation.

Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.

See the POSIX lchown(2) documentation for more detail.

fs.lutimes(path, atime, mtime, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v14.5.0, v12.19.0

Added in: v14.5.0, v12.19.0

Changes the access and modification times of a file in the same way as fs.utimes(), with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

No arguments other than a possible exception are given to the completion callback.

fs.link(existingPath, newPath, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The existingPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.31

Added in: v0.1.31

Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail. No arguments other than a possible exception are given to the completion callback.

fs.lstat(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.30

Added in: v0.1.30

Retrieves the <fs.Stats> for the symbolic link referred to by the path. The callback gets two arguments (err, stats) where stats is a <fs.Stats> object. lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

See the POSIX lstat(2) documentation for more details.

fs.mkdir(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v13.11.0, v12.17.0

In recursive mode, the callback now receives the first created path as an argument.

v10.12.0

The second argument can now be an options object with recursive and mode properties.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.8

Added in: v0.1.8

Asynchronously creates a directory.

The callback is given a possible exception and, if recursive is true, the first directory path created, (err[, path]). path can still be undefined when recursive is true, if no directory was created.

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fs.mkdir() when path is a directory that exists results in an error only when recursive is false.

import { mkdir } from 'node:fs';

// Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist.
mkdir('/tmp/a/apple', { recursive: true }, (err) => {
  if (err) throw err;
});

On Windows, using fs.mkdir() on the root directory even with recursion will result in an error:

import { mkdir } from 'node:fs';

mkdir('/', { recursive: true }, (err) => {
  // => [Error: EPERM: operation not permitted, mkdir 'C:\']
});

See the POSIX mkdir(2) documentation for more details.

fs.mkdtemp(prefix[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.5.0, v14.18.0

The prefix parameter now accepts an empty string.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.2.1

The callback parameter is optional now.

v5.10.0

Added in: v5.10.0

Creates a unique temporary directory.

Generates six random characters to be appended behind a required prefix to create a unique temporary directory. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

The created directory path is passed as a string to the callback's second parameter.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

import { mkdtemp } from 'node:fs';

mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});

The fs.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('node:path').sep).

import { tmpdir } from 'node:os';
import { mkdtemp } from 'node:fs';

// The parent directory for the new temporary directory
const tmpDir = tmpdir();

// This method is *INCORRECT*:
mkdtemp(tmpDir, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmpabc123`.
  // A new temporary directory is created at the file system root
  // rather than *within* the /tmp directory.
});

// This method is *CORRECT*:
import { sep } from 'node:path';
mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

fs.open(path[, flags[, mode]], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v11.1.0

The flags argument is now optional and defaults to 'r'.

v9.9.0

The as and as+ flags are supported now.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.0.2

Added in: v0.0.2

Asynchronous file open. See the POSIX open(2) documentation for more details.

mode sets the file mode (permission and sticky bits), but only if the file was created. On Windows, only the write permission can be manipulated; see fs.chmod().

The callback gets two arguments (err, fd).

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

Functions based on fs.open() exhibit this behavior as well: fs.writeFile(), fs.readFile(), etc.

fs.opendir(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

Asynchronously open a directory. See the POSIX opendir(3) documentation for more details.

Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

fs.read(fd, buffer, offset, length, position, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.10.0

The buffer parameter can now be any TypedArray, or a DataView.

v7.4.0

The buffer parameter can now be a Uint8Array.

v6.0.0

The length parameter can now be 0.

v0.0.2

Added in: v0.0.2

Read data from the file specified by fd.

The callback is given the three arguments, (err, bytesRead, buffer).

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffer properties.

fs.read(fd[, options], callback)#

History
VersionChanges
v13.11.0, v12.17.0

Options object can be passed in to make buffer, offset, length, and position optional.

v13.11.0, v12.17.0

Added in: v13.11.0, v12.17.0

Similar to the fs.read() function, this version takes an optional options object. If no options object is specified, it will default with the above values.

fs.read(fd, buffer[, options], callback)#

Added in: v18.2.0, v16.17.0

Similar to the fs.read() function, this version takes an optional options object. If no options object is specified, it will default with the above values.

fs.readdir(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.10.0

New option withFileTypes was added.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.0.0

The options parameter was added.

v0.1.8

Added in: v0.1.8

Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..'.

See the POSIX readdir(3) documentation for more details.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.

If options.withFileTypes is set to true, the files array will contain <fs.Dirent> objects.

fs.readFile(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.0.0

The error returned may be an AggregateError if more than one error is returned.

v15.2.0, v14.17.0

The options argument may include an AbortSignal to abort an ongoing readFile request.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v5.1.0

The callback will always be called with null as the error parameter in case of success.

v5.0.0

The path parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

Asynchronously reads the entire contents of a file.

import { readFile } from 'node:fs';

readFile('/etc/passwd', (err, data) => {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

If options is a string, then it specifies the encoding:

import { readFile } from 'node:fs';

readFile('/etc/passwd', 'utf8', callback);

When the path is a directory, the behavior of fs.readFile() and fs.readFileSync() is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned.

import { readFile } from 'node:fs';

// macOS, Linux, and Windows
readFile('<directory>', (err, data) => {
  // => [Error: EISDIR: illegal operation on a directory, read <directory>]
});

//  FreeBSD
readFile('<directory>', (err, data) => {
  // => null, <data>
});

It is possible to abort an ongoing request using an AbortSignal. If a request is aborted the callback is called with an AbortError:

import { readFile } from 'node:fs';

const controller = new AbortController();
const signal = controller.signal;
readFile(fileInfo[0].name, { signal }, (err, buf) => {
  // ...
});
// When you want to abort the request
controller.abort();

The fs.readFile() function buffers the entire file. To minimize memory costs, when possible prefer streaming via fs.createReadStream().

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.

File descriptors#
  1. Any specified file descriptor has to support reading.
  2. If a file descriptor is specified as the path, it will not be closed automatically.
  3. The reading will begin at the current position. For example, if the file already had 'Hello World' and six bytes are read with the file descriptor, the call to fs.readFile() with the same file descriptor, would give 'World', rather than 'Hello World'.
Performance Considerations#

The fs.readFile() method asynchronously reads the contents of a file into memory one chunk at a time, allowing the event loop to turn between each chunk. This allows the read operation to have less impact on other activity that may be using the underlying libuv thread pool but means that it will take longer to read a complete file into memory.

The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64 KiB of data. For regular files, each read will process 512 KiB of data.

For applications that require as-fast-as-possible reading of file contents, it is better to use fs.read() directly and for application code to manage reading the full contents of the file itself.

The Node.js GitHub issue #25741 provides more information and a detailed analysis on the performance of fs.readFile() for multiple file sizes in different Node.js versions.

fs.readlink(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.31

Added in: v0.1.31

Reads the contents of the symbolic link referred to by path. The callback gets two arguments (err, linkString).

See the POSIX readlink(2) documentation for more details.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.

fs.readv(fd, buffers[, position], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v13.13.0, v12.17.0

Added in: v13.13.0, v12.17.0

Read from a file specified by fd and write to an array of ArrayBufferViews using readv().

position is the offset from the beginning of the file from where data should be read. If typeof position !== 'number', the data will be read from the current position.

The callback will be given three arguments: err, bytesRead, and buffers. bytesRead is how many bytes were read from the file.

If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesRead and buffers properties.

fs.realpath(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v8.0.0

Pipe/Socket resolve support was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.4.0

Calling realpath now works again for various edge cases on Windows.

v6.0.0

The cache parameter was removed.

v0.1.31

Added in: v0.1.31

Asynchronously computes the canonical pathname by resolving ., .., and symbolic links.

A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.

This function behaves like realpath(3), with some exceptions:

  1. No case conversion is performed on case-insensitive file systems.

  2. The maximum number of symbolic links is platform-independent and generally (much) higher than what the native realpath(3) implementation supports.

The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.

If path resolves to a socket or a pipe, the function will return a system dependent name for that object.

fs.realpath.native(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v9.2.0

Added in: v9.2.0

Asynchronous realpath(3).

The callback gets two arguments (err, resolvedPath).

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fs.rename(oldPath, newPath, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The oldPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronously rename file at oldPath to the pathname provided as newPath. In the case that newPath already exists, it will be overwritten. If there is a directory at newPath, an error will be raised instead. No arguments other than a possible exception are given to the completion callback.

See also: rename(2).

import { rename } from 'node:fs';

rename('oldFile.txt', 'newFile.txt', (err) => {
  if (err) throw err;
  console.log('Rename complete!');
});

fs.rmdir(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.0.0

Using fs.rmdir(path, { recursive: true }) on a path that is a file is no longer permitted and results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

v16.0.0

Using fs.rmdir(path, { recursive: true }) on a path that does not exist is no longer permitted and results in a ENOENT error.

v16.0.0

The recursive option is deprecated, using it triggers a deprecation warning.

v14.14.0

The recursive option is deprecated, use fs.rm instead.

v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameters can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
  • callback <Function>

Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.

Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

To get a behavior similar to the rm -rf Unix command, use fs.rm() with options { recursive: true, force: true }.

fs.rm(path[, options], callback)#

History
VersionChanges
v17.3.0, v16.14.0

The path parameter can be a WHATWG URL object using file: protocol.

v14.14.0

Added in: v14.14.0

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive removal. In recursive mode operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
  • callback <Function>

Asynchronously removes files and directories (modeled on the standard POSIX rm utility). No arguments other than a possible exception are given to the completion callback.

fs.stat(path[, options], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is an <fs.Stats> object.

In case of an error, the err.code will be one of Common System Errors.

Using fs.stat() to check for the existence of a file before calling fs.open(), fs.readFile(), or fs.writeFile() is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.

To check if a file exists without manipulating it afterwards, fs.access() is recommended.

For example, given the following directory structure:

- txtDir
-- file.txt
- app.js

The next program will check for the stats of the given paths:

import { stat } from 'node:fs';

const pathsToCheck = ['./txtDir', './txtDir/file.txt'];

for (let i = 0; i < pathsToCheck.length; i++) {
  stat(pathsToCheck[i], (err, stats) => {
    console.log(stats.isDirectory());
    console.log(stats);
  });
}

The resulting output will resemble:

true
Stats {
  dev: 16777220,
  mode: 16877,
  nlink: 3,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214262,
  size: 96,
  blocks: 0,
  atimeMs: 1561174653071.963,
  mtimeMs: 1561174614583.3518,
  ctimeMs: 1561174626623.5366,
  birthtimeMs: 1561174126937.2893,
  atime: 2019-06-22T03:37:33.072Z,
  mtime: 2019-06-22T03:36:54.583Z,
  ctime: 2019-06-22T03:37:06.624Z,
  birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
  dev: 16777220,
  mode: 33188,
  nlink: 1,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214074,
  size: 8,
  blocks: 8,
  atimeMs: 1561174616618.8555,
  mtimeMs: 1561174614584,
  ctimeMs: 1561174614583.8145,
  birthtimeMs: 1561174007710.7478,
  atime: 2019-06-22T03:36:56.619Z,
  mtime: 2019-06-22T03:36:54.584Z,
  ctime: 2019-06-22T03:36:54.584Z,
  birthtime: 2019-06-22T03:26:47.711Z
}

fs.symlink(target, path[, type], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v12.0.0

If the type argument is left undefined, Node will autodetect target type and automatically select dir or file.

v7.6.0

The target and path parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Creates the link called path pointing to target. No arguments other than a possible exception are given to the completion callback.

See the POSIX symlink(2) documentation for more details.

The type argument is only available on Windows and ignored on other platforms. It can be set to 'dir', 'file', or 'junction'. If the type argument is not a string, Node.js will autodetect target type and use 'file' or 'dir'. If the target does not exist, 'file' will be used. Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Relative targets are relative to the link's parent directory.

import { symlink } from 'node:fs';

symlink('./mew', './mewtwo', callback);

The above example creates a symbolic link mewtwo which points to mew in the same directory:

$ tree .
.
├── mew
└── mewtwo -> ./mew

fs.truncate(path[, len], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v16.0.0

The error returned may be an AggregateError if more than one error is returned.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.8.6

Added in: v0.8.6

Truncates the file. No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.

import { truncate } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was truncated');
});const { truncate } = require('node:fs');
// Assuming that 'path/file.txt' is a regular file.
truncate('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was truncated');
});

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

See the POSIX truncate(2) documentation for more details.

fs.unlink(path, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.

import { unlink } from 'node:fs';
// Assuming that 'path/file.txt' is a regular file.
unlink('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was deleted');
});

fs.unlink() will not work on a directory, empty or otherwise. To remove a directory, use fs.rmdir().

See the POSIX unlink(2) documentation for more details.

fs.unwatchFile(filename[, listener])#

Added in: v0.1.31

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of filename.

Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.

Using fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible.

fs.utimes(path, atime, mtime, callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v8.0.0

NaN, Infinity, and -Infinity are no longer valid time specifiers.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v4.1.0

Numeric strings, NaN, and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Change the file system timestamps of the object referenced by path.

The atime and mtime arguments follow these rules:

  • Values can be either numbers representing Unix epoch time in seconds, Dates, or a numeric string like '123456789.0'.
  • If the value can not be converted to a number, or is NaN, Infinity, or -Infinity, an Error will be thrown.

fs.watch(filename[, options][, listener])#

History
VersionChanges
v19.1.0

Added recursive support for Linux, AIX and IBMi.

v15.9.0, v14.17.0

Added support for closing the watcher with an AbortSignal.

v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol.

v7.0.0

The passed options object will never be modified.

v0.5.10

Added in: v0.5.10

  • filename <string> | <Buffer> | <URL>
  • options <string> | <Object>
    • persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true.
    • recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: false.
    • encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.
    • signal <AbortSignal> allows closing the watcher with an AbortSignal.
  • listener <Function> | <undefined> Default: undefined
  • Returns: <fs.FSWatcher>

Watch for changes on filename, where filename is either a file or a directory.

The second argument is optional. If options is provided as a string, it specifies the encoding. Otherwise options should be passed as an object.

The listener callback gets two arguments (eventType, filename). eventType is either 'rename' or 'change', and filename is the name of the file which triggered the event.

On most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

The listener callback is attached to the 'change' event fired by <fs.FSWatcher>, but it is not the same thing as the 'change' value of eventType.

If a signal is passed, aborting the corresponding AbortController will close the returned <fs.FSWatcher>.

Caveats#

The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations.

On Windows, no events will be emitted if the watched directory is moved or renamed. An EPERM error is reported when the watched directory is deleted.

Availability#

This feature depends on the underlying operating system providing a way to be notified of file system changes.

  • On Linux systems, this uses inotify(7).
  • On BSD systems, this uses kqueue(2).
  • On macOS, this uses kqueue(2) for files and FSEvents for directories.
  • On SunOS systems (including Solaris and SmartOS), this uses event ports.
  • On Windows systems, this feature depends on ReadDirectoryChangesW.
  • On AIX systems, this feature depends on AHAFS, which must be enabled.
  • On IBM i systems, this feature is not supported.

If the underlying functionality is not available for some reason, then fs.watch() will not be able to function and may throw an exception. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc) or host file systems when using virtualization software such as Vagrant or Docker.

It is still possible to use fs.watchFile(), which uses stat polling, but this method is slower and less reliable.

Inodes#

On Linux and macOS systems, fs.watch() resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.

AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).

Filename argument#

Providing filename argument in the callback is only supported on Linux, macOS, Windows, and AIX. Even on supported platforms, filename is not always guaranteed to be provided. Therefore, don't assume that filename argument is always provided in the callback, and have some fallback logic if it is null.

import { watch } from 'node:fs';
watch('somedir', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

fs.watchFile(filename[, options], listener)#

History
VersionChanges
v10.5.0

The bigint option is now supported.

v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol.

v0.1.31

Added in: v0.1.31

Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds.

The listener gets two arguments the current stat object and the previous stat object:

import { watchFile } from 'node:fs';

watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat. If the bigint option is true, the numeric values in these objects are specified as BigInts.

To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtimeMs and prev.mtimeMs.

When an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Using fs.watch() is more efficient than fs.watchFile and fs.unwatchFile. fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible.

When a file being watched by fs.watchFile() disappears and reappears, then the contents of previous in the second callback event (the file's reappearance) will be the same as the contents of previous in the first callback event (its disappearance).

This happens when:

  • the file is deleted, followed by a restore
  • the file is renamed and then renamed a second time back to its original name

fs.write(fd, buffer, offset[, length[, position]], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v14.0.0

The buffer parameter won't coerce unsupported input to strings anymore.

v10.10.0

The buffer parameter can now be any TypedArray or a DataView.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.4.0

The buffer parameter can now be a Uint8Array.

v7.2.0

The offset and length parameters are optional now.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Write buffer to the file specified by fd.

offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position. See pwrite(2).

The callback will be given three arguments (err, bytesWritten, buffer) where bytesWritten specifies how many bytes were written from buffer.

If this method is invoked as its util.promisify()ed version, it returns a promise for an Object with bytesWritten and buffer properties.

It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.write(fd, buffer[, options], callback)#

Added in: v18.3.0, v16.17.0

Write buffer to the file specified by fd.

Similar to the above fs.write function, this version takes an optional options object. If no options object is specified, it will default with the above values.

fs.write(fd, string[, position[, encoding]], callback)#

History
VersionChanges
v19.0.0

Passing to the string parameter an object with an own toString function is no longer supported.

v17.8.0

Passing to the string parameter an object with an own toString function is deprecated.

v14.12.0

The string parameter will stringify an object with an explicit toString function.

v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.2.0

The position parameter is optional now.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.11.5

Added in: v0.11.5

Write string to the file specified by fd. If string is not a string, an exception is thrown.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' the data will be written at the current position. See pwrite(2).

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Bytes written is not necessarily the same as string characters written. See Buffer.byteLength.

It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

On Windows, if the file descriptor is connected to the console (e.g. fd == 1 or stdout) a string containing non-ASCII characters will not be rendered properly by default, regardless of the encoding used. It is possible to configure the console to render UTF-8 properly by changing the active codepage with the chcp 65001 command. See the chcp docs for more details.

fs.writeFile(file, data[, options], callback)#

History
VersionChanges
v19.0.0

Passing to the string parameter an object with an own toString function is no longer supported.

v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v17.8.0

Passing to the string parameter an object with an own toString function is deprecated.

v16.0.0

The error returned may be an AggregateError if more than one error is returned.

v15.2.0, v14.17.0

The options argument may include an AbortSignal to abort an ongoing writeFile request.

v14.12.0

The data parameter will stringify an object with an explicit toString function.

v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.10.0

The data parameter can now be any TypedArray or a DataView.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.4.0

The data parameter can now be a Uint8Array.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v5.0.0

The file parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

When file is a filename, asynchronously writes data to the file, replacing the file if it already exists. data can be a string or a buffer.

When file is a file descriptor, the behavior is similar to calling fs.write() directly (which is recommended). See the notes below on using a file descriptor.

The encoding option is ignored if data is a buffer.

The mode option only affects the newly created file. See fs.open() for more details.

import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';

const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, (err) => {
  if (err) throw err;
  console.log('The file has been saved!');
});

If options is a string, then it specifies the encoding:

import { writeFile } from 'node:fs';

writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

It is unsafe to use fs.writeFile() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

Similarly to fs.readFile - fs.writeFile is a convenience method that performs multiple write calls internally to write the buffer passed to it. For performance sensitive code consider using fs.createWriteStream().

It is possible to use an <AbortSignal> to cancel an fs.writeFile(). Cancelation is "best effort", and some amount of data is likely still to be written.

import { writeFile } from 'node:fs';
import { Buffer } from 'node:buffer';

const controller = new AbortController();
const { signal } = controller;
const data = new Uint8Array(Buffer.from('Hello Node.js'));
writeFile('message.txt', data, { signal }, (err) => {
  // When a request is aborted - the callback is called with an AbortError
});
// When the request should be aborted
controller.abort();

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.writeFile performs.

Using fs.writeFile() with file descriptors#

When file is a file descriptor, the behavior is almost identical to directly calling fs.write() like:

import { write } from 'node:fs';
import { Buffer } from 'node:buffer';

write(fd, Buffer.from(data, options.encoding), callback);

The difference from directly calling fs.write() is that under some unusual conditions, fs.write() might write only part of the buffer and need to be retried to write the remaining data, whereas fs.writeFile() retries until the data is entirely written (or an error occurs).

The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.

For example, if fs.writeFile() is called twice in a row, first to write the string 'Hello', then to write the string ', World', the file would contain 'Hello, World', and might contain some of the file's original data (depending on the size of the original file, and the position of the file descriptor). If a file name had been used instead of a descriptor, the file would be guaranteed to contain only ', World'.

fs.writev(fd, buffers[, position], callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v12.9.0

Added in: v12.9.0

Write an array of ArrayBufferViews to the file specified by fd using writev().

position is the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position.

The callback will be given three arguments: err, bytesWritten, and buffers. bytesWritten is how many bytes were written from buffers.

If this method is util.promisify()ed, it returns a promise for an Object with bytesWritten and buffers properties.

It is unsafe to use fs.writev() multiple times on the same file without waiting for the callback. For this scenario, use fs.createWriteStream().

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Synchronous API#

The synchronous APIs perform all operations synchronously, blocking the event loop until the operation completes or fails.

fs.accessSync(path[, mode])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.11.15

Added in: v0.11.15

Synchronously tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. mode should be either the value fs.constants.F_OK or a mask consisting of the bitwise OR of any of fs.constants.R_OK, fs.constants.W_OK, and fs.constants.X_OK (e.g. fs.constants.W_OK | fs.constants.R_OK). Check File access constants for possible values of mode.

If any of the accessibility checks fail, an Error will be thrown. Otherwise, the method will return undefined.

import { accessSync, constants } from 'node:fs';

try {
  accessSync('etc/passwd', constants.R_OK | constants.W_OK);
  console.log('can read/write');
} catch (err) {
  console.error('no access!');
}

fs.appendFileSync(path, data[, options])#

History
VersionChanges
v7.0.0

The passed options object will never be modified.

v5.0.0

The file parameter can be a file descriptor now.

v0.6.7

Added in: v0.6.7

Synchronously append data to a file, creating the file if it does not yet exist. data can be a string or a <Buffer>.

The mode option only affects the newly created file. See fs.open() for more details.

import { appendFileSync } from 'node:fs';

try {
  appendFileSync('message.txt', 'data to append');
  console.log('The "data to append" was appended to file!');
} catch (err) {
  /* Handle the error */
}

If options is a string, then it specifies the encoding:

import { appendFileSync } from 'node:fs';

appendFileSync('message.txt', 'data to append', 'utf8');

The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync()). The file descriptor will not be closed automatically.

import { openSync, closeSync, appendFileSync } from 'node:fs';

let fd;

try {
  fd = openSync('message.txt', 'a');
  appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
  /* Handle the error */
} finally {
  if (fd !== undefined)
    closeSync(fd);
}

fs.chmodSync(path, mode)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.6.7

Added in: v0.6.7

For detailed information, see the documentation of the asynchronous version of this API: fs.chmod().

See the POSIX chmod(2) documentation for more detail.

fs.chownSync(path, uid, gid)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.97

Added in: v0.1.97

Synchronously changes owner and group of a file. Returns undefined. This is the synchronous version of fs.chown().

See the POSIX chown(2) documentation for more detail.

fs.closeSync(fd)#

Added in: v0.1.21

Closes the file descriptor. Returns undefined.

Calling fs.closeSync() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.

See the POSIX close(2) documentation for more detail.

fs.copyFileSync(src, dest[, mode])#

History
VersionChanges
v14.0.0

Changed flags argument to mode and imposed stricter type validation.

v8.5.0

Added in: v8.5.0

Synchronously copies src to dest. By default, dest is overwritten if it already exists. Returns undefined. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
import { copyFileSync, constants } from 'node:fs';

// destination.txt will be created or overwritten by default.
copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);

fs.cpSync(src, dest[, options])#

History
VersionChanges
v17.6.0, v16.15.0

Accepts an additional verbatimSymlinks option to specify whether to perform path resolution for symlinks.

v16.7.0

Added in: v16.7.0

Stability: 1 - Experimental

  • src <string> | <URL> source path to copy.
  • dest <string> | <URL> destination path to copy to.
  • options <Object>
    • dereference <boolean> dereference symlinks. Default: false.
    • errorOnExist <boolean> when force is false, and the destination exists, throw an error. Default: false.
    • filter <Function> Function to filter copied files/directories. Return true to copy the item, false to ignore it. Default: undefined
    • force <boolean> overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the errorOnExist option to change this behavior. Default: true.
    • preserveTimestamps <boolean> When true timestamps from src will be preserved. Default: false.
    • recursive <boolean> copy directories recursively Default: false
    • verbatimSymlinks <boolean> When true, path resolution for symlinks will be skipped. Default: false

Synchronously copies the entire directory structure from src to dest, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to cp dir1/ dir2/.

fs.existsSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Returns true if the path exists, false otherwise.

For detailed information, see the documentation of the asynchronous version of this API: fs.exists().

fs.exists() is deprecated, but fs.existsSync() is not. The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback.

import { existsSync } from 'node:fs';

if (existsSync('/etc/passwd'))
  console.log('The path exists.');

fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Sets the permissions on the file. Returns undefined.

See the POSIX fchmod(2) documentation for more detail.

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Sets the owner of the file. Returns undefined.

See the POSIX fchown(2) documentation for more detail.

fs.fdatasyncSync(fd)#

Added in: v0.1.96

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX fdatasync(2) documentation for details. Returns undefined.

fs.fstatSync(fd[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v0.1.95

Added in: v0.1.95

Retrieves the <fs.Stats> for the file descriptor.

See the POSIX fstat(2) documentation for more detail.

fs.fsyncSync(fd)#

Added in: v0.1.96

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX fsync(2) documentation for more detail. Returns undefined.

fs.ftruncateSync(fd[, len])#

Added in: v0.8.6

Truncates the file descriptor. Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate().

fs.futimesSync(fd, atime, mtime)#

History
VersionChanges
v4.1.0

Numeric strings, NaN, and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Synchronous version of fs.futimes(). Returns undefined.

fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Changes the permissions on a symbolic link. Returns undefined.

This method is only implemented on macOS.

See the POSIX lchmod(2) documentation for more detail.

fs.lchownSync(path, uid, gid)#

History
VersionChanges
v10.6.0

This API is no longer deprecated.

v0.4.7

Documentation-only deprecation.

Set the owner for the path. Returns undefined.

See the POSIX lchown(2) documentation for more details.

fs.lutimesSync(path, atime, mtime)#

Added in: v14.5.0, v12.19.0

Change the file system timestamps of the symbolic link referenced by path. Returns undefined, or throws an exception when parameters are incorrect or the operation fails. This is the synchronous version of fs.lutimes().

fs.linkSync(existingPath, newPath)#

History
VersionChanges
v7.6.0

The existingPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Creates a new link from the existingPath to the newPath. See the POSIX link(2) documentation for more detail. Returns undefined.

fs.lstatSync(path[, options])#

History
VersionChanges
v15.3.0, v14.17.0

Accepts a throwIfNoEntry option to specify whether an exception should be thrown if the entry does not exist.

v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.30

Added in: v0.1.30

Retrieves the <fs.Stats> for the symbolic link referred to by path.

See the POSIX lstat(2) documentation for more details.

fs.mkdirSync(path[, options])#

History
VersionChanges
v13.11.0, v12.17.0

In recursive mode, the first created path is returned now.

v10.12.0

The second argument can now be an options object with recursive and mode properties.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Synchronously creates a directory. Returns undefined, or if recursive is true, the first directory path created. This is the synchronous version of fs.mkdir().

See the POSIX mkdir(2) documentation for more details.

fs.mkdtempSync(prefix[, options])#

History
VersionChanges
v16.5.0, v14.18.0

The prefix parameter now accepts an empty string.

v5.10.0

Added in: v5.10.0

Returns the created directory path.

For detailed information, see the documentation of the asynchronous version of this API: fs.mkdtemp().

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fs.opendirSync(path[, options])#

History
VersionChanges
v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • encoding <string> | <null> Default: 'utf8'
    • bufferSize <number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: 32
  • Returns: <fs.Dir>

Synchronously open a directory. See opendir(3).

Creates an <fs.Dir>, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

fs.openSync(path[, flags[, mode]])#

History
VersionChanges
v11.1.0

The flags argument is now optional and defaults to 'r'.

v9.9.0

The as and as+ flags are supported now.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Returns an integer representing the file descriptor.

For detailed information, see the documentation of the asynchronous version of this API: fs.open().

fs.readdirSync(path[, options])#

History
VersionChanges
v10.10.0

New option withFileTypes was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Reads the contents of the directory.

See the POSIX readdir(3) documentation for more details.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames returned. If the encoding is set to 'buffer', the filenames returned will be passed as <Buffer> objects.

If options.withFileTypes is set to true, the result will contain <fs.Dirent> objects.

fs.readFileSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v5.0.0

The path parameter can be a file descriptor now.

v0.1.8

Added in: v0.1.8

Returns the contents of the path.

For detailed information, see the documentation of the asynchronous version of this API: fs.readFile().

If the encoding option is specified then this function returns a string. Otherwise it returns a buffer.

Similar to fs.readFile(), when the path is a directory, the behavior of fs.readFileSync() is platform-specific.

import { readFileSync } from 'node:fs';

// macOS, Linux, and Windows
readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]

//  FreeBSD
readFileSync('<directory>'); // => <data>

fs.readlinkSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.31

Added in: v0.1.31

Returns the symbolic link's string value.

See the POSIX readlink(2) documentation for more details.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer', the link path returned will be passed as a <Buffer> object.

fs.readSync(fd, buffer, offset, length[, position])#

History
VersionChanges
v10.10.0

The buffer parameter can now be any TypedArray or a DataView.

v6.0.0

The length parameter can now be 0.

v0.1.21

Added in: v0.1.21

Returns the number of bytesRead.

For detailed information, see the documentation of the asynchronous version of this API: fs.read().

fs.readSync(fd, buffer[, options])#

History
VersionChanges
v13.13.0, v12.17.0

Options object can be passed in to make offset, length, and position optional.

v13.13.0, v12.17.0

Added in: v13.13.0, v12.17.0

Returns the number of bytesRead.

Similar to the above fs.readSync function, this version takes an optional options object. If no options object is specified, it will default with the above values.

For detailed information, see the documentation of the asynchronous version of this API: fs.read().

fs.readvSync(fd, buffers[, position])#

Added in: v13.13.0, v12.17.0

For detailed information, see the documentation of the asynchronous version of this API: fs.readv().

fs.realpathSync(path[, options])#

History
VersionChanges
v8.0.0

Pipe/Socket resolve support was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v6.4.0

Calling realpathSync now works again for various edge cases on Windows.

v6.0.0

The cache parameter was removed.

v0.1.31

Added in: v0.1.31

Returns the resolved pathname.

For detailed information, see the documentation of the asynchronous version of this API: fs.realpath().

fs.realpathSync.native(path[, options])#

Added in: v9.2.0

Synchronous realpath(3).

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path returned. If the encoding is set to 'buffer', the path returned will be passed as a <Buffer> object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fs.renameSync(oldPath, newPath)#

History
VersionChanges
v7.6.0

The oldPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Renames the file from oldPath to newPath. Returns undefined.

See the POSIX rename(2) documentation for more details.

fs.rmdirSync(path[, options])#

History
VersionChanges
v16.0.0

Using fs.rmdirSync(path, { recursive: true }) on a path that is a file is no longer permitted and results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

v16.0.0

Using fs.rmdirSync(path, { recursive: true }) on a path that does not exist is no longer permitted and results in a ENOENT error.

v16.0.0

The recursive option is deprecated, using it triggers a deprecation warning.

v14.14.0

The recursive option is deprecated, use fs.rmSync instead.

v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v7.6.0

The path parameters can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js retries the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, operations are retried on failure. Default: false. Deprecated.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.

Synchronous rmdir(2). Returns undefined.

Using fs.rmdirSync() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

To get a behavior similar to the rm -rf Unix command, use fs.rmSync() with options { recursive: true, force: true }.

fs.rmSync(path[, options])#

History
VersionChanges
v17.3.0, v16.14.0

The path parameter can be a WHATWG URL object using file: protocol.

v14.14.0

Added in: v14.14.0

  • path <string> | <Buffer> | <URL>
  • options <Object>
    • force <boolean> When true, exceptions will be ignored if path does not exist. Default: false.
    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay milliseconds longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.

Synchronously removes files and directories (modeled on the standard POSIX rm utility). Returns undefined.

fs.statSync(path[, options])#

History
VersionChanges
v15.3.0, v14.17.0

Accepts a throwIfNoEntry option to specify whether an exception should be thrown if the entry does not exist.

v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Retrieves the <fs.Stats> for the path.

fs.symlinkSync(target, path[, type])#

History
VersionChanges
v12.0.0

If the type argument is left undefined, Node will autodetect target type and automatically select dir or file.

v7.6.0

The target and path parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.symlink().

fs.truncateSync(path[, len])#

Added in: v0.8.6

Truncates the file. Returns undefined. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called.

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

fs.unlinkSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v0.1.21

Added in: v0.1.21

Synchronous unlink(2). Returns undefined.

fs.utimesSync(path, atime, mtime)#

History
VersionChanges
v8.0.0

NaN, Infinity, and -Infinity are no longer valid time specifiers.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol.

v4.1.0

Numeric strings, NaN, and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.utimes().

fs.writeFileSync(file, data[, options])#

History
VersionChanges
v19.0.0

Passing to the data parameter an object with an own toString function is no longer supported.

v17.8.0

Passing to the data parameter an object with an own toString function is deprecated.

v14.12.0

The data parameter will stringify an object with an explicit toString function.

v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.10.0

The data parameter can now be any TypedArray or a DataView.

v7.4.0

The data parameter can now be a Uint8Array.

v5.0.0

The file parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

Returns undefined.

The mode option only affects the newly created file. See fs.open() for more details.

For detailed information, see the documentation of the asynchronous version of this API: fs.writeFile().

fs.writeSync(fd, buffer, offset[, length[, position]])#

History
VersionChanges
v14.0.0

The buffer parameter won't coerce unsupported input to strings anymore.

v10.10.0

The buffer parameter can now be any TypedArray or a DataView.

v7.4.0

The buffer parameter can now be a Uint8Array.

v7.2.0

The offset and length parameters are optional now.

v0.1.21

Added in: v0.1.21

For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...).

fs.writeSync(fd, buffer[, options])#

Added in: v18.3.0, v16.17.0

For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...).

fs.writeSync(fd, string[, position[, encoding]])#

History
VersionChanges
v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v7.2.0

The position parameter is optional now.

v0.11.5

Added in: v0.11.5

For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, string...).

fs.writevSync(fd, buffers[, position])#

Added in: v12.9.0

For detailed information, see the documentation of the asynchronous version of this API: fs.writev().

Common Objects#

The common objects are shared by all of the file system API variants (promise, callback, and synchronous).

Class: fs.Dir#

Added in: v12.12.0

A class representing a directory stream.

Created by fs.opendir(), fs.opendirSync(), or fsPromises.opendir().

import { opendir } from 'node:fs/promises';

try {
  const dir = await opendir('./');
  for await (const dirent of dir)
    console.log(dirent.name);
} catch (err) {
  console.error(err);
}

When using the async iterator, the <fs.Dir> object will be automatically closed after the iterator exits.

dir.close()#
Added in: v12.12.0

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

A promise is returned that will be resolved after the resource has been closed.

dir.close(callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v12.12.0

Added in: v12.12.0

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

The callback will be called after the resource handle has been closed.

dir.closeSync()#
Added in: v12.12.0

Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

dir.path#
Added in: v12.12.0

The read-only path of this directory as was provided to fs.opendir(), fs.opendirSync(), or fsPromises.opendir().

dir.read()#
Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an <fs.Dirent>.

A promise is returned that will be resolved with an <fs.Dirent>, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir.read(callback)#
Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an <fs.Dirent>.

After the read is completed, the callback will be called with an <fs.Dirent>, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir.readSync()#
Added in: v12.12.0

Synchronously read the next directory entry as an <fs.Dirent>. See the POSIX readdir(3) documentation for more detail.

If there are no more directory entries to read, null will be returned.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

dir[Symbol.asyncIterator]()#
Added in: v12.12.0

Asynchronously iterates over the directory until all entries have been read. Refer to the POSIX readdir(3) documentation for more detail.

Entries returned by the async iterator are always an <fs.Dirent>. The null case from dir.read() is handled internally.

See <fs.Dir> for an example.

Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

Class: fs.Dirent#

Added in: v10.10.0

A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an <fs.Dir>. The directory entry is a combination of the file name and file type pairs.

Additionally, when fs.readdir() or fs.readdirSync() is called with the withFileTypes option set to true, the resulting array is filled with <fs.Dirent> objects, rather than strings or <Buffer>s.

dirent.isBlockDevice()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a block device.

dirent.isCharacterDevice()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a character device.

dirent.isDirectory()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a file system directory.

dirent.isFIFO()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a first-in-first-out (FIFO) pipe.

dirent.isFile()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a regular file.

dirent.isSocket()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a socket.

dirent.isSymbolicLink()#
Added in: v10.10.0

Returns true if the <fs.Dirent> object describes a symbolic link.

dirent.name#
Added in: v10.10.0

The file name that this <fs.Dirent> object refers to. The type of this value is determined by the options.encoding passed to fs.readdir() or fs.readdirSync().

Class: fs.FSWatcher#

Added in: v0.5.8

A successful call to fs.watch() method will return a new <fs.FSWatcher> object.

All <fs.FSWatcher> objects emit a 'change' event whenever a specific watched file is modified.

Event: 'change'#
Added in: v0.5.8
  • eventType <string> The type of change event that has occurred
  • filename <string> | <Buffer> The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in fs.watch().

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a <Buffer> if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a UTF-8 string.

import { watch } from 'node:fs';
// Example when handled through fs.watch() listener
watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
  if (filename) {
    console.log(filename);
    // Prints: <Buffer ...>
  }
});
Event: 'close'#
Added in: v10.0.0

Emitted when the watcher stops watching for changes. The closed <fs.FSWatcher> object is no longer usable in the event handler.

Event: 'error'#
Added in: v0.5.8

Emitted when an error occurs while watching the file. The errored <fs.FSWatcher> object is no longer usable in the event handler.

watcher.close()#
Added in: v0.5.8

Stop watching for changes on the given <fs.FSWatcher>. Once stopped, the <fs.FSWatcher> object is no longer usable.

watcher.ref()#
Added in: v14.3.0, v12.20.0

When called, requests that the Node.js event loop not exit so long as the <fs.FSWatcher> is active. Calling watcher.ref() multiple times will have no effect.

By default, all <fs.FSWatcher> objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#
Added in: v14.3.0, v12.20.0

When called, the active <fs.FSWatcher> object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the <fs.FSWatcher> object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.StatWatcher#

Added in: v14.3.0, v12.20.0

A successful call to fs.watchFile() method will return a new <fs.StatWatcher> object.

watcher.ref()#
Added in: v14.3.0, v12.20.0

When called, requests that the Node.js event loop not exit so long as the <fs.StatWatcher> is active. Calling watcher.ref() multiple times will have no effect.

By default, all <fs.StatWatcher> objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#
Added in: v14.3.0, v12.20.0

When called, the active <fs.StatWatcher> object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the <fs.StatWatcher> object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.ReadStream#

Added in: v0.1.93

Instances of <fs.ReadStream> are created and returned using the fs.createReadStream() function.

Event: 'close'#
Added in: v0.1.93

Emitted when the <fs.ReadStream>'s underlying file descriptor has been closed.

Event: 'open'#
Added in: v0.1.93

Emitted when the <fs.ReadStream>'s file descriptor has been opened.

Event: 'ready'#
Added in: v9.11.0

Emitted when the <fs.ReadStream> is ready to be used.

Fires immediately after 'open'.

readStream.bytesRead#
Added in: v6.4.0

The number of bytes that have been read so far.

readStream.path#
Added in: v0.1.93

The path to the file the stream is reading from as specified in the first argument to fs.createReadStream(). If path is passed as a string, then readStream.path will be a string. If path is passed as a <Buffer>, then readStream.path will be a <Buffer>. If fd is specified, then readStream.path will be undefined.

readStream.pending#
Added in: v11.2.0, v10.16.0

This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.

Class: fs.Stats#

History
VersionChanges
v8.1.0

Added times as numbers.

v0.1.21

Added in: v0.1.21

A <fs.Stats> object provides information about a file.

Objects returned from fs.stat(), fs.lstat(), fs.fstat(), and their synchronous counterparts are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number, and the object will contain additional nanosecond-precision properties suffixed with Ns.

Stats {
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atimeMs: 1318289051000.1,
  mtimeMs: 1318289051000.1,
  ctimeMs: 1318289051000.1,
  birthtimeMs: 1318289051000.1,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

bigint version:

BigIntStats {
  dev: 2114n,
  ino: 48064969n,
  mode: 33188n,
  nlink: 1n,
  uid: 85n,
  gid: 100n,
  rdev: 0n,
  size: 527n,
  blksize: 4096n,
  blocks: 8n,
  atimeMs: 1318289051000n,
  mtimeMs: 1318289051000n,
  ctimeMs: 1318289051000n,
  birthtimeMs: 1318289051000n,
  atimeNs: 1318289051000000000n,
  mtimeNs: 1318289051000000000n,
  ctimeNs: 1318289051000000000n,
  birthtimeNs: 1318289051000000000n,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
stats.isBlockDevice()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a block device.

stats.isCharacterDevice()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a character device.

stats.isDirectory()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a file system directory.

If the <fs.Stats> object was obtained from fs.lstat(), this method will always return false. This is because fs.lstat() returns information about a symbolic link itself and not the path it resolves to.

stats.isFIFO()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a first-in-first-out (FIFO) pipe.

stats.isFile()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a regular file.

stats.isSocket()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a socket.

stats.isSymbolicLink()#
Added in: v0.1.10

Returns true if the <fs.Stats> object describes a symbolic link.

This method is only valid when using fs.lstat().

stats.dev#

The numeric identifier of the device containing the file.

stats.ino#

The file system specific "Inode" number for the file.

stats.mode#

A bit-field describing the file type and mode.

stats.nlink#

The number of hard-links that exist for the file.

stats.uid#

The numeric user identifier of the user that owns the file (POSIX).

stats.gid#

The numeric group identifier of the group that owns the file (POSIX).

stats.rdev#

A numeric device identifier if the file represents a device.

stats.size#

The size of the file in bytes.

If the underlying file system does not support getting the size of the file, this will be 0.

stats.blksize#

The file system block size for i/o operations.

stats.blocks#

The number of blocks allocated for this file.

stats.atimeMs#
Added in: v8.1.0

The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.

stats.mtimeMs#
Added in: v8.1.0

The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.

stats.ctimeMs#
Added in: v8.1.0

The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.

stats.birthtimeMs#
Added in: v8.1.0

The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.

stats.atimeNs#
Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was accessed expressed in nanoseconds since the POSIX Epoch.

stats.mtimeNs#
Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was modified expressed in nanoseconds since the POSIX Epoch.

stats.ctimeNs#
Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time the file status was changed expressed in nanoseconds since the POSIX Epoch.

stats.birthtimeNs#
Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the creation time of this file expressed in nanoseconds since the POSIX Epoch.

stats.atime#
Added in: v0.11.13

The timestamp indicating the last time this file was accessed.

stats.mtime#
Added in: v0.11.13

The timestamp indicating the last time this file was modified.

stats.ctime#
Added in: v0.11.13

The timestamp indicating the last time the file status was changed.

stats.birthtime#
Added in: v0.11.13

The timestamp indicating the creation time of this file.

Stat time values#

The atimeMs, mtimeMs, ctimeMs, birthtimeMs properties are numeric values that hold the corresponding times in milliseconds. Their precision is platform specific. When bigint: true is passed into the method that generates the object, the properties will be bigints, otherwise they will be numbers.

The atimeNs, mtimeNs, ctimeNs, birthtimeNs properties are bigints that hold the corresponding times in nanoseconds. They are only present when bigint: true is passed into the method that generates the object. Their precision is platform specific.

atime, mtime, ctime, and birthtime are Date object alternate representations of the various times. The Date and number values are not connected. Assigning a new number value, or mutating the Date value, will not be reflected in the corresponding alternate representation.

The times in the stat object have the following semantics:

  • atime "Access Time": Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.
  • mtime "Modified Time": Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.
  • ctime "Change Time": Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.
  • birthtime "Birth Time": Time of file creation. Set once when the file is created. On file systems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, Unix epoch timestamp 0). This value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.

Prior to Node.js 0.12, the ctime held the birthtime on Windows systems. As of 0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream#

Added in: v0.1.93

Instances of <fs.WriteStream> are created and returned using the fs.createWriteStream() function.

Event: 'close'#
Added in: v0.1.93

Emitted when the <fs.WriteStream>'s underlying file descriptor has been closed.

Event: 'open'#
Added in: v0.1.93

Emitted when the <fs.WriteStream>'s file is opened.

Event: 'ready'#
Added in: v9.11.0

Emitted when the <fs.WriteStream> is ready to be used.

Fires immediately after 'open'.

writeStream.bytesWritten#
Added in: v0.4.7

The number of bytes written so far. Does not include data that is still queued for writing.

writeStream.close([callback])#
Added in: v0.9.4

Closes writeStream. Optionally accepts a callback that will be executed once the writeStream is closed.

writeStream.path#
Added in: v0.1.93

The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream(). If path is passed as a string, then writeStream.path will be a string. If path is passed as a <Buffer>, then writeStream.path will be a <Buffer>.

writeStream.pending#
Added in: v11.2.0

This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.

fs.constants#

Returns an object containing commonly used constants for file system operations.

FS constants#

The following constants are exported by fs.constants and fsPromises.constants.

Not every constant will be available on every operating system; this is especially important for Windows, where many of the POSIX specific definitions are not available. For portable applications it is recommended to check for their presence before use.

To use more than one constant, use the bitwise OR | operator.

Example:

import { open, constants } from 'node:fs';

const {
  O_RDWR,
  O_CREAT,
  O_EXCL,
} = constants;

open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
  // ...
});
File access constants#

The following constants are meant for use as the mode parameter passed to fsPromises.access(), fs.access(), and fs.accessSync().

Constant Description
F_OK Flag indicating that the file is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
R_OK Flag indicating that the file can be read by the calling process.
W_OK Flag indicating that the file can be written by the calling process.
X_OK Flag indicating that the file can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

The definitions are also available on Windows.

File copy constants#

The following constants are meant for use with fs.copyFile().

Constant Description
COPYFILE_EXCL If present, the copy operation will fail with an error if the destination path already exists.
COPYFILE_FICLONE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used.
COPYFILE_FICLONE_FORCE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error.

The definitions are also available on Windows.

File open constants#

The following constants are meant for use with fs.open().

Constant Description
O_RDONLY Flag indicating to open a file for read-only access.
O_WRONLY Flag indicating to open a file for write-only access.
O_RDWR Flag indicating to open a file for read-write access.
O_CREAT Flag indicating to create the file if it does not already exist.
O_EXCL Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists.
O_NOCTTY Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
O_TRUNC Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
O_APPEND Flag indicating that data will be appended to the end of the file.
O_DIRECTORY Flag indicating that the open should fail if the path is not a directory.
O_NOATIME Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only.
O_NOFOLLOW Flag indicating that the open should fail if the path is a symbolic link.
O_SYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity.
O_DSYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity.
O_SYMLINK Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
O_DIRECT When set, an attempt will be made to minimize caching effects of file I/O.
O_NONBLOCK Flag indicating to open the file in nonblocking mode when possible.
UV_FS_O_FILEMAP When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored.

On Windows, only O_APPEND, O_CREAT, O_EXCL, O_RDONLY, O_RDWR, O_TRUNC, O_WRONLY, and UV_FS_O_FILEMAP are available.

File type constants#

The following constants are meant for use with the <fs.Stats> object's mode property for determining a file's type.

Constant Description
S_IFMT Bit mask used to extract the file type code.
S_IFREG File type constant for a regular file.
S_IFDIR File type constant for a directory.
S_IFCHR File type constant for a character-oriented device file.
S_IFBLK File type constant for a block-oriented device file.
S_IFIFO File type constant for a FIFO/pipe.
S_IFLNK File type constant for a symbolic link.
S_IFSOCK File type constant for a socket.

On Windows, only S_IFCHR, S_IFDIR, S_IFLNK, S_IFMT, and S_IFREG, are available.

File mode constants#

The following constants are meant for use with the <fs.Stats> object's mode property for determining the access permissions for a file.

Constant Description
S_IRWXU File mode indicating readable, writable, and executable by owner.
S_IRUSR File mode indicating readable by owner.
S_IWUSR File mode indicating writable by owner.
S_IXUSR File mode indicating executable by owner.
S_IRWXG File mode indicating readable, writable, and executable by group.
S_IRGRP File mode indicating readable by group.
S_IWGRP File mode indicating writable by group.
S_IXGRP File mode indicating executable by group.
S_IRWXO File mode indicating readable, writable, and executable by others.
S_IROTH File mode indicating readable by others.
S_IWOTH File mode indicating writable by others.
S_IXOTH File mode indicating executable by others.

On Windows, only S_IRUSR and S_IWUSR are available.

Notes#

Ordering of callback and promise-based operations#

Because they are executed asynchronously by the underlying thread pool, there is no guaranteed ordering when using either the callback or promise-based methods.

For example, the following is prone to error because the fs.stat() operation might complete before the fs.rename() operation:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
  if (err) throw err;
  console.log(`stats: ${JSON.stringify(stats)}`);
});

It is important to correctly order the operations by awaiting the results of one before invoking the other:

import { rename, stat } from 'node:fs/promises';

const from = '/tmp/hello';
const to = '/tmp/world';

try {
  await rename(from, to);
  const stats = await stat(to);
  console.log(`stats: ${JSON.stringify(stats)}`);
} catch (error) {
  console.error('there was an error:', error.message);
}const { rename, stat } = require('node:fs/promises');

(async function(from, to) {
  try {
    await rename(from, to);
    const stats = await stat(to);
    console.log(`stats: ${JSON.stringify(stats)}`);
  } catch (error) {
    console.error('there was an error:', error.message);
  }
})('/tmp/hello', '/tmp/world');

Or, when using the callback APIs, move the fs.stat() call into the callback of the fs.rename() operation:

import { rename, stat } from 'node:fs';

rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});const { rename, stat } = require('node:fs/promises');

rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

File paths#

Most fs operations accept file paths that may be specified in the form of a string, a <Buffer>, or a <URL> object using the file: protocol.

String paths#

String paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename. Relative paths will be resolved relative to the current working directory as determined by calling process.cwd().

Example using an absolute path on POSIX:

import { open } from 'node:fs/promises';

let fd;
try {
  fd = await open('/open/some/file.txt', 'r');
  // Do something with the file
} finally {
  await fd.close();
}

Example using a relative path on POSIX (relative to process.cwd()):

import { open } from 'node:fs/promises';

let fd;
try {
  fd = await open('file.txt', 'r');
  // Do something with the file
} finally {
  await fd.close();
}
File URL paths#
Added in: v7.6.0

For most node:fs module functions, the path or filename argument may be passed as a <URL> object using the file: protocol.

import { readFileSync } from 'node:fs';

readFileSync(new URL('file:///tmp/hello'));

file: URLs are always absolute paths.

Platform-specific considerations#

On Windows, file: <URL>s with a host name convert to UNC paths, while file: <URL>s with drive letters convert to local absolute paths. file: <URL>s with no host name and no drive letter will result in an error:

import { readFileSync } from 'node:fs';
// On Windows :

// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
readFileSync(new URL('file://hostname/p/a/t/h/file'));

// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
readFileSync(new URL('file:///C:/tmp/hello'));

// - WHATWG file URLs without hostname must have a drive letters
readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute

file: <URL>s with drive letters must use : as a separator just after the drive letter. Using another separator will result in an error.

On all other platforms, file: <URL>s with a host name are unsupported and will result in an error:

import { readFileSync } from 'node:fs';
// On other platforms:

// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute

// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
readFileSync(new URL('file:///tmp/hello'));

A file: <URL> having encoded slash characters will result in an error on all platforms:

import { readFileSync } from 'node:fs';

// On Windows
readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

// On POSIX
readFileSync(new URL('file:///p/a/t/h/%2F'));
readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */

On Windows, file: <URL>s having encoded backslash will result in an error:

import { readFileSync } from 'node:fs';

// On Windows
readFileSync(new URL('file:///C:/path/%5C'));
readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */
Buffer paths#

Paths specified using a <Buffer> are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, <Buffer> paths may be relative or absolute:

Example using an absolute path on POSIX:

import { open } from 'node:fs/promises';
import { Buffer } from 'node:buffer';

let fd;
try {
  fd = await open(Buffer.from('/open/some/file.txt'), 'r');
  // Do something with the file
} finally {
  await fd.close();
}
Per-drive working directories on Windows#

On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example fs.readdirSync('C:\\') can potentially return a different result than fs.readdirSync('C:'). For more information, see this MSDN page.

File descriptors#

On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.

The callback-based fs.open(), and synchronous fs.openSync() methods open a file and allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.

Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.

import { open, close, fstat } from 'node:fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  try {
    fstat(fd, (err, stat) => {
      if (err) {
        closeFd(fd);
        throw err;
      }

      // use stat

      closeFd(fd);
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

The promise-based APIs use a <FileHandle> object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:

import { open } from 'node:fs/promises';

let file;
try {
  file = await open('/open/some/file.txt', 'r');
  const stat = await file.stat();
  // use stat
} finally {
  await file.close();
}

Threadpool usage#

All callback and promise-based file system APIs (with the exception of fs.FSWatcher()) use libuv's threadpool. This can have surprising and negative performance implications for some applications. See the UV_THREADPOOL_SIZE documentation for more information.

File system flags#

The following flags are available wherever the flag option takes a string.

  • 'a': Open file for appending. The file is created if it does not exist.

  • 'ax': Like 'a' but fails if the path exists.

  • 'a+': Open file for reading and appending. The file is created if it does not exist.

  • 'ax+': Like 'a+' but fails if the path exists.

  • 'as': Open file for appending in synchronous mode. The file is created if it does not exist.

  • 'as+': Open file for reading and appending in synchronous mode. The file is created if it does not exist.

  • 'r': Open file for reading. An exception occurs if the file does not exist.

  • 'r+': Open file for reading and writing. An exception occurs if the file does not exist.

  • 'rs+': Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

    This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.

    This doesn't turn fs.open() or fsPromises.open() into a synchronous blocking call. If synchronous operation is desired, something like fs.openSync() should be used.

  • 'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx': Like 'w' but fails if the path exists.

  • 'w+': Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx+': Like 'w+' but fails if the path exists.

flag can also be a number as documented by open(2); commonly used constants are available from fs.constants. On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY to FILE_GENERIC_WRITE, or O_EXCL|O_CREAT to CREATE_NEW, as accepted by CreateFileW.

The exclusive flag 'x' (O_EXCL flag in open(2)) causes the operation to return an error if the path already exists. On POSIX, if the path is a symbolic link, using O_EXCL returns an error even if the link is to a path that does not exist. The exclusive flag might not work with network file systems.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Modifying a file rather than replacing it may require the flag option to be set to 'r+' rather than the default 'w'.

The behavior of some flags are platform-specific. As such, opening a directory on macOS and Linux with the 'a+' flag, as in the example below, will return an error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle will be returned.

// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
  // => [Error: EISDIR: illegal operation on a directory, open <directory>]
});

// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
  // => null, <fd>
});

On Windows, opening an existing hidden file using the 'w' flag (either through fs.open(), fs.writeFile(), or fsPromises.open()) will fail with EPERM. Existing hidden files can be opened for writing with the 'r+' flag.

A call to fs.ftruncate() or filehandle.truncate() can be used to reset the file contents.

Global objects#

These objects are available in all modules. The following variables may appear to be global but are not. They exist only in the scope of modules, see the module system documentation:

The objects listed here are specific to Node.js. There are built-in objects that are part of the JavaScript language itself, which are also globally accessible.

Class: AbortController#

History
VersionChanges
v15.4.0

No longer experimental.

v15.0.0, v14.17.0

Added in: v15.0.0, v14.17.0

A utility class used to signal cancelation in selected Promise-based APIs. The API is based on the Web API AbortController.

const ac = new AbortController();

ac.signal.addEventListener('abort', () => console.log('Aborted!'),
                           { once: true });

ac.abort();

console.log(ac.signal.aborted);  // Prints True

abortController.abort([reason])#

History
VersionChanges
v17.2.0, v16.14.0

Added the new optional reason argument.

v15.0.0, v14.17.0

Added in: v15.0.0, v14.17.0

  • reason <any> An optional reason, retrievable on the AbortSignal's reason property.

Triggers the abort signal, causing the abortController.signal to emit the 'abort' event.

abortController.signal#

Added in: v15.0.0, v14.17.0

Class: AbortSignal#

Added in: v15.0.0, v14.17.0

The AbortSignal is used to notify observers when the abortController.abort() method is called.

Static method: AbortSignal.abort([reason])#
History
VersionChanges
v17.2.0, v16.14.0

Added the new optional reason argument.

v15.12.0, v14.17.0

Added in: v15.12.0, v14.17.0

Returns a new already aborted AbortSignal.

Static method: AbortSignal.timeout(delay)#
Added in: v17.3.0, v16.14.0
  • delay <number> The number of milliseconds to wait before triggering the AbortSignal.

Returns a new AbortSignal which will be aborted in delay milliseconds.

Event: 'abort'#
Added in: v15.0.0, v14.17.0

The 'abort' event is emitted when the abortController.abort() method is called. The callback is invoked with a single object argument with a single type property set to 'abort':

const ac = new AbortController();

// Use either the onabort property...
ac.signal.onabort = () => console.log('aborted!');

// Or the EventTarget API...
ac.signal.addEventListener('abort', (event) => {
  console.log(event.type);  // Prints 'abort'
}, { once: true });

ac.abort();

The AbortController with which the AbortSignal is associated will only ever trigger the 'abort' event once. We recommended that code check that the abortSignal.aborted attribute is false before adding an 'abort' event listener.

Any event listeners attached to the AbortSignal should use the { once: true } option (or, if using the EventEmitter APIs to attach a listener, use the once() method) to ensure that the event listener is removed as soon as the 'abort' event is handled. Failure to do so may result in memory leaks.

abortSignal.aborted#
Added in: v15.0.0, v14.17.0
  • Type: <boolean> True after the AbortController has been aborted.
abortSignal.onabort#
Added in: v15.0.0, v14.17.0

An optional callback function that may be set by user code to be notified when the abortController.abort() function has been called.

abortSignal.reason#
Added in: v17.2.0, v16.14.0

An optional reason specified when the AbortSignal was triggered.

const ac = new AbortController();
ac.abort(new Error('boom!'));
console.log(ac.signal.reason);  // Error('boom!');
abortSignal.throwIfAborted()#
Added in: v17.3.0, v16.17.0

If abortSignal.aborted is true, throws abortSignal.reason.

Class: Blob#

Added in: v18.0.0

See <Blob>.

Class: Buffer#

Added in: v0.1.103

Used to handle binary data. See the buffer section.

Class: ByteLengthQueuingStrategy#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ByteLengthQueuingStrategy.

__dirname#

This variable may appear to be global but is not. See __dirname.

__filename#

This variable may appear to be global but is not. See __filename.

atob(data)#

Added in: v16.0.0

Stability: 3 - Legacy. Use Buffer.from(data, 'base64') instead.

Global alias for buffer.atob().

BroadcastChannel#

Added in: v18.0.0

See <BroadcastChannel>.

btoa(data)#

Added in: v16.0.0

Stability: 3 - Legacy. Use buf.toString('base64') instead.

Global alias for buffer.btoa().

clearImmediate(immediateObject)#

Added in: v0.9.1

clearImmediate is described in the timers section.

clearInterval(intervalObject)#

Added in: v0.0.1

clearInterval is described in the timers section.

clearTimeout(timeoutObject)#

Added in: v0.0.1

clearTimeout is described in the timers section.

Class: CompressionStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of CompressionStream.

console#

Added in: v0.1.100

Used to print to stdout and stderr. See the console section.

Class: CountQueuingStrategy#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of CountQueuingStrategy.

Crypto#

History
VersionChanges
v19.0.0

No longer behind --experimental-global-webcrypto CLI flag.

v17.6.0, v16.15.0

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-global-webcrypto CLI flag.

A browser-compatible implementation of <Crypto>. This global is available only if the Node.js binary was compiled with including support for the node:crypto module.

crypto#

History
VersionChanges
v19.0.0

No longer behind --experimental-global-webcrypto CLI flag.

v17.6.0, v16.15.0

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-global-webcrypto CLI flag.

A browser-compatible implementation of the Web Crypto API.

CryptoKey#

History
VersionChanges
v19.0.0

No longer behind --experimental-global-webcrypto CLI flag.

v17.6.0, v16.15.0

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-global-webcrypto CLI flag.

A browser-compatible implementation of <CryptoKey>. This global is available only if the Node.js binary was compiled with including support for the node:crypto module.

CustomEvent#

History
VersionChanges
v19.0.0

No longer behind --experimental-global-customevent CLI flag.

v18.7.0, v16.17.0

Added in: v18.7.0, v16.17.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-global-customevent CLI flag.

A browser-compatible implementation of the CustomEvent Web API.

Class: DecompressionStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of DecompressionStream.

Event#

History
VersionChanges
v15.4.0

No longer experimental.

v15.0.0

Added in: v15.0.0

A browser-compatible implementation of the Event class. See EventTarget and Event API for more details.

EventTarget#

History
VersionChanges
v15.4.0

No longer experimental.

v15.0.0

Added in: v15.0.0

A browser-compatible implementation of the EventTarget class. See EventTarget and Event API for more details.

exports#

This variable may appear to be global but is not. See exports.

fetch#

History
VersionChanges
v18.0.0

No longer behind --experimental-global-fetch CLI flag.

v17.5.0, v16.15.0

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-fetch CLI flag.

A browser-compatible implementation of the fetch() function.

Class FormData#

History
VersionChanges
v18.0.0

No longer behind --experimental-global-fetch CLI flag.

v17.6.0, v16.15.0

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-fetch CLI flag.

A browser-compatible implementation of <FormData>.

global#

Added in: v0.1.27

In browsers, the top-level scope is the global scope. This means that within the browser var something will define a new global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside a Node.js module will be local to that module.

Class Headers#

History
VersionChanges
v18.0.0

No longer behind --experimental-global-fetch CLI flag.

v17.5.0, v16.15.0

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-fetch CLI flag.

A browser-compatible implementation of <Headers>.

MessageChannel#

Added in: v15.0.0

The MessageChannel class. See MessageChannel for more details.

MessageEvent#

Added in: v15.0.0

The MessageEvent class. See MessageEvent for more details.

MessagePort#

Added in: v15.0.0

The MessagePort class. See MessagePort for more details.

module#

This variable may appear to be global but is not. See module.

PerformanceEntry#

Added in: v19.0.0

The PerformanceEntry class. See PerformanceEntry for more details.

PerformanceMark#

Added in: v19.0.0

The PerformanceMark class. See PerformanceMark for more details.

PerformanceMeasure#

Added in: v19.0.0

The PerformanceMeasure class. See PerformanceMeasure for more details.

PerformanceObserver#

Added in: v19.0.0

The PerformanceObserver class. See PerformanceObserver for more details.

PerformanceObserverEntryList#

Added in: v19.0.0

The PerformanceObserverEntryList class. See PerformanceObserverEntryList for more details.

PerformanceResourceTiming#

Added in: v19.0.0

The PerformanceResourceTiming class. See PerformanceResourceTiming for more details.

performance#

Added in: v16.0.0

The perf_hooks.performance object.

process#

Added in: v0.1.7

The process object. See the process object section.

queueMicrotask(callback)#

Added in: v11.0.0

The queueMicrotask() method queues a microtask to invoke callback. If callback throws an exception, the process object 'uncaughtException' event will be emitted.

The microtask queue is managed by V8 and may be used in a similar manner to the process.nextTick() queue, which is managed by Node.js. The process.nextTick() queue is always processed before the microtask queue within each turn of the Node.js event loop.

// Here, `queueMicrotask()` is used to ensure the 'load' event is always
// emitted asynchronously, and therefore consistently. Using
// `process.nextTick()` here would result in the 'load' event always emitting
// before any other promise jobs.

DataHandler.prototype.load = async function load(key) {
  const hit = this._cache.get(key);
  if (hit !== undefined) {
    queueMicrotask(() => {
      this.emit('load', hit);
    });
    return;
  }

  const data = await fetchData(key);
  this._cache.set(key, data);
  this.emit('load', data);
};

Class: ReadableByteStreamController#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableByteStreamController.

Class: ReadableStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableStream.

Class: ReadableStreamBYOBReader#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableStreamBYOBReader.

Class: ReadableStreamBYOBRequest#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableStreamBYOBRequest.

Class: ReadableStreamDefaultController#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableStreamDefaultController.

Class: ReadableStreamDefaultReader#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of ReadableStreamDefaultReader.

require()#

This variable may appear to be global but is not. See require().

Response#

History
VersionChanges
v18.0.0

No longer behind --experimental-global-fetch CLI flag.

v17.5.0, v16.15.0

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-fetch CLI flag.

A browser-compatible implementation of <Response>.

Request#

History
VersionChanges
v18.0.0

No longer behind --experimental-global-fetch CLI flag.

v17.5.0, v16.15.0

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-fetch CLI flag.

A browser-compatible implementation of <Request>.

setImmediate(callback[, ...args])#

Added in: v0.9.1

setImmediate is described in the timers section.

setInterval(callback, delay[, ...args])#

Added in: v0.0.1

setInterval is described in the timers section.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1

setTimeout is described in the timers section.

structuredClone(value[, options])#

Added in: v17.0.0

The WHATWG structuredClone method.

SubtleCrypto#

History
VersionChanges
v19.0.0

No longer behind --experimental-global-webcrypto CLI flag.

v17.6.0, v16.15.0

Added in: v17.6.0, v16.15.0

Stability: 1 - Experimental. Disable this API with the --no-experimental-global-webcrypto CLI flag.

A browser-compatible implementation of <SubtleCrypto>. This global is available only if the Node.js binary was compiled with including support for the node:crypto module.

DOMException#

Added in: v17.0.0

The WHATWG DOMException class. See DOMException for more details.

TextDecoder#

Added in: v11.0.0

The WHATWG TextDecoder class. See the TextDecoder section.

Class: TextDecoderStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of TextDecoderStream.

TextEncoder#

Added in: v11.0.0

The WHATWG TextEncoder class. See the TextEncoder section.

Class: TextEncoderStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of TextEncoderStream.

Class: TransformStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of TransformStream.

Class: TransformStreamDefaultController#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of TransformStreamDefaultController.

URL#

Added in: v10.0.0

The WHATWG URL class. See the URL section.

URLSearchParams#

Added in: v10.0.0

The WHATWG URLSearchParams class. See the URLSearchParams section.

WebAssembly#

Added in: v8.0.0

The object that acts as the namespace for all W3C WebAssembly related functionality. See the Mozilla Developer Network for usage and compatibility.

Class: WritableStream#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of WritableStream.

Class: WritableStreamDefaultController#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of WritableStreamDefaultController.

Class: WritableStreamDefaultWriter#

Added in: v18.0.0

Stability: 1 - Experimental.

A browser-compatible implementation of WritableStreamDefaultWriter.

HTTP#

Stability: 2 - Stable

Source Code: lib/http.js

To use the HTTP server and client one must require('node:http').

The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses, so the user is able to stream data.

HTTP message headers are represented by an object like this:

{ 'content-length': '123',
  'content-type': 'text/plain',
  'connection': 'keep-alive',
  'host': 'example.com',
  'accept': '*/*' }

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, the Node.js HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.

See message.headers for details on how duplicate headers are handled.

The raw headers as they were received are retained in the rawHeaders property, which is an array of [key, value, key2, value2, ...]. For example, the previous message header object might have a rawHeaders list like the following:

[ 'ConTent-Length', '123456',
  'content-LENGTH', '123',
  'content-type', 'text/plain',
  'CONNECTION', 'keep-alive',
  'Host', 'example.com',
  'accepT', '*/*' ]

Class: http.Agent#

Added in: v0.3.4

An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).

It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent when the socket emits either a 'close' event or an 'agentRemove' event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:

http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});

An agent may also be used for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection.

agent:false:

http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false,  // Create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});

new Agent([options])#

History
VersionChanges
v15.6.0, v14.17.0

Change the default scheduling from 'fifo' to 'lifo'.

v14.5.0, v12.20.0

Add scheduling option to specify the free socket scheduling strategy.

v14.5.0, v12.19.0

Add maxTotalSockets option to agent constructor.

v0.3.4

Added in: v0.3.4

  • options <Object> Set of configurable options to set on the agent. Can have the following fields:
    • keepAlive <boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Not to be confused with the keep-alive value of the Connection header. The Connection: keep-alive header is always sent when using an agent except when the Connection header is explicitly specified or when the keepAlive and maxSockets options are respectively set to false and Infinity, in which case Connection: close will be used. Default: false.
    • keepAliveMsecs <number> When using the keepAlive option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the keepAlive option is false or undefined. Default: 1000.
    • maxSockets <number> Maximum number of sockets to allow per host. If the same host opens multiple concurrent connections, each request will use new socket until the maxSockets value is reached. If the host attempts to open more connections than maxSockets, the additional requests will enter into a pending request queue, and will enter active connection state when an existing connection terminates. This makes sure there are at most maxSockets active connections at any point in time, from a given host. Default: Infinity.
    • maxTotalSockets <number> Maximum number of sockets allowed for all hosts in total. Each request will use a new socket until the maximum is reached. Default: Infinity.
    • maxFreeSockets <number> Maximum number of sockets per host to leave open in a free state. Only relevant if keepAlive is set to true. Default: 256.
    • scheduling <string> Scheduling strategy to apply when picking the next free socket to use. It can be 'fifo' or 'lifo'. The main difference between the two scheduling strategies is that 'lifo' selects the most recently used socket, while 'fifo' selects the least recently used socket. In case of a low rate of request per second, the 'lifo' scheduling will lower the risk of picking a socket that might have been closed by the server due to inactivity. In case of a high rate of request per second, the 'fifo' scheduling will maximize the number of open sockets, while the 'lifo' scheduling will keep it as low as possible. Default: 'lifo'.
    • timeout <number> Socket timeout in milliseconds. This will set the timeout when the socket is created.

options in socket.connect() are also supported.

The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.

To configure any of them, a custom http.Agent instance must be created.

const http = require('node:http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);

agent.createConnection(options[, callback])#

Added in: v0.11.4

Produces a socket/stream to be used for HTTP requests.

By default, this function is the same as net.createConnection(). However, custom agents may override this method in case greater flexibility is desired.

A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback.

This method is guaranteed to return an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

callback has a signature of (err, stream).

agent.keepSocketAlive(socket)#

Added in: v8.1.0

Called when socket is detached from a request and could be persisted by the Agent. Default behavior is to:

socket.setKeepAlive(true, this.keepAliveMsecs);
socket.unref();
return true;

This method can be overridden by a particular Agent subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request.

The socket argument can be an instance of <net.Socket>, a subclass of <stream.Duplex>.

agent.reuseSocket(socket, request)#

Added in: v8.1.0

Called when socket is attached to request after being persisted because of the keep-alive options. Default behavior is to:

socket.ref();

This method can be overridden by a particular Agent subclass.

The socket argument can be an instance of <net.Socket>, a subclass of <stream.Duplex>.

agent.destroy()#

Added in: v0.11.4

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when it is no longer needed. Otherwise, sockets might stay open for quite a long time before the server terminates them.

agent.freeSockets#

History
VersionChanges
v16.0.0

The property now has a null prototype.

v0.11.4

Added in: v0.11.4

An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.

Sockets in the freeSockets list will be automatically destroyed and removed from the array on 'timeout'.

agent.getName([options])#

History
VersionChanges
v17.7.0, v16.15.0

The options parameter is now optional.

v0.11.4

Added in: v0.11.4

  • options <Object> A set of options providing information for name generation
    • host <string> A domain name or IP address of the server to issue the request to
    • port <number> Port of remote server
    • localAddress <string> Local interface to bind for network connections when issuing the request
    • family <integer> Must be 4 or 6 if this doesn't equal undefined.
  • Returns: <string>

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress or host:port:localAddress:family. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

agent.maxFreeSockets#

Added in: v0.11.7

By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state.

agent.maxSockets#

Added in: v0.3.6

By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of agent.getName().

agent.maxTotalSockets#

Added in: v14.5.0, v12.19.0

By default set to Infinity. Determines how many concurrent sockets the agent can have open. Unlike maxSockets, this parameter applies across all origins.

agent.requests#

History
VersionChanges
v16.0.0

The property now has a null prototype.

v0.5.9

Added in: v0.5.9

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

agent.sockets#

History
VersionChanges
v16.0.0

The property now has a null prototype.

v0.3.6

Added in: v0.3.6

An object which contains arrays of sockets currently in use by the agent. Do not modify.

Class: http.ClientRequest#

Added in: v0.1.17

This object is created internally and returned from http.request(). It represents an in-progress request whose header has already been queued. The header is still mutable using the setHeader(name, value), getHeader(name), removeHeader(name) API. The actual header will be sent along with the first data chunk or when calling request.end().

To get the response, add a listener for 'response' to the request object. 'response' will be emitted from the request object when the response headers have been received. The 'response' event is executed with one argument which is an instance of http.IncomingMessage.

During the 'response' event, one can add listeners to the response object; particularly to listen for the 'data' event.

If no 'response' handler is added, then the response will be entirely discarded. However, if a 'response' event handler is added, then the data from the response object must be consumed, either by calling response.read() whenever there is a 'readable' event, or by adding a 'data' handler, or by calling the .resume() method. Until the data is consumed, the 'end' event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error.

For backward compatibility, res will only emit 'error' if there is an 'error' listener registered.

Set Content-Length header to limit the response body size. Mismatching the Content-Length header value will result in an [Error][] being thrown, identified by code: 'ERR_HTTP_CONTENT_LENGTH_MISMATCH'.

Content-Length value should be in bytes, not characters. Use Buffer.byteLength() to determine the length of the body in bytes.

Event: 'abort'#

Added in: v1.4.1Deprecated since: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Listen for the 'close' event instead.

Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort().

Event: 'close'#

Added in: v0.5.4

Indicates that the request is completed, or its underlying connection was terminated prematurely (before the response completion).

Event: 'connect'#

Added in: v0.7.0

Emitted each time a server responds to a request with a CONNECT method. If this event is not being listened for, clients receiving a CONNECT method will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

A client and server pair demonstrating how to listen for the 'connect' event:

const http = require('node:http');
const net = require('node:net');
const { URL } = require('node:url');

// Create an HTTP tunneling proxy
const proxy = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('okay');
});
proxy.on('connect', (req, clientSocket, head) => {
  // Connect to an origin server
  const { port, hostname } = new URL(`http://${req.url}`);
  const serverSocket = net.connect(port || 80, hostname, () => {
    clientSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    serverSocket.write(head);
    serverSocket.pipe(clientSocket);
    clientSocket.pipe(serverSocket);
  });
});

// Now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // Make a request to a tunneling proxy
  const options = {
    port: 1337,
    host: '127.0.0.1',
    method: 'CONNECT',
    path: 'www.google.com:80',
  };

  const req = http.request(options);
  req.end();

  req.on('connect', (res, socket, head) => {
    console.log('got connected!');

    // Make a request over an HTTP tunnel
    socket.write('GET / HTTP/1.1\r\n' +
                 'Host: www.google.com:80\r\n' +
                 'Connection: close\r\n' +
                 '\r\n');
    socket.on('data', (chunk) => {
      console.log(chunk.toString());
    });
    socket.on('end', () => {
      proxy.close();
    });
  });
});

Event: 'continue'#

Added in: v0.3.2

Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.

Event: 'finish'#

Added in: v0.3.6

Emitted when the request has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the server has received anything yet.

Event: 'information'#

Added in: v10.0.0

Emitted when the server sends a 1xx intermediate response (excluding 101 Upgrade). The listeners of this event will receive an object containing the HTTP version, status code, status message, key-value headers object, and array with the raw header names followed by their respective values.

const http = require('node:http');

const options = {
  host: '127.0.0.1',
  port: 8080,
  path: '/length_request',
};

// Make a request
const req = http.request(options);
req.end();

req.on('information', (info) => {
  console.log(`Got information prior to main response: ${info.statusCode}`);
});

101 Upgrade statuses do not fire this event due to their break from the traditional HTTP request/response chain, such as web sockets, in-place TLS upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the 'upgrade' event instead.

Event: 'response'#

Added in: v0.1.0

Emitted when a response is received to this request. This event is emitted only once.

Event: 'socket'#

Added in: v0.5.3

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Event: 'timeout'#

Added in: v0.7.8

Emitted when the underlying socket times out from inactivity. This only notifies that the socket has been idle. The request must be destroyed manually.

See also: request.setTimeout().

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a server responds to a request with an upgrade. If this event is not being listened for and the response status code is 101 Switching Protocols, clients receiving an upgrade header will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

A client server pair demonstrating how to listen for the 'upgrade' event.

const http = require('node:http');

// Create an HTTP server
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('okay');
});
server.on('upgrade', (req, socket, head) => {
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
               'Upgrade: WebSocket\r\n' +
               'Connection: Upgrade\r\n' +
               '\r\n');

  socket.pipe(socket); // echo back
});

// Now that server is running
server.listen(1337, '127.0.0.1', () => {

  // make a request
  const options = {
    port: 1337,
    host: '127.0.0.1',
    headers: {
      'Connection': 'Upgrade',
      'Upgrade': 'websocket',
    },
  };

  const req = http.request(options);
  req.end();

  req.on('upgrade', (res, socket, upgradeHead) => {
    console.log('got upgraded!');
    socket.end();
    process.exit(0);
  });
});

request.abort()#

Added in: v0.3.8Deprecated since: v14.1.0, v13.14.0

Stability: 0 - Deprecated: Use request.destroy() instead.

Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

request.aborted#

History
VersionChanges
v17.0.0, v16.12.0

Deprecated since: v17.0.0, v16.12.0

v11.0.0

The aborted property is no longer a timestamp number.

v0.11.14

Added in: v0.11.14

Stability: 0 - Deprecated. Check request.destroyed instead.

The request.aborted property will be true if the request has been aborted.

request.connection#

Added in: v0.3.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use request.socket.

See request.socket.

request.cork()#

Added in: v13.2.0, v12.16.0

See writable.cork().

request.end([data[, encoding]][, callback])#

History
VersionChanges
v15.0.0

The data parameter can now be a Uint8Array.

v10.0.0

This method now returns a reference to ClientRequest.

v0.1.90

Added in: v0.1.90

Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n'.

If data is specified, it is equivalent to calling request.write(data, encoding) followed by request.end(callback).

If callback is specified, it will be called when the request stream is finished.

request.destroy([error])#

History
VersionChanges
v14.5.0

The function returns this for consistency with other Readable streams.

v0.3.0

Added in: v0.3.0

  • error <Error> Optional, an error to emit with 'error' event.
  • Returns: <this>

Destroy the request. Optionally emit an 'error' event, and emit a 'close' event. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

See writable.destroy() for further details.

request.destroyed#
Added in: v14.1.0, v13.14.0

Is true after request.destroy() has been called.

See writable.destroyed for further details.

request.finished#

Added in: v0.0.1Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use request.writableEnded.

The request.finished property will be true if request.end() has been called. request.end() will automatically be called if the request was initiated via http.get().

request.flushHeaders()#

Added in: v1.6.0

Flushes the request headers.

For efficiency reasons, Node.js normally buffers the request headers until request.end() is called or the first chunk of request data is written. It then tries to pack the request headers and data into a single TCP packet.

That's usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. request.flushHeaders() bypasses the optimization and kickstarts the request.

request.getHeader(name)#

Added in: v1.6.0

Reads out a header on the request. The name is case-insensitive. The type of the return value depends on the arguments provided to request.setHeader().

request.setHeader('content-type', 'text/html');
request.setHeader('Content-Length', Buffer.byteLength(body));
request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
const contentType = request.getHeader('Content-Type');
// 'contentType' is 'text/html'
const contentLength = request.getHeader('Content-Length');
// 'contentLength' is of type number
const cookie = request.getHeader('Cookie');
// 'cookie' is of type string[]

request.getHeaderNames()#

Added in: v7.7.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

request.setHeader('Foo', 'bar');
request.setHeader('Cookie', ['foo=bar', 'bar=baz']);

const headerNames = request.getHeaderNames();
// headerNames === ['foo', 'cookie']

request.getHeaders()#

Added in: v7.7.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the request.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

request.setHeader('Foo', 'bar');
request.setHeader('Cookie', ['foo=bar', 'bar=baz']);

const headers = request.getHeaders();
// headers === { foo: 'bar', 'cookie': ['foo=bar', 'bar=baz'] }

request.getRawHeaderNames()#

Added in: v15.13.0, v14.17.0

Returns an array containing the unique names of the current outgoing raw headers. Header names are returned with their exact casing being set.

request.setHeader('Foo', 'bar');
request.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headerNames = request.getRawHeaderNames();
// headerNames === ['Foo', 'Set-Cookie']

request.hasHeader(name)#

Added in: v7.7.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const hasContentType = request.hasHeader('content-type');

request.maxHeadersCount#

Limits maximum response headers count. If set to 0, no limit will be applied.

request.path#

Added in: v0.4.0

request.method#

Added in: v0.1.97

request.host#

Added in: v14.5.0, v12.19.0

request.protocol#

Added in: v14.5.0, v12.19.0

request.removeHeader(name)#

Added in: v1.6.0

Removes a header that's already defined into headers object.

request.removeHeader('Content-Type');

request.reusedSocket#

Added in: v13.0.0, v12.16.0
  • <boolean> Whether the request is send through a reused socket.

When sending request through a keep-alive enabled agent, the underlying socket might be reused. But if server closes connection at unfortunate time, client may run into a 'ECONNRESET' error.

const http = require('node:http');

// Server has a 5 seconds keep-alive timeout by default
http
  .createServer((req, res) => {
    res.write('hello\n');
    res.end();
  })
  .listen(3000);

setInterval(() => {
  // Adapting a keep-alive agent
  http.get('http://localhost:3000', { agent }, (res) => {
    res.on('data', (data) => {
      // Do nothing
    });
  });
}, 5000); // Sending request on 5s interval so it's easy to hit idle timeout

By marking a request whether it reused socket or not, we can do automatic error retry base on it.

const http = require('node:http');
const agent = new http.Agent({ keepAlive: true });

function retriableRequest() {
  const req = http
    .get('http://localhost:3000', { agent }, (res) => {
      // ...
    })
    .on('error', (err) => {
      // Check if retry is needed
      if (req.reusedSocket && err.code === 'ECONNRESET') {
        retriableRequest();
      }
    });
}

retriableRequest();

request.setHeader(name, value)#

Added in: v1.6.0

Sets a single header value for headers object. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, request.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission.

request.setHeader('Content-Type', 'application/json');

or

request.setHeader('Cookie', ['type=ninja', 'language=javascript']);

When the value is a string an exception will be thrown if it contains characters outside the latin1 encoding.

If you need to pass UTF-8 characters in the value please encode the value using the RFC 8187 standard.

const filename = 'Rock 🎵.txt';
request.setHeader('Content-Disposition', `attachment; filename*=utf-8''${encodeURIComponent(filename)}`);

request.setNoDelay([noDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setNoDelay() will be called.

request.setSocketKeepAlive([enable][, initialDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setKeepAlive() will be called.

request.setTimeout(timeout[, callback])#

History
VersionChanges
v9.0.0

Consistently set socket timeout only when the socket connects.

v0.5.9

Added in: v0.5.9

  • timeout <number> Milliseconds before a request times out.
  • callback <Function> Optional function to be called when a timeout occurs. Same as binding to the 'timeout' event.
  • Returns: <http.ClientRequest>

Once a socket is assigned to this request and is connected socket.setTimeout() will be called.

request.socket#

Added in: v0.3.0

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket.

const http = require('node:http');
const options = {
  host: 'www.google.com',
};
const req = http.get(options);
req.end();
req.once('response', (res) => {
  const ip = req.socket.localAddress;
  const port = req.socket.localPort;
  console.log(`Your IP address is ${ip} and your source port is ${port}.`);
  // Consume response object
});

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.

request.uncork()#

Added in: v13.2.0, v12.16.0

See writable.uncork().

request.writableEnded#

Added in: v12.9.0

Is true after request.end() has been called. This property does not indicate whether the data has been flushed, for this use request.writableFinished instead.

request.writableFinished#

Added in: v12.7.0

Is true if all data has been flushed to the underlying system, immediately before the 'finish' event is emitted.

request.write(chunk[, encoding][, callback])#

History
VersionChanges
v15.0.0

The chunk parameter can now be a Uint8Array.

v0.1.29

Added in: v0.1.29

Sends a chunk of the body. This method can be called multiple times. If no Content-Length is set, data will automatically be encoded in HTTP Chunked transfer encoding, so that server knows when the data ends. The Transfer-Encoding: chunked header is added. Calling request.end() is necessary to finish sending the request.

The encoding argument is optional and only applies when chunk is a string. Defaults to 'utf8'.

The callback argument is optional and will be called when this chunk of data is flushed, but only if the chunk is non-empty.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

When write function is called with empty string or buffer, it does nothing and waits for more input.

Class: http.Server#

Added in: v0.1.17

Event: 'checkContinue'#

Added in: v0.3.0

Emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'checkExpectation'#

Added in: v5.5.0

Emitted each time a request with an HTTP Expect header is received, where the value is not 100-continue. If this event is not listened for, the server will automatically respond with a 417 Expectation Failed as appropriate.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'clientError'#

History
VersionChanges
v12.0.0

The default behavior will return a 431 Request Header Fields Too Large if a HPE_HEADER_OVERFLOW error occurs.

v9.4.0

The rawPacket is the current buffer that just parsed. Adding this buffer to the error object of 'clientError' event is to make it possible that developers can log the broken packet.

v6.0.0

The default action of calling .destroy() on the socket will no longer take place if there are listeners attached for 'clientError'.

v0.1.94

Added in: v0.1.94

If a client connection emits an 'error' event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with a custom HTTP response instead of abruptly severing the connection.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Default behavior is to try close the socket with a HTTP '400 Bad Request', or a HTTP '431 Request Header Fields Too Large' in the case of a HPE_HEADER_OVERFLOW error. If the socket is not writable or headers of the current attached http.ServerResponse has been sent, it is immediately destroyed.

socket is the net.Socket object that the error originated from.

const http = require('node:http');

const server = http.createServer((req, res) => {
  res.end();
});
server.on('clientError', (err, socket) => {
  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);

When the 'clientError' event occurs, there is no request or response object, so any HTTP response sent, including response headers and payload, must be written directly to the socket object. Care must be taken to ensure the response is a properly formatted HTTP response message.

err is an instance of Error with two extra columns:

  • bytesParsed: the bytes count of request packet that Node.js may have parsed correctly;
  • rawPacket: the raw packet of current request.

In some cases, the client has already received the response and/or the socket has already been destroyed, like in case of ECONNRESET errors. Before trying to send data to the socket, it is better to check that it is still writable.

server.on('clientError', (err, socket) => {
  if (err.code === 'ECONNRESET' || !socket.writable) {
    return;
  }

  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});

Event: 'close'#

Added in: v0.1.4

Emitted when the server closes.

Event: 'connect'#

Added in: v0.7.0

Emitted each time a client requests an HTTP CONNECT method. If this event is not listened for, then clients requesting a CONNECT method will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

Event: 'connection'#

Added in: v0.1.0

This event is emitted when a new TCP stream is established. socket is typically an object of type net.Socket. Usually users will not want to access this event. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket can also be accessed at request.socket.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any Duplex stream can be passed.

If socket.setTimeout() is called here, the timeout will be replaced with server.keepAliveTimeout when the socket has served a request (if server.keepAliveTimeout is non-zero).

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Event: 'dropRequest'#

Added in: v18.7.0, v16.17.0

When the number of requests on a socket reaches the threshold of server.maxRequestsPerSocket, the server will drop new requests and emit 'dropRequest' event instead, then send 503 to client.

Event: 'request'#

Added in: v0.1.0

Emitted each time there is a request. There may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

Event: 'upgrade'#

History
VersionChanges
v10.0.0

Not listening to this event no longer causes the socket to be destroyed if a client sends an Upgrade header.

v0.1.94

Added in: v0.1.94

Emitted each time a client requests an HTTP upgrade. Listening to this event is optional and clients cannot insist on a protocol change.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

server.close([callback])#

History
VersionChanges
v19.0.0

The method closes idle connections before returning.

v0.1.90

Added in: v0.1.90

Stops the server from accepting new connections and closes all connections connected to this server which are not sending a request or waiting for a response. See net.Server.close().

server.closeAllConnections()#

Added in: v18.2.0

Closes all connections connected to this server.

server.closeIdleConnections()#

Added in: v18.2.0

Closes all connections connected to this server which are not sending a request or waiting for a response.

server.headersTimeout#

History
VersionChanges
v19.4.0

The default is now set to the minimum between 60000 (60 seconds) or requestTimeout.

v11.3.0, v10.14.0

Added in: v11.3.0, v10.14.0

Limit the amount of time the parser will wait to receive the complete HTTP headers.

If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.

It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.

server.listen()#

Starts the HTTP server listening for connections. This method is identical to server.listen() from net.Server.

server.listening#

Added in: v5.7.0
  • <boolean> Indicates whether or not the server is listening for connections.

server.maxHeadersCount#

Added in: v0.7.0

Limits maximum incoming headers count. If set to 0, no limit will be applied.

server.requestTimeout#

History
VersionChanges
v18.0.0

The default request timeout changed from no timeout to 300s (5 minutes).

v14.11.0

Added in: v14.11.0

Sets the timeout value in milliseconds for receiving the entire request from the client.

If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.

It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.

server.setTimeout([msecs][, callback])#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.9.12

Added in: v0.9.12

Sets the timeout value for sockets, and emits a 'timeout' event on the Server object, passing the socket as an argument, if a timeout occurs.

If there is a 'timeout' event listener on the Server object, then it will be called with the timed-out socket as an argument.

By default, the Server does not timeout sockets. However, if a callback is assigned to the Server's 'timeout' event, timeouts must be handled explicitly.

server.maxRequestsPerSocket#

Added in: v16.10.0
  • <number> Requests per socket. Default: 0 (no limit)

The maximum number of requests socket can handle before closing keep alive connection.

A value of 0 will disable the limit.

When the limit is reached it will set the Connection header value to close, but will not actually close the connection, subsequent requests sent after the limit is reached will get 503 Service Unavailable as a response.

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.9.12

Added in: v0.9.12

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

server.keepAliveTimeout#

Added in: v8.0.0
  • <number> Timeout in milliseconds. Default: 5000 (5 seconds).

The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. If the server receives new data before the keep-alive timeout has fired, it will reset the regular inactivity timeout, i.e., server.timeout.

A value of 0 will disable the keep-alive timeout behavior on incoming connections. A value of 0 makes the http server behave similarly to Node.js versions prior to 8.0.0, which did not have a keep-alive timeout.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

Class: http.ServerResponse#

Added in: v0.1.17

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the 'request' event.

Event: 'close'#

Added in: v0.6.7

Indicates that the response is completed, or its underlying connection was terminated prematurely (before the response completion).

Event: 'finish'#

Added in: v0.3.6

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

response.addTrailers(headers)#

Added in: v0.3.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.

HTTP requires the Trailer header to be sent in order to emit trailers, with a list of the header fields in its value. E.g.,

response.writeHead(200, { 'Content-Type': 'text/plain',
                          'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
response.end();

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.connection#

Added in: v0.3.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use response.socket.

See response.socket.

response.cork()#

Added in: v13.2.0, v12.16.0

See writable.cork().

response.end([data[, encoding]][, callback])#

History
VersionChanges
v15.0.0

The data parameter can now be a Uint8Array.

v10.0.0

This method now returns a reference to ServerResponse.

v0.1.90

Added in: v0.1.90

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is similar in effect to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#

Added in: v0.0.2Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use response.writableEnded.

The response.finished property will be true if response.end() has been called.

response.flushHeaders()#

Added in: v1.6.0

Flushes the response headers. See also: request.flushHeaders().

response.getHeader(name)#

Added in: v0.4.0

Reads out a header that's already been queued but not sent to the client. The name is case-insensitive. The type of the return value depends on the arguments provided to response.setHeader().

response.setHeader('Content-Type', 'text/html');
response.setHeader('Content-Length', Buffer.byteLength(body));
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
const contentType = response.getHeader('content-type');
// contentType is 'text/html'
const contentLength = response.getHeader('Content-Length');
// contentLength is of type number
const setCookie = response.getHeader('set-cookie');
// setCookie is of type string[]

response.getHeaderNames()#

Added in: v7.7.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']

response.getHeaders()#

Added in: v7.7.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }

response.hasHeader(name)#

Added in: v7.7.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const hasContentType = response.hasHeader('content-type');

response.headersSent#

Added in: v0.9.3

Boolean (read-only). True if headers were sent, false otherwise.

response.removeHeader(name)#

Added in: v0.4.0

Removes a header that's queued for implicit sending.

response.removeHeader('Content-Encoding');

response.req#

Added in: v15.7.0

A reference to the original HTTP request object.

response.sendDate#

Added in: v0.7.5

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#

Added in: v0.4.0

Returns the response object.

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, response.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission. The same response object is returned to the caller, to enable call chaining.

response.setHeader('Content-Type', 'text/html');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

If response.writeHead() method is called and this method has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead of response.writeHead().

response.setTimeout(msecs[, callback])#

Added in: v0.9.12

Sets the Socket's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

response.socket#

Added in: v0.3.0

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. After response.end(), the property is nulled.

const http = require('node:http');
const server = http.createServer((req, res) => {
  const ip = res.socket.remoteAddress;
  const port = res.socket.remotePort;
  res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.

response.statusCode#

Added in: v0.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#

Added in: v0.11.8

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as undefined then the standard message for the status code will be used.

response.statusMessage = 'Not found';

After response header was sent to the client, this property indicates the status message which was sent out.

response.uncork()#

Added in: v13.2.0, v12.16.0

See writable.uncork().

response.writableEnded#

Added in: v12.9.0

Is true after response.end() has been called. This property does not indicate whether the data has been flushed, for this use response.writableFinished instead.

response.writableFinished#

Added in: v12.7.0

Is true if all data has been flushed to the underlying system, immediately before the 'finish' event is emitted.

response.write(chunk[, encoding][, callback])#

History
VersionChanges
v15.0.0

The chunk parameter can now be a Uint8Array.

v0.1.29

Added in: v0.1.29

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

In the node:http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. callback will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#

Added in: v0.3.0

Sends an HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server.

response.writeEarlyHints(hints[, callback])#

History
VersionChanges
v18.11.0

Allow passing hints as an object.

v18.11.0

Added in: v18.11.0

Sends an HTTP/1.1 103 Early Hints message to the client with a Link header, indicating that the user agent can preload/preconnect the linked resources. The hints is an object containing the values of headers to be sent with early hints message. The optional callback argument will be called when the response message has been written.

Example

const earlyHintsLink = '</styles.css>; rel=preload; as=style';
response.writeEarlyHints({
  'link': earlyHintsLink,
});

const earlyHintsLinks = [
  '</styles.css>; rel=preload; as=style',
  '</scripts.js>; rel=preload; as=script',
];
response.writeEarlyHints({
  'link': earlyHintsLinks,
  'x-trace-id': 'id for diagnostics',
});

const earlyHintsCallback = () => console.log('early hints message sent');
response.writeEarlyHints({
  'link': earlyHintsLinks,
}, earlyHintsCallback);

response.writeHead(statusCode[, statusMessage][, headers])#

History
VersionChanges
v14.14.0

Allow passing headers as an array.

v11.10.0, v10.17.0

Return this from writeHead() to allow chaining with end().

v5.11.0, v4.4.5

A RangeError is thrown if statusCode is not a number in the range [100, 999].

v0.1.30

Added in: v0.1.30

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers. Optionally one can give a human-readable statusMessage as the second argument.

headers may be an Array where the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. The array is in the same format as request.rawHeaders.

Returns a reference to the ServerResponse, so that calls can be chained.

const body = 'hello world';
response
  .writeHead(200, {
    'Content-Length': Buffer.byteLength(body),
    'Content-Type': 'text/plain',
  })
  .end(body);

This method must only be called once on a message and it must be called before response.end() is called.

If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

If this method is called and response.setHeader() has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead.

// Returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

Content-Length is read in bytes, not characters. Use Buffer.byteLength() to determine the length of the body in bytes. Node.js will check whether Content-Length and the length of the body which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters will result in a [Error][] being thrown.

response.writeProcessing()#

Added in: v10.0.0

Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.

Class: http.IncomingMessage#

History
VersionChanges
v15.5.0

The destroyed value returns true after the incoming data is consumed.

v13.1.0, v12.16.0

The readableHighWaterMark value mirrors that of the socket.

v0.1.17

Added in: v0.1.17

An IncomingMessage object is created by http.Server or http.ClientRequest and passed as the first argument to the 'request' and 'response' event respectively. It may be used to access response status, headers, and data.

Different from its socket value which is a subclass of <stream.Duplex>, the IncomingMessage itself extends <stream.Readable> and is created separately to parse and emit the incoming HTTP headers and payload, as the underlying socket may be reused multiple times in case of keep-alive.

Event: 'aborted'#

Added in: v0.3.8Deprecated since: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Listen for 'close' event instead.

Emitted when the request has been aborted.

Event: 'close'#

History
VersionChanges
v16.0.0

The close event is now emitted when the request has been completed and not when the underlying socket is closed.

v0.4.2

Added in: v0.4.2

Emitted when the request has been completed.

message.aborted#

Added in: v10.1.0Deprecated since: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Check message.destroyed from <stream.Readable>.

The message.aborted property will be true if the request has been aborted.

message.complete#

Added in: v0.3.0

The message.complete property will be true if a complete HTTP message has been received and successfully parsed.

This property is particularly useful as a means of determining if a client or server fully transmitted a message before a connection was terminated:

const req = http.request({
  host: '127.0.0.1',
  port: 8080,
  method: 'POST',
}, (res) => {
  res.resume();
  res.on('end', () => {
    if (!res.complete)
      console.error(
        'The connection was terminated while the message was still being sent');
  });
});

message.connection#

Added in: v0.1.90Deprecated since: v16.0.0

Stability: 0 - Deprecated. Use message.socket.

Alias for message.socket.

message.destroy([error])#

History
VersionChanges
v14.5.0, v12.19.0

The function returns this for consistency with other Readable streams.

v0.3.0

Added in: v0.3.0

Calls destroy() on the socket that received the IncomingMessage. If error is provided, an 'error' event is emitted on the socket and error is passed as an argument to any listeners on the event.

message.headers#

History
VersionChanges
v19.5.0

The joinDuplicateHeaders option in the http.request() and http.createServer() functions ensures that duplicate headers are not discarded, but rather combined using a comma separator, in accordance with RFC 9110 Section 5.3.

v15.1.0

message.headers is now lazily computed using an accessor property on the prototype and is no longer enumerable.

v0.1.5

Added in: v0.1.5

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

Duplicates in raw headers are handled in the following ways, depending on the header name:

  • Duplicates of age, authorization, content-length, content-type, etag, expires, from, host, if-modified-since, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, referer, retry-after, server, or user-agent are discarded. To allow duplicate values of the headers listed above to be joined, use the option joinDuplicateHeaders in http.request() and http.createServer(). See RFC 9110 Section 5.3 for more information.
  • set-cookie is always an array. Duplicates are added to the array.
  • For duplicate cookie headers, the values are joined together with ; .
  • For all other headers, the values are joined together with , .

message.headersDistinct#

Added in: v18.3.0, v16.17.0

Similar to message.headers, but there is no join logic and the values are always arrays of strings, even for headers received just once.

// Prints something like:
//
// { 'user-agent': ['curl/7.22.0'],
//   host: ['127.0.0.1:8000'],
//   accept: ['*/*'] }
console.log(request.headersDistinct);

message.httpVersion#

Added in: v0.1.1

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either '1.1' or '1.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

message.method#

Added in: v0.1.1

Only valid for request obtained from http.Server.

The request method as a string. Read only. Examples: 'GET', 'DELETE'.

message.rawHeaders#

Added in: v0.11.6

The raw request/response headers list exactly as they were received.

The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);

message.rawTrailers#

Added in: v0.11.6

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

message.setTimeout(msecs[, callback])#

Added in: v0.5.9

Calls message.socket.setTimeout(msecs, callback).

message.socket#

Added in: v0.3.0

The net.Socket object associated with the connection.

With HTTPS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket> or internally nulled.

message.statusCode#

Added in: v0.1.1

Only valid for response obtained from http.ClientRequest.

The 3-digit HTTP response status code. E.G. 404.

message.statusMessage#

Added in: v0.11.10

Only valid for response obtained from http.ClientRequest.

The HTTP response status message (reason phrase). E.G. OK or Internal Server Error.

message.trailers#

Added in: v0.3.0

The request/response trailers object. Only populated at the 'end' event.

message.trailersDistinct#

Added in: v18.3.0, v16.17.0

Similar to message.trailers, but there is no join logic and the values are always arrays of strings, even for headers received just once. Only populated at the 'end' event.

message.url#

Added in: v0.1.90

Only valid for request obtained from http.Server.

Request URL string. This contains only the URL that is present in the actual HTTP request. Take the following request:

GET /status?name=ryan HTTP/1.1
Accept: text/plain

To parse the URL into its parts:

new URL(request.url, `http://${request.headers.host}`);

When request.url is '/status?name=ryan' and request.headers.host is 'localhost:3000':

$ node
> new URL(request.url, `http://${request.headers.host}`)
URL {
  href: 'http://localhost:3000/status?name=ryan',
  origin: 'http://localhost:3000',
  protocol: 'http:',
  username: '',
  password: '',
  host: 'localhost:3000',
  hostname: 'localhost',
  port: '3000',
  pathname: '/status',
  search: '?name=ryan',
  searchParams: URLSearchParams { 'name' => 'ryan' },
  hash: ''
}

Class: http.OutgoingMessage#

Added in: v0.1.17

This class serves as the parent class of http.ClientRequest and http.ServerResponse. It is an abstract outgoing message from the perspective of the participants of an HTTP transaction.

Event: 'drain'#

Added in: v0.3.6

Emitted when the buffer of the message is free again.

Event: 'finish'#

Added in: v0.1.17

Emitted when the transmission is finished successfully.

Event: 'prefinish'#

Added in: v0.11.6

Emitted after outgoingMessage.end() is called. When the event is emitted, all data has been processed but not necessarily completely flushed.

outgoingMessage.addTrailers(headers)#

Added in: v0.3.0

Adds HTTP trailers (headers but at the end of the message) to the message.

Trailers will only be emitted if the message is chunked encoded. If not, the trailers will be silently discarded.

HTTP requires the Trailer header to be sent to emit trailers, with a list of header field names in its value, e.g.

message.writeHead(200, { 'Content-Type': 'text/plain',
                         'Trailer': 'Content-MD5' });
message.write(fileData);
message.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
message.end();

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

outgoingMessage.appendHeader(name, value)#

Added in: v18.3.0, v16.17.0

Append a single header value for the header object.

If the value is an array, this is equivalent of calling this method multiple times.

If there were no previous value for the header, this is equivalent of calling outgoingMessage.setHeader(name, value).

Depending of the value of options.uniqueHeaders when the client request or the server were created, this will end up in the header being sent multiple times or a single time with values joined using ; .

outgoingMessage.connection#

Added in: v0.3.0Deprecated since: v15.12.0, v14.17.1

Stability: 0 - Deprecated: Use outgoingMessage.socket instead.

Alias of outgoingMessage.socket.

outgoingMessage.cork()#

Added in: v13.2.0, v12.16.0

See writable.cork().

outgoingMessage.destroy([error])#

Added in: v0.3.0
  • error <Error> Optional, an error to emit with error event
  • Returns: <this>

Destroys the message. Once a socket is associated with the message and is connected, that socket will be destroyed as well.

outgoingMessage.end(chunk[, encoding][, callback])#

History
VersionChanges
v15.0.0

The chunk parameter can now be a Uint8Array.

v0.11.6

add callback argument.

v0.1.90

Added in: v0.1.90

Finishes the outgoing message. If any parts of the body are unsent, it will flush them to the underlying system. If the message is chunked, it will send the terminating chunk 0\r\n\r\n, and send the trailers (if any).

If chunk is specified, it is equivalent to calling outgoingMessage.write(chunk, encoding), followed by outgoingMessage.end(callback).

If callback is provided, it will be called when the message is finished (equivalent to a listener of the 'finish' event).

outgoingMessage.flushHeaders()#

Added in: v1.6.0

Flushes the message headers.

For efficiency reason, Node.js normally buffers the message headers until outgoingMessage.end() is called or the first chunk of message data is written. It then tries to pack the headers and data into a single TCP packet.

It is usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. outgoingMessage.flushHeaders() bypasses the optimization and kickstarts the message.

outgoingMessage.getHeader(name)#

Added in: v0.4.0

Gets the value of the HTTP header with the given name. If that header is not set, the returned value will be undefined.

outgoingMessage.getHeaderNames()#

Added in: v7.7.0

Returns an array containing the unique names of the current outgoing headers. All names are lowercase.

outgoingMessage.getHeaders()#

Added in: v7.7.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related HTTP module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the outgoingMessage.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

outgoingMessage.setHeader('Foo', 'bar');
outgoingMessage.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headers = outgoingMessage.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }

outgoingMessage.hasHeader(name)#

Added in: v7.7.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name is case-insensitive.

const hasContentType = outgoingMessage.hasHeader('content-type');

outgoingMessage.headersSent#

Added in: v0.9.3

Read-only. true if the headers were sent, otherwise false.

outgoingMessage.pipe()#

Added in: v9.0.0

Overrides the stream.pipe() method inherited from the legacy Stream class which is the parent class of http.OutgoingMessage.

Calling this method will throw an Error because outgoingMessage is a write-only stream.

outgoingMessage.removeHeader(name)#

Added in: v0.4.0

Removes a header that is queued for implicit sending.

outgoingMessage.removeHeader('Content-Encoding');

outgoingMessage.setHeader(name, value)#

Added in: v0.4.0

Sets a single header value. If the header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings to send multiple headers with the same name.

outgoingMessage.setTimeout(msesc[, callback])#

Added in: v0.9.12
  • msesc <number>
  • callback <Function> Optional function to be called when a timeout occurs. Same as binding to the timeout event.
  • Returns: <this>

Once a socket is associated with the message and is connected, socket.setTimeout() will be called with msecs as the first parameter.

outgoingMessage.socket#

Added in: v0.3.0

Reference to the underlying socket. Usually, users will not want to access this property.

After calling outgoingMessage.end(), this property will be nulled.

outgoingMessage.uncork()#

Added in: v13.2.0, v12.16.0

See writable.uncork()

outgoingMessage.writableCorked#

Added in: v13.2.0, v12.16.0

The number of times outgoingMessage.cork() has been called.

outgoingMessage.writableEnded#

Added in: v12.9.0

Is true if outgoingMessage.end() has been called. This property does not indicate whether the data has been flushed. For that purpose, use message.writableFinished instead.

outgoingMessage.writableFinished#

Added in: v12.7.0

Is true if all data has been flushed to the underlying system.

outgoingMessage.writableHighWaterMark#

Added in: v12.9.0

The highWaterMark of the underlying socket if assigned. Otherwise, the default buffer level when writable.write() starts returning false (16384).

outgoingMessage.writableLength#

Added in: v12.9.0

The number of buffered bytes.

outgoingMessage.writableObjectMode#

Added in: v12.9.0

Always false.

outgoingMessage.write(chunk[, encoding][, callback])#

History
VersionChanges
v15.0.0

The chunk parameter can now be a Uint8Array.

v0.11.6

The callback argument was added.

v0.1.29

Added in: v0.1.29

Sends a chunk of the body. This method can be called multiple times.

The encoding argument is only relevant when chunk is a string. Defaults to 'utf8'.

The callback argument is optional and will be called when this chunk of data is flushed.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in the user memory. The 'drain' event will be emitted when the buffer is free again.

http.METHODS#

Added in: v0.11.8

A list of the HTTP methods that are supported by the parser.

http.STATUS_CODES#

Added in: v0.1.22

A collection of all the standard HTTP response status codes, and the short description of each. For example, http.STATUS_CODES[404] === 'Not Found'.

http.createServer([options][, requestListener])#

History
VersionChanges
v18.0.0

The requestTimeout, headersTimeout, keepAliveTimeout, and connectionsCheckingInterval options are supported now.

v18.0.0

The noDelay option now defaults to true.

v17.7.0, v16.15.0

The noDelay, keepAlive and keepAliveInitialDelay options are supported now.

v13.3.0

The maxHeaderSize option is supported now.

v13.8.0, v12.15.0, v10.19.0

The insecureHTTPParser option is supported now.

v9.6.0, v8.12.0

The options argument is supported now.

v0.1.13

Added in: v0.1.13

  • options <Object>

    • connectionsCheckingInterval: Sets the interval value in milliseconds to check for request and headers timeout in incomplete requests. Default: 30000.
    • headersTimeout: Sets the timeout value in milliseconds for receiving the complete HTTP headers from the client. See server.headersTimeout for more information. Default: 60000.
    • insecureHTTPParser <boolean> Use an insecure HTTP parser that accepts invalid HTTP headers when true. Using the insecure parser should be avoided. See --insecure-http-parser for more information. Default: false.
    • IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to be used. Useful for extending the original IncomingMessage. Default: IncomingMessage.
    • keepAlive <boolean> If set to true, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done in [socket.setKeepAlive([enable][, initialDelay])][socket.setKeepAlive(enable, initialDelay)]. Default: false.
    • keepAliveInitialDelay <number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket. Default: 0.
    • keepAliveTimeout: The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. See server.keepAliveTimeout for more information. Default: 5000.
    • maxHeaderSize <number> Optionally overrides the value of --max-http-header-size for requests received by this server, i.e. the maximum length of request headers in bytes. Default: 16384 (16 KiB).
    • noDelay <boolean> If set to true, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default: true.
    • requestTimeout: Sets the timeout value in milliseconds for receiving the entire request from the client. See server.requestTimeout for more information.
    • joinDuplicateHeaders <boolean> It joins the field line values of multiple headers in a request with , instead of discarding the duplicates. See message.headers for more information. Default: false.
    • ServerResponse <http.ServerResponse> Specifies the ServerResponse class to be used. Useful for extending the original ServerResponse. Default: ServerResponse. Default: 300000.
    • uniqueHeaders <Array> A list of response headers that should be sent only once. If the header's value is an array, the items will be joined using ; .

  • requestListener <Function>

  • Returns: <http.Server>

Returns a new instance of http.Server.

The requestListener is a function which is automatically added to the 'request' event.

const http = require('node:http');

// Create a local server to receive data from
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({
    data: 'Hello World!',
  }));
});

server.listen(8000);
const http = require('node:http');

// Create a local server to receive data from
const server = http.createServer();

// Listen to the request event
server.on('request', (request, res) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({
    data: 'Hello World!',
  }));
});

server.listen(8000);

http.get(options[, callback])#

http.get(url[, options][, callback])#

History
VersionChanges
v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and http.request() is that it sets the method to GET and calls req.end() automatically. The callback must take care to consume the response data for reasons stated in http.ClientRequest section.

The callback is invoked with a single argument that is an instance of http.IncomingMessage.

JSON fetching example:

http.get('http://localhost:8000/', (res) => {
  const { statusCode } = res;
  const contentType = res.headers['content-type'];

  let error;
  // Any 2xx status code signals a successful response but
  // here we're only checking for 200.
  if (statusCode !== 200) {
    error = new Error('Request Failed.\n' +
                      `Status Code: ${statusCode}`);
  } else if (!/^application\/json/.test(contentType)) {
    error = new Error('Invalid content-type.\n' +
                      `Expected application/json but received ${contentType}`);
  }
  if (error) {
    console.error(error.message);
    // Consume response data to free up memory
    res.resume();
    return;
  }

  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => { rawData += chunk; });
  res.on('end', () => {
    try {
      const parsedData = JSON.parse(rawData);
      console.log(parsedData);
    } catch (e) {
      console.error(e.message);
    }
  });
}).on('error', (e) => {
  console.error(`Got error: ${e.message}`);
});

// Create a local server to receive data from
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({
    data: 'Hello World!',
  }));
});

server.listen(8000);

http.globalAgent#

History
VersionChanges
v19.0.0

The agent now uses HTTP Keep-Alive by default.

v0.5.9

Added in: v0.5.9

Global instance of Agent which is used as the default for all HTTP client requests.

http.maxHeaderSize#

Added in: v11.6.0, v10.15.0

Read-only property specifying the maximum allowed size of HTTP headers in bytes. Defaults to 16 KiB. Configurable using the --max-http-header-size CLI option.

This can be overridden for servers and client requests by passing the maxHeaderSize option.

http.request(options[, callback])#

http.request(url[, options][, callback])#

History
VersionChanges
v16.7.0, v14.18.0

When using a URL object parsed username and password will now be properly URI decoded.

v15.3.0, v14.17.0

It is possible to abort a request with an AbortSignal.

v13.3.0

The maxHeaderSize option is supported now.

v13.8.0, v12.15.0, v10.19.0

The insecureHTTPParser option is supported now.

v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

  • url <string> | <URL>
  • options <Object>
    • agent <http.Agent> | <boolean> Controls Agent behavior. Possible values:
      • undefined (default): use http.globalAgent for this host and port.
      • Agent object: explicitly use the passed in Agent.
      • false: causes a new Agent with default values to be used.
    • auth <string> Basic authentication ('user:password') to compute an Authorization header.
    • createConnection <Function> A function that produces a socket/stream to use for the request when the agent option is not used. This can be used to avoid creating a custom Agent class just to override the default createConnection function. See agent.createConnection() for more details. Any Duplex stream is a valid return value.
    • defaultPort <number> Default port for the protocol. Default: agent.defaultPort if an Agent is used, else undefined.
    • family <number> IP address family to use when resolving host or hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
    • headers <Object> An object containing request headers.
    • hints <number> Optional dns.lookup() hints.
    • host <string> A domain name or IP address of the server to issue the request to. Default: 'localhost'.
    • hostname <string> Alias for host. To support url.parse(), hostname will be used if both host and hostname are specified.
    • insecureHTTPParser <boolean> Use an insecure HTTP parser that accepts invalid HTTP headers when true. Using the insecure parser should be avoided. See --insecure-http-parser for more information. Default: false
    • localAddress <string> Local interface to bind for network connections.
    • localPort <number> Local port to connect from.
    • lookup <Function> Custom lookup function. Default: dns.lookup().
    • maxHeaderSize <number> Optionally overrides the value of --max-http-header-size (the maximum length of response headers in bytes) for responses received from the server. Default: 16384 (16 KiB).
    • method <string> A string specifying the HTTP request method. Default: 'GET'.
    • path <string> Request path. Should include query string if any. E.G. '/index.html?page=12'. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future. Default: '/'.
    • port <number> Port of remote server. Default: defaultPort if set, else 80.
    • protocol <string> Protocol to use. Default: 'http:'.
    • setHost <boolean>: Specifies whether or not to automatically add the Host header. Defaults to true.
    • signal <AbortSignal>: An AbortSignal that may be used to abort an ongoing request.
    • socketPath <string> Unix domain socket. Cannot be used if one of host or port is specified, as those specify a TCP Socket.
    • timeout <number>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
    • uniqueHeaders <Array> A list of request headers that should be sent only once. If the header's value is an array, the items will be joined using ; .
    • joinDuplicateHeaders <boolean> It joins the field line values of multiple headers in a request with , instead of discarding the duplicates. See message.headers for more information. Default: false.
  • callback <Function>
  • Returns: <http.ClientRequest>

options in socket.connect() are also supported.

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

url can be a string or a URL object. If url is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

If both url and options are specified, the objects are merged, with the options properties taking precedence.

The optional callback parameter will be added as a one-time listener for the 'response' event.

http.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object.

const http = require('node:http');

const postData = JSON.stringify({
  'msg': 'Hello World!',
});

const options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Content-Length': Buffer.byteLength(postData),
  },
};

const req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.error(`problem with request: ${e.message}`);
});

// Write data to request body
req.write(postData);
req.end();

In the example req.end() was called. With http.request() one must always call req.end() to signify the end of the request - even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an 'error' event is emitted on the returned request object. As with all 'error' events, if no listeners are registered the error will be thrown.

There are a few special headers that should be noted.

  • Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.

  • Sending a 'Content-Length' header will disable the default chunked encoding.

  • Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the 'continue' event should be set. See RFC 2616 Section 8.2.3 for more information.

  • Sending an Authorization header will override using the auth option to compute basic authentication.

Example using a URL as options:

const options = new URL('http://abc:xyz@example.com');

const req = http.request(options, (res) => {
  // ...
});

In a successful request, the following events will be emitted in the following order:

  • 'socket'
  • 'response'
    • 'data' any number of times, on the res object ('data' will not be emitted at all if the response body is empty, for instance, in most redirects)
    • 'end' on the res object
  • 'close'

In the case of a connection error, the following events will be emitted:

  • 'socket'
  • 'error'
  • 'close'

In the case of a premature connection close before the response is received, the following events will be emitted in the following order:

  • 'socket'
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

In the case of a premature connection close after the response is received, the following events will be emitted in the following order:

  • 'socket'
  • 'response'
    • 'data' any number of times, on the res object
  • (connection closed here)
  • 'aborted' on the res object
  • 'error' on the res object with an error with message 'Error: aborted' and code 'ECONNRESET'.
  • 'close'
  • 'close' on the res object

If req.destroy() is called before a socket is assigned, the following events will be emitted in the following order:

  • (req.destroy() called here)
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.destroy() is called before the connection succeeds, the following events will be emitted in the following order:

  • 'socket'
  • (req.destroy() called here)
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.destroy() is called after the response is received, the following events will be emitted in the following order:

  • 'socket'
  • 'response'
    • 'data' any number of times, on the res object
  • (req.destroy() called here)
  • 'aborted' on the res object
  • 'error' on the res object with an error with message 'Error: aborted' and code 'ECONNRESET'.
  • 'close'
  • 'close' on the res object

If req.abort() is called before a socket is assigned, the following events will be emitted in the following order:

  • (req.abort() called here)
  • 'abort'
  • 'close'

If req.abort() is called before the connection succeeds, the following events will be emitted in the following order:

  • 'socket'
  • (req.abort() called here)
  • 'abort'
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.abort() is called after the response is received, the following events will be emitted in the following order:

  • 'socket'
  • 'response'
    • 'data' any number of times, on the res object
  • (req.abort() called here)
  • 'abort'
  • 'aborted' on the res object
  • 'error' on the res object with an error with message 'Error: aborted' and code 'ECONNRESET'.
  • 'close'
  • 'close' on the res object

Setting the timeout option or using the setTimeout() function will not abort the request or do anything besides add a 'timeout' event.

Passing an AbortSignal and then calling abort on the corresponding AbortController will behave the same way as calling .destroy() on the request itself.

http.validateHeaderName(name[, label])#

History
VersionChanges
v19.5.0

The label parameter is added.

v14.3.0

Added in: v14.3.0

Performs the low-level validations on the provided name that are done when res.setHeader(name, value) is called.

Passing illegal value as name will result in a TypeError being thrown, identified by code: 'ERR_INVALID_HTTP_TOKEN'.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers. Examples:

Example:

const { validateHeaderName } = require('node:http');

try {
  validateHeaderName('');
} catch (err) {
  console.error(err instanceof TypeError); // --> true
  console.error(err.code); // --> 'ERR_INVALID_HTTP_TOKEN'
  console.error(err.message); // --> 'Header name must be a valid HTTP token [""]'
}

http.validateHeaderValue(name, value)#

Added in: v14.3.0

Performs the low-level validations on the provided value that are done when res.setHeader(name, value) is called.

Passing illegal value as value will result in a TypeError being thrown.

  • Undefined value error is identified by code: 'ERR_HTTP_INVALID_HEADER_VALUE'.
  • Invalid value character error is identified by code: 'ERR_INVALID_CHAR'.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers.

Examples:

const { validateHeaderValue } = require('node:http');

try {
  validateHeaderValue('x-my-header', undefined);
} catch (err) {
  console.error(err instanceof TypeError); // --> true
  console.error(err.code === 'ERR_HTTP_INVALID_HEADER_VALUE'); // --> true
  console.error(err.message); // --> 'Invalid value "undefined" for header "x-my-header"'
}

try {
  validateHeaderValue('x-my-header', 'oʊmɪɡə');
} catch (err) {
  console.error(err instanceof TypeError); // --> true
  console.error(err.code === 'ERR_INVALID_CHAR'); // --> true
  console.error(err.message); // --> 'Invalid character in header content ["x-my-header"]'
}

http.setMaxIdleHTTPParsers(max)#

Added in: v18.8.0, v16.18.0

Set the maximum number of idle HTTP parsers.

HTTP/2#

History
VersionChanges
v15.0.0

Requests with the host header (with or without :authority) can now be sent/received.

v15.3.0, v14.17.0

It is possible to abort a request with an AbortSignal.

v10.10.0

HTTP/2 is now Stable. Previously, it had been Experimental.

v8.4.0

Added in: v8.4.0

Stability: 2 - Stable

Source Code: lib/http2.js

The node:http2 module provides an implementation of the HTTP/2 protocol. It can be accessed using:

const http2 = require('node:http2');

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the node:crypto module. In such cases, attempting to import from node:http2 or calling require('node:http2') will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

let http2;
try {
  http2 = require('node:http2');
} catch (err) {
  console.error('http2 support is disabled!');
}

When using the lexical ESM import keyword, the error can only be caught if a handler for process.on('uncaughtException') is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the import() function instead of the lexical import keyword:

let http2;
try {
  http2 = await import('node:http2');
} catch (err) {
  console.error('http2 support is disabled!');
}

Core API#

The Core API provides a low-level interface designed specifically around support for HTTP/2 protocol features. It is specifically not designed for compatibility with the existing HTTP/1 module API. However, the Compatibility API is.

The http2 Core API is much more symmetric between client and server than the http API. For instance, most events, like 'error', 'connect' and 'stream', can be emitted either by client-side code or server-side code.

Server-side example#

The following illustrates a simple HTTP/2 server using the Core API. Since there are no browsers known that support unencrypted HTTP/2, the use of http2.createSecureServer() is necessary when communicating with browser clients.

const http2 = require('node:http2');
const fs = require('node:fs');

const server = http2.createSecureServer({
  key: fs.readFileSync('localhost-privkey.pem'),
  cert: fs.readFileSync('localhost-cert.pem'),
});
server.on('error', (err) => console.error(err));

server.on('stream', (stream, headers) => {
  // stream is a Duplex
  stream.respond({
    'content-type': 'text/html; charset=utf-8',
    ':status': 200,
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(8443);

To generate the certificate and key for this example, run:

openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
  -keyout localhost-privkey.pem -out localhost-cert.pem

Client-side example#

The following illustrates an HTTP/2 client:

const http2 = require('node:http2');
const fs = require('node:fs');
const client = http2.connect('https://localhost:8443', {
  ca: fs.readFileSync('localhost-cert.pem'),
});
client.on('error', (err) => console.error(err));

const req = client.request({ ':path': '/' });

req.on('response', (headers, flags) => {
  for (const name in headers) {
    console.log(`${name}: ${headers[name]}`);
  }
});

req.setEncoding('utf8');
let data = '';
req.on('data', (chunk) => { data += chunk; });
req.on('end', () => {
  console.log(`\n${data}`);
  client.close();
});
req.end();

Class: Http2Session#

Added in: v8.4.0

Instances of the http2.Http2Session class represent an active communications session between an HTTP/2 client and server. Instances of this class are not intended to be constructed directly by user code.

Each Http2Session instance will exhibit slightly different behaviors depending on whether it is operating as a server or a client. The http2session.type property can be used to determine the mode in which an Http2Session is operating. On the server side, user code should rarely have occasion to work with the Http2Session object directly, with most actions typically taken through interactions with either the Http2Server or Http2Stream objects.

User code will not create Http2Session instances directly. Server-side Http2Session instances are created by the Http2Server instance when a new HTTP/2 connection is received. Client-side Http2Session instances are created using the http2.connect() method.

Http2Session and sockets#

Every Http2Session instance is associated with exactly one net.Socket or tls.TLSSocket when it is created. When either the Socket or the Http2Session are destroyed, both will be destroyed.

Because of the specific serialization and processing requirements imposed by the HTTP/2 protocol, it is not recommended for user code to read data from or write data to a Socket instance bound to a Http2Session. Doing so can put the HTTP/2 session into an indeterminate state causing the session and the socket to become unusable.

Once a Socket has been bound to an Http2Session, user code should rely solely on the API of the Http2Session.

Event: 'close'#
Added in: v8.4.0

The 'close' event is emitted once the Http2Session has been destroyed. Its listener does not expect any arguments.

Event: 'connect'#
Added in: v8.4.0

The 'connect' event is emitted once the Http2Session has been successfully connected to the remote peer and communication may begin.

User code will typically not listen for this event directly.

Event: 'error'#
Added in: v8.4.0

The 'error' event is emitted when an error occurs during the processing of an Http2Session.

Event: 'frameError'#
Added in: v8.4.0
  • type <integer> The frame type.
  • code <integer> The error code.
  • id <integer> The stream id (or 0 if the frame isn't associated with a stream).

The 'frameError' event is emitted when an error occurs while attempting to send a frame on the session. If the frame that could not be sent is associated with a specific Http2Stream, an attempt to emit a 'frameError' event on the Http2Stream is made.

If the 'frameError' event is associated with a stream, the stream will be closed and destroyed immediately following the 'frameError' event. If the event is not associated with a stream, the Http2Session will be shut down immediately following the 'frameError' event.

Event: 'goaway'#
Added in: v8.4.0
  • errorCode <number> The HTTP/2 error code specified in the GOAWAY frame.
  • lastStreamID <number> The ID of the last stream the remote peer successfully processed (or 0 if no ID is specified).
  • opaqueData <Buffer> If additional opaque data was included in the GOAWAY frame, a Buffer instance will be passed containing that data.

The 'goaway' event is emitted when a GOAWAY frame is received.

The Http2Session instance will be shut down automatically when the 'goaway' event is emitted.

Event: 'localSettings'#
Added in: v8.4.0

The 'localSettings' event is emitted when an acknowledgment SETTINGS frame has been received.

When using http2session.settings() to submit new settings, the modified settings do not take effect until the 'localSettings' event is emitted.

session.settings({ enablePush: false });

session.on('localSettings', (settings) => {
  /* Use the new settings */
});
Event: 'ping'#
Added in: v10.12.0
  • payload <Buffer> The PING frame 8-byte payload

The 'ping' event is emitted whenever a PING frame is received from the connected peer.

Event: 'remoteSettings'#
Added in: v8.4.0

The 'remoteSettings' event is emitted when a new SETTINGS frame is received from the connected peer.

session.on('remoteSettings', (settings) => {
  /* Use the new settings */
});
Event: 'stream'#
Added in: v8.4.0

The 'stream' event is emitted when a new Http2Stream is created.

const http2 = require('node:http2');
session.on('stream', (stream, headers, flags) => {
  const method = headers[':method'];
  const path = headers[':path'];
  // ...
  stream.respond({
    ':status': 200,
    'content-type': 'text/plain; charset=utf-8',
  });
  stream.write('hello ');
  stream.end('world');
});

On the server side, user code will typically not listen for this event directly, and would instead register a handler for the 'stream' event emitted by the net.Server or tls.Server instances returned by http2.createServer() and http2.createSecureServer(), respectively, as in the example below:

const http2 = require('node:http2');

// Create an unencrypted HTTP/2 server
const server = http2.createServer();

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html; charset=utf-8',
    ':status': 200,
  });
  stream.on('error', (error) => console.error(error));
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence, a network error will destroy each individual stream and must be handled on the stream level, as shown above.

Event: 'timeout'#
Added in: v8.4.0

After the http2session.setTimeout() method is used to set the timeout period for this Http2Session, the 'timeout' event is emitted if there is no activity on the Http2Session after the configured number of milliseconds. Its listener does not expect any arguments.

session.setTimeout(2000);
session.on('timeout', () => { /* .. */ });
http2session.alpnProtocol#
Added in: v9.4.0

Value will be undefined if the Http2Session is not yet connected to a socket, h2c if the Http2Session is not connected to a TLSSocket, or will return the value of the connected TLSSocket's own alpnProtocol property.

http2session.close([callback])#
Added in: v9.4.0

Gracefully closes the Http2Session, allowing any existing streams to complete on their own and preventing new Http2Stream instances from being created. Once closed, http2session.destroy() might be called if there are no open Http2Stream instances.

If specified, the callback function is registered as a handler for the 'close' event.

http2session.closed#
Added in: v9.4.0

Will be true if this Http2Session instance has been closed, otherwise false.

http2session.connecting#
Added in: v10.0.0

Will be true if this Http2Session instance is still connecting, will be set to false before emitting connect event and/or calling the http2.connect callback.

http2session.destroy([error][, code])#
Added in: v8.4.0
  • error <Error> An Error object if the Http2Session is being destroyed due to an error.
  • code <number> The HTTP/2 error code to send in the final GOAWAY frame. If unspecified, and error is not undefined, the default is INTERNAL_ERROR, otherwise defaults to NO_ERROR.

Immediately terminates the Http2Session and the associated net.Socket or tls.TLSSocket.

Once destroyed, the Http2Session will emit the 'close' event. If error is not undefined, an 'error' event will be emitted immediately before the 'close' event.

If there are any remaining open Http2Streams associated with the Http2Session, those will also be destroyed.

http2session.destroyed#
Added in: v8.4.0

Will be true if this Http2Session instance has been destroyed and must no longer be used, otherwise false.

http2session.encrypted#
Added in: v9.4.0

Value is undefined if the Http2Session session socket has not yet been connected, true if the Http2Session is connected with a TLSSocket, and false if the Http2Session is connected to any other kind of socket or stream.

http2session.goaway([code[, lastStreamID[, opaqueData]]])#
Added in: v9.4.0
  • code <number> An HTTP/2 error code
  • lastStreamID <number> The numeric ID of the last processed Http2Stream
  • opaqueData <Buffer> | <TypedArray> | <DataView> A TypedArray or DataView instance containing additional data to be carried within the GOAWAY frame.

Transmits a GOAWAY frame to the connected peer without shutting down the Http2Session.

http2session.localSettings#
Added in: v8.4.0

A prototype-less object describing the current local settings of this Http2Session. The local settings are local to this Http2Session instance.

http2session.originSet#
Added in: v9.4.0

If the Http2Session is connected to a TLSSocket, the originSet property will return an Array of origins for which the Http2Session may be considered authoritative.

The originSet property is only available when using a secure TLS connection.

http2session.pendingSettingsAck#
Added in: v8.4.0

Indicates whether the Http2Session is currently waiting for acknowledgment of a sent SETTINGS frame. Will be true after calling the http2session.settings() method. Will be false once all sent SETTINGS frames have been acknowledged.

http2session.ping([payload, ]callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.9.3

Added in: v8.9.3

Sends a PING frame to the connected HTTP/2 peer. A callback function must be provided. The method will return true if the PING was sent, false otherwise.

The maximum number of outstanding (unacknowledged) pings is determined by the maxOutstandingPings configuration option. The default maximum is 10.

If provided, the payload must be a Buffer, TypedArray, or DataView containing 8 bytes of data that will be transmitted with the PING and returned with the ping acknowledgment.

The callback will be invoked with three arguments: an error argument that will be null if the PING was successfully acknowledged, a duration argument that reports the number of milliseconds elapsed since the ping was sent and the acknowledgment was received, and a Buffer containing the 8-byte PING payload.

session.ping(Buffer.from('abcdefgh'), (err, duration, payload) => {
  if (!err) {
    console.log(`Ping acknowledged in ${duration} milliseconds`);
    console.log(`With payload '${payload.toString()}'`);
  }
});

If the payload argument is not specified, the default payload will be the 64-bit timestamp (little endian) marking the start of the PING duration.

http2session.ref()#
Added in: v9.4.0

Calls ref() on this Http2Session instance's underlying net.Socket.

http2session.remoteSettings#
Added in: v8.4.0

A prototype-less object describing the current remote settings of this Http2Session. The remote settings are set by the connected HTTP/2 peer.

http2session.setLocalWindowSize(windowSize)#
Added in: v15.3.0, v14.18.0

Sets the local endpoint's window size. The windowSize is the total window size to set, not the delta.

const http2 = require('node:http2');

const server = http2.createServer();
const expectedWindowSize = 2 ** 20;
server.on('connect', (session) => {

  // Set local window size to be 2 ** 20
  session.setLocalWindowSize(expectedWindowSize);
});
http2session.setTimeout(msecs, callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

Used to set a callback function that is called when there is no activity on the Http2Session after msecs milliseconds. The given callback is registered as a listener on the 'timeout' event.

http2session.socket#
Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but limits available methods to ones safe to use with HTTP/2.

destroy, emit, end, pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

setTimeout method will be called on this Http2Session.

All other interactions will be routed directly to the socket.

http2session.state#
Added in: v8.4.0

Provides miscellaneous information about the current state of the Http2Session.

  • <Object>
    • effectiveLocalWindowSize <number> The current local (receive) flow control window size for the Http2Session.
    • effectiveRecvDataLength <number> The current number of bytes that have been received since the last flow control WINDOW_UPDATE.
    • nextStreamID <number> The numeric identifier to be used the next time a new Http2Stream is created by this Http2Session.
    • localWindowSize <number> The number of bytes that the remote peer can send without receiving a WINDOW_UPDATE.
    • lastProcStreamID <number> The numeric id of the Http2Stream for which a HEADERS or DATA frame was most recently received.
    • remoteWindowSize <number> The number of bytes that this Http2Session may send without receiving a WINDOW_UPDATE.
    • outboundQueueSize <number> The number of frames currently within the outbound queue for this Http2Session.
    • deflateDynamicTableSize <number> The current size in bytes of the outbound header compression state table.
    • inflateDynamicTableSize <number> The current size in bytes of the inbound header compression state table.

An object describing the current status of this Http2Session.

http2session.settings([settings][, callback])#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

Updates the current local settings for this Http2Session and sends a new SETTINGS frame to the connected HTTP/2 peer.

Once called, the http2session.pendingSettingsAck property will be true while the session is waiting for the remote peer to acknowledge the new settings.

The new settings will not become effective until the SETTINGS acknowledgment is received and the 'localSettings' event is emitted. It is possible to send multiple SETTINGS frames while acknowledgment is still pending.

http2session.type#
Added in: v8.4.0

The http2session.type will be equal to http2.constants.NGHTTP2_SESSION_SERVER if this Http2Session instance is a server, and http2.constants.NGHTTP2_SESSION_CLIENT if the instance is a client.

http2session.unref()#
Added in: v9.4.0

Calls unref() on this Http2Session instance's underlying net.Socket.

Class: ServerHttp2Session#

Added in: v8.4.0
serverhttp2session.altsvc(alt, originOrStream)#
Added in: v9.4.0
  • alt <string> A description of the alternative service configuration as defined by RFC 7838.
  • originOrStream <number> | <string> | <URL> | <Object> Either a URL string specifying the origin (or an Object with an origin property) or the numeric identifier of an active Http2Stream as given by the http2stream.id property.

Submits an ALTSVC frame (as defined by RFC 7838) to the connected client.

const http2 = require('node:http2');

const server = http2.createServer();
server.on('session', (session) => {
  // Set altsvc for origin https://example.org:80
  session.altsvc('h2=":8000"', 'https://example.org:80');
});

server.on('stream', (stream) => {
  // Set altsvc for a specific stream
  stream.session.altsvc('h2=":8000"', stream.id);
});

Sending an ALTSVC frame with a specific stream ID indicates that the alternate service is associated with the origin of the given Http2Stream.

The alt and origin string must contain only ASCII bytes and are strictly interpreted as a sequence of ASCII bytes. The special value 'clear' may be passed to clear any previously set alternative service for a given domain.

When a string is passed for the originOrStream argument, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org'. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A URL object, or any object with an origin property, may be passed as originOrStream, in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin.

Specifying alternative services#

The format of the alt parameter is strictly defined by RFC 7838 as an ASCII string containing a comma-delimited list of "alternative" protocols associated with a specific host and port.

For example, the value 'h2="example.org:81"' indicates that the HTTP/2 protocol is available on the host 'example.org' on TCP/IP port 81. The host and port must be contained within the quote (") characters.

Multiple alternatives may be specified, for instance: 'h2="example.org:81", h2=":82"'.

The protocol identifier ('h2' in the examples) may be any valid ALPN Protocol ID.

The syntax of these values is not validated by the Node.js implementation and are passed through as provided by the user or received from the peer.

serverhttp2session.origin(...origins)#
Added in: v10.12.0

Submits an ORIGIN frame (as defined by RFC 8336) to the connected client to advertise the set of origins for which the server is capable of providing authoritative responses.

const http2 = require('node:http2');
const options = getSecureOptionsSomehow();
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
  stream.respond();
  stream.end('ok');
});
server.on('session', (session) => {
  session.origin('https://example.com', 'https://example.org');
});

When a string is passed as an origin, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org'. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A URL object, or any object with an origin property, may be passed as an origin, in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin.

Alternatively, the origins option may be used when creating a new HTTP/2 server using the http2.createSecureServer() method:

const http2 = require('node:http2');
const options = getSecureOptionsSomehow();
options.origins = ['https://example.com', 'https://example.org'];
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
  stream.respond();
  stream.end('ok');
});

Class: ClientHttp2Session#

Added in: v8.4.0
Event: 'altsvc'#
Added in: v9.4.0

The 'altsvc' event is emitted whenever an ALTSVC frame is received by the client. The event is emitted with the ALTSVC value, origin, and stream ID. If no origin is provided in the ALTSVC frame, origin will be an empty string.

const http2 = require('node:http2');
const client = http2.connect('https://example.org');

client.on('altsvc', (alt, origin, streamId) => {
  console.log(alt);
  console.log(origin);
  console.log(streamId);
});
Event: 'origin'#
Added in: v10.12.0

The 'origin' event is emitted whenever an ORIGIN frame is received by the client. The event is emitted with an array of origin strings. The http2session.originSet will be updated to include the received origins.

const http2 = require('node:http2');
const client = http2.connect('https://example.org');

client.on('origin', (origins) => {
  for (let n = 0; n < origins.length; n++)
    console.log(origins[n]);
});

The 'origin' event is only emitted when using a secure TLS connection.

clienthttp2session.request(headers[, options])#
Added in: v8.4.0

  • headers <HTTP/2 Headers Object>

  • options <Object>

    • endStream <boolean> true if the Http2Stream writable side should be closed initially, such as when sending a GET request that should not expect a payload body.
    • exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on.
    • weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent. The value is a number between 1 and 256 (inclusive).
    • waitForTrailers <boolean> When true, the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent.
    • signal <AbortSignal> An AbortSignal that may be used to abort an ongoing request.

  • Returns: <ClientHttp2Stream>

For HTTP/2 Client Http2Session instances only, the http2session.request() creates and returns an Http2Stream instance that can be used to send an HTTP/2 request to the connected server.

When a ClientHttp2Session is first created, the socket may not yet be connected. if clienthttp2session.request() is called during this time, the actual request will be deferred until the socket is ready to go. If the session is closed before the actual request be executed, an ERR_HTTP2_GOAWAY_SESSION is thrown.

This method is only available if http2session.type is equal to http2.constants.NGHTTP2_SESSION_CLIENT.

const http2 = require('node:http2');
const clientSession = http2.connect('https://localhost:1234');
const {
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS,
} = http2.constants;

const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' });
req.on('response', (headers) => {
  console.log(headers[HTTP2_HEADER_STATUS]);
  req.on('data', (chunk) => { /* .. */ });
  req.on('end', () => { /* .. */ });
});

When the options.waitForTrailers option is set, the 'wantTrailers' event is emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be called to send trailing headers to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

When options.signal is set with an AbortSignal and then abort on the corresponding AbortController is called, the request will emit an 'error' event with an AbortError error.

The :method and :path pseudo-headers are not specified within headers, they respectively default to:

  • :method = 'GET'
  • :path = /

Class: Http2Stream#

Added in: v8.4.0

Each instance of the Http2Stream class represents a bidirectional HTTP/2 communications stream over an Http2Session instance. Any single Http2Session may have up to 231-1 Http2Stream instances over its lifetime.

User code will not construct Http2Stream instances directly. Rather, these are created, managed, and provided to user code through the Http2Session instance. On the server, Http2Stream instances are created either in response to an incoming HTTP request (and handed off to user code via the 'stream' event), or in response to a call to the http2stream.pushStream() method. On the client, Http2Stream instances are created and returned when either the http2session.request() method is called, or in response to an incoming 'push' event.

The Http2Stream class is a base for the ServerHttp2Stream and ClientHttp2Stream classes, each of which is used specifically by either the Server or Client side, respectively.

All Http2Stream instances are Duplex streams. The Writable side of the Duplex is used to send data to the connected peer, while the Readable side is used to receive data sent by the connected peer.

The default text character encoding for an Http2Stream is UTF-8. When using an Http2Stream to send text, use the 'content-type' header to set the character encoding.

stream.respond({
  'content-type': 'text/html; charset=utf-8',
  ':status': 200,
});
Http2Stream Lifecycle#
Creation#

On the server side, instances of ServerHttp2Stream are created either when:

  • A new HTTP/2 HEADERS frame with a previously unused stream ID is received;
  • The http2stream.pushStream() method is called.

On the client side, instances of ClientHttp2Stream are created when the http2session.request() method is called.

On the client, the Http2Stream instance returned by http2session.request() may not be immediately ready for use if the parent Http2Session has not yet been fully established. In such cases, operations called on the Http2Stream will be buffered until the 'ready' event is emitted. User code should rarely, if ever, need to handle the 'ready' event directly. The ready status of an Http2Stream can be determined by checking the value of http2stream.id. If the value is undefined, the stream is not yet ready for use.

Destruction#

All Http2Stream instances are destroyed either when:

  • An RST_STREAM frame for the stream is received by the connected peer, and (for client streams only) pending data has been read.
  • The http2stream.close() method is called, and (for client streams only) pending data has been read.
  • The http2stream.destroy() or http2session.destroy() methods are called.

When an Http2Stream instance is destroyed, an attempt will be made to send an RST_STREAM frame to the connected peer.

When the Http2Stream instance is destroyed, the 'close' event will be emitted. Because Http2Stream is an instance of stream.Duplex, the 'end' event will also be emitted if the stream data is currently flowing. The 'error' event may also be emitted if http2stream.destroy() was called with an Error passed as the first argument.

After the Http2Stream has been destroyed, the http2stream.destroyed property will be true and the http2stream.rstCode property will specify the RST_STREAM error code. The Http2Stream instance is no longer usable once destroyed.

Event: 'aborted'#
Added in: v8.4.0

The 'aborted' event is emitted whenever a Http2Stream instance is abnormally aborted in mid-communication. Its listener does not expect any arguments.

The 'aborted' event will only be emitted if the Http2Stream writable side has not been ended.

Event: 'close'#
Added in: v8.4.0

The 'close' event is emitted when the Http2Stream is destroyed. Once this event is emitted, the Http2Stream instance is no longer usable.

The HTTP/2 error code used when closing the stream can be retrieved using the http2stream.rstCode property. If the code is any value other than NGHTTP2_NO_ERROR (0), an 'error' event will have also been emitted.

Event: 'error'#
Added in: v8.4.0

The 'error' event is emitted when an error occurs during the processing of an Http2Stream.

Event: 'frameError'#
Added in: v8.4.0
  • type <integer> The frame type.
  • code <integer> The error code.
  • id <integer> The stream id (or 0 if the frame isn't associated with a stream).

The 'frameError' event is emitted when an error occurs while attempting to send a frame. When invoked, the handler function will receive an integer argument identifying the frame type, and an integer argument identifying the error code. The Http2Stream instance will be destroyed immediately after the 'frameError' event is emitted.

Event: 'ready'#
Added in: v8.4.0

The 'ready' event is emitted when the Http2Stream has been opened, has been assigned an id, and can be used. The listener does not expect any arguments.

Event: 'timeout'#
Added in: v8.4.0

The 'timeout' event is emitted after no activity is received for this Http2Stream within the number of milliseconds set using http2stream.setTimeout(). Its listener does not expect any arguments.

Event: 'trailers'#
Added in: v8.4.0

The 'trailers' event is emitted when a block of headers associated with trailing header fields is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

This event might not be emitted if http2stream.end() is called before trailers are received and the incoming data is not being read or listened for.

stream.on('trailers', (headers, flags) => {
  console.log(headers);
});
Event: 'wantTrailers'#
Added in: v10.0.0

The 'wantTrailers' event is emitted when the Http2Stream has queued the final DATA frame to be sent on a frame and the Http2Stream is ready to send trailing headers. When initiating a request or response, the waitForTrailers option must be set for this event to be emitted.

http2stream.aborted#
Added in: v8.4.0

Set to true if the Http2Stream instance was aborted abnormally. When set, the 'aborted' event will have been emitted.

http2stream.bufferSize#
Added in: v11.2.0, v10.16.0

This property shows the number of characters currently buffered to be written. See net.Socket.bufferSize for details.

http2stream.close(code[, callback])#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

  • code <number> Unsigned 32-bit integer identifying the error code. Default: http2.constants.NGHTTP2_NO_ERROR (0x00).
  • callback <Function> An optional function registered to listen for the 'close' event.

Closes the Http2Stream instance by sending an RST_STREAM frame to the connected HTTP/2 peer.

http2stream.closed#
Added in: v9.4.0

Set to true if the Http2Stream instance has been closed.

http2stream.destroyed#
Added in: v8.4.0

Set to true if the Http2Stream instance has been destroyed and is no longer usable.

http2stream.endAfterHeaders#
Added in: v10.11.0

Set to true if the END_STREAM flag was set in the request or response HEADERS frame received, indicating that no additional data should be received and the readable side of the Http2Stream will be closed.

http2stream.id#
Added in: v8.4.0

The numeric stream identifier of this Http2Stream instance. Set to undefined if the stream identifier has not yet been assigned.

http2stream.pending#
Added in: v9.4.0

Set to true if the Http2Stream instance has not yet been assigned a numeric stream identifier.

http2stream.priority(options)#
Added in: v8.4.0
  • options <Object>
    • exclusive <boolean> When true and parent identifies a parent Stream, this stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of this stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream this stream is dependent on.
    • weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent. The value is a number between 1 and 256 (inclusive).
    • silent <boolean> When true, changes the priority locally without sending a PRIORITY frame to the connected peer.

Updates the priority for this Http2Stream instance.

http2stream.rstCode#
Added in: v8.4.0

Set to the RST_STREAM error code reported when the Http2Stream is destroyed after either receiving an RST_STREAM frame from the connected peer, calling http2stream.close(), or http2stream.destroy(). Will be undefined if the Http2Stream has not been closed.

http2stream.sentHeaders#
Added in: v9.5.0

An object containing the outbound headers sent for this Http2Stream.

http2stream.sentInfoHeaders#
Added in: v9.5.0

An array of objects containing the outbound informational (additional) headers sent for this Http2Stream.

http2stream.sentTrailers#
Added in: v9.5.0

An object containing the outbound trailers sent for this HttpStream.

http2stream.session#
Added in: v8.4.0

A reference to the Http2Session instance that owns this Http2Stream. The value will be undefined after the Http2Stream instance is destroyed.

http2stream.setTimeout(msecs, callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

const http2 = require('node:http2');
const client = http2.connect('http://example.org:8000');
const { NGHTTP2_CANCEL } = http2.constants;
const req = client.request({ ':path': '/' });

// Cancel the stream if there's no activity after 5 seconds
req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL));
http2stream.state#
Added in: v8.4.0

Provides miscellaneous information about the current state of the Http2Stream.

  • <Object>
    • localWindowSize <number> The number of bytes the connected peer may send for this Http2Stream without receiving a WINDOW_UPDATE.
    • state <number> A flag indicating the low-level current state of the Http2Stream as determined by nghttp2.
    • localClose <number> 1 if this Http2Stream has been closed locally.
    • remoteClose <number> 1 if this Http2Stream has been closed remotely.
    • sumDependencyWeight <number> The sum weight of all Http2Stream instances that depend on this Http2Stream as specified using PRIORITY frames.
    • weight <number> The priority weight of this Http2Stream.

A current state of this Http2Stream.

http2stream.sendTrailers(headers)#
Added in: v10.0.0

Sends a trailing HEADERS frame to the connected HTTP/2 peer. This method will cause the Http2Stream to be immediately closed and must only be called after the 'wantTrailers' event has been emitted. When sending a request or sending a response, the options.waitForTrailers option must be set in order to keep the Http2Stream open after the final DATA frame so that trailers can be sent.

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond(undefined, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ xyz: 'abc' });
  });
  stream.end('Hello World');
});

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':method', ':path', etc).

Class: ClientHttp2Stream#

Added in: v8.4.0

The ClientHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Clients. Http2Stream instances on the client provide events such as 'response' and 'push' that are only relevant on the client.

Event: 'continue'#
Added in: v8.5.0

Emitted when the server sends a 100 Continue status, usually because the request contained Expect: 100-continue. This is an instruction that the client should send the request body.

Event: 'headers'#
Added in: v8.4.0

The 'headers' event is emitted when an additional block of headers is received for a stream, such as when a block of 1xx informational headers is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

stream.on('headers', (headers, flags) => {
  console.log(headers);
});
Event: 'push'#
Added in: v8.4.0

The 'push' event is emitted when response headers for a Server Push stream are received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

stream.on('push', (headers, flags) => {
  console.log(headers);
});
Event: 'response'#
Added in: v8.4.0

The 'response' event is emitted when a response HEADERS frame has been received for this stream from the connected HTTP/2 server. The listener is invoked with two arguments: an Object containing the received HTTP/2 Headers Object, and flags associated with the headers.

const http2 = require('node:http2');
const client = http2.connect('https://localhost');
const req = client.request({ ':path': '/' });
req.on('response', (headers, flags) => {
  console.log(headers[':status']);
});

Class: ServerHttp2Stream#

Added in: v8.4.0

The ServerHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Servers. Http2Stream instances on the server provide additional methods such as http2stream.pushStream() and http2stream.respond() that are only relevant on the server.

http2stream.additionalHeaders(headers)#
Added in: v8.4.0

Sends an additional informational HEADERS frame to the connected HTTP/2 peer.

http2stream.headersSent#
Added in: v8.4.0

True if headers were sent, false otherwise (read-only).

http2stream.pushAllowed#
Added in: v8.4.0

Read-only property mapped to the SETTINGS_ENABLE_PUSH flag of the remote client's most recent SETTINGS frame. Will be true if the remote peer accepts push streams, false otherwise. Settings are the same for every Http2Stream in the same Http2Session.

http2stream.pushStream(headers[, options], callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

  • headers <HTTP/2 Headers Object>
  • options <Object>
    • exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on.
  • callback <Function> Callback that is called once the push stream has been initiated.

Initiates a push stream. The callback is invoked with the new Http2Stream instance created for the push stream passed as the second argument, or an Error passed as the first argument.

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 });
  stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => {
    if (err) throw err;
    pushStream.respond({ ':status': 200 });
    pushStream.end('some pushed data');
  });
  stream.end('some data');
});

Setting the weight of a push stream is not allowed in the HEADERS frame. Pass a weight value to http2stream.priority with the silent option set to true to enable server-side bandwidth balancing between concurrent streams.

Calling http2stream.pushStream() from within a pushed stream is not permitted and will throw an error.

http2stream.respond([headers[, options]])#
History
VersionChanges
v14.5.0, v12.19.0

Allow explicitly setting date headers.

v8.4.0

Added in: v8.4.0

  • headers <HTTP/2 Headers Object>
  • options <Object>
    • endStream <boolean> Set to true to indicate that the response will not include payload data.
    • waitForTrailers <boolean> When true, the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 });
  stream.end('some data');
});

Initiates a response. When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 }, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });
  stream.end('some data');
});
http2stream.respondWithFD(fd[, headers[, options]])#
History
VersionChanges
v14.5.0, v12.19.0

Allow explicitly setting date headers.

v12.12.0

The fd option may now be a FileHandle.

v10.0.0

Any readable file descriptor, not necessarily for a regular file, is supported now.

v8.4.0

Added in: v8.4.0

Initiates a response whose data is read from the given file descriptor. No validation is performed on the given file descriptor. If an error occurs while attempting to read data using the file descriptor, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code.

When used, the Http2Stream object's Duplex interface will be closed automatically.

const http2 = require('node:http2');
const fs = require('node:fs');

const server = http2.createServer();
server.on('stream', (stream) => {
  const fd = fs.openSync('/some/file', 'r');

  const stat = fs.fstatSync(fd);
  const headers = {
    'content-length': stat.size,
    'last-modified': stat.mtime.toUTCString(),
    'content-type': 'text/plain; charset=utf-8',
  };
  stream.respondWithFD(fd, headers);
  stream.on('close', () => fs.closeSync(fd));
});

The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given fd. If the statCheck function is provided, the http2stream.respondWithFD() method will perform an fs.fstat() call to collect details on the provided file descriptor.

The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The file descriptor or FileHandle is not closed when the stream is closed, so it will need to be closed manually once it is no longer needed. Using the same file descriptor concurrently for multiple streams is not supported and may result in data loss. Re-using a file descriptor after a stream has finished is supported.

When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('node:http2');
const fs = require('node:fs');

const server = http2.createServer();
server.on('stream', (stream) => {
  const fd = fs.openSync('/some/file', 'r');

  const stat = fs.fstatSync(fd);
  const headers = {
    'content-length': stat.size,
    'last-modified': stat.mtime.toUTCString(),
    'content-type': 'text/plain; charset=utf-8',
  };
  stream.respondWithFD(fd, headers, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });

  stream.on('close', () => fs.closeSync(fd));
});
http2stream.respondWithFile(path[, headers[, options]])#
History
VersionChanges
v14.5.0, v12.19.0

Allow explicitly setting date headers.

v10.0.0

Any readable file, not necessarily a regular file, is supported now.

v8.4.0

Added in: v8.4.0

Sends a regular file as the response. The path must specify a regular file or an 'error' event will be emitted on the Http2Stream object.

When used, the Http2Stream object's Duplex interface will be closed automatically.

The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given file:

If an error occurs while attempting to read the file data, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code. If the onError callback is defined, then it will be called. Otherwise the stream will be destroyed.

Example using a file path:

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function statCheck(stat, headers) {
    headers['last-modified'] = stat.mtime.toUTCString();
  }

  function onError(err) {
    // stream.respond() can throw if the stream has been destroyed by
    // the other side.
    try {
      if (err.code === 'ENOENT') {
        stream.respond({ ':status': 404 });
      } else {
        stream.respond({ ':status': 500 });
      }
    } catch (err) {
      // Perform actual error handling.
      console.error(err);
    }
    stream.end();
  }

  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain; charset=utf-8' },
                         { statCheck, onError });
});

The options.statCheck function may also be used to cancel the send operation by returning false. For instance, a conditional request may check the stat results to determine if the file has been modified to return an appropriate 304 response:

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function statCheck(stat, headers) {
    // Check the stat here...
    stream.respond({ ':status': 304 });
    return false; // Cancel the send operation
  }
  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain; charset=utf-8' },
                         { statCheck });
});

The content-length header field will be automatically set.

The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The options.onError function may also be used to handle all the errors that could happen before the delivery of the file is initiated. The default behavior is to destroy the stream.

When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain; charset=utf-8' },
                         { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });
});

Class: Http2Server#

Added in: v8.4.0

Instances of Http2Server are created using the http2.createServer() function. The Http2Server class is not exported directly by the node:http2 module.

Event: 'checkContinue'#
Added in: v8.5.0

If a 'request' listener is registered or http2.createServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'connection'#
Added in: v8.4.0

This event is emitted when a new TCP stream is established. socket is typically an object of type net.Socket. Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any Duplex stream can be passed.

Event: 'request'#
Added in: v8.4.0

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

Event: 'session'#
Added in: v8.4.0

The 'session' event is emitted when a new Http2Session is created by the Http2Server.

Event: 'sessionError'#
Added in: v8.4.0

The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2Server.

Event: 'stream'#
Added in: v8.4.0

The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server.

See also Http2Session's 'stream' event.

const http2 = require('node:http2');
const {
  HTTP2_HEADER_METHOD,
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS,
  HTTP2_HEADER_CONTENT_TYPE,
} = http2.constants;

const server = http2.createServer();
server.on('stream', (stream, headers, flags) => {
  const method = headers[HTTP2_HEADER_METHOD];
  const path = headers[HTTP2_HEADER_PATH];
  // ...
  stream.respond({
    [HTTP2_HEADER_STATUS]: 200,
    [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
  });
  stream.write('hello ');
  stream.end('world');
});
Event: 'timeout'#
History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout(). Default: 0 (no timeout)

server.close([callback])#
Added in: v8.4.0

Stops the server from establishing new sessions. This does not prevent new request streams from being created due to the persistent nature of HTTP/2 sessions. To gracefully shut down the server, call http2session.close() on all active sessions.

If callback is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See net.Server.close() for more details.

server.setTimeout([msecs][, callback])#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

Used to set the timeout value for http2 server requests, and sets a callback function that is called when there is no activity on the Http2Server after msecs milliseconds.

The given callback is registered as a listener on the 'timeout' event.

In case if callback is not a function, a new ERR_INVALID_ARG_TYPE error will be thrown.

server.timeout#
History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

server.updateSettings([settings])#
Added in: v15.1.0, v14.17.0

Used to update the server with the provided settings.

Throws ERR_HTTP2_INVALID_SETTING_VALUE for invalid settings values.

Throws ERR_INVALID_ARG_TYPE for invalid settings argument.

Class: Http2SecureServer#

Added in: v8.4.0

Instances of Http2SecureServer are created using the http2.createSecureServer() function. The Http2SecureServer class is not exported directly by the node:http2 module.

Event: 'checkContinue'#
Added in: v8.5.0

If a 'request' listener is registered or http2.createSecureServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'connection'#
Added in: v8.4.0

This event is emitted when a new TCP stream is established, before the TLS handshake begins. socket is typically an object of type net.Socket. Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any Duplex stream can be passed.

Event: 'request'#
Added in: v8.4.0

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

Event: 'session'#
Added in: v8.4.0

The 'session' event is emitted when a new Http2Session is created by the Http2SecureServer.

Event: 'sessionError'#
Added in: v8.4.0

The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2SecureServer.

Event: 'stream'#
Added in: v8.4.0

The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server.

See also Http2Session's 'stream' event.

const http2 = require('node:http2');
const {
  HTTP2_HEADER_METHOD,
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS,
  HTTP2_HEADER_CONTENT_TYPE,
} = http2.constants;

const options = getOptionsSomehow();

const server = http2.createSecureServer(options);
server.on('stream', (stream, headers, flags) => {
  const method = headers[HTTP2_HEADER_METHOD];
  const path = headers[HTTP2_HEADER_PATH];
  // ...
  stream.respond({
    [HTTP2_HEADER_STATUS]: 200,
    [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
  });
  stream.write('hello ');
  stream.end('world');
});
Event: 'timeout'#
Added in: v8.4.0

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2secureServer.setTimeout(). Default: 2 minutes.

Event: 'unknownProtocol'#
History
VersionChanges
v19.0.0

This event will only be emitted if the client did not transmit an ALPN extension during the TLS handshake.

v8.4.0

Added in: v8.4.0

The 'unknownProtocol' event is emitted when a connecting client fails to negotiate an allowed protocol (i.e. HTTP/2 or HTTP/1.1). The event handler receives the socket for handling. If no listener is registered for this event, the connection is terminated. A timeout may be specified using the 'unknownProtocolTimeout' option passed to http2.createSecureServer().

In earlier versions of Node.js, this event would be emitted if allowHTTP1 is false and, during the TLS handshake, the client either does not send an ALPN extension or sends an ALPN extension that does not include HTTP/2 (h2). Newer versions of Node.js only emit this event if allowHTTP1 is false and the client does not send an ALPN extension. If the client sends an ALPN extension that does not include HTTP/2 (or HTTP/1.1 if allowHTTP1 is true), the TLS handshake will fail and no secure connection will be established.

See the Compatibility API.

server.close([callback])#
Added in: v8.4.0

Stops the server from establishing new sessions. This does not prevent new request streams from being created due to the persistent nature of HTTP/2 sessions. To gracefully shut down the server, call http2session.close() on all active sessions.

If callback is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See tls.Server.close() for more details.

server.setTimeout([msecs][, callback])#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

Used to set the timeout value for http2 secure server requests, and sets a callback function that is called when there is no activity on the Http2SecureServer after msecs milliseconds.

The given callback is registered as a listener on the 'timeout' event.

In case if callback is not a function, a new ERR_INVALID_ARG_TYPE error will be thrown.

server.timeout#
History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

server.updateSettings([settings])#
Added in: v15.1.0, v14.17.0

Used to update the server with the provided settings.

Throws ERR_HTTP2_INVALID_SETTING_VALUE for invalid settings values.

Throws ERR_INVALID_ARG_TYPE for invalid settings argument.

http2.createServer([options][, onRequestHandler])#

History
VersionChanges
v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v13.3.0, v12.16.0

Added maxSessionRejectedStreams option with a default of 100.

v13.3.0, v12.16.0

Added maxSessionInvalidFrames option with a default of 1000.

v12.4.0

The options parameter now supports net.createServer() options.

v15.10.0, v14.16.0, v12.21.0, v10.24.0

Added unknownProtocolTimeout option with a default of 10000.

v14.4.0, v12.18.0, v10.21.0

Added maxSettings option with a default of 32.

v9.6.0

Added the Http1IncomingMessage and Http1ServerResponse option.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • options <Object>
    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. This is similar to server.maxHeadersCount or request.maxHeadersCount in the node:http module. The minimum value is 4. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed. While this sets the maximum allowed size to the entire block of headers, nghttp2 (the internal http2 library) has a limit of 65536 for each decompressed key/value pair.
    • paddingStrategy <number> The strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:
      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • maxSessionInvalidFrames <integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: 1000.
    • maxSessionRejectedStreams <integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an NGHTTP2_ENHANCE_YOUR_CALM error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: 100.
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • Http1IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to used for HTTP/1 fallback. Useful for extending the original http.IncomingMessage. Default: http.IncomingMessage.
    • Http1ServerResponse <http.ServerResponse> Specifies the ServerResponse class to used for HTTP/1 fallback. Useful for extending the original http.ServerResponse. Default: http.ServerResponse.
    • Http2ServerRequest <http2.Http2ServerRequest> Specifies the Http2ServerRequest class to use. Useful for extending the original Http2ServerRequest. Default: Http2ServerRequest.
    • Http2ServerResponse <http2.Http2ServerResponse> Specifies the Http2ServerResponse class to use. Useful for extending the original Http2ServerResponse. Default: Http2ServerResponse.
    • unknownProtocolTimeout <number> Specifies a timeout in milliseconds that a server should wait when an 'unknownProtocol' is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: 10000.
    • ...: Any net.createServer() option can be provided.
  • onRequestHandler <Function> See Compatibility API
  • Returns: <Http2Server>

Returns a net.Server instance that creates and manages Http2Session instances.

Since there are no browsers known that support unencrypted HTTP/2, the use of http2.createSecureServer() is necessary when communicating with browser clients.

const http2 = require('node:http2');

// Create an unencrypted HTTP/2 server.
// Since there are no browsers known that support
// unencrypted HTTP/2, the use of `http2.createSecureServer()`
// is necessary when communicating with browser clients.
const server = http2.createServer();

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html; charset=utf-8',
    ':status': 200,
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.createSecureServer(options[, onRequestHandler])#

History
VersionChanges
v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v13.3.0, v12.16.0

Added maxSessionRejectedStreams option with a default of 100.

v13.3.0, v12.16.0

Added maxSessionInvalidFrames option with a default of 1000.

v15.10.0, v14.16.0, v12.21.0, v10.24.0

Added unknownProtocolTimeout option with a default of 10000.

v14.4.0, v12.18.0, v10.21.0

Added maxSettings option with a default of 32.

v10.12.0

Added the origins option to automatically send an ORIGIN frame on Http2Session startup.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • options <Object>
    • allowHTTP1 <boolean> Incoming client connections that do not support HTTP/2 will be downgraded to HTTP/1.x when set to true. See the 'unknownProtocol' event. See ALPN negotiation. Default: false.
    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. This is similar to server.maxHeadersCount or request.maxHeadersCount in the node:http module. The minimum value is 4. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed.
    • paddingStrategy <number> Strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:
      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • maxSessionInvalidFrames <integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: 1000.
    • maxSessionRejectedStreams <integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an NGHTTP2_ENHANCE_YOUR_CALM error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: 100.
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • ...: Any tls.createServer() options can be provided. For servers, the identity options (pfx or key/cert) are usually required.
    • origins <string[]> An array of origin strings to send within an ORIGIN frame immediately following creation of a new server Http2Session.
    • unknownProtocolTimeout <number> Specifies a timeout in milliseconds that a server should wait when an 'unknownProtocol' event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: 10000.
  • onRequestHandler <Function> See Compatibility API
  • Returns: <Http2SecureServer>

Returns a tls.Server instance that creates and manages Http2Session instances.

const http2 = require('node:http2');
const fs = require('node:fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),
};

// Create a secure HTTP/2 server
const server = http2.createSecureServer(options);

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html; charset=utf-8',
    ':status': 200,
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.connect(authority[, options][, listener])#

History
VersionChanges
v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v15.10.0, v14.16.0, v12.21.0, v10.24.0

Added unknownProtocolTimeout option with a default of 10000.

v14.4.0, v12.18.0, v10.21.0

Added maxSettings option with a default of 32.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • authority <string> | <URL> The remote HTTP/2 server to connect to. This must be in the form of a minimal, valid URL with the http:// or https:// prefix, host name, and IP port (if a non-default port is used). Userinfo (user ID and password), path, querystring, and fragment details in the URL will be ignored.
  • options <Object>
    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. This is similar to server.maxHeadersCount or request.maxHeadersCount in the node:http module. The minimum value is 1. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxReservedRemoteStreams <number> Sets the maximum number of reserved push streams the client will accept at any given time. Once the current number of currently reserved push streams exceeds reaches this limit, new push streams sent by the server will be automatically rejected. The minimum allowed value is 0. The maximum allowed value is 232-1. A negative value sets this option to the maximum allowed value. Default: 200.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed.
    • paddingStrategy <number> Strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:
      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • protocol <string> The protocol to connect with, if not set in the authority. Value may be either 'http:' or 'https:'. Default: 'https:'
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • createConnection <Function> An optional callback that receives the URL instance passed to connect and the options object, and returns any Duplex stream that is to be used as the connection for this session.
    • ...: Any net.connect() or tls.connect() options can be provided.
    • unknownProtocolTimeout <number> Specifies a timeout in milliseconds that a server should wait when an 'unknownProtocol' event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: 10000.
  • listener <Function> Will be registered as a one-time listener of the 'connect' event.
  • Returns: <ClientHttp2Session>

Returns a ClientHttp2Session instance.

const http2 = require('node:http2');
const client = http2.connect('https://localhost:1234');

/* Use the client */

client.close();

http2.constants#

Added in: v8.4.0
Error codes for RST_STREAM and GOAWAY#
ValueNameConstant
0x00No Errorhttp2.constants.NGHTTP2_NO_ERROR
0x01Protocol Errorhttp2.constants.NGHTTP2_PROTOCOL_ERROR
0x02Internal Errorhttp2.constants.NGHTTP2_INTERNAL_ERROR
0x03Flow Control Errorhttp2.constants.NGHTTP2_FLOW_CONTROL_ERROR
0x04Settings Timeouthttp2.constants.NGHTTP2_SETTINGS_TIMEOUT
0x05Stream Closedhttp2.constants.NGHTTP2_STREAM_CLOSED
0x06Frame Size Errorhttp2.constants.NGHTTP2_FRAME_SIZE_ERROR
0x07Refused Streamhttp2.constants.NGHTTP2_REFUSED_STREAM
0x08Cancelhttp2.constants.NGHTTP2_CANCEL
0x09Compression Errorhttp2.constants.NGHTTP2_COMPRESSION_ERROR
0x0aConnect Errorhttp2.constants.NGHTTP2_CONNECT_ERROR
0x0bEnhance Your Calmhttp2.constants.NGHTTP2_ENHANCE_YOUR_CALM
0x0cInadequate Securityhttp2.constants.NGHTTP2_INADEQUATE_SECURITY
0x0dHTTP/1.1 Requiredhttp2.constants.NGHTTP2_HTTP_1_1_REQUIRED

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout().

http2.getDefaultSettings()#

Added in: v8.4.0

Returns an object containing the default settings for an Http2Session instance. This method returns a new object instance every time it is called so instances returned may be safely modified for use.

http2.getPackedSettings([settings])#

Added in: v8.4.0

Returns a Buffer instance containing serialized representation of the given HTTP/2 settings as specified in the HTTP/2 specification. This is intended for use with the HTTP2-Settings header field.

const http2 = require('node:http2');

const packed = http2.getPackedSettings({ enablePush: false });

console.log(packed.toString('base64'));
// Prints: AAIAAAAA

http2.getUnpackedSettings(buf)#

Added in: v8.4.0

Returns a HTTP/2 Settings Object containing the deserialized settings from the given Buffer as generated by http2.getPackedSettings().

http2.sensitiveHeaders#

Added in: v15.0.0, v14.18.0

This symbol can be set as a property on the HTTP/2 headers object with an array value in order to provide a list of headers considered sensitive. See Sensitive headers for more details.

Headers object#

Headers are represented as own-properties on JavaScript objects. The property keys will be serialized to lower-case. Property values should be strings (if they are not they will be coerced to strings) or an Array of strings (in order to send more than one value per header field).

const headers = {
  ':status': '200',
  'content-type': 'text-plain',
  'ABC': ['has', 'more', 'than', 'one', 'value'],
};

stream.respond(headers);

Header objects passed to callback functions will have a null prototype. This means that normal JavaScript object methods such as Object.prototype.toString() and Object.prototype.hasOwnProperty() will not work.

For incoming headers:

  • The :status header is converted to number.
  • Duplicates of :status, :method, :authority, :scheme, :path, :protocol, age, authorization, access-control-allow-credentials, access-control-max-age, access-control-request-method, content-encoding, content-language, content-length, content-location, content-md5, content-range, content-type, date, dnt, etag, expires, from, host, if-match, if-modified-since, if-none-match, if-range, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, range, referer,retry-after, tk, upgrade-insecure-requests, user-agent or x-content-type-options are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For duplicate cookie headers, the values are joined together with '; '.
  • For all other headers, the values are joined together with ', '.
const http2 = require('node:http2');
const server = http2.createServer();
server.on('stream', (stream, headers) => {
  console.log(headers[':path']);
  console.log(headers.ABC);
});
Sensitive headers#

HTTP2 headers can be marked as sensitive, which means that the HTTP/2 header compression algorithm will never index them. This can make sense for header values with low entropy and that may be considered valuable to an attacker, for example Cookie or Authorization. To achieve this, add the header name to the [http2.sensitiveHeaders] property as an array:

const headers = {
  ':status': '200',
  'content-type': 'text-plain',
  'cookie': 'some-cookie',
  'other-sensitive-header': 'very secret data',
  [http2.sensitiveHeaders]: ['cookie', 'other-sensitive-header'],
};

stream.respond(headers);

For some headers, such as Authorization and short Cookie headers, this flag is set automatically.

This property is also set for received headers. It will contain the names of all headers marked as sensitive, including ones marked that way automatically.

Settings object#

History
VersionChanges
v12.12.0

The maxConcurrentStreams setting is stricter.

v8.9.3

The maxHeaderListSize setting is now strictly enforced.

v8.4.0

Added in: v8.4.0

The http2.getDefaultSettings(), http2.getPackedSettings(), http2.createServer(), http2.createSecureServer(), http2session.settings(), http2session.localSettings, and http2session.remoteSettings APIs either return or receive as input an object that defines configuration settings for an Http2Session object. These objects are ordinary JavaScript objects containing the following properties.

  • headerTableSize <number> Specifies the maximum number of bytes used for header compression. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 4096.
  • enablePush <boolean> Specifies true if HTTP/2 Push Streams are to be permitted on the Http2Session instances. Default: true.
  • initialWindowSize <number> Specifies the sender's initial window size in bytes for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65535.
  • maxFrameSize <number> Specifies the size in bytes of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default: 16384.
  • maxConcurrentStreams <number> Specifies the maximum number of concurrent streams permitted on an Http2Session. There is no default value which implies, at least theoretically, 232-1 streams may be open concurrently at any given time in an Http2Session. The minimum value is 0. The maximum allowed value is 232-1. Default: 4294967295.
  • maxHeaderListSize <number> Specifies the maximum size (uncompressed octets) of header list that will be accepted. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65535.
  • maxHeaderSize <number> Alias for maxHeaderListSize.
  • enableConnectProtocol<boolean> Specifies true if the "Extended Connect Protocol" defined by RFC 8441 is to be enabled. This setting is only meaningful if sent by the server. Once the enableConnectProtocol setting has been enabled for a given Http2Session, it cannot be disabled. Default: false.

All additional properties on the settings object are ignored.

Error handling#

There are several types of error conditions that may arise when using the node:http2 module:

Validation errors occur when an incorrect argument, option, or setting value is passed in. These will always be reported by a synchronous throw.

State errors occur when an action is attempted at an incorrect time (for instance, attempting to send data on a stream after it has closed). These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream, Http2Session or HTTP/2 Server objects, depending on where and when the error occurs.

Internal errors occur when an HTTP/2 session fails unexpectedly. These will be reported via an 'error' event on the Http2Session or HTTP/2 Server objects.

Protocol errors occur when various HTTP/2 protocol constraints are violated. These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream, Http2Session or HTTP/2 Server objects, depending on where and when the error occurs.

Invalid character handling in header names and values#

The HTTP/2 implementation applies stricter handling of invalid characters in HTTP header names and values than the HTTP/1 implementation.

Header field names are case-insensitive and are transmitted over the wire strictly as lower-case strings. The API provided by Node.js allows header names to be set as mixed-case strings (e.g. Content-Type) but will convert those to lower-case (e.g. content-type) upon transmission.

Header field-names must only contain one or more of the following ASCII characters: a-z, A-Z, 0-9, !, #, $, %, &, ', *, +, -, ., ^, _, ` (backtick), |, and ~.

Using invalid characters within an HTTP header field name will cause the stream to be closed with a protocol error being reported.

Header field values are handled with more leniency but should not contain new-line or carriage return characters and should be limited to US-ASCII characters, per the requirements of the HTTP specification.

Push streams on the client#

To receive pushed streams on the client, set a listener for the 'stream' event on the ClientHttp2Session:

const http2 = require('node:http2');

const client = http2.connect('http://localhost');

client.on('stream', (pushedStream, requestHeaders) => {
  pushedStream.on('push', (responseHeaders) => {
    // Process response headers
  });
  pushedStream.on('data', (chunk) => { /* handle pushed data */ });
});

const req = client.request({ ':path': '/' });

Supporting the CONNECT method#

The CONNECT method is used to allow an HTTP/2 server to be used as a proxy for TCP/IP connections.

A simple TCP Server:

const net = require('node:net');

const server = net.createServer((socket) => {
  let name = '';
  socket.setEncoding('utf8');
  socket.on('data', (chunk) => name += chunk);
  socket.on('end', () => socket.end(`hello ${name}`));
});

server.listen(8000);

An HTTP/2 CONNECT proxy:

const http2 = require('node:http2');
const { NGHTTP2_REFUSED_STREAM } = http2.constants;
const net = require('node:net');

const proxy = http2.createServer();
proxy.on('stream', (stream, headers) => {
  if (headers[':method'] !== 'CONNECT') {
    // Only accept CONNECT requests
    stream.close(NGHTTP2_REFUSED_STREAM);
    return;
  }
  const auth = new URL(`tcp://${headers[':authority']}`);
  // It's a very good idea to verify that hostname and port are
  // things this proxy should be connecting to.
  const socket = net.connect(auth.port, auth.hostname, () => {
    stream.respond();
    socket.pipe(stream);
    stream.pipe(socket);
  });
  socket.on('error', (error) => {
    stream.close(http2.constants.NGHTTP2_CONNECT_ERROR);
  });
});

proxy.listen(8001);

An HTTP/2 CONNECT client:

const http2 = require('node:http2');

const client = http2.connect('http://localhost:8001');

// Must not specify the ':path' and ':scheme' headers
// for CONNECT requests or an error will be thrown.
const req = client.request({
  ':method': 'CONNECT',
  ':authority': `localhost:${port}`,
});

req.on('response', (headers) => {
  console.log(headers[http2.constants.HTTP2_HEADER_STATUS]);
});
let data = '';
req.setEncoding('utf8');
req.on('data', (chunk) => data += chunk);
req.on('end', () => {
  console.log(`The server says: ${data}`);
  client.close();
});
req.end('Jane');

The extended CONNECT protocol#

RFC 8441 defines an "Extended CONNECT Protocol" extension to HTTP/2 that may be used to bootstrap the use of an Http2Stream using the CONNECT method as a tunnel for other communication protocols (such as WebSockets).

The use of the Extended CONNECT Protocol is enabled by HTTP/2 servers by using the enableConnectProtocol setting:

const http2 = require('node:http2');
const settings = { enableConnectProtocol: true };
const server = http2.createServer({ settings });

Once the client receives the SETTINGS frame from the server indicating that the extended CONNECT may be used, it may send CONNECT requests that use the ':protocol' HTTP/2 pseudo-header:

const http2 = require('node:http2');
const client = http2.connect('http://localhost:8080');
client.on('remoteSettings', (settings) => {
  if (settings.enableConnectProtocol) {
    const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' });
    // ...
  }
});

Compatibility API#

The Compatibility API has the goal of providing a similar developer experience of HTTP/1 when using HTTP/2, making it possible to develop applications that support both HTTP/1 and HTTP/2. This API targets only the public API of the HTTP/1. However many modules use internal methods or state, and those are not supported as it is a completely different implementation.

The following example creates an HTTP/2 server using the compatibility API:

const http2 = require('node:http2');
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
  res.end('ok');
});

In order to create a mixed HTTPS and HTTP/2 server, refer to the ALPN negotiation section. Upgrading from non-tls HTTP/1 servers is not supported.

The HTTP/2 compatibility API is composed of Http2ServerRequest and Http2ServerResponse. They aim at API compatibility with HTTP/1, but they do not hide the differences between the protocols. As an example, the status message for HTTP codes is ignored.

ALPN negotiation#

ALPN negotiation allows supporting both HTTPS and HTTP/2 over the same socket. The req and res objects can be either HTTP/1 or HTTP/2, and an application must restrict itself to the public API of HTTP/1, and detect if it is possible to use the more advanced features of HTTP/2.

The following example creates a server that supports both protocols:

const { createSecureServer } = require('node:http2');
const { readFileSync } = require('node:fs');

const cert = readFileSync('./cert.pem');
const key = readFileSync('./key.pem');

const server = createSecureServer(
  { cert, key, allowHTTP1: true },
  onRequest,
).listen(4443);

function onRequest(req, res) {
  // Detects if it is a HTTPS request or HTTP/2
  const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ?
    req.stream.session : req;
  res.writeHead(200, { 'content-type': 'application/json' });
  res.end(JSON.stringify({
    alpnProtocol,
    httpVersion: req.httpVersion,
  }));
}

The 'request' event works identically on both HTTPS and HTTP/2.

Class: http2.Http2ServerRequest#

Added in: v8.4.0

A Http2ServerRequest object is created by http2.Server or http2.SecureServer and passed as the first argument to the 'request' event. It may be used to access a request status, headers, and data.

Event: 'aborted'#
Added in: v8.4.0

The 'aborted' event is emitted whenever a Http2ServerRequest instance is abnormally aborted in mid-communication.

The 'aborted' event will only be emitted if the Http2ServerRequest writable side has not been ended.

Event: 'close'#
Added in: v8.4.0

Indicates that the underlying Http2Stream was closed. Just like 'end', this event occurs only once per response.

request.aborted#
Added in: v10.1.0

The request.aborted property will be true if the request has been aborted.

request.authority#
Added in: v8.4.0

The request authority pseudo header field. Because HTTP/2 allows requests to set either :authority or host, this value is derived from req.headers[':authority'] if present. Otherwise, it is derived from req.headers['host'].

request.complete#
Added in: v12.10.0

The request.complete property will be true if the request has been completed, aborted, or destroyed.

request.connection#
Added in: v8.4.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use request.socket.

See request.socket.

request.destroy([error])#
Added in: v8.4.0

Calls destroy() on the Http2Stream that received the Http2ServerRequest. If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event.

It does nothing if the stream was already destroyed.

request.headers#
Added in: v8.4.0

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

See HTTP/2 Headers Object.

In HTTP/2, the request path, host name, protocol, and method are represented as special headers prefixed with the : character (e.g. ':path'). These special headers will be included in the request.headers object. Care must be taken not to inadvertently modify these special headers or errors may occur. For instance, removing all headers from the request will cause errors to occur:

removeAllHeaders(request.headers);
assert(request.url);   // Fails because the :path header has been removed
request.httpVersion#
Added in: v8.4.0

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Returns '2.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

request.method#
Added in: v8.4.0

The request method as a string. Read-only. Examples: 'GET', 'DELETE'.

request.rawHeaders#
Added in: v8.4.0

The raw request/response headers list exactly as they were received.

The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);
request.rawTrailers#
Added in: v8.4.0

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

request.scheme#
Added in: v8.4.0

The request scheme pseudo header field indicating the scheme portion of the target URL.

request.setTimeout(msecs, callback)#
Added in: v8.4.0

Sets the Http2Stream's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then Http2Streams are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

request.socket#
Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but applies getters, setters, and methods based on HTTP/2 logic.

destroyed, readable, and writable properties will be retrieved from and set on request.stream.

destroy, emit, end, on and once methods will be called on request.stream.

setTimeout method will be called on request.stream.session.

pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

All other interactions will be routed directly to the socket. With TLS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

request.stream#
Added in: v8.4.0

The Http2Stream object backing the request.

request.trailers#
Added in: v8.4.0

The request/response trailers object. Only populated at the 'end' event.

request.url#
Added in: v8.4.0

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

GET /status?name=ryan HTTP/1.1
Accept: text/plain

Then request.url will be:

'/status?name=ryan'

To parse the url into its parts, new URL() can be used:

$ node
> new URL('/status?name=ryan', 'http://example.com')
URL {
  href: 'http://example.com/status?name=ryan',
  origin: 'http://example.com',
  protocol: 'http:',
  username: '',
  password: '',
  host: 'example.com',
  hostname: 'example.com',
  port: '',
  pathname: '/status',
  search: '?name=ryan',
  searchParams: URLSearchParams { 'name' => 'ryan' },
  hash: ''
}

Class: http2.Http2ServerResponse#

Added in: v8.4.0

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the 'request' event.

Event: 'close'#
Added in: v8.4.0

Indicates that the underlying Http2Stream was terminated before response.end() was called or able to flush.

Event: 'finish'#
Added in: v8.4.0

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the HTTP/2 multiplexing for transmission over the network. It does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

response.addTrailers(headers)#
Added in: v8.4.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.connection#
Added in: v8.4.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use response.socket.

See response.socket.

response.createPushResponse(headers, callback)#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.4.0

Added in: v8.4.0

  • headers <HTTP/2 Headers Object> An object describing the headers
  • callback <Function> Called once http2stream.pushStream() is finished, or either when the attempt to create the pushed Http2Stream has failed or has been rejected, or the state of Http2ServerRequest is closed prior to calling the http2stream.pushStream() method

Call http2stream.pushStream() with the given headers, and wrap the given Http2Stream on a newly created Http2ServerResponse as the callback parameter if successful. When Http2ServerRequest is closed, the callback is called with an error ERR_HTTP2_INVALID_STREAM.

response.end([data[, encoding]][, callback])#
History
VersionChanges
v10.0.0

This method now returns a reference to ServerResponse.

v8.4.0

Added in: v8.4.0

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is equivalent to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#
Added in: v8.4.0Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use response.writableEnded.

Boolean value that indicates whether the response has completed. Starts as false. After response.end() executes, the value will be true.

response.getHeader(name)#
Added in: v8.4.0

Reads out a header that has already been queued but not sent to the client. The name is case-insensitive.

const contentType = response.getHeader('content-type');
response.getHeaderNames()#
Added in: v8.4.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
response.getHeaders()#
Added in: v8.4.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
response.hasHeader(name)#
Added in: v8.4.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const hasContentType = response.hasHeader('content-type');
response.headersSent#
Added in: v8.4.0

True if headers were sent, false otherwise (read-only).

response.removeHeader(name)#
Added in: v8.4.0

Removes a header that has been queued for implicit sending.

response.removeHeader('Content-Encoding');
response.req#
Added in: v15.7.0

A reference to the original HTTP2 request object.

response.sendDate#
Added in: v8.4.0

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#
Added in: v8.4.0

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name.

response.setHeader('Content-Type', 'text/html; charset=utf-8');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html; charset=utf-8');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
  res.end('ok');
});
response.setTimeout(msecs[, callback])#
Added in: v8.4.0

Sets the Http2Stream's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then Http2Streams are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

response.socket#
Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but applies getters, setters, and methods based on HTTP/2 logic.

destroyed, readable, and writable properties will be retrieved from and set on response.stream.

destroy, emit, end, on and once methods will be called on response.stream.

setTimeout method will be called on response.stream.session.

pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

All other interactions will be routed directly to the socket.

const http2 = require('node:http2');
const server = http2.createServer((req, res) => {
  const ip = req.socket.remoteAddress;
  const port = req.socket.remotePort;
  res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
response.statusCode#
Added in: v8.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#
Added in: v8.4.0

Status message is not supported by HTTP/2 (RFC 7540 8.1.2.4). It returns an empty string.

response.stream#
Added in: v8.4.0

The Http2Stream object backing the response.

response.writableEnded#
Added in: v12.9.0

Is true after response.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

response.write(chunk[, encoding][, callback])#
Added in: v8.4.0

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

In the node:http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. By default the encoding is 'utf8'. callback will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#
Added in: v8.4.0

Sends a status 100 Continue to the client, indicating that the request body should be sent. See the 'checkContinue' event on Http2Server and Http2SecureServer.

response.writeEarlyHints(hints)#
Added in: v18.11.0

Sends a status 103 Early Hints to the client with a Link header, indicating that the user agent can preload/preconnect the linked resources. The hints is an object containing the values of headers to be sent with early hints message.

Example

const earlyHintsLink = '</styles.css>; rel=preload; as=style';
response.writeEarlyHints({
  'link': earlyHintsLink,
});

const earlyHintsLinks = [
  '</styles.css>; rel=preload; as=style',
  '</scripts.js>; rel=preload; as=script',
];
response.writeEarlyHints({
  'link': earlyHintsLinks,
});
response.writeHead(statusCode[, statusMessage][, headers])#
History
VersionChanges
v11.10.0, v10.17.0

Return this from writeHead() to allow chaining with end().

v8.4.0

Added in: v8.4.0

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers.

Returns a reference to the Http2ServerResponse, so that calls can be chained.

For compatibility with HTTP/1, a human-readable statusMessage may be passed as the second argument. However, because the statusMessage has no meaning within HTTP/2, the argument will have no effect and a process warning will be emitted.

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain; charset=utf-8',
});

Content-Length is given in bytes not characters. The Buffer.byteLength() API may be used to determine the number of bytes in a given encoding. On outbound messages, Node.js does not check if Content-Length and the length of the body being transmitted are equal or not. However, when receiving messages, Node.js will automatically reject messages when the Content-Length does not match the actual payload size.

This method may be called at most one time on a message before response.end() is called.

If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html; charset=utf-8');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
  res.end('ok');
});

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

Collecting HTTP/2 performance metrics#

The Performance Observer API can be used to collect basic performance metrics for each Http2Session and Http2Stream instance.

const { PerformanceObserver } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  const entry = items.getEntries()[0];
  console.log(entry.entryType);  // prints 'http2'
  if (entry.name === 'Http2Session') {
    // Entry contains statistics about the Http2Session
  } else if (entry.name === 'Http2Stream') {
    // Entry contains statistics about the Http2Stream
  }
});
obs.observe({ entryTypes: ['http2'] });

The entryType property of the PerformanceEntry will be equal to 'http2'.

The name property of the PerformanceEntry will be equal to either 'Http2Stream' or 'Http2Session'.

If name is equal to Http2Stream, the PerformanceEntry will contain the following additional properties:

  • bytesRead <number> The number of DATA frame bytes received for this Http2Stream.
  • bytesWritten <number> The number of DATA frame bytes sent for this Http2Stream.
  • id <number> The identifier of the associated Http2Stream
  • timeToFirstByte <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first DATA frame.
  • timeToFirstByteSent <number> The number of milliseconds elapsed between the PerformanceEntry startTime and sending of the first DATA frame.
  • timeToFirstHeader <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first header.

If name is equal to Http2Session, the PerformanceEntry will contain the following additional properties:

  • bytesRead <number> The number of bytes received for this Http2Session.
  • bytesWritten <number> The number of bytes sent for this Http2Session.
  • framesReceived <number> The number of HTTP/2 frames received by the Http2Session.
  • framesSent <number> The number of HTTP/2 frames sent by the Http2Session.
  • maxConcurrentStreams <number> The maximum number of streams concurrently open during the lifetime of the Http2Session.
  • pingRTT <number> The number of milliseconds elapsed since the transmission of a PING frame and the reception of its acknowledgment. Only present if a PING frame has been sent on the Http2Session.
  • streamAverageDuration <number> The average duration (in milliseconds) for all Http2Stream instances.
  • streamCount <number> The number of Http2Stream instances processed by the Http2Session.
  • type <string> Either 'server' or 'client' to identify the type of Http2Session.

Note on :authority and host#

HTTP/2 requires requests to have either the :authority pseudo-header or the host header. Prefer :authority when constructing an HTTP/2 request directly, and host when converting from HTTP/1 (in proxies, for instance).

The compatibility API falls back to host if :authority is not present. See request.authority for more information. However, if you don't use the compatibility API (or use req.headers directly), you need to implement any fall-back behavior yourself.

HTTPS#

Stability: 2 - Stable

Source Code: lib/https.js

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the node:crypto module. In such cases, attempting to import from https or calling require('node:https') will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

let https;
try {
  https = require('node:https');
} catch (err) {
  console.error('https support is disabled!');
}

When using the lexical ESM import keyword, the error can only be caught if a handler for process.on('uncaughtException') is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the import() function instead of the lexical import keyword:

let https;
try {
  https = await import('node:https');
} catch (err) {
  console.error('https support is disabled!');
}

Class: https.Agent#

History
VersionChanges
v5.3.0

support 0 maxCachedSessions to disable TLS session caching.

v2.5.0

parameter maxCachedSessions added to options for TLS sessions reuse.

v0.4.5

Added in: v0.4.5

An Agent object for HTTPS similar to http.Agent. See https.request() for more information.

new Agent([options])#

History
VersionChanges
v12.5.0

do not automatically set servername if the target host was specified using an IP address.

  • options <Object> Set of configurable options to set on the agent. Can have the same fields as for http.Agent(options), and

    • maxCachedSessions <number> maximum number of TLS cached sessions. Use 0 to disable TLS session caching. Default: 100.

    • servername <string> the value of Server Name Indication extension to be sent to the server. Use empty string '' to disable sending the extension. Default: host name of the target server, unless the target server is specified using an IP address, in which case the default is '' (no extension).

      See Session Resumption for information about TLS session reuse.

Event: 'keylog'#
Added in: v13.2.0, v12.16.0
  • line <Buffer> Line of ASCII text, in NSS SSLKEYLOGFILE format.
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance on which it was generated.

The keylog event is emitted when key material is generated or received by a connection managed by this agent (typically before handshake has completed, but not necessarily). This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times for each socket.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

// ...
https.globalAgent.on('keylog', (line, tlsSocket) => {
  fs.appendFileSync('/tmp/ssl-keys.log', line, { mode: 0o600 });
});

Class: https.Server#

Added in: v0.3.4

See http.Server for more information.

server.close([callback])#

Added in: v0.1.90

See server.close() in the node:http module.

server.closeAllConnections()#

Added in: v18.2.0

See server.closeAllConnections() in the node:http module.

server.closeIdleConnections()#

Added in: v18.2.0

See server.closeIdleConnections() in the node:http module.

server.headersTimeout#

Added in: v11.3.0

See server.headersTimeout in the node:http module.

server.listen()#

Starts the HTTPS server listening for encrypted connections. This method is identical to server.listen() from net.Server.

server.maxHeadersCount#

See server.maxHeadersCount in the node:http module.

server.requestTimeout#

History
VersionChanges
v18.0.0

The default request timeout changed from no timeout to 300s (5 minutes).

v14.11.0

Added in: v14.11.0

See server.requestTimeout in the node:http module.

server.setTimeout([msecs][, callback])#

Added in: v0.11.2

See server.setTimeout() in the node:http module.

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.11.2

Added in: v0.11.2

See server.timeout in the node:http module.

server.keepAliveTimeout#

Added in: v8.0.0

See server.keepAliveTimeout in the node:http module.

https.createServer([options][, requestListener])#

Added in: v0.3.4 
// curl -k https://localhost:8000/
const https = require('node:https');
const fs = require('node:fs');

const options = {
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

Or

const https = require('node:https');
const fs = require('node:fs');

const options = {
  pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
  passphrase: 'sample',
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

https.get(options[, callback])#

https.get(url[, options][, callback])#

History
VersionChanges
v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Like http.get() but for HTTPS.

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

const https = require('node:https');

https.get('https://encrypted.google.com/', (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });

}).on('error', (e) => {
  console.error(e);
});

https.globalAgent#

History
VersionChanges
v19.0.0

The agent now uses HTTP Keep-Alive by default.

v0.5.9

Added in: v0.5.9

Global instance of https.Agent for all HTTPS client requests.

https.request(options[, callback])#

https.request(url[, options][, callback])#

History
VersionChanges
v16.7.0, v14.18.0

When using a URL object parsed username and password will now be properly URI decoded.

v14.1.0, v13.14.0

The highWaterMark option is accepted now.

v10.9.0

The url parameter can now be passed along with a separate options object.

v9.3.0

The options parameter can now include clientCertEngine.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Makes a request to a secure web server.

The following additional options from tls.connect() are also accepted: ca, cert, ciphers, clientCertEngine, crl, dhparam, ecdhCurve, honorCipherOrder, key, passphrase, pfx, rejectUnauthorized, secureOptions, secureProtocol, servername, sessionIdContext, highWaterMark.

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

https.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object.

const https = require('node:https');

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
};

const req = https.request(options, (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });
});

req.on('error', (e) => {
  console.error(e);
});
req.end();

Example using options from tls.connect():

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
};
options.agent = new https.Agent(options);

const req = https.request(options, (res) => {
  // ...
});

Alternatively, opt out of connection pooling by not using an Agent.

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
  agent: false,
};

const req = https.request(options, (res) => {
  // ...
});

Example using a URL as options:

const options = new URL('https://abc:xyz@example.com');

const req = https.request(options, (res) => {
  // ...
});

Example pinning on certificate fingerprint, or the public key (similar to pin-sha256):

const tls = require('node:tls');
const https = require('node:https');
const crypto = require('node:crypto');

function sha256(s) {
  return crypto.createHash('sha256').update(s).digest('base64');
}
const options = {
  hostname: 'github.com',
  port: 443,
  path: '/',
  method: 'GET',
  checkServerIdentity: function(host, cert) {
    // Make sure the certificate is issued to the host we are connected to
    const err = tls.checkServerIdentity(host, cert);
    if (err) {
      return err;
    }

    // Pin the public key, similar to HPKP pin-sha256 pinning
    const pubkey256 = 'pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=';
    if (sha256(cert.pubkey) !== pubkey256) {
      const msg = 'Certificate verification error: ' +
        `The public key of '${cert.subject.CN}' ` +
        'does not match our pinned fingerprint';
      return new Error(msg);
    }

    // Pin the exact certificate, rather than the pub key
    const cert256 = '25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:' +
      'D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16';
    if (cert.fingerprint256 !== cert256) {
      const msg = 'Certificate verification error: ' +
        `The certificate of '${cert.subject.CN}' ` +
        'does not match our pinned fingerprint';
      return new Error(msg);
    }

    // This loop is informational only.
    // Print the certificate and public key fingerprints of all certs in the
    // chain. Its common to pin the public key of the issuer on the public
    // internet, while pinning the public key of the service in sensitive
    // environments.
    do {
      console.log('Subject Common Name:', cert.subject.CN);
      console.log('  Certificate SHA256 fingerprint:', cert.fingerprint256);

      hash = crypto.createHash('sha256');
      console.log('  Public key ping-sha256:', sha256(cert.pubkey));

      lastprint256 = cert.fingerprint256;
      cert = cert.issuerCertificate;
    } while (cert.fingerprint256 !== lastprint256);

  },
};

options.agent = new https.Agent(options);
const req = https.request(options, (res) => {
  console.log('All OK. Server matched our pinned cert or public key');
  console.log('statusCode:', res.statusCode);
  // Print the HPKP values
  console.log('headers:', res.headers['public-key-pins']);

  res.on('data', (d) => {});
});

req.on('error', (e) => {
  console.error(e.message);
});
req.end();

Outputs for example:

Subject Common Name: github.com
  Certificate SHA256 fingerprint: 25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16
  Public key ping-sha256: pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=
Subject Common Name: DigiCert SHA2 Extended Validation Server CA
  Certificate SHA256 fingerprint: 40:3E:06:2A:26:53:05:91:13:28:5B:AF:80:A0:D4:AE:42:2C:84:8C:9F:78:FA:D0:1F:C9:4B:C5:B8:7F:EF:1A
  Public key ping-sha256: RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho=
Subject Common Name: DigiCert High Assurance EV Root CA
  Certificate SHA256 fingerprint: 74:31:E5:F4:C3:C1:CE:46:90:77:4F:0B:61:E0:54:40:88:3B:A9:A0:1E:D0:0B:A6:AB:D7:80:6E:D3:B1:18:CF
  Public key ping-sha256: WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18=
All OK. Server matched our pinned cert or public key
statusCode: 200
headers: max-age=0; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho="; pin-sha256="k2v657xBsOVe1PQRwOsHsw3bsGT2VzIqz5K+59sNQws="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="IQBnNBEiFuhj+8x6X8XLgh01V9Ic5/V3IRQLNFFc7v4="; pin-sha256="iie1VXtL7HzAMF+/PVPR9xzT80kQxdZeJ+zduCB3uj0="; pin-sha256="LvRiGEjRqfzurezaWuj8Wie2gyHMrW5Q06LspMnox7A="; includeSubDomains

Inspector#

Stability: 2 - Stable

Source Code: lib/inspector.js

The node:inspector module provides an API for interacting with the V8 inspector.

It can be accessed using:

import * as inspector from 'node:inspector/promises';const inspector = require('node:inspector/promises');

or

import * as inspector from 'node:inspector';const inspector = require('node:inspector');

Promises API#

Stability: 1 - Experimental

Added in: v19.0.0

Class: inspector.Session#

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

new inspector.Session()#
Added in: v8.0.0

Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend.

Event: 'inspectorNotification'#
Added in: v8.0.0
  • <Object> The notification message object

Emitted when any notification from the V8 Inspector is received.

session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed

It is also possible to subscribe only to notifications with specific method:

Event: <inspector-protocol-method>;#
Added in: v8.0.0
  • <Object> The notification message object

Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value.

The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

session.on('Debugger.paused', ({ params }) => {
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]
session.connect()#
Added in: v8.0.0

Connects a session to the inspector back-end.

session.connectToMainThread()#
Added in: v12.11.0

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

session.disconnect()#
Added in: v8.0.0

Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

session.post(method[, params])#
Added in: v19.0.0

Posts a message to the inspector back-end.

import { Session } from 'node:inspector/promises';
try {
  const session = new Session();
  session.connect();
  const result = await session.post('Runtime.evaluate', { expression: '2 + 2' });
  console.log(result);
} catch (error) {
  console.error(error);
}
// Output: { result: { type: 'number', value: 4, description: '4' } }

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

Example usage#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

CPU profiler#

Here's an example showing how to use the CPU Profiler:

import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();
session.connect();

await session.post('Profiler.enable');
await session.post('Profiler.start');
// Invoke business logic under measurement here...

// some time later...
const { profile } = await session.post('Profiler.stop');

// Write profile to disk, upload, etc.
fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
Heap profiler#

Here's an example showing how to use the Heap Profiler:

import { Session } from 'node:inspector/promises';
import fs from 'node:fs';
const session = new Session();

const fd = fs.openSync('profile.heapsnapshot', 'w');

session.connect();

session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
  fs.writeSync(fd, m.params.chunk);
});

const result = await session.post('HeapProfiler.takeHeapSnapshot', null);
console.log('HeapProfiler.takeHeapSnapshot done:', result);
session.disconnect();
fs.closeSync(fd);

Callback API#

Class: inspector.Session#

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

new inspector.Session()#
Added in: v8.0.0

Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend.

Event: 'inspectorNotification'#
Added in: v8.0.0
  • <Object> The notification message object

Emitted when any notification from the V8 Inspector is received.

session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed

It is also possible to subscribe only to notifications with specific method:

Event: <inspector-protocol-method>;#
Added in: v8.0.0
  • <Object> The notification message object

Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value.

The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

session.on('Debugger.paused', ({ params }) => {
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]
session.connect()#
Added in: v8.0.0

Connects a session to the inspector back-end.

session.connectToMainThread()#
Added in: v12.11.0

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

session.disconnect()#
Added in: v8.0.0

Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

session.post(method[, params][, callback])#
History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.0.0

Added in: v8.0.0

Posts a message to the inspector back-end. callback will be notified when a response is received. callback is a function that accepts two optional arguments: error and message-specific result.

session.post('Runtime.evaluate', { expression: '2 + 2' },
             (error, { result }) => console.log(result));
// Output: { type: 'number', value: 4, description: '4' }

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

Example usage#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

CPU profiler#

Here's an example showing how to use the CPU Profiler:

const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();
session.connect();

session.post('Profiler.enable', () => {
  session.post('Profiler.start', () => {
    // Invoke business logic under measurement here...

    // some time later...
    session.post('Profiler.stop', (err, { profile }) => {
      // Write profile to disk, upload, etc.
      if (!err) {
        fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
      }
    });
  });
});
Heap profiler#

Here's an example showing how to use the Heap Profiler:

const inspector = require('node:inspector');
const fs = require('node:fs');
const session = new inspector.Session();

const fd = fs.openSync('profile.heapsnapshot', 'w');

session.connect();

session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
  fs.writeSync(fd, m.params.chunk);
});

session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => {
  console.log('HeapProfiler.takeHeapSnapshot done:', err, r);
  session.disconnect();
  fs.closeSync(fd);
});

Common Objects#

inspector.close()#

History
VersionChanges
v18.10.0

The API is exposed in the worker threads.

v9.0.0

Added in: v9.0.0

Deactivate the inspector. Blocks until there are no active connections.

inspector.console#

  • <Object> An object to send messages to the remote inspector console.
require('node:inspector').console.log('a message');

The inspector console does not have API parity with Node.js console.

inspector.open([port[, host[, wait]]])#

  • port <number> Port to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • host <string> Host to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • wait <boolean> Block until a client has connected. Optional. Default: false.

Activate inspector on host and port. Equivalent to node --inspect=[[host:]port], but can be done programmatically after node has started.

If wait is true, will block until a client has connected to the inspect port and flow control has been passed to the debugger client.

See the security warning regarding the host parameter usage.

inspector.url()#

Return the URL of the active inspector, or undefined if there is none.

$ node --inspect -p 'inspector.url()'
Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
For help, see: https://nodejs.org/en/docs/inspector
ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34

$ node --inspect=localhost:3000 -p 'inspector.url()'
Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
For help, see: https://nodejs.org/en/docs/inspector
ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a

$ node -p 'inspector.url()'
undefined

inspector.waitForDebugger()#

Added in: v12.7.0

Blocks until a client (existing or connected later) has sent Runtime.runIfWaitingForDebugger command.

An exception will be thrown if there is no active inspector.

Internationalization support#

Node.js has many features that make it easier to write internationalized programs. Some of them are:

Node.js and the underlying V8 engine use International Components for Unicode (ICU) to implement these features in native C/C++ code. The full ICU data set is provided by Node.js by default. However, due to the size of the ICU data file, several options are provided for customizing the ICU data set either when building or running Node.js.

Options for building Node.js#

To control how ICU is used in Node.js, four configure options are available during compilation. Additional details on how to compile Node.js are documented in BUILDING.md.

  • --with-intl=none/--without-intl
  • --with-intl=system-icu
  • --with-intl=small-icu
  • --with-intl=full-icu (default)

An overview of available Node.js and JavaScript features for each configure option:

Featurenonesystem-icusmall-icufull-icu
String.prototype.normalize()none (function is no-op)fullfullfull
String.prototype.to*Case()fullfullfullfull
Intlnone (object does not exist)partial/full (depends on OS)partial (English-only)full
String.prototype.localeCompare()partial (not locale-aware)fullfullfull
String.prototype.toLocale*Case()partial (not locale-aware)fullfullfull
Number.prototype.toLocaleString()partial (not locale-aware)partial/full (depends on OS)partial (English-only)full
Date.prototype.toLocale*String()partial (not locale-aware)partial/full (depends on OS)partial (English-only)full
Legacy URL Parserpartial (no IDN support)fullfullfull
WHATWG URL Parserpartial (no IDN support)fullfullfull
require('node:buffer').transcode()none (function does not exist)fullfullfull
REPLpartial (inaccurate line editing)fullfullfull
require('node:util').TextDecoderpartial (basic encodings support)partial/full (depends on OS)partial (Unicode-only)full
RegExp Unicode Property Escapesnone (invalid RegExp error)fullfullfull

The "(not locale-aware)" designation denotes that the function carries out its operation just like the non-Locale version of the function, if one exists. For example, under none mode, Date.prototype.toLocaleString()'s operation is identical to that of Date.prototype.toString().

Disable all internationalization features (none)#

If this option is chosen, ICU is disabled and most internationalization features mentioned above will be unavailable in the resulting node binary.

Build with a pre-installed ICU (system-icu)#

Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.

Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser, are fully supported under system-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat may be fully or partially supported, depending on the completeness of the ICU data installed on the system.

Embed a limited set of ICU data (small-icu)#

This option makes the resulting binary link against the ICU library statically, and includes a subset of ICU data (typically only the English locale) within the node executable.

Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser, are fully supported under small-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat, generally only work with the English locale:

const january = new Date(9e8);
const english = new Intl.DateTimeFormat('en', { month: 'long' });
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });

console.log(english.format(january));
// Prints "January"
console.log(spanish.format(january));
// Prints either "M01" or "January" on small-icu, depending on the user’s default locale
// Should print "enero"

This mode provides a balance between features and binary size.

Providing ICU data at runtime#

If the small-icu option is used, one can still provide additional locale data at runtime so that the JS methods would work for all ICU locales. Assuming the data file is stored at /some/directory, it can be made available to ICU through either:

  • The NODE_ICU_DATA environment variable:

    env NODE_ICU_DATA=/some/directory node

  • The --icu-data-dir CLI parameter:

    node --icu-data-dir=/some/directory

(If both are specified, the --icu-data-dir CLI parameter takes precedence.)

ICU is able to automatically find and load a variety of data formats, but the data must be appropriate for the ICU version, and the file correctly named. The most common name for the data file is icudt6X[bl].dat, where 6X denotes the intended ICU version, and b or l indicates the system's endianness. Check "ICU Data" article in the ICU User Guide for other supported formats and more details on ICU data in general.

The full-icu npm module can greatly simplify ICU data installation by detecting the ICU version of the running node executable and downloading the appropriate data file. After installing the module through npm i full-icu, the data file will be available at ./node_modules/full-icu. This path can be then passed either to NODE_ICU_DATA or --icu-data-dir as shown above to enable full Intl support.

Embed the entire ICU (full-icu)#

This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. This is the default behavior if no --with-intl flag is passed. The official binaries are also built in this mode.

Detecting internationalization support#

To verify that ICU is enabled at all (system-icu, small-icu, or full-icu), simply checking the existence of Intl should suffice:

const hasICU = typeof Intl === 'object';

Alternatively, checking for process.versions.icu, a property defined only when ICU is enabled, works too:

const hasICU = typeof process.versions.icu === 'string';

To check for support for a non-English locale (i.e. full-icu or system-icu), Intl.DateTimeFormat can be a good distinguishing factor:

const hasFullICU = (() => {
  try {
    const january = new Date(9e8);
    const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
    return spanish.format(january) === 'enero';
  } catch (err) {
    return false;
  }
})();

For more verbose tests for Intl support, the following resources may be found to be helpful:

  • btest402: Generally used to check whether Node.js with Intl support is built correctly.
  • Test262: ECMAScript's official conformance test suite includes a section dedicated to ECMA-402.

Modules: CommonJS modules#

Stability: 2 - Stable

CommonJS modules are the original way to package JavaScript code for Node.js. Node.js also supports the ECMAScript modules standard used by browsers and other JavaScript runtimes.

In Node.js, each file is treated as a separate module. For example, consider a file named foo.js:

const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);

On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.

Here are the contents of circle.js:

const { PI } = Math;

exports.area = (r) => PI * r ** 2;

exports.circumference = (r) => 2 * PI * r;

The module circle.js has exported the functions area() and circumference(). Functions and objects are added to the root of a module by specifying additional properties on the special exports object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.

The module.exports property can be assigned a new value (such as a function or object).

Below, bar.js makes use of the square module, which exports a Square class:

const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);

The square module is defined in square.js:

// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
  constructor(width) {
    this.width = width;
  }

  area() {
    return this.width ** 2;
  }
};

The CommonJS module system is implemented in the module core module.

Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

By default, Node.js will treat the following as CommonJS modules:

  • Files with a .cjs extension;

  • Files with a .js extension when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

  • Files with a .js extension when the nearest parent package.json file doesn't contain a top-level field "type". Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will make things easier for build tools and loaders to determine how the files in the package should be interpreted.

  • Files with an extension that is not .mjs, .cjs, .json, .node, or .js (when the nearest parent package.json file contains a top-level field "type" with a value of "module", those files will be recognized as CommonJS modules only if they are being included via require(), not when used as the command-line entry point of the program).

See Determining module system for more details.

Calling require() always use the CommonJS module loader. Calling import() always use the ECMAScript module loader.

Accessing the main module#

When a file is run directly from Node.js, require.main is set to its module. That means that it is possible to determine whether a file has been run directly by testing require.main === module.

For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').

When the entry point is not a CommonJS module, require.main is undefined, and the main module is out of reach.

Package manager tips#

The semantics of the Node.js require() function were designed to be general enough to support reasonable directory structures. Package manager programs such as dpkg, rpm, and npm will hopefully find it possible to build native packages from Node.js modules without modification.

Below we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at /usr/lib/node/<some-package>/<some-version> hold the contents of a specific version of a package.

Packages can depend on one another. In order to install package foo, it may be necessary to install a specific version of package bar. The bar package may itself have dependencies, and in some cases, these may even collide or form cyclic dependencies.

Because Node.js looks up the realpath of any modules it loads (that is, it resolves symlinks) and then looks for their dependencies in node_modules folders, this situation can be resolved with the following architecture:

  • /usr/lib/node/foo/1.2.3/: Contents of the foo package, version 1.2.3.
  • /usr/lib/node/bar/4.3.2/: Contents of the bar package that foo depends on.
  • /usr/lib/node/foo/1.2.3/node_modules/bar: Symbolic link to /usr/lib/node/bar/4.3.2/.
  • /usr/lib/node/bar/4.3.2/node_modules/*: Symbolic links to the packages that bar depends on.

Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.

When the code in the foo package does require('bar'), it will get the version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar. Then, when the code in the bar package calls require('quux'), it'll get the version that is symlinked into /usr/lib/node/bar/4.3.2/node_modules/quux.

Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in /usr/lib/node, we could put them in /usr/lib/node_modules/<name>/<version>. Then Node.js will not bother looking for missing dependencies in /usr/node_modules or /node_modules.

In order to make modules available to the Node.js REPL, it might be useful to also add the /usr/lib/node_modules folder to the $NODE_PATH environment variable. Since the module lookups using node_modules folders are all relative, and based on the real path of the files making the calls to require(), the packages themselves can be anywhere.

The .mjs extension#

Due to the synchronous nature of require(), it is not possible to use it to load ECMAScript module files. Attempting to do so will throw a ERR_REQUIRE_ESM error. Use import() instead.

The .mjs extension is reserved for ECMAScript Modules which cannot be loaded via require(). See Determining module system section for more info regarding which files are parsed as ECMAScript modules.

All together#

To get the exact filename that will be loaded when require() is called, use the require.resolve() function.

Putting together all of the above, here is the high-level algorithm in pseudocode of what require() does:

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to be the file system root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
   c. THROW "not found"
4. If X begins with '#'
   a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
5. LOAD_PACKAGE_SELF(X, dirname(Y))
6. LOAD_NODE_MODULES(X, dirname(Y))
7. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as its file extension format. STOP
2. If X.js is a file, load X.js as JavaScript text. STOP
3. If X.json is a file, parse X.json to a JavaScript Object. STOP
4. If X.node is a file, load X.node as binary addon. STOP

LOAD_INDEX(X)
1. If X/index.js is a file, load X/index.js as JavaScript text. STOP
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.node as binary addon. STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. If "main" is a falsy value, GOTO 2.
   c. let M = X + (json main field)
   d. LOAD_AS_FILE(M)
   e. LOAD_INDEX(M)
   f. LOAD_INDEX(X) DEPRECATED
   g. THROW "not found"
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS = NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_PACKAGE_EXPORTS(X, DIR)
   b. LOAD_AS_FILE(DIR/X)
   c. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
   a. if PARTS[I] = "node_modules" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIR + DIRS
   d. let I = I - 1
5. return DIRS + GLOBAL_FOLDERS

LOAD_PACKAGE_IMPORTS(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "imports" is null or undefined, return.
4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
  ["node", "require"]) defined in the ESM resolver.
5. RESOLVE_ESM_MATCH(MATCH).

LOAD_PACKAGE_EXPORTS(X, DIR)
1. Try to interpret X as a combination of NAME and SUBPATH where the name
   may have a @scope/ prefix and the subpath begins with a slash (`/`).
2. If X does not match this pattern or DIR/NAME/package.json is not a file,
   return.
3. Parse DIR/NAME/package.json, and look for "exports" field.
4. If "exports" is null or undefined, return.
5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
   `package.json` "exports", ["node", "require"]) defined in the ESM resolver.
6. RESOLVE_ESM_MATCH(MATCH)

LOAD_PACKAGE_SELF(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "exports" is null or undefined, return.
4. If the SCOPE/package.json "name" is not the first segment of X, return.
5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE),
   "." + X.slice("name".length), `package.json` "exports", ["node", "require"])
   defined in the ESM resolver.
6. RESOLVE_ESM_MATCH(MATCH)

RESOLVE_ESM_MATCH(MATCH)
1. let RESOLVED_PATH = fileURLToPath(MATCH)
2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension
   format. STOP
3. THROW "not found"

Caching#

Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.

Provided require.cache is not modified, multiple calls to require('foo') will not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

To have a module execute code multiple times, export a function, and call that function.

Module caching caveats#

Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file.

Core modules#

History
VersionChanges
v16.0.0, v14.18.0

Added node: import support to require(...).

Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.

The core modules are defined within the Node.js source and are located in the lib/ folder.

Core modules can be identified using the node: prefix, in which case it bypasses the require cache. For instance, require('node:http') will always return the built in HTTP module, even if there is require.cache entry by that name.

Some core modules are always preferentially loaded if their identifier is passed to require(). For instance, require('http') will always return the built-in HTTP module, even if there is a file by that name. The list of core modules that can be loaded without using the node: prefix is exposed as module.builtinModules.

Cycles#

When there are circular require() calls, a module might not have finished executing when it is returned.

Consider this situation:

a.js:

console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

b.js:

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

main.js:

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

When main.js loads a.js, then a.js in turn loads b.js. At that point, b.js tries to load a.js. In order to prevent an infinite loop, an unfinished copy of the a.js exports object is returned to the b.js module. b.js then finishes loading, and its exports object is provided to the a.js module.

By the time main.js has loaded both modules, they're both finished. The output of this program would thus be:

$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done = true, b.done = true

Careful planning is required to allow cyclic module dependencies to work correctly within an application.

File modules#

If the exact filename is not found, then Node.js will attempt to load the required filename with the added extensions: .js, .json, and finally .node. When loading a file that has a different extension (e.g. .cjs), its full name must be passed to require(), including its file extension (e.g. require('./file.cjs')).

.json files are parsed as JSON text files, .node files are interpreted as compiled addon modules loaded with process.dlopen(). Files using any other extension (or no extension at all) are parsed as JavaScript text files. Refer to the Determining module system section to understand what parse goal will be used.

A required module prefixed with '/' is an absolute path to the file. For example, require('/home/marco/foo.js') will load the file at /home/marco/foo.js.

A required module prefixed with './' is relative to the file calling require(). That is, circle.js must be in the same directory as foo.js for require('./circle') to find it.

Without a leading '/', './', or '../' to indicate a file, the module must either be a core module or is loaded from a node_modules folder.

If the given path does not exist, require() will throw a MODULE_NOT_FOUND error.

Folders as modules#

Stability: 3 - Legacy: Use subpath exports or subpath imports instead.

There are three ways in which a folder may be passed to require() as an argument.

The first is to create a package.json file in the root of the folder, which specifies a main module. An example package.json file might look like this:

{ "name" : "some-library",
  "main" : "./lib/some-library.js" }

If this was in a folder at ./some-library, then require('./some-library') would attempt to load ./some-library/lib/some-library.js.

If there is no package.json file present in the directory, or if the "main" entry is missing or cannot be resolved, then Node.js will attempt to load an index.js or index.node file out of that directory. For example, if there was no package.json file in the previous example, then require('./some-library') would attempt to load:

  • ./some-library/index.js
  • ./some-library/index.node

If these attempts fail, then Node.js will report the entire module as missing with the default error:

Error: Cannot find module 'some-library'

In all three above cases, an import('./some-library') call would result in a ERR_UNSUPPORTED_DIR_IMPORT error. Using package subpath exports or subpath imports can provide the same containment organization benefits as folders as modules, and work for both require and import.

Loading from node_modules folders#

If the module identifier passed to require() is not a core module, and does not begin with '/', '../', or './', then Node.js starts at the directory of the current module, and adds /node_modules, and attempts to load the module from that location. Node.js will not append node_modules to a path already ending in node_modules.

If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.

For example, if the file at '/home/ry/projects/foo.js' called require('bar.js'), then Node.js would look in the following locations, in this order:

  • /home/ry/projects/node_modules/bar.js
  • /home/ry/node_modules/bar.js
  • /home/node_modules/bar.js
  • /node_modules/bar.js

This allows programs to localize their dependencies, so that they do not clash.

It is possible to require specific files or sub modules distributed with a module by including a path suffix after the module name. For instance require('example-module/path/to/file') would resolve path/to/file relative to where example-module is located. The suffixed path follows the same module resolution semantics.

Loading from the global folders#

If the NODE_PATH environment variable is set to a colon-delimited list of absolute paths, then Node.js will search those paths for modules if they are not found elsewhere.

On Windows, NODE_PATH is delimited by semicolons (;) instead of colons.

NODE_PATH was originally created to support loading modules from varying paths before the current module resolution algorithm was defined.

NODE_PATH is still supported, but is less necessary now that the Node.js ecosystem has settled on a convention for locating dependent modules. Sometimes deployments that rely on NODE_PATH show surprising behavior when people are unaware that NODE_PATH must be set. Sometimes a module's dependencies change, causing a different version (or even a different module) to be loaded as the NODE_PATH is searched.

Additionally, Node.js will search in the following list of GLOBAL_FOLDERS:

  • 1: $HOME/.node_modules
  • 2: $HOME/.node_libraries
  • 3: $PREFIX/lib/node

Where $HOME is the user's home directory, and $PREFIX is the Node.js configured node_prefix.

These are mostly for historic reasons.

It is strongly encouraged to place dependencies in the local node_modules folder. These will be loaded faster, and more reliably.

The module wrapper#

Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:

(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});

By doing this, Node.js achieves a few things:

  • It keeps top-level variables (defined with var, const, or let) scoped to the module rather than the global object.
  • It helps to provide some global-looking variables that are actually specific to the module, such as:
    • The module and exports objects that the implementor can use to export values from the module.
    • The convenience variables __filename and __dirname, containing the module's absolute filename and directory path.

The module scope#

__dirname#

Added in: v0.1.27

The directory name of the current module. This is the same as the path.dirname() of the __filename.

Example: running node example.js from /Users/mjr

console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr

__filename#

Added in: v0.0.1

The file name of the current module. This is the current module file's absolute path with symlinks resolved.

For a main program this is not necessarily the same as the file name used in the command line.

See __dirname for the directory name of the current module.

Examples:

Running node example.js from /Users/mjr

console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr

Given two modules: a and b, where b is a dependency of a and there is a directory structure of:

  • /Users/mjr/app/a.js
  • /Users/mjr/app/node_modules/b/b.js

References to __filename within b.js will return /Users/mjr/app/node_modules/b/b.js while references to __filename within a.js will return /Users/mjr/app/a.js.

exports#

Added in: v0.1.12

A reference to the module.exports that is shorter to type. See the section about the exports shortcut for details on when to use exports and when to use module.exports.

module#

Added in: v0.1.16

A reference to the current module, see the section about the module object. In particular, module.exports is used for defining what a module exports and makes available through require().

require(id)#

Added in: v0.1.13
  • id <string> module name or path
  • Returns: <any> exported module content

Used to import modules, JSON, and local files. Modules can be imported from node_modules. Local modules and JSON files can be imported using a relative path (e.g. ./, ./foo, ./bar/baz, ../foo) that will be resolved against the directory named by __dirname (if defined) or the current working directory. The relative paths of POSIX style are resolved in an OS independent fashion, meaning that the examples above will work on Windows in the same way they would on Unix systems.

// Importing a local module with a path relative to the `__dirname` or current
// working directory. (On Windows, this would resolve to .\path\myLocalModule.)
const myLocalModule = require('./path/myLocalModule');

// Importing a JSON file:
const jsonData = require('./path/filename.json');

// Importing a module from node_modules or Node.js built-in module:
const crypto = require('node:crypto');
require.cache#
Added in: v0.3.0

Modules are cached in this object when they are required. By deleting a key value from this object, the next require will reload the module. This does not apply to native addons, for which reloading will result in an error.

Adding or replacing entries is also possible. This cache is checked before built-in modules and if a name matching a built-in module is added to the cache, only node:-prefixed require calls are going to receive the built-in module. Use with care!

const assert = require('node:assert');
const realFs = require('node:fs');

const fakeFs = {};
require.cache.fs = { exports: fakeFs };

assert.strictEqual(require('fs'), fakeFs);
assert.strictEqual(require('node:fs'), realFs);
require.extensions#
Added in: v0.3.0Deprecated since: v0.10.6

Stability: 0 - Deprecated

Instruct require on how to handle certain file extensions.

Process files with the extension .sjs as .js:

require.extensions['.sjs'] = require.extensions['.js'];

Deprecated. In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.

Avoid using require.extensions. Use could cause subtle bugs and resolving the extensions gets slower with each registered extension.

require.main#
Added in: v0.1.17

The Module object representing the entry script loaded when the Node.js process launched, or undefined if the entry point of the program is not a CommonJS module. See "Accessing the main module".

In entry.js script:

console.log(require.main);
node entry.js
Module {
  id: '.',
  path: '/absolute/path/to',
  exports: {},
  filename: '/absolute/path/to/entry.js',
  loaded: false,
  children: [],
  paths:
   [ '/absolute/path/to/node_modules',
     '/absolute/path/node_modules',
     '/absolute/node_modules',
     '/node_modules' ] }
require.resolve(request[, options])#
History
VersionChanges
v8.9.0

The paths option is now supported.

v0.3.0

Added in: v0.3.0

  • request <string> The module path to resolve.
  • options <Object>
    • paths <string[]> Paths to resolve module location from. If present, these paths are used instead of the default resolution paths, with the exception of GLOBAL_FOLDERS like $HOME/.node_modules, which are always included. Each of these paths is used as a starting point for the module resolution algorithm, meaning that the node_modules hierarchy is checked from this location.
  • Returns: <string>

Use the internal require() machinery to look up the location of a module, but rather than loading the module, just return the resolved filename.

If the module can not be found, a MODULE_NOT_FOUND error is thrown.

require.resolve.paths(request)#
Added in: v8.9.0

Returns an array containing the paths searched during resolution of request or null if the request string references a core module, for example http or fs.

The module object#

Added in: v0.1.16

In each module, the module free variable is a reference to the object representing the current module. For convenience, module.exports is also accessible via the exports module-global. module is not actually a global but rather local to each module.

module.children#

Added in: v0.1.16

The module objects required for the first time by this one.

module.exports#

Added in: v0.1.16

The module.exports object is created by the Module system. Sometimes this is not acceptable; many want their module to be an instance of some class. To do this, assign the desired export object to module.exports. Assigning the desired object to exports will simply rebind the local exports variable, which is probably not what is desired.

For example, suppose we were making a module called a.js:

const EventEmitter = require('node:events');

module.exports = new EventEmitter();

// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
  module.exports.emit('ready');
}, 1000);

Then in another file we could do:

const a = require('./a');
a.on('ready', () => {
  console.log('module "a" is ready');
});

Assignment to module.exports must be done immediately. It cannot be done in any callbacks. This does not work:

x.js:

setTimeout(() => {
  module.exports = { a: 'hello' };
}, 0);

y.js:

const x = require('./x');
console.log(x.a);
exports shortcut#
Added in: v0.1.16

The exports variable is available within a module's file-level scope, and is assigned the value of module.exports before the module is evaluated.

It allows a shortcut, so that module.exports.f = ... can be written more succinctly as exports.f = .... However, be aware that like any variable, if a new value is assigned to exports, it is no longer bound to module.exports:

module.exports.hello = true; // Exported from require of module
exports = { hello: false };  // Not exported, only available in the module

When the module.exports property is being completely replaced by a new object, it is common to also reassign exports:

module.exports = exports = function Constructor() {
  // ... etc.
};

To illustrate the behavior, imagine this hypothetical implementation of require(), which is quite similar to what is actually done by require():

function require(/* ... */) {
  const module = { exports: {} };
  ((module, exports) => {
    // Module code here. In this example, define a function.
    function someFunc() {}
    exports = someFunc;
    // At this point, exports is no longer a shortcut to module.exports, and
    // this module will still export an empty default object.
    module.exports = someFunc;
    // At this point, the module will now export someFunc, instead of the
    // default object.
  })(module, module.exports);
  return module.exports;
}

module.filename#

Added in: v0.1.16

The fully resolved filename of the module.

module.id#

Added in: v0.1.16

The identifier for the module. Typically this is the fully resolved filename.

module.isPreloading#

Added in: v15.4.0, v14.17.0
  • Type: <boolean> true if the module is running during the Node.js preload phase.

module.loaded#

Added in: v0.1.16

Whether or not the module is done loading, or is in the process of loading.

module.parent#

Added in: v0.1.16Deprecated since: v14.6.0, v12.19.0

Stability: 0 - Deprecated: Please use require.main and module.children instead.

The module that first required this one, or null if the current module is the entry point of the current process, or undefined if the module was loaded by something that is not a CommonJS module (E.G.: REPL or import).

module.path#

Added in: v11.14.0

The directory name of the module. This is usually the same as the path.dirname() of the module.id.

module.paths#

Added in: v0.4.0

The search paths for the module.

module.require(id)#

Added in: v0.5.1

The module.require() method provides a way to load a module as if require() was called from the original module.

In order to do this, it is necessary to get a reference to the module object. Since require() returns the module.exports, and the module is typically only available within a specific module's code, it must be explicitly exported in order to be used.

The Module object#

This section was moved to Modules: module core module.

Source map v3 support#

This section was moved to Modules: module core module.

Modules: ECMAScript modules#

History
VersionChanges
v18.6.0, v16.17.0

Add support for chaining loaders.

v17.1.0, v16.14.0

Add support for import assertions.

v17.0.0, v16.12.0

Consolidate loader hooks, removed getFormat, getSource, transformSource, and getGlobalPreloadCode hooks added load and globalPreload hooks allowed returning format from either resolve or load hooks.

v14.8.0

Unflag Top-Level Await.

v15.3.0, v14.17.0, v12.22.0

Stabilize modules implementation.

v14.13.0, v12.20.0

Support for detection of CommonJS named exports.

v14.0.0, v13.14.0, v12.20.0

Remove experimental modules warning.

v13.2.0, v12.17.0

Loading ECMAScript modules no longer requires a command-line flag.

v12.0.0

Add support for ES modules using .js file extension via package.json "type" field.

v8.5.0

Added in: v8.5.0

Stability: 2 - Stable

Introduction#

ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.

The following example of an ES module exports a function:

// addTwo.mjs
function addTwo(num) {
  return num + 2;
}

export { addTwo };

The following example of an ES module imports the function from addTwo.mjs:

// app.mjs
import { addTwo } from './addTwo.mjs';

// Prints: 6
console.log(addTwo(4));

Node.js fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.

Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

Authors can tell Node.js to use the ECMAScript modules loader via the .mjs file extension, the package.json "type" field, or the --input-type flag. Outside of those cases, Node.js will use the CommonJS module loader. See Determining module system for more details.

Packages#

This section was moved to Modules: Packages.

import Specifiers#

Terminology#

The specifier of an import statement is the string after the from keyword, e.g. 'node:path' in import { sep } from 'node:path'. Specifiers are also used in export from statements, and as the argument to an import() expression.

There are three types of specifiers:

  • Relative specifiers like './startup.js' or '../config.mjs'. They refer to a path relative to the location of the importing file. The file extension is always necessary for these.

  • Bare specifiers like 'some-package' or 'some-package/shuffle'. They can refer to the main entry point of a package by the package name, or a specific feature module within a package prefixed by the package name as per the examples respectively. Including the file extension is only necessary for packages without an "exports" field.

  • Absolute specifiers like 'file:///opt/nodejs/config.js'. They refer directly and explicitly to a full path.

Bare specifier resolutions are handled by the Node.js module resolution algorithm. All other specifier resolutions are always only resolved with the standard relative URL resolution semantics.

Like in CommonJS, module files within packages can be accessed by appending a path to the package name unless the package's package.json contains an "exports" field, in which case files within packages can only be accessed via the paths defined in "exports".

For details on these package resolution rules that apply to bare specifiers in the Node.js module resolution, see the packages documentation.

Mandatory file extensions#

A file extension must be provided when using the import keyword to resolve relative or absolute specifiers. Directory indexes (e.g. './startup/index.js') must also be fully specified.

This behavior matches how import behaves in browser environments, assuming a typically configured server.

URLs#

ES modules are resolved and cached as URLs. This means that special characters must be percent-encoded, such as # with %23 and ? with %3F.

file:, node:, and data: URL schemes are supported. A specifier like 'https://example.com/app.js' is not supported natively in Node.js unless using a custom HTTPS loader.

file: URLs#

Modules are loaded multiple times if the import specifier used to resolve them has a different query or fragment.

import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"

The volume root may be referenced via /, //, or file:///. Given the differences between URL and path resolution (such as percent encoding details), it is recommended to use url.pathToFileURL when importing a path.

data: imports#
Added in: v12.10.0

data: URLs are supported for importing with the following MIME types:

  • text/javascript for ES modules
  • application/json for JSON
  • application/wasm for Wasm
import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"' assert { type: 'json' };

data: URLs only resolve bare specifiers for builtin modules and absolute specifiers. Resolving relative specifiers does not work because data: is not a special scheme. For example, attempting to load ./foo from data:text/javascript,import "./foo"; fails to resolve because there is no concept of relative resolution for data: URLs.

node: imports#
History
VersionChanges
v16.0.0, v14.18.0

Added node: import support to require(...).

v14.13.1, v12.20.0

Added in: v14.13.1, v12.20.0

node: URLs are supported as an alternative means to load Node.js builtin modules. This URL scheme allows for builtin modules to be referenced by valid absolute URL strings.

import fs from 'node:fs/promises';

Import assertions#

Added in: v17.1.0, v16.14.0

Stability: 1 - Experimental

The Import Assertions proposal adds an inline syntax for module import statements to pass on more information alongside the module specifier.

import fooData from './foo.json' assert { type: 'json' };

const { default: barData } =
  await import('./bar.json', { assert: { type: 'json' } });

Node.js supports the following type values, for which the assertion is mandatory:

Assertion typeNeeded for
'json'JSON modules

Builtin modules#

Core modules provide named exports of their public API. A default export is also provided which is the value of the CommonJS exports. The default export can be used for, among other things, modifying the named exports. Named exports of builtin modules are updated only by calling module.syncBuiltinESMExports().

import EventEmitter from 'node:events';
const e = new EventEmitter();
import { readFile } from 'node:fs';
readFile('./foo.txt', (err, source) => {
  if (err) {
    console.error(err);
  } else {
    console.log(source);
  }
});
import fs, { readFileSync } from 'node:fs';
import { syncBuiltinESMExports } from 'node:module';
import { Buffer } from 'node:buffer';

fs.readFileSync = () => Buffer.from('Hello, ESM');
syncBuiltinESMExports();

fs.readFileSync === readFileSync;

import() expressions#

Dynamic import() is supported in both CommonJS and ES modules. In CommonJS modules it can be used to load ES modules.

import.meta#

The import.meta meta property is an Object that contains the following properties.

import.meta.url#

  • <string> The absolute file: URL of the module.

This is defined exactly the same as it is in browsers providing the URL of the current module file.

This enables useful patterns such as relative file loading:

import { readFileSync } from 'node:fs';
const buffer = readFileSync(new URL('./data.proto', import.meta.url));

import.meta.resolve(specifier[, parent])#

Stability: 1 - Experimental

This feature is only available with the --experimental-import-meta-resolve command flag enabled.

  • specifier <string> The module specifier to resolve relative to parent.
  • parent <string> | <URL> The absolute parent module URL to resolve from. If none is specified, the value of import.meta.url is used as the default.
  • Returns: <Promise>

Provides a module-relative resolution function scoped to each module, returning the URL string.

const dependencyAsset = await import.meta.resolve('component-lib/asset.css');

import.meta.resolve also accepts a second argument which is the parent module from which to resolve from:

await import.meta.resolve('./dep', import.meta.url);

This function is asynchronous because the ES module resolver in Node.js is allowed to be asynchronous.

Interoperability with CommonJS#

import statements#

An import statement can reference an ES module or a CommonJS module. import statements are permitted only in ES modules, but dynamic import() expressions are supported in CommonJS for loading ES modules.

When importing CommonJS modules, the module.exports object is provided as the default export. Named exports may be available, provided by static analysis as a convenience for better ecosystem compatibility.

require#

The CommonJS module require always treats the files it references as CommonJS.

Using require to load an ES module is not supported because ES modules have asynchronous execution. Instead, use import() to load an ES module from a CommonJS module.

CommonJS Namespaces#

CommonJS modules consist of a module.exports object which can be of any type.

When importing a CommonJS module, it can be reliably imported using the ES module default import or its corresponding sugar syntax:

import { default as cjs } from 'cjs';

// The following import statement is "syntax sugar" (equivalent but sweeter)
// for `{ default as cjsSugar }` in the above import statement:
import cjsSugar from 'cjs';

console.log(cjs);
console.log(cjs === cjsSugar);
// Prints:
//   <module.exports>
//   true

The ECMAScript Module Namespace representation of a CommonJS module is always a namespace with a default export key pointing to the CommonJS module.exports value.

This Module Namespace Exotic Object can be directly observed either when using import * as m from 'cjs' or a dynamic import:

import * as m from 'cjs';
console.log(m);
console.log(m === await import('cjs'));
// Prints:
//   [Module] { default: <module.exports> }
//   true

For better compatibility with existing usage in the JS ecosystem, Node.js in addition attempts to determine the CommonJS named exports of every imported CommonJS module to provide them as separate ES module exports using a static analysis process.

For example, consider a CommonJS module written:

// cjs.cjs
exports.name = 'exported';

The preceding module supports named imports in ES modules:

import { name } from './cjs.cjs';
console.log(name);
// Prints: 'exported'

import cjs from './cjs.cjs';
console.log(cjs);
// Prints: { name: 'exported' }

import * as m from './cjs.cjs';
console.log(m);
// Prints: [Module] { default: { name: 'exported' }, name: 'exported' }

As can be seen from the last example of the Module Namespace Exotic Object being logged, the name export is copied off of the module.exports object and set directly on the ES module namespace when the module is imported.

Live binding updates or new exports added to module.exports are not detected for these named exports.

The detection of named exports is based on common syntax patterns but does not always correctly detect named exports. In these cases, using the default import form described above can be a better option.

Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See cjs-module-lexer for the exact semantics implemented.

Differences between ES modules and CommonJS#

No require, exports, or module.exports#

In most cases, the ES module import can be used to load CommonJS modules.

If needed, a require function can be constructed within an ES module using module.createRequire().

No __filename or __dirname#

These CommonJS variables are not available in ES modules.

__filename and __dirname use cases can be replicated via import.meta.url.

No Addon Loading#

Addons are not currently supported with ES module imports.

They can instead be loaded with module.createRequire() or process.dlopen.

No require.resolve#

Relative resolution can be handled via new URL('./local', import.meta.url).

For a complete require.resolve replacement, there is a flagged experimental import.meta.resolve API.

Alternatively module.createRequire() can be used.

No NODE_PATH#

NODE_PATH is not part of resolving import specifiers. Please use symlinks if this behavior is desired.

No require.extensions#

require.extensions is not used by import. The expectation is that loader hooks can provide this workflow in the future.

No require.cache#

require.cache is not used by import as the ES module loader has its own separate cache.

JSON modules#

Stability: 1 - Experimental

JSON files can be referenced by import:

import packageConfig from './package.json' assert { type: 'json' };

The assert { type: 'json' } syntax is mandatory; see Import Assertions.

The imported JSON only exposes a default export. There is no support for named exports. A cache entry is created in the CommonJS cache to avoid duplication. The same object is returned in CommonJS if the JSON module has already been imported from the same path.

Wasm modules#

Stability: 1 - Experimental

Importing WebAssembly modules is supported under the --experimental-wasm-modules flag, allowing any .wasm files to be imported as normal modules while also supporting their module imports.

This integration is in line with the ES Module Integration Proposal for WebAssembly.

For example, an index.mjs containing:

import * as M from './module.wasm';
console.log(M);

executed under:

node --experimental-wasm-modules index.mjs

would provide the exports interface for the instantiation of module.wasm.

Top-level await#

Added in: v14.8.0

The await keyword may be used in the top level body of an ECMAScript module.

Assuming an a.mjs with

export const five = await Promise.resolve(5);

And a b.mjs with

import { five } from './a.mjs';

console.log(five); // Logs `5`
node b.mjs # works

If a top level await expression never resolves, the node process will exit with a 13 status code.

import { spawn } from 'node:child_process';
import { execPath } from 'node:process';

spawn(execPath, [
  '--input-type=module',
  '--eval',
  // Never-resolving Promise:
  'await new Promise(() => {})',
]).once('exit', (code) => {
  console.log(code); // Logs `13`
});

HTTPS and HTTP imports#

Stability: 1 - Experimental

Importing network based modules using https: and http: is supported under the --experimental-network-imports flag. This allows web browser-like imports to work in Node.js with a few differences due to application stability and security concerns that are different when running in a privileged environment instead of a browser sandbox.

Imports are limited to HTTP/1#

Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.

HTTP is limited to loopback addresses#

http: is vulnerable to man-in-the-middle attacks and is not allowed to be used for addresses outside of the IPv4 address 127.0.0.0/8 (127.0.0.1 to 127.255.255.255) and the IPv6 address ::1. Support for http: is intended to be used for local development.

Authentication is never sent to the destination server.#

Authorization, Cookie, and Proxy-Authorization headers are not sent to the server. Avoid including user info in parts of imported URLs. A security model for safely using these on the server is being worked on.

CORS is never checked on the destination server#

CORS is designed to allow a server to limit the consumers of an API to a specific set of hosts. This is not supported as it does not make sense for a server-based implementation.

Cannot load non-network dependencies#

These modules cannot access other modules that are not over http: or https:. To still access local modules while avoiding the security concern, pass in references to the local dependencies:

// file.mjs
import worker_threads from 'node:worker_threads';
import { configure, resize } from 'https://example.com/imagelib.mjs';
configure({ worker_threads });
// https://example.com/imagelib.mjs
let worker_threads;
export function configure(opts) {
  worker_threads = opts.worker_threads;
}
export function resize(img, size) {
  // Perform resizing in worker_thread to avoid main thread blocking
}

Network-based loading is not enabled by default#

For now, the --experimental-network-imports flag is required to enable loading resources over http: or https:. In the future, a different mechanism will be used to enforce this. Opt-in is required to prevent transitive dependencies inadvertently using potentially mutable state that could affect reliability of Node.js applications.

Loaders#

History
VersionChanges
v18.6.0, v16.17.0

Add support for chaining loaders.

v16.12.0

Removed getFormat, getSource, transformSource, and globalPreload; added load hook and getGlobalPreload hook.

v8.8.0

Added in: v8.8.0

Stability: 1 - Experimental

This API is currently being redesigned and will still change.

To customize the default module resolution, loader hooks can optionally be provided via a --experimental-loader ./loader-name.mjs argument to Node.js.

When hooks are used they apply to the entry point and all import calls. They won't apply to require calls; those still follow CommonJS rules.

Loaders follow the pattern of --require:

node \
  --experimental-loader unpkg \
  --experimental-loader http-to-https \
  --experimental-loader cache-buster

These are called in the following sequence: cache-buster calls http-to-https which calls unpkg.

Hooks#

Hooks are part of a chain, even if that chain consists of only one custom (user-provided) hook and the default hook, which is always present. Hook functions nest: each one must always return a plain object, and chaining happens as a result of each function calling next<hookName>(), which is a reference to the subsequent loader's hook.

A hook that returns a value lacking a required property triggers an exception. A hook that returns without calling next<hookName>() and without returning shortCircuit: true also triggers an exception. These errors are to help prevent unintentional breaks in the chain.

resolve(specifier, context, nextResolve)#
History
VersionChanges
v18.6.0, v16.17.0

Add support for chaining resolve hooks. Each hook must either call nextResolve() or include a shortCircuit property set to true in its return.

v17.1.0, v16.14.0

Add support for import assertions.

The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

  • specifier <string>
  • context <Object>
    • conditions <string[]> Export conditions of the relevant package.json
    • importAssertions <Object>
    • parentURL <string> | <undefined> The module importing this one, or undefined if this is the Node.js entry point
  • nextResolve <Function> The subsequent resolve hook in the chain, or the Node.js default resolve hook after the last user-supplied resolve hook
  • Returns: <Object>
    • format <string> | <null> | <undefined> A hint to the load hook (it might be ignored) 'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'
    • shortCircuit <undefined> | <boolean> A signal that this hook intends to terminate the chain of resolve hooks. Default: false
    • url <string> The absolute URL to which this input resolves

The resolve hook chain is responsible for resolving file URL for a given module specifier and parent URL, and optionally its format (such as 'module') as a hint to the load hook. If a format is specified, the load hook is ultimately responsible for providing the final format value (and it is free to ignore the hint provided by resolve); if resolve provides a format, a custom load hook is required even if only to pass the value to the Node.js default load hook.

The module specifier is the string in an import statement or import() expression.

The parent URL is the URL of the module that imported this one, or undefined if this is the main entry point for the application.

The conditions property in context is an array of conditions for package exports conditions that apply to this resolution request. They can be used for looking up conditional mappings elsewhere or to modify the list when calling the default resolution logic.

The current package exports conditions are always in the context.conditions array passed into the hook. To guarantee default Node.js module specifier resolution behavior when calling defaultResolve, the context.conditions array passed to it must include all elements of the context.conditions array originally passed into the resolve hook.

export async function resolve(specifier, context, nextResolve) {
  const { parentURL = null } = context;

  if (Math.random() > 0.5) { // Some condition.
    // For some or all specifiers, do some custom logic for resolving.
    // Always return an object of the form {url: <string>}.
    return {
      shortCircuit: true,
      url: parentURL ?
        new URL(specifier, parentURL).href :
        new URL(specifier).href,
    };
  }

  if (Math.random() < 0.5) { // Another condition.
    // When calling `defaultResolve`, the arguments can be modified. In this
    // case it's adding another value for matching conditional exports.
    return nextResolve(specifier, {
      ...context,
      conditions: [...context.conditions, 'another-condition'],
    });
  }

  // Defer to the next hook in the chain, which would be the
  // Node.js default resolve if this is the last user-specified loader.
  return nextResolve(specifier);
}
load(url, context, nextLoad)#
History
VersionChanges
v18.6.0, v16.17.0

Add support for chaining load hooks. Each hook must either call nextLoad() or include a shortCircuit property set to true in its return.

The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

In a previous version of this API, this was split across 3 separate, now deprecated, hooks (getFormat, getSource, and transformSource).

The load hook provides a way to define a custom method of determining how a URL should be interpreted, retrieved, and parsed. It is also in charge of validating the import assertion.

The final value of format must be one of the following:

formatDescriptionAcceptable types for source returned by load
'builtin'Load a Node.js builtin moduleNot applicable
'commonjs'Load a Node.js CommonJS moduleNot applicable
'json'Load a JSON file{ string, ArrayBuffer, TypedArray }
'module'Load an ES module{ string, ArrayBuffer, TypedArray }
'wasm'Load a WebAssembly module{ ArrayBuffer, TypedArray }

The value of source is ignored for type 'builtin' because currently it is not possible to replace the value of a Node.js builtin (core) module. The value of source is ignored for type 'commonjs' because the CommonJS module loader does not provide a mechanism for the ES module loader to override the CommonJS module return value. This limitation might be overcome in the future.

Caveat: The ESM load hook and namespaced exports from CommonJS modules are incompatible. Attempting to use them together will result in an empty object from the import. This may be addressed in the future.

These types all correspond to classes defined in ECMAScript.

If the source value of a text-based format (i.e., 'json', 'module') is not a string, it is converted to a string using util.TextDecoder.

The load hook provides a way to define a custom method for retrieving the source code of an ES module specifier. This would allow a loader to potentially avoid reading files from disk. It could also be used to map an unrecognized format to a supported one, for example yaml to module.

export async function load(url, context, nextLoad) {
  const { format } = context;

  if (Math.random() > 0.5) { // Some condition
    /*
      For some or all URLs, do some custom logic for retrieving the source.
      Always return an object of the form {
        format: <string>,
        source: <string|buffer>,
      }.
    */
    return {
      format,
      shortCircuit: true,
      source: '...',
    };
  }

  // Defer to the next hook in the chain.
  return nextLoad(url);
}

In a more advanced scenario, this can also be used to transform an unsupported source to a supported one (see Examples below).

globalPreload()#
History
VersionChanges
v18.6.0, v16.17.0

Add support for chaining globalPreload hooks.

The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

In a previous version of this API, this hook was named getGlobalPreloadCode.

Sometimes it might be necessary to run some code inside of the same global scope that the application runs in. This hook allows the return of a string that is run as a sloppy-mode script on startup.

Similar to how CommonJS wrappers work, the code runs in an implicit function scope. The only argument is a require-like function that can be used to load builtins like "fs": getBuiltin(request: string).

If the code needs more advanced require features, it has to construct its own require using module.createRequire().

export function globalPreload(context) {
  return `\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');

const { createRequire } = getBuiltin('module');
const { cwd } = getBuiltin('process');

const require = createRequire(cwd() + '/<preload>');
// [...]
`;
}

In order to allow communication between the application and the loader, another argument is provided to the preload code: port. This is available as a parameter to the loader hook and inside of the source text returned by the hook. Some care must be taken in order to properly call port.ref() and port.unref() to prevent a process from being in a state where it won't close normally.

/**
 * This example has the application context send a message to the loader
 * and sends the message back to the application context
 */
export function globalPreload({ port }) {
  port.onmessage = (evt) => {
    port.postMessage(evt.data);
  };
  return `\
    port.postMessage('console.log("I went to the Loader and back");');
    port.onmessage = (evt) => {
      eval(evt.data);
    };
  `;
}

Examples#

The various loader hooks can be used together to accomplish wide-ranging customizations of the Node.js code loading and evaluation behaviors.

HTTPS loader#

In current Node.js, specifiers starting with https:// are experimental (see HTTPS and HTTP imports).

The loader below registers hooks to enable rudimentary support for such specifiers. While this may seem like a significant improvement to Node.js core functionality, there are substantial downsides to actually using this loader: performance is much slower than loading files from disk, there is no caching, and there is no security.

// https-loader.mjs
import { get } from 'node:https';

export function resolve(specifier, context, nextResolve) {
  const { parentURL = null } = context;

  // Normally Node.js would error on specifiers starting with 'https://', so
  // this hook intercepts them and converts them into absolute URLs to be
  // passed along to the later hooks below.
  if (specifier.startsWith('https://')) {
    return {
      shortCircuit: true,
      url: specifier,
    };
  } else if (parentURL && parentURL.startsWith('https://')) {
    return {
      shortCircuit: true,
      url: new URL(specifier, parentURL).href,
    };
  }

  // Let Node.js handle all other specifiers.
  return nextResolve(specifier);
}

export function load(url, context, nextLoad) {
  // For JavaScript to be loaded over the network, we need to fetch and
  // return it.
  if (url.startsWith('https://')) {
    return new Promise((resolve, reject) => {
      get(url, (res) => {
        let data = '';
        res.on('data', (chunk) => data += chunk);
        res.on('end', () => resolve({
          // This example assumes all network-provided JavaScript is ES module
          // code.
          format: 'module',
          shortCircuit: true,
          source: data,
        }));
      }).on('error', (err) => reject(err));
    });
  }

  // Let Node.js handle all other URLs.
  return nextLoad(url);
}
// main.mjs
import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';

console.log(VERSION);

With the preceding loader, running node --experimental-loader ./https-loader.mjs ./main.mjs prints the current version of CoffeeScript per the module at the URL in main.mjs.

Transpiler loader#

Sources that are in formats Node.js doesn't understand can be converted into JavaScript using the load hook. Before that hook gets called, however, a resolve hook needs to tell Node.js not to throw an error on unknown file types.

This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.

// coffeescript-loader.mjs
import { readFile } from 'node:fs/promises';
import { dirname, extname, resolve as resolvePath } from 'node:path';
import { cwd } from 'node:process';
import { fileURLToPath, pathToFileURL } from 'node:url';
import CoffeeScript from 'coffeescript';

const baseURL = pathToFileURL(`${cwd()}/`).href;

// CoffeeScript files end in .coffee, .litcoffee, or .coffee.md.
const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;

export async function resolve(specifier, context, nextResolve) {
  if (extensionsRegex.test(specifier)) {
    const { parentURL = baseURL } = context;

    // Node.js normally errors on unknown file extensions, so return a URL for
    // specifiers ending in the CoffeeScript file extensions.
    return {
      shortCircuit: true,
      url: new URL(specifier, parentURL).href,
    };
  }

  // Let Node.js handle all other specifiers.
  return nextResolve(specifier);
}

export async function load(url, context, nextLoad) {
  if (extensionsRegex.test(url)) {
    // Now that we patched resolve to let CoffeeScript URLs through, we need to
    // tell Node.js what format such URLs should be interpreted as. Because
    // CoffeeScript transpiles into JavaScript, it should be one of the two
    // JavaScript formats: 'commonjs' or 'module'.

    // CoffeeScript files can be either CommonJS or ES modules, so we want any
    // CoffeeScript file to be treated by Node.js the same as a .js file at the
    // same location. To determine how Node.js would interpret an arbitrary .js
    // file, search up the file system for the nearest parent package.json file
    // and read its "type" field.
    const format = await getPackageType(url);
    // When a hook returns a format of 'commonjs', `source` is be ignored.
    // To handle CommonJS files, a handler needs to be registered with
    // `require.extensions` in order to process the files with the CommonJS
    // loader. Avoiding the need for a separate CommonJS handler is a future
    // enhancement planned for ES module loaders.
    if (format === 'commonjs') {
      return {
        format,
        shortCircuit: true,
      };
    }

    const { source: rawSource } = await nextLoad(url, { ...context, format });
    // This hook converts CoffeeScript source code into JavaScript source code
    // for all imported CoffeeScript files.
    const transformedSource = coffeeCompile(rawSource.toString(), url);

    return {
      format,
      shortCircuit: true,
      source: transformedSource,
    };
  }

  // Let Node.js handle all other URLs.
  return nextLoad(url);
}

async function getPackageType(url) {
  // `url` is only a file path during the first iteration when passed the
  // resolved url from the load() hook
  // an actual file path from load() will contain a file extension as it's
  // required by the spec
  // this simple truthy check for whether `url` contains a file extension will
  // work for most projects but does not cover some edge-cases (such as
  // extensionless files or a url ending in a trailing space)
  const isFilePath = !!extname(url);
  // If it is a file path, get the directory it's in
  const dir = isFilePath ?
    dirname(fileURLToPath(url)) :
    url;
  // Compose a file path to a package.json in the same directory,
  // which may or may not exist
  const packagePath = resolvePath(dir, 'package.json');
  // Try to read the possibly nonexistent package.json
  const type = await readFile(packagePath, { encoding: 'utf8' })
    .then((filestring) => JSON.parse(filestring).type)
    .catch((err) => {
      if (err?.code !== 'ENOENT') console.error(err);
    });
  // Ff package.json existed and contained a `type` field with a value, voila
  if (type) return type;
  // Otherwise, (if not at the root) continue checking the next directory up
  // If at the root, stop and return false
  return dir.length > 1 && getPackageType(resolvePath(dir, '..'));
}
# main.coffee
import { scream } from './scream.coffee'
console.log scream 'hello, world'

import { version } from 'node:process'
console.log "Brought to you by Node.js version #{version}"
# scream.coffee
export scream = (str) -> str.toUpperCase()

With the preceding loader, running node --experimental-loader ./coffeescript-loader.mjs main.coffee causes main.coffee to be turned into JavaScript after its source code is loaded from disk but before Node.js executes it; and so on for any .coffee, .litcoffee or .coffee.md files referenced via import statements of any loaded file.

Resolution algorithm#

Features#

The resolver has the following properties:

  • FileURL-based resolution as is used by ES modules
  • Support for builtin module loading
  • Relative and absolute URL resolution
  • No default extensions
  • No folder mains
  • Bare specifier package resolution lookup through node_modules

Resolver algorithm#

The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.

The algorithm to determine the module format of a resolved URL is provided by ESM_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.

In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.

defaultConditions is the conditional environment name array, ["node", "import"].

The resolver can throw the following errors:

  • Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
  • Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
  • Invalid Package Target: Package exports or imports define a target module for the package that is an invalid type or string target.
  • Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
  • Package Import Not Defined: Package imports do not define the specifier.
  • Module Not Found: The package or module requested does not exist.
  • Unsupported Directory Import: The resolved path corresponds to a directory, which is not a supported target for module imports.

Resolver Algorithm Specification#

ESM_RESOLVE(specifier, parentURL)

  1. Let resolved be undefined.
  2. If specifier is a valid URL, then
    1. Set resolved to the result of parsing and reserializing specifier as a URL.
  3. Otherwise, if specifier starts with "/", "./", or "../", then
    1. Set resolved to the URL resolution of specifier relative to parentURL.
  4. Otherwise, if specifier starts with "#", then
    1. Set resolved to the result of PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, defaultConditions).
  5. Otherwise,
    1. Note: specifier is now a bare specifier.
    2. Set resolved the result of PACKAGE_RESOLVE(specifier, parentURL).
  6. Let format be undefined.
  7. If resolved is a "file:" URL, then
    1. If resolved contains any percent encodings of "/" or "\" ("%2F" and "%5C" respectively), then
      1. Throw an Invalid Module Specifier error.
    2. If the file at resolved is a directory, then
      1. Throw an Unsupported Directory Import error.
    3. If the file at resolved does not exist, then
      1. Throw a Module Not Found error.
    4. Set resolved to the real path of resolved, maintaining the same URL querystring and fragment components.
    5. Set format to the result of ESM_FILE_FORMAT(resolved).
  8. Otherwise,
    1. Set format the module format of the content type associated with the URL resolved.
  9. Load resolved as module format, format.

PACKAGE_RESOLVE(packageSpecifier, parentURL)

  1. Let packageName be undefined.
  2. If packageSpecifier is an empty string, then
    1. Throw an Invalid Module Specifier error.
  3. If packageSpecifier is a Node.js builtin module name, then
    1. Return the string "node:" concatenated with packageSpecifier.
  4. If packageSpecifier does not start with "@", then
    1. Set packageName to the substring of packageSpecifier until the first "/" separator or the end of the string.
  5. Otherwise,
    1. If packageSpecifier does not contain a "/" separator, then
      1. Throw an Invalid Module Specifier error.
    2. Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.
  6. If packageName starts with "." or contains "\" or "%", then
    1. Throw an Invalid Module Specifier error.
  7. Let packageSubpath be "." concatenated with the substring of packageSpecifier from the position at the length of packageName.
  8. If packageSubpath ends in "/", then
    1. Throw an Invalid Module Specifier error.
  9. Let selfUrl be the result of PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL).
  10. If selfUrl is not undefined, return selfUrl.
  11. While parentURL is not the file system root,
    1. Let packageURL be the URL resolution of "node_modules/" concatenated with packageSpecifier, relative to parentURL.
    2. Set parentURL to the parent folder URL of parentURL.
    3. If the folder at packageURL does not exist, then
      1. Continue the next loop iteration.
    4. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
    5. If pjson is not null and pjson.exports is not null or undefined, then
      1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
    6. Otherwise, if packageSubpath is equal to ".", then
      1. If pjson.main is a string, then
        1. Return the URL resolution of main in packageURL.
    7. Otherwise,
      1. Return the URL resolution of packageSubpath in packageURL.
  12. Throw a Module Not Found error.

PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL)

  1. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
  2. If packageURL is null, then
    1. Return undefined.
  3. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
  4. If pjson is null or if pjson.exports is null or undefined, then
    1. Return undefined.
  5. If pjson.name is equal to packageName, then
    1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
  6. Otherwise, return undefined.

PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, exports, conditions)

  1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.
  2. If subpath is equal to ".", then
    1. Let mainExport be undefined.
    2. If exports is a String or Array, or an Object containing no keys starting with ".", then
      1. Set mainExport to exports.
    3. Otherwise if exports is an Object containing a "." property, then
      1. Set mainExport to exports["."].
    4. If mainExport is not undefined, then
      1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, mainExport, null, false, conditions).
      2. If resolved is not null or undefined, return resolved.
  3. Otherwise, if exports is an Object and all keys of exports start with ".", then
    1. Let matchKey be the string "./" concatenated with subpath.
    2. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( matchKey, exports, packageURL, false, conditions).
    3. If resolved is not null or undefined, return resolved.
  4. Throw a Package Path Not Exported error.

PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)

  1. Assert: specifier begins with "#".
  2. If specifier is exactly equal to "#" or starts with "#/", then
    1. Throw an Invalid Module Specifier error.
  3. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
  4. If packageURL is not null, then
    1. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
    2. If pjson.imports is a non-null Object, then
      1. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( specifier, pjson.imports, packageURL, true, conditions).
      2. If resolved is not null or undefined, return resolved.
  5. Throw a Package Import Not Defined error.

PACKAGE_IMPORTS_EXPORTS_RESOLVE(matchKey, matchObj, packageURL, isImports, conditions)

  1. If matchKey is a key of matchObj and does not contain "*", then
    1. Let target be the value of matchObj[matchKey].
    2. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, null, isImports, conditions).
  2. Let expansionKeys be the list of keys of matchObj containing only a single "*", sorted by the sorting function PATTERN_KEY_COMPARE which orders in descending order of specificity.
  3. For each key expansionKey in expansionKeys, do
    1. Let patternBase be the substring of expansionKey up to but excluding the first "*" character.
    2. If matchKey starts with but is not equal to patternBase, then
      1. Let patternTrailer be the substring of expansionKey from the index after the first "*" character.
      2. If patternTrailer has zero length, or if matchKey ends with patternTrailer and the length of matchKey is greater than or equal to the length of expansionKey, then
        1. Let target be the value of matchObj[expansionKey].
        2. Let patternMatch be the substring of matchKey starting at the index of the length of patternBase up to the length of matchKey minus the length of patternTrailer.
        3. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions).
  4. Return null.

PATTERN_KEY_COMPARE(keyA, keyB)

  1. Assert: keyA ends with "/" or contains only a single "*".
  2. Assert: keyB ends with "/" or contains only a single "*".
  3. Let baseLengthA be the index of "*" in keyA plus one, if keyA contains "*", or the length of keyA otherwise.
  4. Let baseLengthB be the index of "*" in keyB plus one, if keyB contains "*", or the length of keyB otherwise.
  5. If baseLengthA is greater than baseLengthB, return -1.
  6. If baseLengthB is greater than baseLengthA, return 1.
  7. If keyA does not contain "*", return 1.
  8. If keyB does not contain "*", return -1.
  9. If the length of keyA is greater than the length of keyB, return -1.
  10. If the length of keyB is greater than the length of keyA, return 1.
  11. Return 0.

PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions)

  1. If target is a String, then
    1. If target does not start with "./", then
      1. If isImports is false, or if target starts with "../" or "/", or if target is a valid URL, then
        1. Throw an Invalid Package Target error.
      2. If patternMatch is a String, then
        1. Return PACKAGE_RESOLVE(target with every instance of "*" replaced by patternMatch, packageURL + "/").
      3. Return PACKAGE_RESOLVE(target, packageURL + "/").
    2. If target split on "/" or "\" contains any "", ".", "..", or "node_modules" segments after the first "." segment, case insensitive and including percent encoded variants, throw an Invalid Package Target error.
    3. Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
    4. Assert: resolvedTarget is contained in packageURL.
    5. If patternMatch is null, then
      1. Return resolvedTarget.
    6. If patternMatch split on "/" or "\" contains any "", ".", "..", or "node_modules" segments, case insensitive and including percent encoded variants, throw an Invalid Module Specifier error.
    7. Return the URL resolution of resolvedTarget with every instance of "*" replaced with patternMatch.
  2. Otherwise, if target is a non-null Object, then
    1. If exports contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.
    2. For each property p of target, in object insertion order as,
      1. If p equals "default" or conditions contains an entry for p, then
        1. Let targetValue be the value of the p property in target.
        2. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions).
        3. If resolved is equal to undefined, continue the loop.
        4. Return resolved.
    3. Return undefined.
  3. Otherwise, if target is an Array, then
    1. If _target.length is zero, return null.
    2. For each item targetValue in target, do
      1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions), continuing the loop on any Invalid Package Target error.
      2. If resolved is undefined, continue the loop.
      3. Return resolved.
    3. Return or throw the last fallback resolution null return or error.
  4. Otherwise, if target is null, return null.
  5. Otherwise throw an Invalid Package Target error.

ESM_FILE_FORMAT(url)

  1. Assert: url corresponds to an existing file.
  2. If url ends in ".mjs", then
    1. Return "module".
  3. If url ends in ".cjs", then
    1. Return "commonjs".
  4. If url ends in ".json", then
    1. Return "json".
  5. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(url).
  6. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
  7. If pjson?.type exists and is "module", then
    1. If url ends in ".js", then
      1. Return "module".
    2. Throw an Unsupported File Extension error.
  8. Otherwise,
    1. Throw an Unsupported File Extension error.

LOOKUP_PACKAGE_SCOPE(url)

  1. Let scopeURL be url.
  2. While scopeURL is not the file system root,
    1. Set scopeURL to the parent URL of scopeURL.
    2. If scopeURL ends in a "node_modules" path segment, return null.
    3. Let pjsonURL be the resolution of "package.json" within scopeURL.
    4. if the file at pjsonURL exists, then
      1. Return scopeURL.
  3. Return null.

READ_PACKAGE_JSON(packageURL)

  1. Let pjsonURL be the resolution of "package.json" within packageURL.
  2. If the file at pjsonURL does not exist, then
    1. Return null.
  3. If the file at packageURL does not parse as valid JSON, then
    1. Throw an Invalid Package Configuration error.
  4. Return the parsed JSON source of the file at pjsonURL.

Customizing ESM specifier resolution algorithm#

The Loaders API provides a mechanism for customizing the ESM specifier resolution algorithm. An example loader that provides CommonJS-style resolution for ESM specifiers is commonjs-extension-resolution-loader.

Modules: node:module API#

Added in: v0.3.7

The Module object#

Provides general utility methods when interacting with instances of Module, the module variable often seen in CommonJS modules. Accessed via import 'node:module' or require('node:module').

module.builtinModules#

Added in: v9.3.0, v8.10.0, v6.13.0

A list of the names of all modules provided by Node.js. Can be used to verify if a module is maintained by a third party or not.

module in this context isn't the same object that's provided by the module wrapper. To access it, require the Module module:

// module.mjs
// In an ECMAScript module
import { builtinModules as builtin } from 'node:module';// module.cjs
// In a CommonJS module
const builtin = require('node:module').builtinModules;

module.createRequire(filename)#

Added in: v12.2.0
  • filename <string> | <URL> Filename to be used to construct the require function. Must be a file URL object, file URL string, or absolute path string.
  • Returns: <require> Require function
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);

// sibling-module.js is a CommonJS module.
const siblingModule = require('./sibling-module');

module.isBuiltin(moduleName)#

Added in: v18.6.0, v16.17.0
  • moduleName <string> name of the module
  • Returns: <boolean> returns true if the module is builtin else returns false
import { isBuiltin } from 'node:module';
isBuiltin('node:fs'); // true
isBuiltin('fs'); // true
isBuiltin('wss'); // false

module.syncBuiltinESMExports()#

Added in: v12.12.0

The module.syncBuiltinESMExports() method updates all the live bindings for builtin ES Modules to match the properties of the CommonJS exports. It does not add or remove exported names from the ES Modules.

const fs = require('node:fs');
const assert = require('node:assert');
const { syncBuiltinESMExports } = require('node:module');

fs.readFile = newAPI;

delete fs.readFileSync;

function newAPI() {
  // ...
}

fs.newAPI = newAPI;

syncBuiltinESMExports();

import('node:fs').then((esmFS) => {
  // It syncs the existing readFile property with the new value
  assert.strictEqual(esmFS.readFile, newAPI);
  // readFileSync has been deleted from the required fs
  assert.strictEqual('readFileSync' in fs, false);
  // syncBuiltinESMExports() does not remove readFileSync from esmFS
  assert.strictEqual('readFileSync' in esmFS, true);
  // syncBuiltinESMExports() does not add names
  assert.strictEqual(esmFS.newAPI, undefined);
});

Source map v3 support#

Added in: v13.7.0, v12.17.0

Stability: 1 - Experimental

Helpers for interacting with the source map cache. This cache is populated when source map parsing is enabled and source map include directives are found in a modules' footer.

To enable source map parsing, Node.js must be run with the flag --enable-source-maps, or with code coverage enabled by setting NODE_V8_COVERAGE=dir.

// module.mjs
// In an ECMAScript module
import { findSourceMap, SourceMap } from 'node:module';// module.cjs
// In a CommonJS module
const { findSourceMap, SourceMap } = require('node:module');

module.findSourceMap(path)#

Added in: v13.7.0, v12.17.0

path is the resolved path for the file for which a corresponding source map should be fetched.

Class: module.SourceMap#

Added in: v13.7.0, v12.17.0
new SourceMap(payload)#

Creates a new sourceMap instance.

payload is an object with keys matching the Source map v3 format:

sourceMap.payload#

Getter for the payload used to construct the SourceMap instance.

sourceMap.findEntry(lineNumber, columnNumber)#

Given a line number and column number in the generated source file, returns an object representing the position in the original file. The object returned consists of the following keys:

Modules: Packages#

History
VersionChanges
v14.13.0, v12.20.0

Add support for "exports" patterns.

v14.6.0, v12.19.0

Add package "imports" field.

v13.7.0, v12.17.0

Unflag conditional exports.

v13.7.0, v12.16.0

Remove the --experimental-conditional-exports option. In 12.16.0, conditional exports are still behind --experimental-modules.

v13.6.0, v12.16.0

Unflag self-referencing a package using its name.

v12.7.0

Introduce "exports" package.json field as a more powerful alternative to the classic "main" field.

v12.0.0

Add support for ES modules using .js file extension via package.json "type" field.

Introduction#

A package is a folder tree described by a package.json file. The package consists of the folder containing the package.json file and all subfolders until the next folder containing another package.json file, or a folder named node_modules.

This page provides guidance for package authors writing package.json files along with a reference for the package.json fields defined by Node.js.

Determining module system#

Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements or import() expressions:

  • Files with an .mjs extension.

  • Files with a .js extension when the nearest parent package.json file contains a top-level "type" field with a value of "module".

  • Strings passed in as an argument to --eval, or piped to node via STDIN, with the flag --input-type=module.

Node.js will treat as CommonJS all other forms of input, such as .js files where the nearest parent package.json file contains no top-level "type" field, or string input without the flag --input-type. This behavior is to preserve backward compatibility. However, now that Node.js supports both CommonJS and ES modules, it is best to be explicit whenever possible. Node.js will treat the following as CommonJS when passed to node as the initial input, or when referenced by import statements, import() expressions, or require() expressions:

  • Files with a .cjs extension.

  • Files with a .js extension when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

  • Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=commonjs.

Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

Modules loaders#

Node.js has two systems for resolving a specifier and loading modules.

There is the CommonJS module loader:

  • It is fully synchronous.
  • It is responsible for handling require() calls.
  • It is monkey patchable.
  • It supports folders as modules.
  • When resolving a specifier, if no exact match is found, it will try to add extensions (.js, .json, and finally .node) and then attempt to resolve folders as modules.
  • It treats .json as JSON text files.
  • .node files are interpreted as compiled addon modules loaded with process.dlopen().
  • It treats all files that lack .json or .node extensions as JavaScript text files.
  • It cannot be used to load ECMAScript modules (although it is possible to load ECMASCript modules from CommonJS modules). When used to load a JavaScript text file that is not an ECMAScript module, it loads it as a CommonJS module.

There is the ECMAScript module loader:

  • It is asynchronous.
  • It is responsible for handling import statements and import() expressions.
  • It is not monkey patchable, can be customized using loader hooks.
  • It does not support folders as modules, directory indexes (e.g. './startup/index.js') must be fully specified.
  • It does no extension searching. A file extension must be provided when the specifier is a relative or absolute file URL.
  • It can load JSON modules, but an import assertion is required.
  • It accepts only .js, .mjs, and .cjs extensions for JavaScript text files.
  • It can be used to load JavaScript CommonJS modules. Such modules are passed through the cjs-module-lexer to try to identify named exports, which are available if they can be determined through static analysis. Imported CommonJS modules have their URLs converted to absolute paths and are then loaded via the CommonJS module loader.

package.json and file extensions#

Within a package, the package.json "type" field defines how Node.js should interpret .js files. If a package.json file does not have a "type" field, .js files are treated as CommonJS.

A package.json "type" value of "module" tells Node.js to interpret .js files within that package as using ES module syntax.

The "type" field applies not only to initial entry points (node my-app.js) but also to files referenced by import statements and import() expressions.

// my-app.js, treated as an ES module because there is a package.json
// file in the same folder with "type": "module".

import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the "type" value from one level up.

import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

import './node_modules/commonjs-package/index.js';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

Files ending with .mjs are always loaded as ES modules regardless of the nearest parent package.json.

Files ending with .cjs are always loaded as CommonJS regardless of the nearest parent package.json.

import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always loaded as CommonJS.

import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always loaded as ES module.

The .mjs and .cjs extensions can be used to mix types within the same package:

  • Within a "type": "module" package, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a .cjs extension (since both .js and .mjs files are treated as ES modules within a "module" package).

  • Within a "type": "commonjs" package, Node.js can be instructed to interpret a particular file as an ES module by naming it with an .mjs extension (since both .js and .cjs files are treated as CommonJS within a "commonjs" package).

--input-type flag#

Added in: v12.0.0

Strings passed in as an argument to --eval (or -e), or piped to node via STDIN, are treated as ES modules when the --input-type=module flag is set.

node --input-type=module --eval "import { sep } from 'node:path'; console.log(sep);"

echo "import { sep } from 'node:path'; console.log(sep);" | node --input-type=module

For completeness there is also --input-type=commonjs, for explicitly running string input as CommonJS. This is the default behavior if --input-type is unspecified.

Determining package manager#

Stability: 1 - Experimental

While all Node.js projects are expected to be installable by all package managers once published, their development teams are often required to use one specific package manager. To make this process easier, Node.js ships with a tool called Corepack that aims to make all package managers transparently available in your environment - provided you have Node.js installed.

By default Corepack won't enforce any specific package manager and will use the generic "Last Known Good" versions associated with each Node.js release, but you can improve this experience by setting the "packageManager" field in your project's package.json.

Package entry points#

In a package's package.json file, two fields can define entry points for a package: "main" and "exports". Both fields apply to both ES module and CommonJS module entry points.

The "main" field is supported in all versions of Node.js, but its capabilities are limited: it only defines the main entry point of the package.

The "exports" provides a modern alternative to "main" allowing multiple entry points to be defined, conditional entry resolution support between environments, and preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to clearly define the public interface for their package.

For new packages targeting the currently supported versions of Node.js, the "exports" field is recommended. For packages supporting Node.js 10 and below, the "main" field is required. If both "exports" and "main" are defined, the "exports" field takes precedence over "main" in supported versions of Node.js.

Conditional exports can be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import. For more information about supporting both CommonJS and ES modules in a single package please consult the dual CommonJS/ES module packages section.

Existing packages introducing the "exports" field will prevent consumers of the package from using any entry points that are not defined, including the package.json (e.g. require('your-package/package.json'). This will likely be a breaking change.

To make the introduction of "exports" non-breaking, ensure that every previously supported entry point is exported. It is best to explicitly specify entry points so that the package's public API is well-defined. For example, a project that previously exported main, lib, feature, and the package.json could use the following package.exports:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/index": "./lib/index.js",
    "./lib/index.js": "./lib/index.js",
    "./feature": "./feature/index.js",
    "./feature/index": "./feature/index.js",
    "./feature/index.js": "./feature/index.js",
    "./package.json": "./package.json"
  }
}

Alternatively a project could choose to export entire folders both with and without extensioned subpaths using export patterns:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/*": "./lib/*.js",
    "./lib/*.js": "./lib/*.js",
    "./feature": "./feature/index.js",
    "./feature/*": "./feature/*.js",
    "./feature/*.js": "./feature/*.js",
    "./package.json": "./package.json"
  }
}

With the above providing backwards-compatibility for any minor package versions, a future major change for the package can then properly restrict the exports to only the specific feature exports exposed:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./feature/*.js": "./feature/*.js",
    "./feature/internal/*": null
  }
}

Main entry point export#

When writing a new package, it is recommended to use the "exports" field:

{
  "exports": "./index.js"
}

When the "exports" field is defined, all subpaths of the package are encapsulated and no longer available to importers. For example, require('pkg/subpath.js') throws an ERR_PACKAGE_PATH_NOT_EXPORTED error.

This encapsulation of exports provides more reliable guarantees about package interfaces for tools and when handling semver upgrades for a package. It is not a strong encapsulation since a direct require of any absolute subpath of the package such as require('/path/to/node_modules/pkg/subpath.js') will still load subpath.js.

All currently supported versions of Node.js and modern build tools support the "exports" field. For projects using an older version of Node.js or a related build tool, compatibility can be achieved by including the "main" field alongside "exports" pointing to the same module:

{
  "main": "./index.js",
  "exports": "./index.js"
}

Subpath exports#

Added in: v12.7.0

When using the "exports" field, custom subpaths can be defined along with the main entry point by treating the main entry point as the "." subpath:

{
  "exports": {
    ".": "./index.js",
    "./submodule.js": "./src/submodule.js"
  }
}

Now only the defined subpath in "exports" can be imported by a consumer:

import submodule from 'es-module-package/submodule.js';
// Loads ./node_modules/es-module-package/src/submodule.js

While other subpaths will error:

import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED
Extensions in subpaths#

Package authors should provide either extensioned (import 'pkg/subpath.js') or extensionless (import 'pkg/subpath') subpaths in their exports. This ensures that there is only one subpath for each exported module so that all dependents import the same consistent specifier, keeping the package contract clear for consumers and simplifying package subpath completions.

Traditionally, packages tended to use the extensionless style, which has the benefits of readability and of masking the true path of the file within the package.

With import maps now providing a standard for package resolution in browsers and other JavaScript runtimes, using the extensionless style can result in bloated import map definitions. Explicit file extensions can avoid this issue by enabling the import map to utilize a packages folder mapping to map multiple subpaths where possible instead of a separate map entry per package subpath export. This also mirrors the requirement of using the full specifier path in relative and absolute import specifiers.

Exports sugar#

Added in: v12.11.0

If the "." export is the only export, the "exports" field provides sugar for this case being the direct "exports" field value.

{
  "exports": {
    ".": "./index.js"
  }
}

can be written:

{
  "exports": "./index.js"
}

Subpath imports#

Added in: v14.6.0, v12.19.0

In addition to the "exports" field, there is a package "imports" field to create private mappings that only apply to import specifiers from within the package itself.

Entries in the "imports" field must always start with # to ensure they are disambiguated from external package specifiers.

For example, the imports field can be used to gain the benefits of conditional exports for internal modules:

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
}

where import '#dep' does not get the resolution of the external package dep-node-native (including its exports in turn), and instead gets the local file ./dep-polyfill.js relative to the package in other environments.

Unlike the "exports" field, the "imports" field permits mapping to external packages.

The resolution rules for the imports field are otherwise analogous to the exports field.

Subpath patterns#

History
VersionChanges
v16.10.0, v14.19.0

Support pattern trailers in "imports" field.

v16.9.0, v14.19.0

Support pattern trailers.

v14.13.0, v12.20.0

Added in: v14.13.0, v12.20.0

For packages with a small number of exports or imports, we recommend explicitly listing each exports subpath entry. But for packages that have large numbers of subpaths, this might cause package.json bloat and maintenance issues.

For these use cases, subpath export patterns can be used instead:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js"
  },
  "imports": {
    "#internal/*.js": "./src/internal/*.js"
  }
}

* maps expose nested subpaths as it is a string replacement syntax only.

All instances of * on the right hand side will then be replaced with this value, including if it contains any / separators.

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

import featureY from 'es-module-package/features/y/y.js';
// Loads ./node_modules/es-module-package/src/features/y/y.js

import internalZ from '#internal/z.js';
// Loads ./node_modules/es-module-package/src/internal/z.js

This is a direct static matching and replacement without any special handling for file extensions. Including the "*.js" on both sides of the mapping restricts the exposed package exports to only JS files.

The property of exports being statically enumerable is maintained with exports patterns since the individual exports for a package can be determined by treating the right hand side target pattern as a ** glob against the list of files within the package. Because node_modules paths are forbidden in exports targets, this expansion is dependent on only the files of the package itself.

To exclude private subfolders from patterns, null targets can be used:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js",
    "./features/private-internal/*": null
  }
}
import featureInternal from 'es-module-package/features/private-internal/m.js';
// Throws: ERR_PACKAGE_PATH_NOT_EXPORTED

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

Conditional exports#

History
VersionChanges
v13.7.0, v12.16.0

Unflag conditional exports.

v13.2.0, v12.16.0

Added in: v13.2.0, v12.16.0

Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports for require() and import can be written:

// package.json
{
  "exports": {
    "import": "./index-module.js",
    "require": "./index-require.cjs"
  },
  "type": "module"
}

Node.js implements the following conditions, listed in order from most specific to least specific as conditions should be defined:

  • "node-addons" - similar to "node" and matches for any Node.js environment. This condition can be used to provide an entry point which uses native C++ addons as opposed to an entry point which is more universal and doesn't rely on native addons. This condition can be disabled via the --no-addons flag.
  • "node" - matches for any Node.js environment. Can be a CommonJS or ES module file. In most cases explicitly calling out the Node.js platform is not necessary.
  • "import" - matches when the package is loaded via import or import(), or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with "require".
  • "require" - matches when the package is loaded via require(). The referenced file should be loadable with require() although the condition matches regardless of the module format of the target file. Expected formats include CommonJS, JSON, and native addons but not ES modules as require() doesn't support them. Always mutually exclusive with "import".
  • "default" - the generic fallback that always matches. Can be a CommonJS or ES module file. This condition should always come last.

Within the "exports" object, key order is significant. During condition matching, earlier entries have higher priority and take precedence over later entries. The general rule is that conditions should be from most specific to least specific in object order.

Using the "import" and "require" conditions can lead to some hazards, which are further explained in the dual CommonJS/ES module packages section.

The "node-addons" condition can be used to provide an entry point which uses native C++ addons. However, this condition can be disabled via the --no-addons flag. When using "node-addons", it's recommended to treat "default" as an enhancement that provides a more universal entry point, e.g. using WebAssembly instead of a native addon.

Conditional exports can also be extended to exports subpaths, for example:

{
  "exports": {
    ".": "./index.js",
    "./feature.js": {
      "node": "./feature-node.js",
      "default": "./feature.js"
    }
  }
}

Defines a package where require('pkg/feature.js') and import 'pkg/feature.js' could provide different implementations between Node.js and other JS environments.

When using environment branches, always include a "default" condition where possible. Providing a "default" condition ensures that any unknown JS environments are able to use this universal implementation, which helps avoid these JS environments from having to pretend to be existing environments in order to support packages with conditional exports. For this reason, using "node" and "default" condition branches is usually preferable to using "node" and "browser" condition branches.

Nested conditions#

In addition to direct mappings, Node.js also supports nested condition objects.

For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:

{
  "exports": {
    "node": {
      "import": "./feature-node.mjs",
      "require": "./feature-node.cjs"
    },
    "default": "./feature.mjs"
  }
}

Conditions continue to be matched in order as with flat conditions. If a nested condition does not have any mapping it will continue checking the remaining conditions of the parent condition. In this way nested conditions behave analogously to nested JavaScript if statements.

Resolving user conditions#

Added in: v14.9.0, v12.19.0

When running Node.js, custom user conditions can be added with the --conditions flag:

node --conditions=development index.js

which would then resolve the "development" condition in package imports and exports, while resolving the existing "node", "node-addons", "default", "import", and "require" conditions as appropriate.

Any number of custom conditions can be set with repeat flags.

Community Conditions Definitions#

Condition strings other than the "import", "require", "node", "node-addons" and "default" conditions implemented in Node.js core are ignored by default.

Other platforms may implement other conditions and user conditions can be enabled in Node.js via the --conditions / -C flag.

Since custom package conditions require clear definitions to ensure correct usage, a list of common known package conditions and their strict definitions is provided below to assist with ecosystem coordination.

  • "types" - can be used by typing systems to resolve the typing file for the given export. This condition should always be included first.
  • "deno" - indicates a variation for the Deno platform.
  • "browser" - any web browser environment.
  • "react-native" - will be matched by the React Native framework (all platforms). To target React Native for Web, "browser" should be specified before this condition.
  • "development" - can be used to define a development-only environment entry point, for example to provide additional debugging context such as better error messages when running in a development mode. Must always be mutually exclusive with "production".
  • "production" - can be used to define a production environment entry point. Must always be mutually exclusive with "development".

New conditions definitions may be added to this list by creating a pull request to the Node.js documentation for this section. The requirements for listing a new condition definition here are that:

  • The definition should be clear and unambiguous for all implementers.
  • The use case for why the condition is needed should be clearly justified.
  • There should exist sufficient existing implementation usage.
  • The condition name should not conflict with another condition definition or condition in wide usage.
  • The listing of the condition definition should provide a coordination benefit to the ecosystem that wouldn't otherwise be possible. For example, this would not necessarily be the case for company-specific or application-specific conditions.

The above definitions may be moved to a dedicated conditions registry in due course.

Self-referencing a package using its name#

History
VersionChanges
v13.6.0, v12.16.0

Unflag self-referencing a package using its name.

v13.1.0, v12.16.0

Added in: v13.1.0, v12.16.0

Within a package, the values defined in the package's package.json "exports" field can be referenced via the package's name. For example, assuming the package.json is:

// package.json
{
  "name": "a-package",
  "exports": {
    ".": "./index.mjs",
    "./foo.js": "./foo.js"
  }
}

Then any module in that package can reference an export in the package itself:

// ./a-module.mjs
import { something } from 'a-package'; // Imports "something" from ./index.mjs.

Self-referencing is available only if package.json has "exports", and will allow importing only what that "exports" (in the package.json) allows. So the code below, given the previous package, will generate a runtime error:

// ./another-module.mjs

// Imports "another" from ./m.mjs. Fails because
// the "package.json" "exports" field
// does not provide an export named "./m.mjs".
import { another } from 'a-package/m.mjs';

Self-referencing is also available when using require, both in an ES module, and in a CommonJS one. For example, this code will also work:

// ./a-module.js
const { something } = require('a-package/foo.js'); // Loads from ./foo.js.

Finally, self-referencing also works with scoped packages. For example, this code will also work:

// package.json
{
  "name": "@my/package",
  "exports": "./index.js"
}
// ./index.js
module.exports = 42;
// ./other.js
console.log(require('@my/package'));
$ node other.js
42

Dual CommonJS/ES module packages#

Prior to the introduction of support for ES modules in Node.js, it was a common pattern for package authors to include both CommonJS and ES module JavaScript sources in their package, with package.json "main" specifying the CommonJS entry point and package.json "module" specifying the ES module entry point. This enabled Node.js to run the CommonJS entry point while build tools such as bundlers used the ES module entry point, since Node.js ignored (and still ignores) the top-level "module" field.

Node.js can now run ES module entry points, and a package can contain both CommonJS and ES module entry points (either via separate specifiers such as 'pkg' and 'pkg/es-module', or both at the same specifier via Conditional exports). Unlike in the scenario where "module" is only used by bundlers, or ES module files are transpiled into CommonJS on the fly before evaluation by Node.js, the files referenced by the ES module entry point are evaluated as ES modules.

Dual package hazard#

When an application is using a package that provides both CommonJS and ES module sources, there is a risk of certain bugs if both versions of the package get loaded. This potential comes from the fact that the pkgInstance created by const pkgInstance = require('pkg') is not the same as the pkgInstance created by import pkgInstance from 'pkg' (or an alternative main path like 'pkg/module'). This is the “dual package hazard,” where two versions of the same package can be loaded within the same runtime environment. While it is unlikely that an application or package would intentionally load both versions directly, it is common for an application to load one version while a dependency of the application loads the other version. This hazard can happen because Node.js supports intermixing CommonJS and ES modules, and can lead to unexpected behavior.

If the package main export is a constructor, an instanceof comparison of instances created by the two versions returns false, and if the export is an object, properties added to one (like pkgInstance.foo = 3) are not present on the other. This differs from how import and require statements work in all-CommonJS or all-ES module environments, respectively, and therefore is surprising to users. It also differs from the behavior users are familiar with when using transpilation via tools like Babel or esm.

Writing dual packages while avoiding or minimizing hazards#

First, the hazard described in the previous section occurs when a package contains both CommonJS and ES module sources and both sources are provided for use in Node.js, either via separate main entry points or exported paths. A package might instead be written where any version of Node.js receives only CommonJS sources, and any separate ES module sources the package might contain are intended only for other environments such as browsers. Such a package would be usable by any version of Node.js, since import can refer to CommonJS files; but it would not provide any of the advantages of using ES module syntax.

A package might also switch from CommonJS to ES module syntax in a breaking change version bump. This has the disadvantage that the newest version of the package would only be usable in ES module-supporting versions of Node.js.

Every pattern has tradeoffs, but there are two broad approaches that satisfy the following conditions:

  1. The package is usable via both require and import.
  2. The package is usable in both current Node.js and older versions of Node.js that lack support for ES modules.
  3. The package main entry point, e.g. 'pkg' can be used by both require to resolve to a CommonJS file and by import to resolve to an ES module file. (And likewise for exported paths, e.g. 'pkg/feature'.)
  4. The package provides named exports, e.g. import { name } from 'pkg' rather than import pkg from 'pkg'; pkg.name.
  5. The package is potentially usable in other ES module environments such as browsers.
  6. The hazards described in the previous section are avoided or minimized.
Approach #1: Use an ES module wrapper#

Write the package in CommonJS or transpile ES module sources into CommonJS, and create an ES module wrapper file that defines the named exports. Using Conditional exports, the ES module wrapper is used for import and the CommonJS entry point for require.

// ./node_modules/pkg/package.json
{
  "type": "module",
  "exports": {
    "import": "./wrapper.mjs",
    "require": "./index.cjs"
  }
}

The preceding example uses explicit extensions .mjs and .cjs. If your files use the .js extension, "type": "module" will cause such files to be treated as ES modules, just as "type": "commonjs" would cause them to be treated as CommonJS. See Enabling.

// ./node_modules/pkg/index.cjs
exports.name = 'value';
// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;

In this example, the name from import { name } from 'pkg' is the same singleton as the name from const { name } = require('pkg'). Therefore === returns true when comparing the two names and the divergent specifier hazard is avoided.

If the module is not simply a list of named exports, but rather contains a unique function or object export like module.exports = function () { ... }, or if support in the wrapper for the import pkg from 'pkg' pattern is desired, then the wrapper would instead be written to export the default optionally along with any named exports as well:

import cjsModule from './index.cjs';
export const name = cjsModule.name;
export default cjsModule;

This approach is appropriate for any of the following use cases:

  • The package is currently written in CommonJS and the author would prefer not to refactor it into ES module syntax, but wishes to provide named exports for ES module consumers.
  • The package has other packages that depend on it, and the end user might install both this package and those other packages. For example a utilities package is used directly in an application, and a utilities-plus package adds a few more functions to utilities. Because the wrapper exports underlying CommonJS files, it doesn't matter if utilities-plus is written in CommonJS or ES module syntax; it will work either way.
  • The package stores internal state, and the package author would prefer not to refactor the package to isolate its state management. See the next section.

A variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package. This could be used via import 'pkg/module' by users who are certain that the CommonJS version will not be loaded anywhere in the application, such as by dependencies; or if the CommonJS version can be loaded but doesn't affect the ES module version (for example, because the package is stateless):

// ./node_modules/pkg/package.json
{
  "type": "module",
  "exports": {
    ".": "./index.cjs",
    "./module": "./wrapper.mjs"
  }
}
Approach #2: Isolate state#

A package.json file can define the separate CommonJS and ES module entry points directly:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "exports": {
    "import": "./index.mjs",
    "require": "./index.cjs"
  }
}

This can be done if both the CommonJS and ES module versions of the package are equivalent, for example because one is the transpiled output of the other; and the package's management of state is carefully isolated (or the package is stateless).

The reason that state is an issue is because both the CommonJS and ES module versions of the package might get used within an application; for example, the user's application code could import the ES module version while a dependency requires the CommonJS version. If that were to occur, two copies of the package would be loaded in memory and therefore two separate states would be present. This would likely cause hard-to-troubleshoot bugs.

Aside from writing a stateless package (if JavaScript's Math were a package, for example, it would be stateless as all of its methods are static), there are some ways to isolate state so that it's shared between the potentially loaded CommonJS and ES module instances of the package:

  1. If possible, contain all state within an instantiated object. JavaScript's Date, for example, needs to be instantiated to contain state; if it were a package, it would be used like this:

    import Date from 'date';
const someDate = new Date();
// someDate contains state; Date does not

    The new keyword isn't required; a package's function can return a new object, or modify a passed-in object, to keep the state external to the package.

  2. Isolate the state in one or more CommonJS files that are shared between the CommonJS and ES module versions of the package. For example, if the CommonJS and ES module entry points are index.cjs and index.mjs, respectively:

    // ./node_modules/pkg/index.cjs
const state = require('./state.cjs');
module.exports.state = state;
    // ./node_modules/pkg/index.mjs
import state from './state.cjs';
export {
  state,
};

    Even if pkg is used via both require and import in an application (for example, via import in application code and via require by a dependency) each reference of pkg will contain the same state; and modifying that state from either module system will apply to both.

Any plugins that attach to the package's singleton would need to separately attach to both the CommonJS and ES module singletons.

This approach is appropriate for any of the following use cases:

  • The package is currently written in ES module syntax and the package author wants that version to be used wherever such syntax is supported.
  • The package is stateless or its state can be isolated without too much difficulty.
  • The package is unlikely to have other public packages that depend on it, or if it does, the package is stateless or has state that need not be shared between dependencies or with the overall application.

Even with isolated state, there is still the cost of possible extra code execution between the CommonJS and ES module versions of a package.

As with the previous approach, a variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "exports": {
    ".": "./index.cjs",
    "./module": "./index.mjs"
  }
}

Node.js package.json field definitions#

This section describes the fields used by the Node.js runtime. Other tools (such as npm) use additional fields which are ignored by Node.js and not documented here.

The following fields in package.json files are used in Node.js:

  • "name" - Relevant when using named imports within a package. Also used by package managers as the name of the package.
  • "main" - The default module when loading the package, if exports is not specified, and in versions of Node.js prior to the introduction of exports.
  • "packageManager" - The package manager recommended when contributing to the package. Leveraged by the Corepack shims.
  • "type" - The package type determining whether to load .js files as CommonJS or ES modules.
  • "exports" - Package exports and conditional exports. When present, limits which submodules can be loaded from within the package.
  • "imports" - Package imports, for use by modules within the package itself.

"name"#

History
VersionChanges
v13.6.0, v12.16.0

Remove the --experimental-resolve-self option.

v13.1.0, v12.16.0

Added in: v13.1.0, v12.16.0

{
  "name": "package-name"
}

The "name" field defines your package's name. Publishing to the npm registry requires a name that satisfies certain requirements.

The "name" field can be used in addition to the "exports" field to self-reference a package using its name.

"main"#

Added in: v0.4.0 
{
  "main": "./index.js"
}

The "main" field defines the entry point of a package when imported by name via a node_modules lookup. Its value is a path.

When a package has an "exports" field, this will take precedence over the "main" field when importing the package by name.

It also defines the script that is used when the package directory is loaded via require().

// This resolves to ./path/to/directory/index.js.
require('./path/to/directory');

"packageManager"#

Added in: v16.9.0, v14.19.0

Stability: 1 - Experimental

{
  "packageManager": "<package manager name>@<version>"
}

The "packageManager" field defines which package manager is expected to be used when working on the current project. It can be set to any of the supported package managers, and will ensure that your teams use the exact same package manager versions without having to install anything else other than Node.js.

This field is currently experimental and needs to be opted-in; check the Corepack page for details about the procedure.

"type"#

History
VersionChanges
v13.2.0, v12.17.0

Unflag --experimental-modules.

v12.0.0

Added in: v12.0.0

The "type" field defines the module format that Node.js uses for all .js files that have that package.json file as their nearest parent.

Files ending with .js are loaded as ES modules when the nearest parent package.json file contains a top-level field "type" with a value of "module".

The nearest parent package.json is defined as the first package.json found when searching in the current folder, that folder's parent, and so on up until a node_modules folder or the volume root is reached.

// package.json
{
  "type": "module"
}
# In same folder as preceding package.json
node my-app.js # Runs as ES module

If the nearest parent package.json lacks a "type" field, or contains "type": "commonjs", .js files are treated as CommonJS. If the volume root is reached and no package.json is found, .js files are treated as CommonJS.

import statements of .js files are treated as ES modules if the nearest parent package.json contains "type": "module".

// my-app.js, part of the same example as above
import './startup.js'; // Loaded as ES module because of package.json

Regardless of the value of the "type" field, .mjs files are always treated as ES modules and .cjs files are always treated as CommonJS.

"exports"#

History
VersionChanges
v14.13.0, v12.20.0

Add support for "exports" patterns.

v13.7.0, v12.17.0

Unflag conditional exports.

v13.7.0, v12.16.0

Implement logical conditional exports ordering.

v13.7.0, v12.16.0

Remove the --experimental-conditional-exports option. In 12.16.0, conditional exports are still behind --experimental-modules.

v13.2.0, v12.16.0

Implement conditional exports.

v12.7.0

Added in: v12.7.0

{
  "exports": "./index.js"
}

The "exports" field allows defining the entry points of a package when imported by name loaded either via a node_modules lookup or a self-reference to its own name. It is supported in Node.js 12+ as an alternative to the "main" that can support defining subpath exports and conditional exports while encapsulating internal unexported modules.

Conditional Exports can also be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import.

All paths defined in the "exports" must be relative file URLs starting with ./.

"imports"#

Added in: v14.6.0, v12.19.0 
// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
}

Entries in the imports field must be strings starting with #.

Package imports permit mapping to external packages.

This field defines subpath imports for the current package.

Net#

Stability: 2 - Stable

Source Code: lib/net.js

The node:net module provides an asynchronous network API for creating stream-based TCP or IPC servers (net.createServer()) and clients (net.createConnection()).

It can be accessed using:

const net = require('node:net');

IPC support#

The node:net module supports IPC with named pipes on Windows, and Unix domain sockets on other operating systems.

Identifying paths for IPC connections#

net.connect(), net.createConnection(), server.listen(), and socket.connect() take a path parameter to identify IPC endpoints.

On Unix, the local domain is also known as the Unix domain. The path is a file system pathname. It gets truncated to an OS-dependent length of sizeof(sockaddr_un.sun_path) - 1. Typical values are 107 bytes on Linux and 103 bytes on macOS. If a Node.js API abstraction creates the Unix domain socket, it will unlink the Unix domain socket as well. For example, net.createServer() may create a Unix domain socket and server.close() will unlink it. But if a user creates the Unix domain socket outside of these abstractions, the user will need to remove it. The same applies when a Node.js API creates a Unix domain socket but the program then crashes. In short, a Unix domain socket will be visible in the file system and will persist until unlinked.

On Windows, the local domain is implemented using a named pipe. The path must refer to an entry in \\?\pipe\ or \\.\pipe\. Any characters are permitted, but the latter may do some processing of pipe names, such as resolving .. sequences. Despite how it might look, the pipe namespace is flat. Pipes will not persist. They are removed when the last reference to them is closed. Unlike Unix domain sockets, Windows will close and remove the pipe when the owning process exits.

JavaScript string escaping requires paths to be specified with extra backslash escaping such as:

net.createServer().listen(
  path.join('\\\\?\\pipe', process.cwd(), 'myctl'));

Class: net.BlockList#

Added in: v15.0.0, v14.18.0

The BlockList object can be used with some network APIs to specify rules for disabling inbound or outbound access to specific IP addresses, IP ranges, or IP subnets.

blockList.addAddress(address[, type])#

Added in: v15.0.0, v14.18.0

Adds a rule to block the given IP address.

blockList.addRange(start, end[, type])#

Added in: v15.0.0, v14.18.0

Adds a rule to block a range of IP addresses from start (inclusive) to end (inclusive).

blockList.addSubnet(net, prefix[, type])#

Added in: v15.0.0, v14.18.0
  • net <string> | <net.SocketAddress> The network IPv4 or IPv6 address.
  • prefix <number> The number of CIDR prefix bits. For IPv4, this must be a value between 0 and 32. For IPv6, this must be between 0 and 128.
  • type <string> Either 'ipv4' or 'ipv6'. Default: 'ipv4'.

Adds a rule to block a range of IP addresses specified as a subnet mask.

blockList.check(address[, type])#

Added in: v15.0.0, v14.18.0

Returns true if the given IP address matches any of the rules added to the BlockList.

const blockList = new net.BlockList();
blockList.addAddress('123.123.123.123');
blockList.addRange('10.0.0.1', '10.0.0.10');
blockList.addSubnet('8592:757c:efae:4e45::', 64, 'ipv6');

console.log(blockList.check('123.123.123.123'));  // Prints: true
console.log(blockList.check('10.0.0.3'));  // Prints: true
console.log(blockList.check('222.111.111.222'));  // Prints: false

// IPv6 notation for IPv4 addresses works:
console.log(blockList.check('::ffff:7b7b:7b7b', 'ipv6')); // Prints: true
console.log(blockList.check('::ffff:123.123.123.123', 'ipv6')); // Prints: true

blockList.rules#

Added in: v15.0.0, v14.18.0

The list of rules added to the blocklist.

Class: net.SocketAddress#

Added in: v15.14.0, v14.18.0

new net.SocketAddress([options])#

Added in: v15.14.0, v14.18.0
  • options <Object>
    • address <string> The network address as either an IPv4 or IPv6 string. Default: '127.0.0.1' if family is 'ipv4'; '::' if family is 'ipv6'.
    • family <string> One of either 'ipv4' or 'ipv6'. Default: 'ipv4'.
    • flowlabel <number> An IPv6 flow-label used only if family is 'ipv6'.
    • port <number> An IP port.

socketaddress.address#

Added in: v15.14.0, v14.18.0

socketaddress.family#

Added in: v15.14.0, v14.18.0
  • Type <string> Either 'ipv4' or 'ipv6'.

socketaddress.flowlabel#

Added in: v15.14.0, v14.18.0

socketaddress.port#

Added in: v15.14.0, v14.18.0

Class: net.Server#

Added in: v0.1.90

This class is used to create a TCP or IPC server.

new net.Server([options][, connectionListener])#

net.Server is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.5.0

Emitted when the server closes. If connections exist, this event is not emitted until all connections are ended.

Event: 'connection'#

Added in: v0.1.90

Emitted when a new connection is made. socket is an instance of net.Socket.

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. Unlike net.Socket, the 'close' event will not be emitted directly following this event unless server.close() is manually called. See the example in discussion of server.listen().

Event: 'listening'#

Added in: v0.1.90

Emitted when the server has been bound after calling server.listen().

Event: 'drop'#

Added in: v18.6.0, v16.17.0

When the number of connections reaches the threshold of server.maxConnections, the server will drop new connections and emit 'drop' event instead. If it is a TCP server, the argument is as follows, otherwise the argument is undefined.

  • data <Object> The argument passed to event listener.

server.address()#

History
VersionChanges
v18.4.0

The family property now returns a string instead of a number.

v18.0.0

The family property now returns a number instead of a string.

v0.1.90

Added in: v0.1.90

Returns the bound address, the address family name, and port of the server as reported by the operating system if listening on an IP socket (useful to find which port was assigned when getting an OS-assigned address): { port: 12346, family: 'IPv4', address: '127.0.0.1' }.

For a server listening on a pipe or Unix domain socket, the name is returned as a string.

const server = net.createServer((socket) => {
  socket.end('goodbye\n');
}).on('error', (err) => {
  // Handle errors here.
  throw err;
});

// Grab an arbitrary unused port.
server.listen(() => {
  console.log('opened server on', server.address());
});

server.address() returns null before the 'listening' event has been emitted or after calling server.close().

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.

server.getConnections(callback)#

Added in: v0.9.7

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments err and count.

server.listen()#

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All net.Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requested port/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
  if (e.code === 'EADDRINUSE') {
    console.error('Address in use, retrying...');
    setTimeout(() => {
      server.close();
      server.listen(PORT, HOST);
    }, 1000);
  }
});
server.listen(handle[, backlog][, callback])#
Added in: v0.5.10

Start a server listening for connections on a given handle that has already been bound to a port, a Unix domain socket, or a Windows named pipe.

The handle object can be either a server, a socket (anything with an underlying _handle member), or an object with an fd member that is a valid file descriptor.

Listening on a file descriptor is not supported on Windows.

server.listen(options[, callback])#
History
VersionChanges
v15.6.0

AbortSignal support was added.

v11.4.0

The ipv6Only option is supported.

v0.11.14

Added in: v0.11.14

If port is specified, it behaves the same as server.listen([port[, host[, backlog]]][, callback]). Otherwise, if path is specified, it behaves the same as server.listen(path[, backlog][, callback]). If none of them is specified, an error will be thrown.

If exclusive is false (default), then cluster workers will use the same underlying handle, allowing connection handling duties to be shared. When exclusive is true, the handle is not shared, and attempted port sharing results in an error. An example which listens on an exclusive port is shown below.

server.listen({
  host: 'localhost',
  port: 80,
  exclusive: true,
});

When exclusive is true and the underlying handle is shared, it is possible that several workers query a handle with different backlogs. In this case, the first backlog passed to the master process will be used.

Starting an IPC server as root may cause the server path to be inaccessible for unprivileged users. Using readableAll and writableAll will make the server accessible for all users.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .close() on the server:

const controller = new AbortController();
server.listen({
  host: 'localhost',
  port: 80,
  signal: controller.signal,
});
// Later, when you want to close the server.
controller.abort();
server.listen(path[, backlog][, callback])#
Added in: v0.1.90

Start an IPC server listening for connections on the given path.

server.listen([port[, host[, backlog]]][, callback])#
Added in: v0.1.90

Start a TCP server listening for connections on the given port and host.

If port is omitted or is 0, the operating system will assign an arbitrary unused port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

If host is omitted, the server will accept connections on the unspecified IPv6 address (::) when IPv6 is available, or the unspecified IPv4 address (0.0.0.0) otherwise.

In most operating systems, listening to the unspecified IPv6 address (::) may cause the net.Server to also listen on the unspecified IPv4 address (0.0.0.0).

server.listening#

Added in: v5.7.0
  • <boolean> Indicates whether or not the server is listening for connections.

server.maxConnections#

Added in: v0.2.0

Set this property to reject connections when the server's connection count gets high.

It is not recommended to use this option once a socket has been sent to a child with child_process.fork().

server.ref()#

Added in: v0.9.1

Opposite of unref(), calling ref() on a previously unrefed server will not let the program exit if it's the only server left (the default behavior). If the server is refed calling ref() again will have no effect.

server.unref()#

Added in: v0.9.1

Calling unref() on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefed calling unref() again will have no effect.

Class: net.Socket#

Added in: v0.3.4

This class is an abstraction of a TCP socket or a streaming IPC endpoint (uses named pipes on Windows, and Unix domain sockets otherwise). It is also an EventEmitter.

A net.Socket can be created by the user and used directly to interact with a server. For example, it is returned by net.createConnection(), so the user can use it to talk to the server.

It can also be created by Node.js and passed to the user when a connection is received. For example, it is passed to the listeners of a 'connection' event emitted on a net.Server, so the user can use it to interact with the client.

new net.Socket([options])#

History
VersionChanges
v15.14.0

AbortSignal support was added.

v0.3.4

Added in: v0.3.4

  • options <Object> Available options are:
    • fd <number> If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.
    • allowHalfOpen <boolean> If set to false, then the socket will automatically end the writable side when the readable side ends. See net.createServer() and the 'end' event for details. Default: false.
    • readable <boolean> Allow reads on the socket when an fd is passed, otherwise ignored. Default: false.
    • writable <boolean> Allow writes on the socket when an fd is passed, otherwise ignored. Default: false.
    • signal <AbortSignal> An Abort signal that may be used to destroy the socket.
  • Returns: <net.Socket>

Creates a new socket object.

The newly created socket can be either a TCP socket or a streaming IPC endpoint, depending on what it connect() to.

Event: 'close'#

Added in: v0.1.90
  • hadError <boolean> true if the socket had a transmission error.

Emitted once the socket is fully closed. The argument hadError is a boolean which says if the socket was closed due to a transmission error.

Event: 'connect'#

Added in: v0.1.90

Emitted when a socket connection is successfully established. See net.createConnection().

Event: 'data'#

Added in: v0.1.90

Emitted when data is received. The argument data will be a Buffer or String. Encoding of data is set by socket.setEncoding().

The data will be lost if there is no listener when a Socket emits a 'data' event.

Event: 'drain'#

Added in: v0.1.90

Emitted when the write buffer becomes empty. Can be used to throttle uploads.

See also: the return values of socket.write().

Event: 'end'#

Added in: v0.1.90

Emitted when the other end of the socket signals the end of transmission, thus ending the readable side of the socket.

By default (allowHalfOpen is false) the socket will send an end of transmission packet back and destroy its file descriptor once it has written out its pending write queue. However, if allowHalfOpen is set to true, the socket will not automatically end() its writable side, allowing the user to write arbitrary amounts of data. The user must call end() explicitly to close the connection (i.e. sending a FIN packet back).

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. The 'close' event will be called directly following this event.

Event: 'lookup'#

History
VersionChanges
v5.10.0

The host parameter is supported now.

v0.11.3

Added in: v0.11.3

Emitted after resolving the host name but before connecting. Not applicable to Unix sockets.

Event: 'ready'#

Added in: v9.11.0

Emitted when a socket is ready to be used.

Triggered immediately after 'connect'.

Event: 'timeout'#

Added in: v0.1.90

Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.

See also: socket.setTimeout().

socket.address()#

History
VersionChanges
v18.4.0

The family property now returns a string instead of a number.

v18.0.0

The family property now returns a number instead of a string.

v0.1.90

Added in: v0.1.90

Returns the bound address, the address family name and port of the socket as reported by the operating system: { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.autoSelectFamilyAttemptedAddresses#

Added in: v19.4.0

This property is only present if the family autoselection algorithm is enabled in socket.connect(options) and it is an array of the addresses that have been attempted.

Each address is a string in the form of $IP:$PORT. If the connection was successful, then the last address is the one that the socket is currently connected to.

socket.bufferSize#

Added in: v0.3.8Deprecated since: v14.6.0

Stability: 0 - Deprecated: Use writable.writableLength instead.

This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.

net.Socket has the property that socket.write() always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket. The network connection simply might be too slow. Node.js will internally queue up the data written to a socket and send it out over the wire when it is possible.

The consequence of this internal buffering is that memory may grow. Users who experience large or growing bufferSize should attempt to "throttle" the data flows in their program with socket.pause() and socket.resume().

socket.bytesRead#

Added in: v0.5.3

The amount of received bytes.

socket.bytesWritten#

Added in: v0.5.3

The amount of bytes sent.

socket.connect()#

Initiate a connection on a given socket.

Possible signatures:

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after 'close' has been emitted or otherwise it may lead to undefined behavior.

socket.connect(options[, connectListener])#
History
VersionChanges
v19.4.0

The default value for autoSelectFamily option can be changed at runtime using setDefaultAutoSelectFamily or via the command line option --enable-network-family-autoselection.

v19.3.0

Added the autoSelectFamily option.

v17.7.0, v16.15.0

The noDelay, keepAlive, and keepAliveInitialDelay options are supported now.

v12.10.0

Added onread option.

v6.0.0

The hints option defaults to 0 in all cases now. Previously, in the absence of the family option it would default to dns.ADDRCONFIG | dns.V4MAPPED.

v5.11.0

The hints option is supported now.

v0.1.90

Added in: v0.1.90

Initiate a connection on a given socket. Normally this method is not needed, the socket should be created and opened with net.createConnection(). Use this only when implementing a custom Socket.

For TCP connections, available options are:

  • port <number> Required. Port the socket should connect to.
  • host <string> Host the socket should connect to. Default: 'localhost'.
  • localAddress <string> Local address the socket should connect from.
  • localPort <number> Local port the socket should connect from.
  • family <number>: Version of IP stack. Must be 4, 6, or 0. The value 0 indicates that both IPv4 and IPv6 addresses are allowed. Default: 0.
  • hints <number> Optional dns.lookup() hints.
  • lookup <Function> Custom lookup function. Default: dns.lookup().
  • noDelay <boolean> If set to true, it disables the use of Nagle's algorithm immediately after the socket is established. Default: false.
  • keepAlive <boolean> If set to true, it enables keep-alive functionality on the socket immediately after the connection is established, similarly on what is done in socket.setKeepAlive([enable][, initialDelay]). Default: false.
  • keepAliveInitialDelay <number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket.Default: 0.
  • autoSelectFamily <boolean>: If set to true, it enables a family autodetection algorithm that loosely implements section 5 of RFC 8305. The all option passed to lookup is set to true and the sockets attempts to connect to all obtained IPv6 and IPv4 addresses, in sequence, until a connection is established. The first returned AAAA address is tried first, then the first returned A address, then the second returned AAAA address and so on. Each connection attempt is given the amount of time specified by the autoSelectFamilyAttemptTimeout option before timing out and trying the next address. Ignored if the family option is not 0 or if localAddress is set. Connection errors are not emitted if at least one connection succeeds. Default: initially false, but it can be changed at runtime using net.setDefaultAutoSelectFamily(value) or via the command line option --enable-network-family-autoselection.
  • autoSelectFamilyAttemptTimeout <number>: The amount of time in milliseconds to wait for a connection attempt to finish before trying the next address when using the autoSelectFamily option. If set to a positive integer less than 10, then the value 10 will be used instead. Default: 250.

For IPC connections, available options are:

For both types, available options include:

  • onread <Object> If specified, incoming data is stored in a single buffer and passed to the supplied callback when data arrives on the socket. This will cause the streaming functionality to not provide any data. The socket will emit events like 'error', 'end', and 'close' as usual. Methods like pause() and resume() will also behave as expected.
    • buffer <Buffer> | <Uint8Array> | <Function> Either a reusable chunk of memory to use for storing incoming data or a function that returns such.
    • callback <Function> This function is called for every chunk of incoming data. Two arguments are passed to it: the number of bytes written to buffer and a reference to buffer. Return false from this function to implicitly pause() the socket. This function will be executed in the global context.

Following is an example of a client using the onread option:

const net = require('node:net');
net.connect({
  port: 80,
  onread: {
    // Reuses a 4KiB Buffer for every read from the socket.
    buffer: Buffer.alloc(4 * 1024),
    callback: function(nread, buf) {
      // Received data is available in `buf` from 0 to `nread`.
      console.log(buf.toString('utf8', 0, nread));
    },
  },
});
socket.connect(path[, connectListener])#

Initiate an IPC connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with { path: path } as options.

socket.connect(port[, host][, connectListener])#
Added in: v0.1.90

Initiate a TCP connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with {port: port, host: host} as options.

socket.connecting#

Added in: v6.1.0

If true, socket.connect(options[, connectListener]) was called and has not yet finished. It will stay true until the socket becomes connected, then it is set to false and the 'connect' event is emitted. Note that the socket.connect(options[, connectListener]) callback is a listener for the 'connect' event.

socket.destroy([error])#

Added in: v0.1.90

Ensures that no more I/O activity happens on this socket. Destroys the stream and closes the connection.

See writable.destroy() for further details.

socket.destroyed#

  • <boolean> Indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

See writable.destroyed for further details.

socket.end([data[, encoding]][, callback])#

Added in: v0.1.90

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See writable.end() for further details.

socket.localAddress#

Added in: v0.9.6

The string representation of the local IP address the remote client is connecting on. For example, in a server listening on '0.0.0.0', if a client connects on '192.168.1.1', the value of socket.localAddress would be '192.168.1.1'.

socket.localPort#

Added in: v0.9.6

The numeric representation of the local port. For example, 80 or 21.

socket.localFamily#

Added in: v18.8.0, v16.18.0

The string representation of the local IP family. 'IPv4' or 'IPv6'.

socket.pause()#

Pauses the reading of data. That is, 'data' events will not be emitted. Useful to throttle back an upload.

socket.pending#

Added in: v11.2.0, v10.16.0

This is true if the socket is not connected yet, either because .connect() has not yet been called or because it is still in the process of connecting (see socket.connecting).

socket.ref()#

Added in: v0.9.1

Opposite of unref(), calling ref() on a previously unrefed socket will not let the program exit if it's the only socket left (the default behavior). If the socket is refed calling ref again will have no effect.

socket.remoteAddress#

Added in: v0.5.10

The string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.remoteFamily#

Added in: v0.11.14

The string representation of the remote IP family. 'IPv4' or 'IPv6'.

socket.remotePort#

Added in: v0.5.10

The numeric representation of the remote port. For example, 80 or 21.

socket.resetAndDestroy()#

Added in: v18.3.0, v16.17.0

Close the TCP connection by sending an RST packet and destroy the stream. If this TCP socket is in connecting status, it will send an RST packet and destroy this TCP socket once it is connected. Otherwise, it will call socket.destroy with an ERR_SOCKET_CLOSED Error. If this is not a TCP socket (for example, a pipe), calling this method will immediately throw an ERR_INVALID_HANDLE_TYPE Error.

socket.resume()#

Resumes reading after a call to socket.pause().

socket.setEncoding([encoding])#

Added in: v0.1.90

Set the encoding for the socket as a Readable Stream. See readable.setEncoding() for more information.

socket.setKeepAlive([enable][, initialDelay])#

History
VersionChanges
v13.12.0, v12.17.0

New defaults for TCP_KEEPCNT and TCP_KEEPINTVL socket options were added.

v0.1.92

Added in: v0.1.92

Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket.

Set initialDelay (in milliseconds) to set the delay between the last data packet received and the first keepalive probe. Setting 0 for initialDelay will leave the value unchanged from the default (or previous) setting.

Enabling the keep-alive functionality will set the following socket options:

  • SO_KEEPALIVE=1
  • TCP_KEEPIDLE=initialDelay
  • TCP_KEEPCNT=10
  • TCP_KEEPINTVL=1

socket.setNoDelay([noDelay])#

Added in: v0.1.90

Enable/disable the use of Nagle's algorithm.

When a TCP connection is created, it will have Nagle's algorithm enabled.

Nagle's algorithm delays data before it is sent via the network. It attempts to optimize throughput at the expense of latency.

Passing true for noDelay or not passing an argument will disable Nagle's algorithm for the socket. Passing false for noDelay will enable Nagle's algorithm.

socket.setTimeout(timeout[, callback])#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v0.1.90

Added in: v0.1.90

Sets the socket to timeout after timeout milliseconds of inactivity on the socket. By default net.Socket do not have a timeout.

When an idle timeout is triggered the socket will receive a 'timeout' event but the connection will not be severed. The user must manually call socket.end() or socket.destroy() to end the connection.

socket.setTimeout(3000);
socket.on('timeout', () => {
  console.log('socket timeout');
  socket.end();
});

If timeout is 0, then the existing idle timeout is disabled.

The optional callback parameter will be added as a one-time listener for the 'timeout' event.

socket.timeout#

Added in: v10.7.0

The socket timeout in milliseconds as set by socket.setTimeout(). It is undefined if a timeout has not been set.

socket.unref()#

Added in: v0.9.1

Calling unref() on a socket will allow the program to exit if this is the only active socket in the event system. If the socket is already unrefed calling unref() again will have no effect.

socket.write(data[, encoding][, callback])#

Added in: v0.1.90

Sends data on the socket. The second parameter specifies the encoding in the case of a string. It defaults to UTF8 encoding.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is again free.

The optional callback parameter will be executed when the data is finally written out, which may not be immediately.

See Writable stream write() method for more information.

socket.readyState#

Added in: v0.5.0

This property represents the state of the connection as a string.

  • If the stream is connecting socket.readyState is opening.
  • If the stream is readable and writable, it is open.
  • If the stream is readable and not writable, it is readOnly.
  • If the stream is not readable and writable, it is writeOnly.

net.connect()#

Aliases to net.createConnection().

Possible signatures:

net.connect(options[, connectListener])#

Added in: v0.7.0

Alias to net.createConnection(options[, connectListener]).

net.connect(path[, connectListener])#

Added in: v0.1.90

Alias to net.createConnection(path[, connectListener]).

net.connect(port[, host][, connectListener])#

Added in: v0.1.90

Alias to net.createConnection(port[, host][, connectListener]).

net.createConnection()#

A factory function, which creates a new net.Socket, immediately initiates connection with socket.connect(), then returns the net.Socket that starts the connection.

When the connection is established, a 'connect' event will be emitted on the returned socket. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

Possible signatures:

The net.connect() function is an alias to this function.

net.createConnection(options[, connectListener])#

Added in: v0.1.90

For available options, see new net.Socket([options]) and socket.connect(options[, connectListener]).

Additional options:

Following is an example of a client of the echo server described in the net.createServer() section:

const net = require('node:net');
const client = net.createConnection({ port: 8124 }, () => {
  // 'connect' listener.
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock:

const client = net.createConnection({ path: '/tmp/echo.sock' });

net.createConnection(path[, connectListener])#

Added in: v0.1.90

Initiates an IPC connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(path[, connectListener]), then returns the net.Socket that starts the connection.

net.createConnection(port[, host][, connectListener])#

Added in: v0.1.90

Initiates a TCP connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(port[, host][, connectListener]), then returns the net.Socket that starts the connection.

net.setDefaultAutoSelectFamily(value)#

Added in: v19.4.0

Sets the default value of the autoSelectFamily option of socket.connect(options).

  • value <boolean> The new default value. The initial default value is false.

net.getDefaultAutoSelectFamily()#

Added in: v19.4.0

Gets the current default value of the autoSelectFamily option of socket.connect(options).

  • Returns: <boolean> The current default value of the autoSelectFamily option.

net.createServer([options][, connectionListener])#

History
VersionChanges
v17.7.0, v16.15.0

The noDelay, keepAlive, and keepAliveInitialDelay options are supported now.

v0.5.0

Added in: v0.5.0

  • options <Object>

    • allowHalfOpen <boolean> If set to false, then the socket will automatically end the writable side when the readable side ends. Default: false.
    • pauseOnConnect <boolean> Indicates whether the socket should be paused on incoming connections. Default: false.
    • noDelay <boolean> If set to true, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default: false.
    • keepAlive <boolean> If set to true, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done in socket.setKeepAlive([enable][, initialDelay]). Default: false.
    • keepAliveInitialDelay <number> If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket.Default: 0.

  • connectionListener <Function> Automatically set as a listener for the 'connection' event.

  • Returns: <net.Server>

Creates a new TCP or IPC server.

If allowHalfOpen is set to true, when the other end of the socket signals the end of transmission, the server will only send back the end of transmission when socket.end() is explicitly called. For example, in the context of TCP, when a FIN packed is received, a FIN packed is sent back only when socket.end() is explicitly called. Until then the connection is half-closed (non-readable but still writable). See 'end' event and RFC 1122 (section 4.2.2.13) for more information.

If pauseOnConnect is set to true, then the socket associated with each incoming connection will be paused, and no data will be read from its handle. This allows connections to be passed between processes without any data being read by the original process. To begin reading data from a paused socket, call socket.resume().

The server can be a TCP server or an IPC server, depending on what it listen() to.

Here is an example of a TCP echo server which listens for connections on port 8124:

const net = require('node:net');
const server = net.createServer((c) => {
  // 'connection' listener.
  console.log('client connected');
  c.on('end', () => {
    console.log('client disconnected');
  });
  c.write('hello\r\n');
  c.pipe(c);
});
server.on('error', (err) => {
  throw err;
});
server.listen(8124, () => {
  console.log('server bound');
});

Test this by using telnet:

$ telnet localhost 8124

To listen on the socket /tmp/echo.sock:

server.listen('/tmp/echo.sock', () => {
  console.log('server bound');
});

Use nc to connect to a Unix domain socket server:

$ nc -U /tmp/echo.sock

net.isIP(input)#

Added in: v0.3.0

Returns 6 if input is an IPv6 address. Returns 4 if input is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns 0.

net.isIP('::1'); // returns 6
net.isIP('127.0.0.1'); // returns 4
net.isIP('127.000.000.001'); // returns 0
net.isIP('127.0.0.1/24'); // returns 0
net.isIP('fhqwhgads'); // returns 0

net.isIPv4(input)#

Added in: v0.3.0

Returns true if input is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns false.

net.isIPv4('127.0.0.1'); // returns true
net.isIPv4('127.000.000.001'); // returns false
net.isIPv4('127.0.0.1/24'); // returns false
net.isIPv4('fhqwhgads'); // returns false

net.isIPv6(input)#

Added in: v0.3.0

Returns true if input is an IPv6 address. Otherwise, returns false.

net.isIPv6('::1'); // returns true
net.isIPv6('fhqwhgads'); // returns false

OS#

Stability: 2 - Stable

Source Code: lib/os.js

The node:os module provides operating system-related utility methods and properties. It can be accessed using:

const os = require('node:os');

os.EOL#

Added in: v0.7.8

The operating system-specific end-of-line marker.

  • \n on POSIX
  • \r\n on Windows

os.availableParallelism()#

Added in: v19.4.0

Returns an estimate of the default amount of parallelism a program should use. Always returns a value greater than zero.

This function is a small wrapper about libuv's uv_available_parallelism().

os.arch()#

Added in: v0.5.0

Returns the operating system CPU architecture for which the Node.js binary was compiled. Possible values are 'arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 's390', 's390x', and 'x64'.

The return value is equivalent to process.arch.

os.constants#

Added in: v6.3.0

Contains commonly used operating system-specific constants for error codes, process signals, and so on. The specific constants defined are described in OS constants.

os.cpus()#

Added in: v0.3.3

Returns an array of objects containing information about each logical CPU core.

The properties included on each object include:

  • model <string>
  • speed <number> (in MHz)
  • times <Object>
    • user <number> The number of milliseconds the CPU has spent in user mode.
    • nice <number> The number of milliseconds the CPU has spent in nice mode.
    • sys <number> The number of milliseconds the CPU has spent in sys mode.
    • idle <number> The number of milliseconds the CPU has spent in idle mode.
    • irq <number> The number of milliseconds the CPU has spent in irq mode.
[
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 252020,
      nice: 0,
      sys: 30340,
      idle: 1070356870,
      irq: 0,
    },
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 306960,
      nice: 0,
      sys: 26980,
      idle: 1071569080,
      irq: 0,
    },
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 248450,
      nice: 0,
      sys: 21750,
      idle: 1070919370,
      irq: 0,
    },
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 256880,
      nice: 0,
      sys: 19430,
      idle: 1070905480,
      irq: 20,
    },
  },
]

nice values are POSIX-only. On Windows, the nice values of all processors are always 0.

os.cpus().length should not be used to calculate the amount of parallelism available to an application. Use os.availableParallelism() for this purpose.

os.devNull#

Added in: v16.3.0, v14.18.0

The platform-specific file path of the null device.

  • \\.\nul on Windows
  • /dev/null on POSIX

os.endianness()#

Added in: v0.9.4

Returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.

Possible values are 'BE' for big endian and 'LE' for little endian.

os.freemem()#

Added in: v0.3.3

Returns the amount of free system memory in bytes as an integer.

os.getPriority([pid])#

Added in: v10.10.0
  • pid <integer> The process ID to retrieve scheduling priority for. Default: 0.
  • Returns: <integer>

Returns the scheduling priority for the process specified by pid. If pid is not provided or is 0, the priority of the current process is returned.

os.homedir()#

Added in: v2.3.0

Returns the string path of the current user's home directory.

On POSIX, it uses the $HOME environment variable if defined. Otherwise it uses the effective UID to look up the user's home directory.

On Windows, it uses the USERPROFILE environment variable if defined. Otherwise it uses the path to the profile directory of the current user.

os.hostname()#

Added in: v0.3.3

Returns the host name of the operating system as a string.

os.loadavg()#

Added in: v0.3.3

Returns an array containing the 1, 5, and 15 minute load averages.

The load average is a measure of system activity calculated by the operating system and expressed as a fractional number.

The load average is a Unix-specific concept. On Windows, the return value is always [0, 0, 0].

os.machine()#

Added in: v18.9.0, v16.18.0

Returns the machine type as a string, such as arm, arm64, aarch64, mips, mips64, ppc64, ppc64le, s390, s390x, i386, i686, x86_64.

On POSIX systems, the machine type is determined by calling uname(3). On Windows, RtlGetVersion() is used, and if it is not available, GetVersionExW() will be used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

os.networkInterfaces()#

History
VersionChanges
v18.4.0

The family property now returns a string instead of a number.

v18.0.0

The family property now returns a number instead of a string.

v0.6.0

Added in: v0.6.0

Returns an object containing network interfaces that have been assigned a network address.

Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.

The properties available on the assigned network address object include:

  • address <string> The assigned IPv4 or IPv6 address
  • netmask <string> The IPv4 or IPv6 network mask
  • family <string> Either IPv4 or IPv6
  • mac <string> The MAC address of the network interface
  • internal <boolean> true if the network interface is a loopback or similar interface that is not remotely accessible; otherwise false
  • scopeid <number> The numeric IPv6 scope ID (only specified when family is IPv6)
  • cidr <string> The assigned IPv4 or IPv6 address with the routing prefix in CIDR notation. If the netmask is invalid, this property is set to null.
{
  lo: [
    {
      address: '127.0.0.1',
      netmask: '255.0.0.0',
      family: 'IPv4',
      mac: '00:00:00:00:00:00',
      internal: true,
      cidr: '127.0.0.1/8'
    },
    {
      address: '::1',
      netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff',
      family: 'IPv6',
      mac: '00:00:00:00:00:00',
      scopeid: 0,
      internal: true,
      cidr: '::1/128'
    }
  ],
  eth0: [
    {
      address: '192.168.1.108',
      netmask: '255.255.255.0',
      family: 'IPv4',
      mac: '01:02:03:0a:0b:0c',
      internal: false,
      cidr: '192.168.1.108/24'
    },
    {
      address: 'fe80::a00:27ff:fe4e:66a1',
      netmask: 'ffff:ffff:ffff:ffff::',
      family: 'IPv6',
      mac: '01:02:03:0a:0b:0c',
      scopeid: 1,
      internal: false,
      cidr: 'fe80::a00:27ff:fe4e:66a1/64'
    }
  ]
}

os.platform()#

Added in: v0.5.0

Returns a string identifying the operating system platform for which the Node.js binary was compiled. The value is set at compile time. Possible values are 'aix', 'darwin', 'freebsd','linux', 'openbsd', 'sunos', and 'win32'.

The return value is equivalent to process.platform.

The value 'android' may also be returned if Node.js is built on the Android operating system. Android support is experimental.

os.release()#

Added in: v0.3.3

Returns the operating system as a string.

On POSIX systems, the operating system release is determined by calling uname(3). On Windows, GetVersionExW() is used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

os.setPriority([pid, ]priority)#

Added in: v10.10.0
  • pid <integer> The process ID to set scheduling priority for. Default: 0.
  • priority <integer> The scheduling priority to assign to the process.

Attempts to set the scheduling priority for the process specified by pid. If pid is not provided or is 0, the process ID of the current process is used.

The priority input must be an integer between -20 (high priority) and 19 (low priority). Due to differences between Unix priority levels and Windows priority classes, priority is mapped to one of six priority constants in os.constants.priority. When retrieving a process priority level, this range mapping may cause the return value to be slightly different on Windows. To avoid confusion, set priority to one of the priority constants.

On Windows, setting priority to PRIORITY_HIGHEST requires elevated user privileges. Otherwise the set priority will be silently reduced to PRIORITY_HIGH.

os.tmpdir()#

History
VersionChanges
v2.0.0

This function is now cross-platform consistent and no longer returns a path with a trailing slash on any platform.

v0.9.9

Added in: v0.9.9

Returns the operating system's default directory for temporary files as a string.

os.totalmem()#

Added in: v0.3.3

Returns the total amount of system memory in bytes as an integer.

os.type()#

Added in: v0.3.3

Returns the operating system name as returned by uname(3). For example, it returns 'Linux' on Linux, 'Darwin' on macOS, and 'Windows_NT' on Windows.

See https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running uname(3) on various operating systems.

os.uptime()#

History
VersionChanges
v10.0.0

The result of this function no longer contains a fraction component on Windows.

v0.3.3

Added in: v0.3.3

Returns the system uptime in number of seconds.

os.userInfo([options])#

Added in: v6.0.0
  • options <Object>
    • encoding <string> Character encoding used to interpret resulting strings. If encoding is set to 'buffer', the username, shell, and homedir values will be Buffer instances. Default: 'utf8'.
  • Returns: <Object>

Returns information about the currently effective user. On POSIX platforms, this is typically a subset of the password file. The returned object includes the username, uid, gid, shell, and homedir. On Windows, the uid and gid fields are -1, and shell is null.

The value of homedir returned by os.userInfo() is provided by the operating system. This differs from the result of os.homedir(), which queries environment variables for the home directory before falling back to the operating system response.

Throws a SystemError if a user has no username or homedir.

os.version()#

Added in: v13.11.0, v12.17.0

Returns a string identifying the kernel version.

On POSIX systems, the operating system release is determined by calling uname(3). On Windows, RtlGetVersion() is used, and if it is not available, GetVersionExW() will be used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

OS constants#

The following constants are exported by os.constants.

Not all constants will be available on every operating system.

Signal constants#

History
VersionChanges
v5.11.0

Added support for SIGINFO.

The following signal constants are exported by os.constants.signals.

Constant Description
SIGHUP Sent to indicate when a controlling terminal is closed or a parent process exits.
SIGINT Sent to indicate when a user wishes to interrupt a process (Ctrl+C).
SIGQUIT Sent to indicate when a user wishes to terminate a process and perform a core dump.
SIGILL Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown, or privileged instruction.
SIGTRAP Sent to a process when an exception has occurred.
SIGABRT Sent to a process to request that it abort.
SIGIOT Synonym for SIGABRT
SIGBUS Sent to a process to notify that it has caused a bus error.
SIGFPE Sent to a process to notify that it has performed an illegal arithmetic operation.
SIGKILL Sent to a process to terminate it immediately.
SIGUSR1 SIGUSR2 Sent to a process to identify user-defined conditions.
SIGSEGV Sent to a process to notify of a segmentation fault.
SIGPIPE Sent to a process when it has attempted to write to a disconnected pipe.
SIGALRM Sent to a process when a system timer elapses.
SIGTERM Sent to a process to request termination.
SIGCHLD Sent to a process when a child process terminates.
SIGSTKFLT Sent to a process to indicate a stack fault on a coprocessor.
SIGCONT Sent to instruct the operating system to continue a paused process.
SIGSTOP Sent to instruct the operating system to halt a process.
SIGTSTP Sent to a process to request it to stop.
SIGBREAK Sent to indicate when a user wishes to interrupt a process.
SIGTTIN Sent to a process when it reads from the TTY while in the background.
SIGTTOU Sent to a process when it writes to the TTY while in the background.
SIGURG Sent to a process when a socket has urgent data to read.
SIGXCPU Sent to a process when it has exceeded its limit on CPU usage.
SIGXFSZ Sent to a process when it grows a file larger than the maximum allowed.
SIGVTALRM Sent to a process when a virtual timer has elapsed.
SIGPROF Sent to a process when a system timer has elapsed.
SIGWINCH Sent to a process when the controlling terminal has changed its size.
SIGIO Sent to a process when I/O is available.
SIGPOLL Synonym for SIGIO
SIGLOST Sent to a process when a file lock has been lost.
SIGPWR Sent to a process to notify of a power failure.
SIGINFO Synonym for SIGPWR
SIGSYS Sent to a process to notify of a bad argument.
SIGUNUSED Synonym for SIGSYS

Error constants#

The following error constants are exported by os.constants.errno.

POSIX error constants#
Constant Description
E2BIG Indicates that the list of arguments is longer than expected.
EACCES Indicates that the operation did not have sufficient permissions.
EADDRINUSE Indicates that the network address is already in use.
EADDRNOTAVAIL Indicates that the network address is currently unavailable for use.
EAFNOSUPPORT Indicates that the network address family is not supported.
EAGAIN Indicates that there is no data available and to try the operation again later.
EALREADY Indicates that the socket already has a pending connection in progress.
EBADF Indicates that a file descriptor is not valid.
EBADMSG Indicates an invalid data message.
EBUSY Indicates that a device or resource is busy.
ECANCELED Indicates that an operation was canceled.
ECHILD Indicates that there are no child processes.
ECONNABORTED Indicates that the network connection has been aborted.
ECONNREFUSED Indicates that the network connection has been refused.
ECONNRESET Indicates that the network connection has been reset.
EDEADLK Indicates that a resource deadlock has been avoided.
EDESTADDRREQ Indicates that a destination address is required.
EDOM Indicates that an argument is out of the domain of the function.
EDQUOT Indicates that the disk quota has been exceeded.
EEXIST Indicates that the file already exists.
EFAULT Indicates an invalid pointer address.
EFBIG Indicates that the file is too large.
EHOSTUNREACH Indicates that the host is unreachable.
EIDRM Indicates that the identifier has been removed.
EILSEQ Indicates an illegal byte sequence.
EINPROGRESS Indicates that an operation is already in progress.
EINTR Indicates that a function call was interrupted.
EINVAL Indicates that an invalid argument was provided.
EIO Indicates an otherwise unspecified I/O error.
EISCONN Indicates that the socket is connected.
EISDIR Indicates that the path is a directory.
ELOOP Indicates too many levels of symbolic links in a path.
EMFILE Indicates that there are too many open files.
EMLINK Indicates that there are too many hard links to a file.
EMSGSIZE Indicates that the provided message is too long.
EMULTIHOP Indicates that a multihop was attempted.
ENAMETOOLONG Indicates that the filename is too long.
ENETDOWN Indicates that the network is down.
ENETRESET Indicates that the connection has been aborted by the network.
ENETUNREACH Indicates that the network is unreachable.
ENFILE Indicates too many open files in the system.
ENOBUFS Indicates that no buffer space is available.
ENODATA Indicates that no message is available on the stream head read queue.
ENODEV Indicates that there is no such device.
ENOENT Indicates that there is no such file or directory.
ENOEXEC Indicates an exec format error.
ENOLCK Indicates that there are no locks available.
ENOLINK Indications that a link has been severed.
ENOMEM Indicates that there is not enough space.
ENOMSG Indicates that there is no message of the desired type.
ENOPROTOOPT Indicates that a given protocol is not available.
ENOSPC Indicates that there is no space available on the device.
ENOSR Indicates that there are no stream resources available.
ENOSTR Indicates that a given resource is not a stream.
ENOSYS Indicates that a function has not been implemented.
ENOTCONN Indicates that the socket is not connected.
ENOTDIR Indicates that the path is not a directory.
ENOTEMPTY Indicates that the directory is not empty.
ENOTSOCK Indicates that the given item is not a socket.
ENOTSUP Indicates that a given operation is not supported.
ENOTTY Indicates an inappropriate I/O control operation.
ENXIO Indicates no such device or address.
EOPNOTSUPP Indicates that an operation is not supported on the socket. Although ENOTSUP and EOPNOTSUPP have the same value on Linux, according to POSIX.1 these error values should be distinct.)
EOVERFLOW Indicates that a value is too large to be stored in a given data type.
EPERM Indicates that the operation is not permitted.
EPIPE Indicates a broken pipe.
EPROTO Indicates a protocol error.
EPROTONOSUPPORT Indicates that a protocol is not supported.
EPROTOTYPE Indicates the wrong type of protocol for a socket.
ERANGE Indicates that the results are too large.
EROFS Indicates that the file system is read only.
ESPIPE Indicates an invalid seek operation.
ESRCH Indicates that there is no such process.
ESTALE Indicates that the file handle is stale.
ETIME Indicates an expired timer.
ETIMEDOUT Indicates that the connection timed out.
ETXTBSY Indicates that a text file is busy.
EWOULDBLOCK Indicates that the operation would block.
EXDEV Indicates an improper link.
Windows-specific error constants#

The following error codes are specific to the Windows operating system.

Constant Description
WSAEINTR Indicates an interrupted function call.
WSAEBADF Indicates an invalid file handle.
WSAEACCES Indicates insufficient permissions to complete the operation.
WSAEFAULT Indicates an invalid pointer address.
WSAEINVAL Indicates that an invalid argument was passed.
WSAEMFILE Indicates that there are too many open files.
WSAEWOULDBLOCK Indicates that a resource is temporarily unavailable.
WSAEINPROGRESS Indicates that an operation is currently in progress.
WSAEALREADY Indicates that an operation is already in progress.
WSAENOTSOCK Indicates that the resource is not a socket.
WSAEDESTADDRREQ Indicates that a destination address is required.
WSAEMSGSIZE Indicates that the message size is too long.
WSAEPROTOTYPE Indicates the wrong protocol type for the socket.
WSAENOPROTOOPT Indicates a bad protocol option.
WSAEPROTONOSUPPORT Indicates that the protocol is not supported.
WSAESOCKTNOSUPPORT Indicates that the socket type is not supported.
WSAEOPNOTSUPP Indicates that the operation is not supported.
WSAEPFNOSUPPORT Indicates that the protocol family is not supported.
WSAEAFNOSUPPORT Indicates that the address family is not supported.
WSAEADDRINUSE Indicates that the network address is already in use.
WSAEADDRNOTAVAIL Indicates that the network address is not available.
WSAENETDOWN Indicates that the network is down.
WSAENETUNREACH Indicates that the network is unreachable.
WSAENETRESET Indicates that the network connection has been reset.
WSAECONNABORTED Indicates that the connection has been aborted.
WSAECONNRESET Indicates that the connection has been reset by the peer.
WSAENOBUFS Indicates that there is no buffer space available.
WSAEISCONN Indicates that the socket is already connected.
WSAENOTCONN Indicates that the socket is not connected.
WSAESHUTDOWN Indicates that data cannot be sent after the socket has been shutdown.
WSAETOOMANYREFS Indicates that there are too many references.
WSAETIMEDOUT Indicates that the connection has timed out.
WSAECONNREFUSED Indicates that the connection has been refused.
WSAELOOP Indicates that a name cannot be translated.
WSAENAMETOOLONG Indicates that a name was too long.
WSAEHOSTDOWN Indicates that a network host is down.
WSAEHOSTUNREACH Indicates that there is no route to a network host.
WSAENOTEMPTY Indicates that the directory is not empty.
WSAEPROCLIM Indicates that there are too many processes.
WSAEUSERS Indicates that the user quota has been exceeded.
WSAEDQUOT Indicates that the disk quota has been exceeded.
WSAESTALE Indicates a stale file handle reference.
WSAEREMOTE Indicates that the item is remote.
WSASYSNOTREADY Indicates that the network subsystem is not ready.
WSAVERNOTSUPPORTED Indicates that the winsock.dll version is out of range.
WSANOTINITIALISED Indicates that successful WSAStartup has not yet been performed.
WSAEDISCON Indicates that a graceful shutdown is in progress.
WSAENOMORE Indicates that there are no more results.
WSAECANCELLED Indicates that an operation has been canceled.
WSAEINVALIDPROCTABLE Indicates that the procedure call table is invalid.
WSAEINVALIDPROVIDER Indicates an invalid service provider.
WSAEPROVIDERFAILEDINIT Indicates that the service provider failed to initialized.
WSASYSCALLFAILURE Indicates a system call failure.
WSASERVICE_NOT_FOUND Indicates that a service was not found.
WSATYPE_NOT_FOUND Indicates that a class type was not found.
WSA_E_NO_MORE Indicates that there are no more results.
WSA_E_CANCELLED Indicates that the call was canceled.
WSAEREFUSED Indicates that a database query was refused.

dlopen constants#

If available on the operating system, the following constants are exported in os.constants.dlopen. See dlopen(3) for detailed information.

Constant Description
RTLD_LAZY Perform lazy binding. Node.js sets this flag by default.
RTLD_NOW Resolve all undefined symbols in the library before dlopen(3) returns.
RTLD_GLOBAL Symbols defined by the library will be made available for symbol resolution of subsequently loaded libraries.
RTLD_LOCAL The converse of RTLD_GLOBAL. This is the default behavior if neither flag is specified.
RTLD_DEEPBIND Make a self-contained library use its own symbols in preference to symbols from previously loaded libraries.

Priority constants#

Added in: v10.10.0

The following process scheduling constants are exported by os.constants.priority.

Constant Description
PRIORITY_LOW The lowest process scheduling priority. This corresponds to IDLE_PRIORITY_CLASS on Windows, and a nice value of 19 on all other platforms.
PRIORITY_BELOW_NORMAL The process scheduling priority above PRIORITY_LOW and below PRIORITY_NORMAL. This corresponds to BELOW_NORMAL_PRIORITY_CLASS on Windows, and a nice value of 10 on all other platforms.
PRIORITY_NORMAL The default process scheduling priority. This corresponds to NORMAL_PRIORITY_CLASS on Windows, and a nice value of 0 on all other platforms.
PRIORITY_ABOVE_NORMAL The process scheduling priority above PRIORITY_NORMAL and below PRIORITY_HIGH. This corresponds to ABOVE_NORMAL_PRIORITY_CLASS on Windows, and a nice value of -7 on all other platforms.
PRIORITY_HIGH The process scheduling priority above PRIORITY_ABOVE_NORMAL and below PRIORITY_HIGHEST. This corresponds to HIGH_PRIORITY_CLASS on Windows, and a nice value of -14 on all other platforms.
PRIORITY_HIGHEST The highest process scheduling priority. This corresponds to REALTIME_PRIORITY_CLASS on Windows, and a nice value of -20 on all other platforms.

libuv constants#

Constant Description
UV_UDP_REUSEADDR

Path#

Stability: 2 - Stable

Source Code: lib/path.js

The node:path module provides utilities for working with file and directory paths. It can be accessed using:

const path = require('node:path');

Windows vs. POSIX#

The default operation of the node:path module varies based on the operating system on which a Node.js application is running. Specifically, when running on a Windows operating system, the node:path module will assume that Windows-style paths are being used.

So using path.basename() might yield different results on POSIX and Windows:

On POSIX:

path.basename('C:\\temp\\myfile.html');
// Returns: 'C:\\temp\\myfile.html'

On Windows:

path.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with Windows file paths on any operating system, use path.win32:

On POSIX and Windows:

path.win32.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with POSIX file paths on any operating system, use path.posix:

On POSIX and Windows:

path.posix.basename('/tmp/myfile.html');
// Returns: 'myfile.html'

On Windows Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example, path.resolve('C:\\') can potentially return a different result than path.resolve('C:'). For more information, see this MSDN page.

path.basename(path[, suffix])#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.25

Added in: v0.1.25

The path.basename() method returns the last portion of a path, similar to the Unix basename command. Trailing directory separators are ignored.

path.basename('/foo/bar/baz/asdf/quux.html');
// Returns: 'quux.html'

path.basename('/foo/bar/baz/asdf/quux.html', '.html');
// Returns: 'quux'

Although Windows usually treats file names, including file extensions, in a case-insensitive manner, this function does not. For example, C:\\foo.html and C:\\foo.HTML refer to the same file, but basename treats the extension as a case-sensitive string:

path.win32.basename('C:\\foo.html', '.html');
// Returns: 'foo'

path.win32.basename('C:\\foo.HTML', '.html');
// Returns: 'foo.HTML'

A TypeError is thrown if path is not a string or if suffix is given and is not a string.

path.delimiter#

Added in: v0.9.3

Provides the platform-specific path delimiter:

  • ; for Windows
  • : for POSIX

For example, on POSIX:

console.log(process.env.PATH);
// Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'

process.env.PATH.split(path.delimiter);
// Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']

On Windows:

console.log(process.env.PATH);
// Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'

process.env.PATH.split(path.delimiter);
// Returns ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']

path.dirname(path)#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.16

Added in: v0.1.16

The path.dirname() method returns the directory name of a path, similar to the Unix dirname command. Trailing directory separators are ignored, see path.sep.

path.dirname('/foo/bar/baz/asdf/quux');
// Returns: '/foo/bar/baz/asdf'

A TypeError is thrown if path is not a string.

path.extname(path)#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.25

Added in: v0.1.25

The path.extname() method returns the extension of the path, from the last occurrence of the . (period) character to end of string in the last portion of the path. If there is no . in the last portion of the path, or if there are no . characters other than the first character of the basename of path (see path.basename()) , an empty string is returned.

path.extname('index.html');
// Returns: '.html'

path.extname('index.coffee.md');
// Returns: '.md'

path.extname('index.');
// Returns: '.'

path.extname('index');
// Returns: ''

path.extname('.index');
// Returns: ''

path.extname('.index.md');
// Returns: '.md'

A TypeError is thrown if path is not a string.

path.format(pathObject)#

History
VersionChanges
v19.0.0

The dot will be added if it is not specified in ext.

v0.11.15

Added in: v0.11.15

The path.format() method returns a path string from an object. This is the opposite of path.parse().

When providing properties to the pathObject remember that there are combinations where one property has priority over another:

  • pathObject.root is ignored if pathObject.dir is provided
  • pathObject.ext and pathObject.name are ignored if pathObject.base exists

For example, on POSIX:

// If `dir`, `root` and `base` are provided,
// `${dir}${path.sep}${base}`
// will be returned. `root` is ignored.
path.format({
  root: '/ignored',
  dir: '/home/user/dir',
  base: 'file.txt',
});
// Returns: '/home/user/dir/file.txt'

// `root` will be used if `dir` is not specified.
// If only `root` is provided or `dir` is equal to `root` then the
// platform separator will not be included. `ext` will be ignored.
path.format({
  root: '/',
  base: 'file.txt',
  ext: 'ignored',
});
// Returns: '/file.txt'

// `name` + `ext` will be used if `base` is not specified.
path.format({
  root: '/',
  name: 'file',
  ext: '.txt',
});
// Returns: '/file.txt'

// The dot will be added if it is not specified in `ext`.
path.format({
  root: '/',
  name: 'file',
  ext: 'txt',
});
// Returns: '/file.txt'

On Windows:

path.format({
  dir: 'C:\\path\\dir',
  base: 'file.txt',
});
// Returns: 'C:\\path\\dir\\file.txt'

path.isAbsolute(path)#

Added in: v0.11.2

The path.isAbsolute() method determines if path is an absolute path.

If the given path is a zero-length string, false will be returned.

For example, on POSIX:

path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..');  // true
path.isAbsolute('qux/');     // false
path.isAbsolute('.');        // false

On Windows:

path.isAbsolute('//server');    // true
path.isAbsolute('\\\\server');  // true
path.isAbsolute('C:/foo/..');   // true
path.isAbsolute('C:\\foo\\..'); // true
path.isAbsolute('bar\\baz');    // false
path.isAbsolute('bar/baz');     // false
path.isAbsolute('.');           // false

A TypeError is thrown if path is not a string.

path.join([...paths])#

Added in: v0.1.16

The path.join() method joins all given path segments together using the platform-specific separator as a delimiter, then normalizes the resulting path.

Zero-length path segments are ignored. If the joined path string is a zero-length string then '.' will be returned, representing the current working directory.

path.join('/foo', 'bar', 'baz/asdf', 'quux', '..');
// Returns: '/foo/bar/baz/asdf'

path.join('foo', {}, 'bar');
// Throws 'TypeError: Path must be a string. Received {}'

A TypeError is thrown if any of the path segments is not a string.

path.normalize(path)#

Added in: v0.1.23

The path.normalize() method normalizes the given path, resolving '..' and '.' segments.

When multiple, sequential path segment separation characters are found (e.g. / on POSIX and either \ or / on Windows), they are replaced by a single instance of the platform-specific path segment separator (/ on POSIX and \ on Windows). Trailing separators are preserved.

If the path is a zero-length string, '.' is returned, representing the current working directory.

For example, on POSIX:

path.normalize('/foo/bar//baz/asdf/quux/..');
// Returns: '/foo/bar/baz/asdf'

On Windows:

path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'

Since Windows recognizes multiple path separators, both separators will be replaced by instances of the Windows preferred separator (\):

path.win32.normalize('C:////temp\\\\/\\/\\/foo/bar');
// Returns: 'C:\\temp\\foo\\bar'

A TypeError is thrown if path is not a string.

path.parse(path)#

Added in: v0.11.15

The path.parse() method returns an object whose properties represent significant elements of the path. Trailing directory separators are ignored, see path.sep.

The returned object will have the following properties:

For example, on POSIX:

path.parse('/home/user/dir/file.txt');
// Returns:
// { root: '/',
//   dir: '/home/user/dir',
//   base: 'file.txt',
//   ext: '.txt',
//   name: 'file' }
┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
"  /    home/user/dir / file  .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)

On Windows:

path.parse('C:\\path\\dir\\file.txt');
// Returns:
// { root: 'C:\\',
//   dir: 'C:\\path\\dir',
//   base: 'file.txt',
//   ext: '.txt',
//   name: 'file' }
┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
" C:\      path\dir   \ file  .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)

A TypeError is thrown if path is not a string.

path.posix#

History
VersionChanges
v15.3.0

Exposed as require('path/posix').

v0.11.15

Added in: v0.11.15

The path.posix property provides access to POSIX specific implementations of the path methods.

The API is accessible via require('node:path').posix or require('node:path/posix').

path.relative(from, to)#

History
VersionChanges
v6.8.0

On Windows, the leading slashes for UNC paths are now included in the return value.

v0.5.0

Added in: v0.5.0

The path.relative() method returns the relative path from from to to based on the current working directory. If from and to each resolve to the same path (after calling path.resolve() on each), a zero-length string is returned.

If a zero-length string is passed as from or to, the current working directory will be used instead of the zero-length strings.

For example, on POSIX:

path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb');
// Returns: '../../impl/bbb'

On Windows:

path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb');
// Returns: '..\\..\\impl\\bbb'

A TypeError is thrown if either from or to is not a string.

path.resolve([...paths])#

Added in: v0.3.4

The path.resolve() method resolves a sequence of paths or path segments into an absolute path.

The given sequence of paths is processed from right to left, with each subsequent path prepended until an absolute path is constructed. For instance, given the sequence of path segments: /foo, /bar, baz, calling path.resolve('/foo', '/bar', 'baz') would return /bar/baz because 'baz' is not an absolute path but '/bar' + '/' + 'baz' is.

If, after processing all given path segments, an absolute path has not yet been generated, the current working directory is used.

The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.

Zero-length path segments are ignored.

If no path segments are passed, path.resolve() will return the absolute path of the current working directory.

path.resolve('/foo/bar', './baz');
// Returns: '/foo/bar/baz'

path.resolve('/foo/bar', '/tmp/file/');
// Returns: '/tmp/file'

path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif');
// If the current working directory is /home/myself/node,
// this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'

A TypeError is thrown if any of the arguments is not a string.

path.sep#

Added in: v0.7.9

Provides the platform-specific path segment separator:

  • \ on Windows
  • / on POSIX

For example, on POSIX:

'foo/bar/baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']

On Windows:

'foo\\bar\\baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']

On Windows, both the forward slash (/) and backward slash (\) are accepted as path segment separators; however, the path methods only add backward slashes (\).

path.toNamespacedPath(path)#

Added in: v9.0.0

On Windows systems only, returns an equivalent namespace-prefixed path for the given path. If path is not a string, path will be returned without modifications.

This method is meaningful only on Windows systems. On POSIX systems, the method is non-operational and always returns path without modifications.

path.win32#

History
VersionChanges
v15.3.0

Exposed as require('path/win32').

v0.11.15

Added in: v0.11.15

The path.win32 property provides access to Windows-specific implementations of the path methods.

The API is accessible via require('node:path').win32 or require('node:path/win32').

Performance measurement APIs#

Stability: 2 - Stable

Source Code: lib/perf_hooks.js

This module provides an implementation of a subset of the W3C Web Performance APIs as well as additional APIs for Node.js-specific performance measurements.

Node.js supports the following Web Performance APIs:

const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});

perf_hooks.performance#

Added in: v8.5.0

An object that can be used to collect performance metrics from the current Node.js instance. It is similar to window.performance in browsers.

performance.clearMarks([name])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v8.5.0

Added in: v8.5.0

If name is not provided, removes all PerformanceMark objects from the Performance Timeline. If name is provided, removes only the named mark.

performance.clearMeasures([name])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.7.0

Added in: v16.7.0

If name is not provided, removes all PerformanceMeasure objects from the Performance Timeline. If name is provided, removes only the named measure.

performance.clearResourceTimings([name])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

If name is not provided, removes all PerformanceResourceTiming objects from the Resource Timeline. If name is provided, removes only the named resource.

performance.eventLoopUtilization([utilization1[, utilization2]])#

Added in: v14.10.0, v12.19.0
  • utilization1 <Object> The result of a previous call to eventLoopUtilization().
  • utilization2 <Object> The result of a previous call to eventLoopUtilization() prior to utilization1.
  • Returns <Object>

The eventLoopUtilization() method returns an object that contains the cumulative duration of time the event loop has been both idle and active as a high resolution milliseconds timer. The utilization value is the calculated Event Loop Utilization (ELU).

If bootstrapping has not yet finished on the main thread the properties have the value of 0. The ELU is immediately available on Worker threads since bootstrap happens within the event loop.

Both utilization1 and utilization2 are optional parameters.

If utilization1 is passed, then the delta between the current call's active and idle times, as well as the corresponding utilization value are calculated and returned (similar to process.hrtime()).

If utilization1 and utilization2 are both passed, then the delta is calculated between the two arguments. This is a convenience option because, unlike process.hrtime(), calculating the ELU is more complex than a single subtraction.

ELU is similar to CPU utilization, except that it only measures event loop statistics and not CPU usage. It represents the percentage of time the event loop has spent outside the event loop's event provider (e.g. epoll_wait). No other CPU idle time is taken into consideration. The following is an example of how a mostly idle process will have a high ELU.

'use strict';
const { eventLoopUtilization } = require('node:perf_hooks').performance;
const { spawnSync } = require('node:child_process');

setImmediate(() => {
  const elu = eventLoopUtilization();
  spawnSync('sleep', ['5']);
  console.log(eventLoopUtilization(elu).utilization);
});

Although the CPU is mostly idle while running this script, the value of utilization is 1. This is because the call to child_process.spawnSync() blocks the event loop from proceeding.

Passing in a user-defined object instead of the result of a previous call to eventLoopUtilization() will lead to undefined behavior. The return values are not guaranteed to reflect any correct state of the event loop.

performance.getEntries()#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.7.0

Added in: v16.7.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime. If you are only interested in performance entries of certain types or that have certain names, see performance.getEntriesByType() and performance.getEntriesByName().

performance.getEntriesByName(name[, type])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.7.0

Added in: v16.7.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.name is equal to name, and optionally, whose performanceEntry.entryType is equal to type.

performance.getEntriesByType(type)#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.7.0

Added in: v16.7.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.entryType is equal to type.

performance.mark([name[, options]])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.0.0

Updated to conform to the User Timing Level 3 specification.

v8.5.0

Added in: v8.5.0

  • name <string>
  • options <Object>
    • detail <any> Additional optional detail to include with the mark.
    • startTime <number> An optional timestamp to be used as the mark time. Default: performance.now().

Creates a new PerformanceMark entry in the Performance Timeline. A PerformanceMark is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'mark', and whose performanceEntry.duration is always 0. Performance marks are used to mark specific significant moments in the Performance Timeline.

The created PerformanceMark entry is put in the global Performance Timeline and can be queried with performance.getEntries, performance.getEntriesByName, and performance.getEntriesByType. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with performance.clearMarks.

performance.markResourceTiming(timingInfo, requestedUrl, initiatorType, global, cacheMode)#

Added in: v18.2.0, v16.17.0

This property is an extension by Node.js. It is not available in Web browsers.

Creates a new PerformanceResourceTiming entry in the Resource Timeline. A PerformanceResourceTiming is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'resource'. Performance resources are used to mark moments in the Resource Timeline.

The created PerformanceMark entry is put in the global Resource Timeline and can be queried with performance.getEntries, performance.getEntriesByName, and performance.getEntriesByType. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with performance.clearResourceTimings.

performance.measure(name[, startMarkOrOptions[, endMark]])#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.0.0

Updated to conform to the User Timing Level 3 specification.

v13.13.0, v12.16.3

Make startMark and endMark parameters optional.

v8.5.0

Added in: v8.5.0

  • name <string>
  • startMarkOrOptions <string> | <Object> Optional.
    • detail <any> Additional optional detail to include with the measure.
    • duration <number> Duration between start and end times.
    • end <number> | <string> Timestamp to be used as the end time, or a string identifying a previously recorded mark.
    • start <number> | <string> Timestamp to be used as the start time, or a string identifying a previously recorded mark.
  • endMark <string> Optional. Must be omitted if startMarkOrOptions is an <Object>.

Creates a new PerformanceMeasure entry in the Performance Timeline. A PerformanceMeasure is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'measure', and whose performanceEntry.duration measures the number of milliseconds elapsed since startMark and endMark.

The startMark argument may identify any existing PerformanceMark in the Performance Timeline, or may identify any of the timestamp properties provided by the PerformanceNodeTiming class. If the named startMark does not exist, an error is thrown.

The optional endMark argument must identify any existing PerformanceMark in the Performance Timeline or any of the timestamp properties provided by the PerformanceNodeTiming class. endMark will be performance.now() if no parameter is passed, otherwise if the named endMark does not exist, an error will be thrown.

The created PerformanceMeasure entry is put in the global Performance Timeline and can be queried with performance.getEntries, performance.getEntriesByName, and performance.getEntriesByType. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with performance.clearMeasures.

performance.nodeTiming#

Added in: v8.5.0

This property is an extension by Node.js. It is not available in Web browsers.

An instance of the PerformanceNodeTiming class that provides performance metrics for specific Node.js operational milestones.

performance.now()#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v8.5.0

Added in: v8.5.0

Returns the current high resolution millisecond timestamp, where 0 represents the start of the current node process.

performance.setResourceTimingBufferSize(maxSize)#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v18.8.0

Added in: v18.8.0

Sets the global performance resource timing buffer size to the specified number of "resource" type performance entry objects.

By default the max buffer size is set to 250.

performance.timeOrigin#

Added in: v8.5.0

The timeOrigin specifies the high resolution millisecond timestamp at which the current node process began, measured in Unix time.

performance.timerify(fn[, options])#

History
VersionChanges
v16.0.0

Added the histogram option.

v16.0.0

Re-implemented to use pure-JavaScript and the ability to time async functions.

v8.5.0

Added in: v8.5.0

This property is an extension by Node.js. It is not available in Web browsers.

Wraps a function within a new function that measures the running time of the wrapped function. A PerformanceObserver must be subscribed to the 'function' event type in order for the timing details to be accessed.

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

function someFunction() {
  console.log('hello world');
}

const wrapped = performance.timerify(someFunction);

const obs = new PerformanceObserver((list) => {
  console.log(list.getEntries()[0].duration);

  performance.clearMarks();
  performance.clearMeasures();
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

// A performance timeline entry will be created
wrapped();

If the wrapped function returns a promise, a finally handler will be attached to the promise and the duration will be reported once the finally handler is invoked.

performance.toJSON()#

History
VersionChanges
v19.0.0

This method must be called with the performance object as the receiver.

v16.1.0

Added in: v16.1.0

An object which is JSON representation of the performance object. It is similar to window.performance.toJSON in browsers.

Event: 'resourcetimingbufferfull'#
Added in: v18.8.0

The 'resourcetimingbufferfull' event is fired when the global performance resource timing buffer is full. Adjust resource timing buffer size with performance.setResourceTimingBufferSize() or clear the buffer with performance.clearResourceTimings() in the event listener to allow more entries to be added to the performance timeline buffer.

Class: PerformanceEntry#

Added in: v8.5.0

The constructor of this class is not exposed to users directly.

performanceEntry.duration#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceEntry object as the receiver.

v8.5.0

Added in: v8.5.0

The total number of milliseconds elapsed for this entry. This value will not be meaningful for all Performance Entry types.

performanceEntry.entryType#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceEntry object as the receiver.

v8.5.0

Added in: v8.5.0

The type of the performance entry. It may be one of:

  • 'node' (Node.js only)
  • 'mark' (available on the Web)
  • 'measure' (available on the Web)
  • 'gc' (Node.js only)
  • 'function' (Node.js only)
  • 'http2' (Node.js only)
  • 'http' (Node.js only)

performanceEntry.name#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceEntry object as the receiver.

v8.5.0

Added in: v8.5.0

The name of the performance entry.

performanceEntry.startTime#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceEntry object as the receiver.

v8.5.0

Added in: v8.5.0

The high resolution millisecond timestamp marking the starting time of the Performance Entry.

Class: PerformanceMark#

Added in: v18.2.0, v16.17.0

Exposes marks created via the Performance.mark() method.

performanceMark.detail#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceMark object as the receiver.

v16.0.0

Added in: v16.0.0

Additional detail specified when creating with Performance.mark() method.

Class: PerformanceMeasure#

Added in: v18.2.0, v16.17.0

Exposes measures created via the Performance.measure() method.

The constructor of this class is not exposed to users directly.

performanceMeasure.detail#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceMeasure object as the receiver.

v16.0.0

Added in: v16.0.0

Additional detail specified when creating with Performance.measure() method.

Class: PerformanceNodeEntry#

Added in: v19.0.0

This class is an extension by Node.js. It is not available in Web browsers.

Provides detailed Node.js timing data.

The constructor of this class is not exposed to users directly.

performanceNodeEntry.detail#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceNodeEntry object as the receiver.

v16.0.0

Added in: v16.0.0

Additional detail specific to the entryType.

performanceNodeEntry.flags#

History
VersionChanges
v16.0.0

Runtime deprecated. Now moved to the detail property when entryType is 'gc'.

v13.9.0, v12.17.0

Added in: v13.9.0, v12.17.0

Stability: 0 - Deprecated: Use performanceNodeEntry.detail instead.

When performanceEntry.entryType is equal to 'gc', the performance.flags property contains additional information about garbage collection operation. The value may be one of:

  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE

performanceNodeEntry.kind#

History
VersionChanges
v16.0.0

Runtime deprecated. Now moved to the detail property when entryType is 'gc'.

v8.5.0

Added in: v8.5.0

Stability: 0 - Deprecated: Use performanceNodeEntry.detail instead.

When performanceEntry.entryType is equal to 'gc', the performance.kind property identifies the type of garbage collection operation that occurred. The value may be one of:

  • perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
  • perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
  • perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
  • perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB

Garbage Collection ('gc') Details#

When performanceEntry.type is equal to 'gc', the performanceNodeEntry.detail property will be an <Object> with two properties:

  • kind <number> One of:
    • perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
    • perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
    • perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
    • perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB
  • flags <number> One of:
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY
    • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE

HTTP ('http') Details#

When performanceEntry.type is equal to 'http', the performanceNodeEntry.detail property will be an <Object> containing additional information.

If performanceEntry.name is equal to HttpClient, the detail will contain the following properties: req, res. And the req property will be an <Object> containing method, url, headers, the res property will be an <Object> containing statusCode, statusMessage, headers.

If performanceEntry.name is equal to HttpRequest, the detail will contain the following properties: req, res. And the req property will be an <Object> containing method, url, headers, the res property will be an <Object> containing statusCode, statusMessage, headers.

This could add additional memory overhead and should only be used for diagnostic purposes, not left turned on in production by default.

HTTP/2 ('http2') Details#

When performanceEntry.type is equal to 'http2', the performanceNodeEntry.detail property will be an <Object> containing additional performance information.

If performanceEntry.name is equal to Http2Stream, the detail will contain the following properties:

  • bytesRead <number> The number of DATA frame bytes received for this Http2Stream.
  • bytesWritten <number> The number of DATA frame bytes sent for this Http2Stream.
  • id <number> The identifier of the associated Http2Stream
  • timeToFirstByte <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first DATA frame.
  • timeToFirstByteSent <number> The number of milliseconds elapsed between the PerformanceEntry startTime and sending of the first DATA frame.
  • timeToFirstHeader <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first header.

If performanceEntry.name is equal to Http2Session, the detail will contain the following properties:

  • bytesRead <number> The number of bytes received for this Http2Session.
  • bytesWritten <number> The number of bytes sent for this Http2Session.
  • framesReceived <number> The number of HTTP/2 frames received by the Http2Session.
  • framesSent <number> The number of HTTP/2 frames sent by the Http2Session.
  • maxConcurrentStreams <number> The maximum number of streams concurrently open during the lifetime of the Http2Session.
  • pingRTT <number> The number of milliseconds elapsed since the transmission of a PING frame and the reception of its acknowledgment. Only present if a PING frame has been sent on the Http2Session.
  • streamAverageDuration <number> The average duration (in milliseconds) for all Http2Stream instances.
  • streamCount <number> The number of Http2Stream instances processed by the Http2Session.
  • type <string> Either 'server' or 'client' to identify the type of Http2Session.

Timerify ('function') Details#

When performanceEntry.type is equal to 'function', the performanceNodeEntry.detail property will be an <Array> listing the input arguments to the timed function.

Net ('net') Details#

When performanceEntry.type is equal to 'net', the performanceNodeEntry.detail property will be an <Object> containing additional information.

If performanceEntry.name is equal to connect, the detail will contain the following properties: host, port.

DNS ('dns') Details#

When performanceEntry.type is equal to 'dns', the performanceNodeEntry.detail property will be an <Object> containing additional information.

If performanceEntry.name is equal to lookup, the detail will contain the following properties: hostname, family, hints, verbatim, addresses.

If performanceEntry.name is equal to lookupService, the detail will contain the following properties: host, port, hostname, service.

If performanceEntry.name is equal to queryxxx or getHostByAddr, the detail will contain the following properties: host, ttl, result. The value of result is same as the result of queryxxx or getHostByAddr.

Class: PerformanceNodeTiming#

Added in: v8.5.0

This property is an extension by Node.js. It is not available in Web browsers.

Provides timing details for Node.js itself. The constructor of this class is not exposed to users.

performanceNodeTiming.bootstrapComplete#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js process completed bootstrapping. If bootstrapping has not yet finished, the property has the value of -1.

performanceNodeTiming.environment#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js environment was initialized.

performanceNodeTiming.idleTime#

Added in: v14.10.0, v12.19.0

The high resolution millisecond timestamp of the amount of time the event loop has been idle within the event loop's event provider (e.g. epoll_wait). This does not take CPU usage into consideration. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of 0.

performanceNodeTiming.loopExit#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js event loop exited. If the event loop has not yet exited, the property has the value of -1. It can only have a value of not -1 in a handler of the 'exit' event.

performanceNodeTiming.loopStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js event loop started. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of -1.

performanceNodeTiming.nodeStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js process was initialized.

performanceNodeTiming.v8Start#

Added in: v8.5.0

The high resolution millisecond timestamp at which the V8 platform was initialized.

Class: PerformanceResourceTiming#

Added in: v18.2.0, v16.17.0

Provides detailed network timing data regarding the loading of an application's resources.

The constructor of this class is not exposed to users directly.

performanceResourceTiming.workerStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp at immediately before dispatching the fetch request. If the resource is not intercepted by a worker the property will always return 0.

performanceResourceTiming.redirectStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp that represents the start time of the fetch which initiates the redirect.

performanceResourceTiming.redirectEnd#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp that will be created immediately after receiving the last byte of the response of the last redirect.

performanceResourceTiming.fetchStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp immediately before the Node.js starts to fetch the resource.

performanceResourceTiming.domainLookupStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp immediately before the Node.js starts the domain name lookup for the resource.

performanceResourceTiming.domainLookupEnd#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately after the Node.js finished the domain name lookup for the resource.

performanceResourceTiming.connectStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately before Node.js starts to establish the connection to the server to retrieve the resource.

performanceResourceTiming.connectEnd#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately after Node.js finishes establishing the connection to the server to retrieve the resource.

performanceResourceTiming.secureConnectionStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately before Node.js starts the handshake process to secure the current connection.

performanceResourceTiming.requestStart#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately before Node.js receives the first byte of the response from the server.

performanceResourceTiming.responseEnd#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

The high resolution millisecond timestamp representing the time immediately after Node.js receives the last byte of the resource or immediately before the transport connection is closed, whichever comes first.

performanceResourceTiming.transferSize#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body.

performanceResourceTiming.encodedBodySize#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

A number representing the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings.

performanceResourceTiming.decodedBodySize#

History
VersionChanges
v19.0.0

This property getter must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

A number representing the size (in octets) received from the fetch (HTTP or cache), of the message body, after removing any applied content-codings.

performanceResourceTiming.toJSON()#

History
VersionChanges
v19.0.0

This method must be called with the PerformanceResourceTiming object as the receiver.

v18.2.0, v16.17.0

Added in: v18.2.0, v16.17.0

Returns a object that is the JSON representation of the PerformanceResourceTiming object

Class: PerformanceObserver#

Added in: v8.5.0

PerformanceObserver.supportedEntryTypes#

Added in: v16.0.0

Get supported types.

new PerformanceObserver(callback)#

History
VersionChanges
v18.0.0

Passing an invalid callback to the callback argument now throws ERR_INVALID_ARG_TYPE instead of ERR_INVALID_CALLBACK.

v8.5.0

Added in: v8.5.0

PerformanceObserver objects provide notifications when new PerformanceEntry instances have been added to the Performance Timeline.

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());

  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });

performance.mark('test');

Because PerformanceObserver instances introduce their own additional performance overhead, instances should not be left subscribed to notifications indefinitely. Users should disconnect observers as soon as they are no longer needed.

The callback is invoked when a PerformanceObserver is notified about new PerformanceEntry instances. The callback receives a PerformanceObserverEntryList instance and a reference to the PerformanceObserver.

performanceObserver.disconnect()#

Added in: v8.5.0

Disconnects the PerformanceObserver instance from all notifications.

performanceObserver.observe(options)#

History
VersionChanges
v16.7.0

Updated to conform to Performance Timeline Level 2. The buffered option has been added back.

v16.0.0

Updated to conform to User Timing Level 3. The buffered option has been removed.

v8.5.0

Added in: v8.5.0

  • options <Object>
    • type <string> A single <PerformanceEntry> type. Must not be given if entryTypes is already specified.
    • entryTypes <string[]> An array of strings identifying the types of <PerformanceEntry> instances the observer is interested in. If not provided an error will be thrown.
    • buffered <boolean> If true, the observer callback is called with a list global PerformanceEntry buffered entries. If false, only PerformanceEntrys created after the time point are sent to the observer callback. Default: false.

Subscribes the <PerformanceObserver> instance to notifications of new <PerformanceEntry> instances identified either by options.entryTypes or options.type:

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // Called once asynchronously. `list` contains three items.
});
obs.observe({ type: 'mark' });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

Class: PerformanceObserverEntryList#

Added in: v8.5.0

The PerformanceObserverEntryList class is used to provide access to the PerformanceEntry instances passed to a PerformanceObserver. The constructor of this class is not exposed to users.

performanceObserverEntryList.getEntries()#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime.

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntries());
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 81.465639,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 81.860064,
   *     duration: 0
   *   }
   * ]
   */

  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ type: 'mark' });

performance.mark('test');
performance.mark('meow');

performanceObserverEntryList.getEntriesByName(name[, type])#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.name is equal to name, and optionally, whose performanceEntry.entryType is equal to type.

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntriesByName('meow'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 98.545991,
   *     duration: 0
   *   }
   * ]
   */
  console.log(perfObserverList.getEntriesByName('nope')); // []

  console.log(perfObserverList.getEntriesByName('test', 'mark'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 63.518931,
   *     duration: 0
   *   }
   * ]
   */
  console.log(perfObserverList.getEntriesByName('test', 'measure')); // []

  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark', 'measure'] });

performance.mark('test');
performance.mark('meow');

performanceObserverEntryList.getEntriesByType(type)#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.entryType is equal to type.

const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntriesByType('mark'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 55.897834,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 56.350146,
   *     duration: 0
   *   }
   * ]
   */
  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ type: 'mark' });

performance.mark('test');
performance.mark('meow');

perf_hooks.createHistogram([options])#

Added in: v15.9.0, v14.18.0
  • options <Object>
    • lowest <number> | <bigint> The lowest discernible value. Must be an integer value greater than 0. Default: 1.
    • highest <number> | <bigint> The highest recordable value. Must be an integer value that is equal to or greater than two times lowest. Default: Number.MAX_SAFE_INTEGER.
    • figures <number> The number of accuracy digits. Must be a number between 1 and 5. Default: 3.
  • Returns <RecordableHistogram>

Returns a <RecordableHistogram>.

perf_hooks.monitorEventLoopDelay([options])#

Added in: v11.10.0

This property is an extension by Node.js. It is not available in Web browsers.

Creates an IntervalHistogram object that samples and reports the event loop delay over time. The delays will be reported in nanoseconds.

Using a timer to detect approximate event loop delay works because the execution of timers is tied specifically to the lifecycle of the libuv event loop. That is, a delay in the loop will cause a delay in the execution of the timer, and those delays are specifically what this API is intended to detect.

const { monitorEventLoopDelay } = require('node:perf_hooks');
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
// Do something.
h.disable();
console.log(h.min);
console.log(h.max);
console.log(h.mean);
console.log(h.stddev);
console.log(h.percentiles);
console.log(h.percentile(50));
console.log(h.percentile(99));

Class: Histogram#

Added in: v11.10.0

histogram.count#

Added in: v17.4.0, v16.14.0

The number of samples recorded by the histogram.

histogram.countBigInt#

Added in: v17.4.0, v16.14.0

The number of samples recorded by the histogram.

histogram.exceeds#

Added in: v11.10.0

The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

histogram.exceedsBigInt#

Added in: v17.4.0, v16.14.0

The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

histogram.max#

Added in: v11.10.0

The maximum recorded event loop delay.

histogram.maxBigInt#

Added in: v17.4.0, v16.14.0

The maximum recorded event loop delay.

histogram.mean#

Added in: v11.10.0

The mean of the recorded event loop delays.

histogram.min#

Added in: v11.10.0

The minimum recorded event loop delay.

histogram.minBigInt#

Added in: v17.4.0, v16.14.0

The minimum recorded event loop delay.

histogram.percentile(percentile)#

Added in: v11.10.0
  • percentile <number> A percentile value in the range (0, 100].
  • Returns: <number>

Returns the value at the given percentile.

histogram.percentileBigInt(percentile)#

Added in: v17.4.0, v16.14.0
  • percentile <number> A percentile value in the range (0, 100].
  • Returns: <bigint>

Returns the value at the given percentile.

histogram.percentiles#

Added in: v11.10.0

Returns a Map object detailing the accumulated percentile distribution.

histogram.percentilesBigInt#

Added in: v17.4.0, v16.14.0

Returns a Map object detailing the accumulated percentile distribution.

histogram.reset()#

Added in: v11.10.0

Resets the collected histogram data.

histogram.stddev#

Added in: v11.10.0

The standard deviation of the recorded event loop delays.

Class: IntervalHistogram extends Histogram#

A Histogram that is periodically updated on a given interval.

histogram.disable()#

Added in: v11.10.0

Disables the update interval timer. Returns true if the timer was stopped, false if it was already stopped.

histogram.enable()#

Added in: v11.10.0

Enables the update interval timer. Returns true if the timer was started, false if it was already started.

Cloning an IntervalHistogram#

<IntervalHistogram> instances can be cloned via <MessagePort>. On the receiving end, the histogram is cloned as a plain <Histogram> object that does not implement the enable() and disable() methods.

Class: RecordableHistogram extends Histogram#

Added in: v15.9.0, v14.18.0

histogram.add(other)#

Added in: v17.4.0, v16.14.0

Adds the values from other to this histogram.

histogram.record(val)#

Added in: v15.9.0, v14.18.0

histogram.recordDelta()#

Added in: v15.9.0, v14.18.0

Calculates the amount of time (in nanoseconds) that has passed since the previous call to recordDelta() and records that amount in the histogram.

Examples#

Measuring the duration of async operations#

The following example uses the Async Hooks and Performance APIs to measure the actual duration of a Timeout operation (including the amount of time it took to execute the callback).

'use strict';
const async_hooks = require('node:async_hooks');
const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');

const set = new Set();
const hook = async_hooks.createHook({
  init(id, type) {
    if (type === 'Timeout') {
      performance.mark(`Timeout-${id}-Init`);
      set.add(id);
    }
  },
  destroy(id) {
    if (set.has(id)) {
      set.delete(id);
      performance.mark(`Timeout-${id}-Destroy`);
      performance.measure(`Timeout-${id}`,
                          `Timeout-${id}-Init`,
                          `Timeout-${id}-Destroy`);
    }
  },
});
hook.enable();

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries()[0]);
  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ entryTypes: ['measure'], buffered: true });

setTimeout(() => {}, 1000);

Measuring how long it takes to load dependencies#

The following example measures the duration of require() operations to load dependencies:

'use strict';
const {
  performance,
  PerformanceObserver,
} = require('node:perf_hooks');
const mod = require('node:module');

// Monkey patch the require function
mod.Module.prototype.require =
  performance.timerify(mod.Module.prototype.require);
require = performance.timerify(require);

// Activate the observer
const obs = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  entries.forEach((entry) => {
    console.log(`require('${entry[0]}')`, entry.duration);
  });
  performance.clearMarks();
  performance.clearMeasures();
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');

Measuring how long one HTTP round-trip takes#

The following example is used to trace the time spent by HTTP client (OutgoingMessage) and HTTP request (IncomingMessage). For HTTP client, it means the time interval between starting the request and receiving the response, and for HTTP request, it means the time interval between receiving the request and sending the response:

'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const http = require('node:http');

const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});

obs.observe({ entryTypes: ['http'] });

const PORT = 8080;

http.createServer((req, res) => {
  res.end('ok');
}).listen(PORT, () => {
  http.get(`http://127.0.0.1:${PORT}`);
});

Measuring how long the net.connect (only for TCP) takes when the connection is successful#

'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const net = require('node:net');
const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});
obs.observe({ entryTypes: ['net'] });
const PORT = 8080;
net.createServer((socket) => {
  socket.destroy();
}).listen(PORT, () => {
  net.connect(PORT);
});

Measuring how long the DNS takes when the request is successful#

'use strict';
const { PerformanceObserver } = require('node:perf_hooks');
const dns = require('node:dns');
const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});
obs.observe({ entryTypes: ['dns'] });
dns.lookup('localhost', () => {});
dns.promises.resolve('localhost');

Permissions#

Permissions can be used to control what system resources the Node.js process has access to or what actions the process can take with those resources. Permissions can also control what modules can be accessed by other modules.

  • Module-based permissions control which files or URLs are available to other modules during application execution. This can be used to control what modules can be accessed by third-party dependencies, for example.

If you find a potential security vulnerability, please refer to our Security Policy.

Module-based permissions#

Policies#

Stability: 1 - Experimental

Node.js contains experimental support for creating policies on loading code.

Policies are a security feature intended to allow guarantees about what code Node.js is able to load. The use of policies assumes safe practices for the policy files such as ensuring that policy files cannot be overwritten by the Node.js application by using file permissions.

A best practice would be to ensure that the policy manifest is read-only for the running Node.js application and that the file cannot be changed by the running Node.js application in any way. A typical setup would be to create the policy file as a different user id than the one running Node.js and granting read permissions to the user id running Node.js.

Enabling#

The --experimental-policy flag can be used to enable features for policies when loading modules.

Once this has been set, all modules must conform to a policy manifest file passed to the flag:

node --experimental-policy=policy.json app.js

The policy manifest will be used to enforce constraints on code loaded by Node.js.

To mitigate tampering with policy files on disk, an integrity for the policy file itself may be provided via --policy-integrity. This allows running node and asserting the policy file contents even if the file is changed on disk.

node --experimental-policy=policy.json --policy-integrity="sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0" app.js
Features#
Error behavior#

When a policy check fails, Node.js by default will throw an error. It is possible to change the error behavior to one of a few possibilities by defining an "onerror" field in a policy manifest. The following values are available to change the behavior:

  • "exit": will exit the process immediately. No cleanup code will be allowed to run.
  • "log": will log the error at the site of the failure.
  • "throw": will throw a JS error at the site of the failure. This is the default.
{
  "onerror": "log",
  "resources": {
    "./app/checked.js": {
      "integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
    }
  }
}
Integrity checks#

Policy files must use integrity checks with Subresource Integrity strings compatible with the browser integrity attribute associated with absolute URLs.

When using require() or import all resources involved in loading are checked for integrity if a policy manifest has been specified. If a resource does not match the integrity listed in the manifest, an error will be thrown.

An example policy file that would allow loading a file checked.js:

{
  "resources": {
    "./app/checked.js": {
      "integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
    }
  }
}

Each resource listed in the policy manifest can be of one the following formats to determine its location:

  1. A relative-URL string to a resource from the manifest such as ./resource.js, ../resource.js, or /resource.js.
  2. A complete URL string to a resource such as file:///resource.js.

When loading resources the entire URL must match including search parameters and hash fragment. ./a.js?b will not be used when attempting to load ./a.js and vice versa.

To generate integrity strings, a script such as node -e 'process.stdout.write("sha256-");process.stdin.pipe(crypto.createHash("sha256").setEncoding("base64")).pipe(process.stdout)' < FILE can be used.

Integrity can be specified as the boolean value true to accept any body for the resource which can be useful for local development. It is not recommended in production since it would allow unexpected alteration of resources to be considered valid.

Dependency redirection#

An application may need to ship patched versions of modules or to prevent modules from allowing all modules access to all other modules. Redirection can be used by intercepting attempts to load the modules wishing to be replaced.

{
  "resources": {
    "./app/checked.js": {
      "dependencies": {
        "fs": true,
        "os": "./app/node_modules/alt-os",
        "http": { "import": true }
      }
    }
  }
}

The dependencies are keyed by the requested specifier string and have values of either true, null, a string pointing to a module to be resolved, or a conditions object.

The specifier string does not perform any searching and must match exactly what is provided to the require() or import except for a canonicalization step. Therefore, multiple specifiers may be needed in the policy if it uses multiple different strings to point to the same module (such as excluding the extension).

Specifier strings are canonicalized but not resolved prior to be used for matching in order to have some compatibility with import maps, for example if a resource file:///C:/app/server.js was given the following redirection from a policy located at file:///C:/app/policy.json:

{
  "resources": {
    "file:///C:/app/utils.js": {
      "dependencies": {
        "./utils.js": "./utils-v2.js"
      }
    }
  }
}

Any specifier used to load file:///C:/app/utils.js would then be intercepted and redirected to file:///C:/app/utils-v2.js instead regardless of using an absolute or relative specifier. However, if a specifier that is not an absolute or relative URL string is used, it would not be intercepted. So, if an import such as import('#utils') was used, it would not be intercepted.

If the value of the redirection is true, a "dependencies" field at the top of the policy file will be used. If that field at the top of the policy file is true the default node searching algorithms are used to find the module.

If the value of the redirection is a string, it is resolved relative to the manifest and then immediately used without searching.

Any specifier string for which resolution is attempted and that is not listed in the dependencies results in an error according to the policy.

Redirection does not prevent access to APIs through means such as direct access to require.cache or through module.constructor which allow access to loading modules. Policy redirection only affects specifiers to require() and import. Other means, such as to prevent undesired access to APIs through variables, are necessary to lock down that path of loading modules.

A boolean value of true for the dependencies map can be specified to allow a module to load any specifier without redirection. This can be useful for local development and may have some valid usage in production, but should be used only with care after auditing a module to ensure its behavior is valid.

Similar to "exports" in package.json, dependencies can also be specified to be objects containing conditions which branch how dependencies are loaded. In the preceding example, "http" is allowed when the "import" condition is part of loading it.

A value of null for the resolved value causes the resolution to fail. This can be used to ensure some kinds of dynamic access are explicitly prevented.

Unknown values for the resolved module location cause failures but are not guaranteed to be forward compatible.

Example: Patched dependency#

Redirected dependencies can provide attenuated or modified functionality as fits the application. For example, log data about timing of function durations by wrapping the original:

const original = require('fn');
module.exports = function fn(...args) {
  console.time();
  try {
    return new.target ?
      Reflect.construct(original, args) :
      Reflect.apply(original, this, args);
  } finally {
    console.timeEnd();
  }
};
Scopes#

Use the "scopes" field of a manifest to set configuration for many resources at once. The "scopes" field works by matching resources by their segments. If a scope or resource includes "cascade": true, unknown specifiers will be searched for in their containing scope. The containing scope for cascading is found by recursively reducing the resource URL by removing segments for special schemes, keeping trailing "/" suffixes, and removing the query and hash fragment. This leads to the eventual reduction of the URL to its origin. If the URL is non-special the scope will be located by the URL's origin. If no scope is found for the origin or in the case of opaque origins, a protocol string can be used as a scope. If no scope is found for the URL's protocol, a final empty string "" scope will be used.

Note, blob: URLs adopt their origin from the path they contain, and so a scope of "blob:https://nodejs.org" will have no effect since no URL can have an origin of blob:https://nodejs.org; URLs starting with blob:https://nodejs.org/ will use https://nodejs.org for its origin and thus https: for its protocol scope. For opaque origin blob: URLs they will have blob: for their protocol scope since they do not adopt origins.

Example#
{
  "scopes": {
    "file:///C:/app/": {},
    "file:": {},
    "": {}
  }
}

Given a file located at file:///C:/app/bin/main.js, the following scopes would be checked in order:

  1. "file:///C:/app/bin/"

This determines the policy for all file based resources within "file:///C:/app/bin/". This is not in the "scopes" field of the policy and would be skipped. Adding this scope to the policy would cause it to be used prior to the "file:///C:/app/" scope.

  1. "file:///C:/app/"

This determines the policy for all file based resources within "file:///C:/app/". This is in the "scopes" field of the policy and it would determine the policy for the resource at file:///C:/app/bin/main.js. If the scope has "cascade": true, any unsatisfied queries about the resource would delegate to the next relevant scope for file:///C:/app/bin/main.js, "file:".

  1. "file:///C:/"

This determines the policy for all file based resources within "file:///C:/". This is not in the "scopes" field of the policy and would be skipped. It would not be used for file:///C:/app/bin/main.js unless "file:///" is set to cascade or is not in the "scopes" of the policy.

  1. "file:///"

This determines the policy for all file based resources on the localhost. This is not in the "scopes" field of the policy and would be skipped. It would not be used for file:///C:/app/bin/main.js unless "file:///" is set to cascade or is not in the "scopes" of the policy.

  1. "file:"

This determines the policy for all file based resources. It would not be used for file:///C:/app/bin/main.js unless "file:///" is set to cascade or is not in the "scopes" of the policy.

  1. ""

This determines the policy for all resources. It would not be used for file:///C:/app/bin/main.js unless "file:" is set to cascade.

Integrity using scopes#

Setting an integrity to true on a scope will set the integrity for any resource not found in the manifest to true.

Setting an integrity to null on a scope will set the integrity for any resource not found in the manifest to fail matching.

Not including an integrity is the same as setting the integrity to null.

"cascade" for integrity checks will be ignored if "integrity" is explicitly set.

The following example allows loading any file:

{
  "scopes": {
    "file:": {
      "integrity": true
    }
  }
}
Dependency redirection using scopes#

The following example, would allow access to fs for all resources within ./app/:

{
  "resources": {
    "./app/checked.js": {
      "cascade": true,
      "integrity": true
    }
  },
  "scopes": {
    "./app/": {
      "dependencies": {
        "fs": true
      }
    }
  }
}

The following example, would allow access to fs for all data: resources:

{
  "resources": {
    "data:text/javascript,import('node:fs');": {
      "cascade": true,
      "integrity": true
    }
  },
  "scopes": {
    "data:": {
      "dependencies": {
        "fs": true
      }
    }
  }
}
Example: import maps emulation#

Given an import map:

{
  "imports": {
    "react": "./app/node_modules/react/index.js"
  },
  "scopes": {
    "./ssr/": {
      "react": "./app/node_modules/server-side-react/index.js"
    }
  }
}
{
  "dependencies": true,
  "scopes": {
    "": {
      "cascade": true,
      "dependencies": {
        "react": "./app/node_modules/react/index.js"
      }
    },
    "./ssr/": {
      "cascade": true,
      "dependencies": {
        "react": "./app/node_modules/server-side-react/index.js"
      }
    }
  }
}

Import maps assume you can get any resource by default. This means "dependencies" at the top level of the policy should be set to true. Policies require this to be opt-in since it enables all resources of the application cross linkage which doesn't make sense for many scenarios. They also assume any given scope has access to any scope above its allowed dependencies; all scopes emulating import maps must set "cascade": true.

Import maps only have a single top level scope for their "imports". So for emulating "imports" use the "" scope. For emulating "scopes" use the "scopes" in a similar manner to how "scopes" works in import maps.

Caveats: Policies do not use string matching for various finding of scope. They do URL traversals. This means things like blob: and data: URLs might not be entirely interoperable between the two systems. For example import maps can partially match a data: or blob: URL by partitioning the URL on a / character, policies intentionally cannot. For blob: URLs import map scopes do not adopt the origin of the blob: URL.

Additionally, import maps only work on import so it may be desirable to add a "import" condition to all dependency mappings.

Process#

Source Code: lib/process.js

The process object provides information about, and control over, the current Node.js process.

import process from 'node:process';const process = require('node:process');

Process events#

The process object is an instance of EventEmitter.

Event: 'beforeExit'#

Added in: v0.11.12

The 'beforeExit' event is emitted when Node.js empties its event loop and has no additional work to schedule. Normally, the Node.js process will exit when there is no work scheduled, but a listener registered on the 'beforeExit' event can make asynchronous calls, and thereby cause the Node.js process to continue.

The listener callback function is invoked with the value of process.exitCode passed as the only argument.

The 'beforeExit' event is not emitted for conditions causing explicit termination, such as calling process.exit() or uncaught exceptions.

The 'beforeExit' should not be used as an alternative to the 'exit' event unless the intention is to schedule additional work.

import process from 'node:process';

process.on('beforeExit', (code) => {
  console.log('Process beforeExit event with code: ', code);
});

process.on('exit', (code) => {
  console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0const process = require('node:process');

process.on('beforeExit', (code) => {
  console.log('Process beforeExit event with code: ', code);
});

process.on('exit', (code) => {
  console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0

Event: 'disconnect'#

Added in: v0.7.7

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'disconnect' event will be emitted when the IPC channel is closed.

Event: 'exit'#

Added in: v0.1.7

The 'exit' event is emitted when the Node.js process is about to exit as a result of either:

  • The process.exit() method being called explicitly;
  • The Node.js event loop no longer having any additional work to perform.

There is no way to prevent the exiting of the event loop at this point, and once all 'exit' listeners have finished running the Node.js process will terminate.

The listener callback function is invoked with the exit code specified either by the process.exitCode property, or the exitCode argument passed to the process.exit() method.

import process from 'node:process';

process.on('exit', (code) => {
  console.log(`About to exit with code: ${code}`);
});const process = require('node:process');

process.on('exit', (code) => {
  console.log(`About to exit with code: ${code}`);
});

Listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the 'exit' event listeners causing any additional work still queued in the event loop to be abandoned. In the following example, for instance, the timeout will never occur:

import process from 'node:process';

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});const process = require('node:process');

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});

Event: 'message'#

Added in: v0.5.10

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'message' event is emitted whenever a message sent by a parent process using childprocess.send() is received by the child process.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the serialization option was set to advanced used when spawning the process, the message argument can contain data that JSON is not able to represent. See Advanced serialization for child_process for more details.

Event: 'multipleResolves'#

Added in: v10.12.0Deprecated since: v17.6.0, v16.15.0

Stability: 0 - Deprecated

  • type <string> The resolution type. One of 'resolve' or 'reject'.
  • promise <Promise> The promise that resolved or rejected more than once.
  • value <any> The value with which the promise was either resolved or rejected after the original resolve.

The 'multipleResolves' event is emitted whenever a Promise has been either:

  • Resolved more than once.
  • Rejected more than once.
  • Rejected after resolve.
  • Resolved after reject.

This is useful for tracking potential errors in an application while using the Promise constructor, as multiple resolutions are silently swallowed. However, the occurrence of this event does not necessarily indicate an error. For example, Promise.race() can trigger a 'multipleResolves' event.

Because of the unreliability of the event in cases like the Promise.race() example above it has been deprecated.

import process from 'node:process';

process.on('multipleResolves', (type, promise, reason) => {
  console.error(type, promise, reason);
  setImmediate(() => process.exit(1));
});

async function main() {
  try {
    return await new Promise((resolve, reject) => {
      resolve('First call');
      resolve('Swallowed resolve');
      reject(new Error('Swallowed reject'));
    });
  } catch {
    throw new Error('Failed');
  }
}

main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
//     at Promise (*)
//     at new Promise (<anonymous>)
//     at main (*)
// First callconst process = require('node:process');

process.on('multipleResolves', (type, promise, reason) => {
  console.error(type, promise, reason);
  setImmediate(() => process.exit(1));
});

async function main() {
  try {
    return await new Promise((resolve, reject) => {
      resolve('First call');
      resolve('Swallowed resolve');
      reject(new Error('Swallowed reject'));
    });
  } catch {
    throw new Error('Failed');
  }
}

main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
//     at Promise (*)
//     at new Promise (<anonymous>)
//     at main (*)
// First call

Event: 'rejectionHandled'#

Added in: v1.4.1
  • promise <Promise> The late handled promise.

The 'rejectionHandled' event is emitted whenever a Promise has been rejected and an error handler was attached to it (using promise.catch(), for example) later than one turn of the Node.js event loop.

The Promise object would have previously been emitted in an 'unhandledRejection' event, but during the course of processing gained a rejection handler.

There is no notion of a top level for a Promise chain at which rejections can always be handled. Being inherently asynchronous in nature, a Promise rejection can be handled at a future point in time, possibly much later than the event loop turn it takes for the 'unhandledRejection' event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.

In synchronous code, the 'uncaughtException' event is emitted when the list of unhandled exceptions grows.

In asynchronous code, the 'unhandledRejection' event is emitted when the list of unhandled rejections grows, and the 'rejectionHandled' event is emitted when the list of unhandled rejections shrinks.

import process from 'node:process';

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
  unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
  unhandledRejections.delete(promise);
});const process = require('node:process');

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
  unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
  unhandledRejections.delete(promise);
});

In this example, the unhandledRejections Map will grow and shrink over time, reflecting rejections that start unhandled and then become handled. It is possible to record such errors in an error log, either periodically (which is likely best for long-running application) or upon process exit (which is likely most convenient for scripts).

Event: 'uncaughtException'#

History
VersionChanges
v12.0.0, v10.17.0

Added the origin argument.

v0.1.18

Added in: v0.1.18

  • err <Error> The uncaught exception.
  • origin <string> Indicates if the exception originates from an unhandled rejection or from a synchronous error. Can either be 'uncaughtException' or 'unhandledRejection'. The latter is used when an exception happens in a Promise based async context (or if a Promise is rejected) and --unhandled-rejections flag set to strict or throw (which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.

The 'uncaughtException' event is emitted when an uncaught JavaScript exception bubbles all the way back to the event loop. By default, Node.js handles such exceptions by printing the stack trace to stderr and exiting with code 1, overriding any previously set process.exitCode. Adding a handler for the 'uncaughtException' event overrides this default behavior. Alternatively, change the process.exitCode in the 'uncaughtException' handler which will result in the process exiting with the provided exit code. Otherwise, in the presence of such handler the process will exit with 0.

import process from 'node:process';

process.on('uncaughtException', (err, origin) => {
  fs.writeSync(
    process.stderr.fd,
    `Caught exception: ${err}\n` +
    `Exception origin: ${origin}`,
  );
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');const process = require('node:process');

process.on('uncaughtException', (err, origin) => {
  fs.writeSync(
    process.stderr.fd,
    `Caught exception: ${err}\n` +
    `Exception origin: ${origin}`,
  );
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

It is possible to monitor 'uncaughtException' events without overriding the default behavior to exit the process by installing a 'uncaughtExceptionMonitor' listener.

Warning: Using 'uncaughtException' correctly#

'uncaughtException' is a crude mechanism for exception handling intended to be used only as a last resort. The event should not be used as an equivalent to On Error Resume Next. Unhandled exceptions inherently mean that an application is in an undefined state. Attempting to resume application code without properly recovering from the exception can cause additional unforeseen and unpredictable issues.

Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.

Attempting to resume normally after an uncaught exception can be similar to pulling out the power cord when upgrading a computer. Nine out of ten times, nothing happens. But the tenth time, the system becomes corrupted.

The correct use of 'uncaughtException' is to perform synchronous cleanup of allocated resources (e.g. file descriptors, handles, etc) before shutting down the process. It is not safe to resume normal operation after 'uncaughtException'.

To restart a crashed application in a more reliable way, whether 'uncaughtException' is emitted or not, an external monitor should be employed in a separate process to detect application failures and recover or restart as needed.

Event: 'uncaughtExceptionMonitor'#

Added in: v13.7.0, v12.17.0
  • err <Error> The uncaught exception.
  • origin <string> Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be 'uncaughtException' or 'unhandledRejection'. The latter is used when an exception happens in a Promise based async context (or if a Promise is rejected) and --unhandled-rejections flag set to strict or throw (which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.

The 'uncaughtExceptionMonitor' event is emitted before an 'uncaughtException' event is emitted or a hook installed via process.setUncaughtExceptionCaptureCallback() is called.

Installing an 'uncaughtExceptionMonitor' listener does not change the behavior once an 'uncaughtException' event is emitted. The process will still crash if no 'uncaughtException' listener is installed.

import process from 'node:process';

process.on('uncaughtExceptionMonitor', (err, origin) => {
  MyMonitoringTool.logSync(err, origin);
});

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.jsconst process = require('node:process');

process.on('uncaughtExceptionMonitor', (err, origin) => {
  MyMonitoringTool.logSync(err, origin);
});

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js

Event: 'unhandledRejection'#

History
VersionChanges
v7.0.0

Not handling Promise rejections is deprecated.

v6.6.0

Unhandled Promise rejections will now emit a process warning.

v1.4.1

Added in: v1.4.1

  • reason <Error> | <any> The object with which the promise was rejected (typically an Error object).
  • promise <Promise> The rejected promise.

The 'unhandledRejection' event is emitted whenever a Promise is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with Promises, exceptions are encapsulated as "rejected promises". Rejections can be caught and handled using promise.catch() and are propagated through a Promise chain. The 'unhandledRejection' event is useful for detecting and keeping track of promises that were rejected whose rejections have not yet been handled.

import process from 'node:process';

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, 'reason:', reason);
  // Application specific logging, throwing an error, or other logic here
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`const process = require('node:process');

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, 'reason:', reason);
  // Application specific logging, throwing an error, or other logic here
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`

The following will also trigger the 'unhandledRejection' event to be emitted:

import process from 'node:process';

function SomeResource() {
  // Initially set the loaded status to a rejected promise
  this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}

const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turnconst process = require('node:process');

function SomeResource() {
  // Initially set the loaded status to a rejected promise
  this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}

const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other 'unhandledRejection' events. To address such failures, a non-operational .catch(() => { }) handler may be attached to resource.loaded, which would prevent the 'unhandledRejection' event from being emitted.

Event: 'warning'#

Added in: v6.0.0
  • warning <Error> Key properties of the warning are:
    • name <string> The name of the warning. Default: 'Warning'.
    • message <string> A system-provided description of the warning.
    • stack <string> A stack trace to the location in the code where the warning was issued.

The 'warning' event is emitted whenever Node.js emits a process warning.

A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs, or security vulnerabilities.

import process from 'node:process';

process.on('warning', (warning) => {
  console.warn(warning.name);    // Print the warning name
  console.warn(warning.message); // Print the warning message
  console.warn(warning.stack);   // Print the stack trace
});const process = require('node:process');

process.on('warning', (warning) => {
  console.warn(warning.name);    // Print the warning name
  console.warn(warning.message); // Print the warning message
  console.warn(warning.stack);   // Print the stack trace
});

By default, Node.js will print process warnings to stderr. The --no-warnings command-line option can be used to suppress the default console output but the 'warning' event will still be emitted by the process object.

The following example illustrates the warning that is printed to stderr when too many listeners have been added to an event:

$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit

In contrast, the following example turns off the default warning output and adds a custom handler to the 'warning' event:

$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

The --trace-warnings command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Launching Node.js using the --throw-deprecation command-line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the --trace-deprecation command-line flag will cause the custom deprecation to be printed to stderr along with the stack trace.

Using the --no-deprecation command-line flag will suppress all reporting of the custom deprecation.

The *-deprecation command-line flags only affect warnings that use the name 'DeprecationWarning'.

Event: 'worker'#

Added in: v16.2.0, v14.18.0

The 'worker' event is emitted after a new <Worker> thread has been created.

Emitting custom warnings#

See the process.emitWarning() method for issuing custom or application-specific warnings.

Node.js warning names#

There are no strict guidelines for warning types (as identified by the name property) emitted by Node.js. New types of warnings can be added at any time. A few of the warning types that are most common include:

  • 'DeprecationWarning' - Indicates use of a deprecated Node.js API or feature. Such warnings must include a 'code' property identifying the deprecation code.
  • 'ExperimentalWarning' - Indicates use of an experimental Node.js API or feature. Such features must be used with caution as they may change at any time and are not subject to the same strict semantic-versioning and long-term support policies as supported features.
  • 'MaxListenersExceededWarning' - Indicates that too many listeners for a given event have been registered on either an EventEmitter or EventTarget. This is often an indication of a memory leak.
  • 'TimeoutOverflowWarning' - Indicates that a numeric value that cannot fit within a 32-bit signed integer has been provided to either the setTimeout() or setInterval() functions.
  • 'UnsupportedWarning' - Indicates use of an unsupported option or feature that will be ignored rather than treated as an error. One example is use of the HTTP response status message when using the HTTP/2 compatibility API.

Signal events#

Signal events will be emitted when the Node.js process receives a signal. Please refer to signal(7) for a listing of standard POSIX signal names such as 'SIGINT', 'SIGHUP', etc.

Signals are not available on Worker threads.

The signal handler will receive the signal's name ('SIGINT', 'SIGTERM', etc.) as the first argument.

The name of each event will be the uppercase common name for the signal (e.g. 'SIGINT' for SIGINT signals).

import process from 'node:process';

// Begin reading from stdin so the process does not exit.
process.stdin.resume();

process.on('SIGINT', () => {
  console.log('Received SIGINT. Press Control-D to exit.');
});

// Using a single function to handle multiple signals
function handle(signal) {
  console.log(`Received ${signal}`);
}

process.on('SIGINT', handle);
process.on('SIGTERM', handle);const process = require('node:process');

// Begin reading from stdin so the process does not exit.
process.stdin.resume();

process.on('SIGINT', () => {
  console.log('Received SIGINT. Press Control-D to exit.');
});

// Using a single function to handle multiple signals
function handle(signal) {
  console.log(`Received ${signal}`);
}

process.on('SIGINT', handle);
process.on('SIGTERM', handle);
  • 'SIGUSR1' is reserved by Node.js to start the debugger. It's possible to install a listener but doing so might interfere with the debugger.
  • 'SIGTERM' and 'SIGINT' have default handlers on non-Windows platforms that reset the terminal mode before exiting with code 128 + signal number. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).
  • 'SIGPIPE' is ignored by default. It can have a listener installed.
  • 'SIGHUP' is generated on Windows when the console window is closed, and on other platforms under various similar conditions. See signal(7). It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior of SIGHUP is to terminate Node.js, but once a listener has been installed its default behavior will be removed.
  • 'SIGTERM' is not supported on Windows, it can be listened on.
  • 'SIGINT' from the terminal is supported on all platforms, and can usually be generated with Ctrl+C (though this may be configurable). It is not generated when terminal raw mode is enabled and Ctrl+C is used.
  • 'SIGBREAK' is delivered on Windows when Ctrl+Break is pressed. On non-Windows platforms, it can be listened on, but there is no way to send or generate it.
  • 'SIGWINCH' is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.
  • 'SIGKILL' cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.
  • 'SIGSTOP' cannot have a listener installed.
  • 'SIGBUS', 'SIGFPE', 'SIGSEGV', and 'SIGILL', when not raised artificially using kill(2), inherently leave the process in a state from which it is not safe to call JS listeners. Doing so might cause the process to stop responding.
  • 0 can be sent to test for the existence of a process, it has no effect if the process exists, but will throw an error if the process does not exist.

Windows does not support signals so has no equivalent to termination by signal, but Node.js offers some emulation with process.kill(), and subprocess.kill():

  • Sending SIGINT, SIGTERM, and SIGKILL will cause the unconditional termination of the target process, and afterwards, subprocess will report that the process was terminated by signal.
  • Sending signal 0 can be used as a platform independent way to test for the existence of a process.

process.abort()#

Added in: v0.7.0

The process.abort() method causes the Node.js process to exit immediately and generate a core file.

This feature is not available in Worker threads.

process.allowedNodeEnvironmentFlags#

Added in: v10.10.0

The process.allowedNodeEnvironmentFlags property is a special, read-only Set of flags allowable within the NODE_OPTIONS environment variable.

process.allowedNodeEnvironmentFlags extends Set, but overrides Set.prototype.has to recognize several different possible flag representations. process.allowedNodeEnvironmentFlags.has() will return true in the following cases:

  • Flags may omit leading single (-) or double (--) dashes; e.g., inspect-brk for --inspect-brk, or r for -r.
  • Flags passed through to V8 (as listed in --v8-options) may replace one or more non-leading dashes for an underscore, or vice-versa; e.g., --perf_basic_prof, --perf-basic-prof, --perf_basic-prof, etc.
  • Flags may contain one or more equals (=) characters; all characters after and including the first equals will be ignored; e.g., --stack-trace-limit=100.
  • Flags must be allowable within NODE_OPTIONS.

When iterating over process.allowedNodeEnvironmentFlags, flags will appear only once; each will begin with one or more dashes. Flags passed through to V8 will contain underscores instead of non-leading dashes:

import { allowedNodeEnvironmentFlags } from 'node:process';

allowedNodeEnvironmentFlags.forEach((flag) => {
  // -r
  // --inspect-brk
  // --abort_on_uncaught_exception
  // ...
});const { allowedNodeEnvironmentFlags } = require('node:process');

allowedNodeEnvironmentFlags.forEach((flag) => {
  // -r
  // --inspect-brk
  // --abort_on_uncaught_exception
  // ...
});

The methods add(), clear(), and delete() of process.allowedNodeEnvironmentFlags do nothing, and will fail silently.

If Node.js was compiled without NODE_OPTIONS support (shown in process.config), process.allowedNodeEnvironmentFlags will contain what would have been allowable.

process.arch#

Added in: v0.5.0

The operating system CPU architecture for which the Node.js binary was compiled. Possible values are: 'arm', 'arm64', 'ia32', 'mips','mipsel', 'ppc', 'ppc64', 's390', 's390x', and 'x64'.

import { arch } from 'node:process';

console.log(`This processor architecture is ${arch}`);const { arch } = require('node:process');

console.log(`This processor architecture is ${arch}`);

process.argv#

Added in: v0.1.27

The process.argv property returns an array containing the command-line arguments passed when the Node.js process was launched. The first element will be process.execPath. See process.argv0 if access to the original value of argv[0] is needed. The second element will be the path to the JavaScript file being executed. The remaining elements will be any additional command-line arguments.

For example, assuming the following script for process-args.js:

import { argv } from 'node:process';

// print process.argv
argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});const { argv } = require('node:process');

// print process.argv
argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});

Launching the Node.js process as:

$ node process-args.js one two=three four

Would generate the output:

0: /usr/local/bin/node
1: /Users/mjr/work/node/process-args.js
2: one
3: two=three
4: four

process.argv0#

Added in: v6.4.0

The process.argv0 property stores a read-only copy of the original value of argv[0] passed when Node.js starts.

$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'

process.channel#

History
VersionChanges
v14.0.0

The object no longer accidentally exposes native C++ bindings.

v7.1.0

Added in: v7.1.0

If the Node.js process was spawned with an IPC channel (see the Child Process documentation), the process.channel property is a reference to the IPC channel. If no IPC channel exists, this property is undefined.

process.channel.ref()#

Added in: v7.1.0

This method makes the IPC channel keep the event loop of the process running if .unref() has been called before.

Typically, this is managed through the number of 'disconnect' and 'message' listeners on the process object. However, this method can be used to explicitly request a specific behavior.

process.channel.unref()#

Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the process running, and lets it finish even while the channel is open.

Typically, this is managed through the number of 'disconnect' and 'message' listeners on the process object. However, this method can be used to explicitly request a specific behavior.

process.chdir(directory)#

Added in: v0.1.17

The process.chdir() method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified directory does not exist).

import { chdir, cwd } from 'node:process';

console.log(`Starting directory: ${cwd()}`);
try {
  chdir('/tmp');
  console.log(`New directory: ${cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}const { chdir, cwd } = require('node:process');

console.log(`Starting directory: ${cwd()}`);
try {
  chdir('/tmp');
  console.log(`New directory: ${cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}

This feature is not available in Worker threads.

process.config#

History
VersionChanges
v19.0.0

The process.config object is now frozen.

v16.0.0

Modifying process.config has been deprecated.

v0.7.7

Added in: v0.7.7

The process.config property returns a frozen Object containing the JavaScript representation of the configure options used to compile the current Node.js executable. This is the same as the config.gypi file that was produced when running the ./configure script.

An example of the possible output looks like:

{
  target_defaults:
   { cflags: [],
     default_configuration: 'Release',
     defines: [],
     include_dirs: [],
     libraries: [] },
  variables:
   {
     host_arch: 'x64',
     napi_build_version: 5,
     node_install_npm: 'true',
     node_prefix: '',
     node_shared_cares: 'false',
     node_shared_http_parser: 'false',
     node_shared_libuv: 'false',
     node_shared_zlib: 'false',
     node_use_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 1
   }
}

process.connected#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.connected property will return true so long as the IPC channel is connected and will return false after process.disconnect() is called.

Once process.connected is false, it is no longer possible to send messages over the IPC channel using process.send().

process.cpuUsage([previousValue])#

Added in: v6.1.0

The process.cpuUsage() method returns the user and system CPU time usage of the current process, in an object with properties user and system, whose values are microsecond values (millionth of a second). These values measure time spent in user and system code respectively, and may end up being greater than actual elapsed time if multiple CPU cores are performing work for this process.

The result of a previous call to process.cpuUsage() can be passed as the argument to the function, to get a diff reading.

import { cpuUsage } from 'node:process';

const startUsage = cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }const { cpuUsage } = require('node:process');

const startUsage = cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(cpuUsage(startUsage));
// { user: 514883, system: 11226 }

process.cwd()#

Added in: v0.1.8

The process.cwd() method returns the current working directory of the Node.js process.

import { cwd } from 'node:process';

console.log(`Current directory: ${cwd()}`);const { cwd } = require('node:process');

console.log(`Current directory: ${cwd()}`);

process.debugPort#

Added in: v0.7.2

The port used by the Node.js debugger when enabled.

import process from 'node:process';

process.debugPort = 5858;const process = require('node:process');

process.debugPort = 5858;

process.disconnect()#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.disconnect() method will close the IPC channel to the parent process, allowing the child process to exit gracefully once there are no other connections keeping it alive.

The effect of calling process.disconnect() is the same as calling ChildProcess.disconnect() from the parent process.

If the Node.js process was not spawned with an IPC channel, process.disconnect() will be undefined.

process.dlopen(module, filename[, flags])#

History
VersionChanges
v9.0.0

Added support for the flags argument.

v0.1.16

Added in: v0.1.16

The process.dlopen() method allows dynamically loading shared objects. It is primarily used by require() to load C++ Addons, and should not be used directly, except in special cases. In other words, require() should be preferred over process.dlopen() unless there are specific reasons such as custom dlopen flags or loading from ES modules.

The flags argument is an integer that allows to specify dlopen behavior. See the os.constants.dlopen documentation for details.

An important requirement when calling process.dlopen() is that the module instance must be passed. Functions exported by the C++ Addon are then accessible via module.exports.

The example below shows how to load a C++ Addon, named local.node, that exports a foo function. All the symbols are loaded before the call returns, by passing the RTLD_NOW constant. In this example the constant is assumed to be available.

import { dlopen } from 'node:process';
import { constants } from 'node:os';
import { fileURLToPath } from 'node:url';

const module = { exports: {} };
dlopen(module, fileURLToPath(new URL('local.node', import.meta.url)),
       constants.dlopen.RTLD_NOW);
module.exports.foo();const { dlopen } = require('node:process');
const { constants } = require('node:os');
const { join } = require('node:path');

const module = { exports: {} };
dlopen(module, join(__dirname, 'local.node'), constants.dlopen.RTLD_NOW);
module.exports.foo();

process.emitWarning(warning[, options])#

Added in: v8.0.0
  • warning <string> | <Error> The warning to emit.
  • options <Object>
    • type <string> When warning is a String, type is the name to use for the type of warning being emitted. Default: 'Warning'.
    • code <string> A unique identifier for the warning instance being emitted.
    • ctor <Function> When warning is a String, ctor is an optional function used to limit the generated stack trace. Default: process.emitWarning.
    • detail <string> Additional text to include with the error.

The process.emitWarning() method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the 'warning' event.

import { emitWarning } from 'node:process';

// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
  code: 'MY_WARNING',
  detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional informationconst { emitWarning } = require('node:process');

// Emit a warning with a code and additional detail.
emitWarning('Something happened!', {
  code: 'MY_WARNING',
  detail: 'This is some additional information',
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information

In this example, an Error object is generated internally by process.emitWarning() and passed through to the 'warning' handler.

import process from 'node:process';

process.on('warning', (warning) => {
  console.warn(warning.name);    // 'Warning'
  console.warn(warning.message); // 'Something happened!'
  console.warn(warning.code);    // 'MY_WARNING'
  console.warn(warning.stack);   // Stack trace
  console.warn(warning.detail);  // 'This is some additional information'
});const process = require('node:process');

process.on('warning', (warning) => {
  console.warn(warning.name);    // 'Warning'
  console.warn(warning.message); // 'Something happened!'
  console.warn(warning.code);    // 'MY_WARNING'
  console.warn(warning.stack);   // Stack trace
  console.warn(warning.detail);  // 'This is some additional information'
});

If warning is passed as an Error object, the options argument is ignored.

process.emitWarning(warning[, type[, code]][, ctor])#

Added in: v6.0.0
  • warning <string> | <Error> The warning to emit.
  • type <string> When warning is a String, type is the name to use for the type of warning being emitted. Default: 'Warning'.
  • code <string> A unique identifier for the warning instance being emitted.
  • ctor <Function> When warning is a String, ctor is an optional function used to limit the generated stack trace. Default: process.emitWarning.

The process.emitWarning() method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the 'warning' event.

import { emitWarning } from 'node:process';

// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!const { emitWarning } = require('node:process');

// Emit a warning using a string.
emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!
import { emitWarning } from 'node:process';

// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!const { emitWarning } = require('node:process');

// Emit a warning using a string and a type.
emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!
import { emitWarning } from 'node:process';

emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!const { emitWarning } = require('node:process');

process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

In each of the previous examples, an Error object is generated internally by process.emitWarning() and passed through to the 'warning' handler.

import process from 'node:process';

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});const process = require('node:process');

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});

If warning is passed as an Error object, it will be passed through to the 'warning' event handler unmodified (and the optional type, code and ctor arguments will be ignored):

import { emitWarning } from 'node:process';

// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!const { emitWarning } = require('node:process');

// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

A TypeError is thrown if warning is anything other than a string or Error object.

While process warnings use Error objects, the process warning mechanism is not a replacement for normal error handling mechanisms.

The following additional handling is implemented if the warning type is 'DeprecationWarning':

  • If the --throw-deprecation command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event.
  • If the --no-deprecation command-line flag is used, the deprecation warning is suppressed.
  • If the --trace-deprecation command-line flag is used, the deprecation warning is printed to stderr along with the full stack trace.

Avoiding duplicate warnings#

As a best practice, warnings should be emitted only once per process. To do so, place the emitWarning() behind a boolean.

import { emitWarning } from 'node:process';

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothingconst { emitWarning } = require('node:process');

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

process.env#

History
VersionChanges
v11.14.0

Worker threads will now use a copy of the parent thread's process.env by default, configurable through the env option of the Worker constructor.

v10.0.0

Implicit conversion of variable value to string is deprecated.

v0.1.27

Added in: v0.1.27

The process.env property returns an object containing the user environment. See environ(7).

An example of this object looks like:

{
  TERM: 'xterm-256color',
  SHELL: '/usr/local/bin/bash',
  USER: 'maciej',
  PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
  PWD: '/Users/maciej',
  EDITOR: 'vim',
  SHLVL: '1',
  HOME: '/Users/maciej',
  LOGNAME: 'maciej',
  _: '/usr/local/bin/node'
}

It is possible to modify this object, but such modifications will not be reflected outside the Node.js process, or (unless explicitly requested) to other Worker threads. In other words, the following example would not work:

$ node -e 'process.env.foo = "bar"' && echo $foo

While the following will:

import { env } from 'node:process';

env.foo = 'bar';
console.log(env.foo);const { env } = require('node:process');

env.foo = 'bar';
console.log(env.foo);

Assigning a property on process.env will implicitly convert the value to a string. This behavior is deprecated. Future versions of Node.js may throw an error when the value is not a string, number, or boolean.

import { env } from 'node:process';

env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'const { env } = require('node:process');

env.test = null;
console.log(env.test);
// => 'null'
env.test = undefined;
console.log(env.test);
// => 'undefined'

Use delete to delete a property from process.env.

import { env } from 'node:process';

env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefinedconst { env } = require('node:process');

env.TEST = 1;
delete env.TEST;
console.log(env.TEST);
// => undefined

On Windows operating systems, environment variables are case-insensitive.

import { env } from 'node:process';

env.TEST = 1;
console.log(env.test);
// => 1const { env } = require('node:process');

env.TEST = 1;
console.log(env.test);
// => 1

Unless explicitly specified when creating a Worker instance, each Worker thread has its own copy of process.env, based on its parent thread's process.env, or whatever was specified as the env option to the Worker constructor. Changes to process.env will not be visible across Worker threads, and only the main thread can make changes that are visible to the operating system or to native add-ons.

process.execArgv#

Added in: v0.7.7

The process.execArgv property returns the set of Node.js-specific command-line options passed when the Node.js process was launched. These options do not appear in the array returned by the process.argv property, and do not include the Node.js executable, the name of the script, or any options following the script name. These options are useful in order to spawn child processes with the same execution environment as the parent.

$ node --harmony script.js --version

Results in process.execArgv:

['--harmony']

And process.argv:

['/usr/local/bin/node', 'script.js', '--version']

Refer to Worker constructor for the detailed behavior of worker threads with this property.

process.execPath#

Added in: v0.1.100

The process.execPath property returns the absolute pathname of the executable that started the Node.js process. Symbolic links, if any, are resolved.

'/usr/local/bin/node'

process.exit([code])#

Added in: v0.1.13

The process.exit() method instructs Node.js to terminate the process synchronously with an exit status of code. If code is omitted, exit uses either the 'success' code 0 or the value of process.exitCode if it has been set. Node.js will not terminate until all the 'exit' event listeners are called.

To exit with a 'failure' code:

import { exit } from 'node:process';

exit(1);const { exit } = require('node:process');

exit(1);

The shell that executed Node.js should see the exit code as 1.

Calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

In most situations, it is not actually necessary to call process.exit() explicitly. The Node.js process will exit on its own if there is no additional work pending in the event loop. The process.exitCode property can be set to tell the process which exit code to use when the process exits gracefully.

For instance, the following example illustrates a misuse of the process.exit() method that could lead to data printed to stdout being truncated and lost:

import { exit } from 'node:process';

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  exit(1);
}const { exit } = require('node:process');

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  exit(1);
}

The reason this is problematic is because writes to process.stdout in Node.js are sometimes asynchronous and may occur over multiple ticks of the Node.js event loop. Calling process.exit(), however, forces the process to exit before those additional writes to stdout can be performed.

Rather than calling process.exit() directly, the code should set the process.exitCode and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

import process from 'node:process';

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exitCode = 1;
}const process = require('node:process');

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exitCode = 1;
}

If it is necessary to terminate the Node.js process due to an error condition, throwing an uncaught error and allowing the process to terminate accordingly is safer than calling process.exit().

In Worker threads, this function stops the current thread rather than the current process.

process.exitCode#

Added in: v0.11.8

A number which will be the process exit code, when the process either exits gracefully, or is exited via process.exit() without specifying a code.

Specifying a code to process.exit(code) will override any previous setting of process.exitCode.

process.getActiveResourcesInfo()#

Added in: v17.3.0, v16.14.0

Stability: 1 - Experimental

The process.getActiveResourcesInfo() method returns an array of strings containing the types of the active resources that are currently keeping the event loop alive.

import { getActiveResourcesInfo } from 'node:process';
import { setTimeout } from 'node:timers';

console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
//   Before: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
//   After: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]const { getActiveResourcesInfo } = require('node:process');
const { setTimeout } = require('node:timers');

console.log('Before:', getActiveResourcesInfo());
setTimeout(() => {}, 1000);
console.log('After:', getActiveResourcesInfo());
// Prints:
//   Before: [ 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
//   After: [ 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]

process.getegid()#

Added in: v2.0.0

The process.getegid() method returns the numerical effective group identity of the Node.js process. (See getegid(2).)

import process from 'node:process';

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}const process = require('node:process');

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.geteuid()#

Added in: v2.0.0

The process.geteuid() method returns the numerical effective user identity of the process. (See geteuid(2).)

import process from 'node:process';

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}const process = require('node:process');

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getgid()#

Added in: v0.1.31

The process.getgid() method returns the numerical group identity of the process. (See getgid(2).)

import process from 'node:process';

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}const process = require('node:process');

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getgroups()#

Added in: v0.9.4

The process.getgroups() method returns an array with the supplementary group IDs. POSIX leaves it unspecified if the effective group ID is included but Node.js ensures it always is.

import process from 'node:process';

if (process.getgroups) {
  console.log(process.getgroups()); // [ 16, 21, 297 ]
}const process = require('node:process');

if (process.getgroups) {
  console.log(process.getgroups()); // [ 16, 21, 297 ]
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getuid()#

Added in: v0.1.28

The process.getuid() method returns the numeric user identity of the process. (See getuid(2).)

import process from 'node:process';

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}const process = require('node:process');

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.hasUncaughtExceptionCaptureCallback()#

Added in: v9.3.0

Indicates whether a callback has been set using process.setUncaughtExceptionCaptureCallback().

process.hrtime([time])#

Added in: v0.7.6

Stability: 3 - Legacy. Use process.hrtime.bigint() instead.

This is the legacy version of process.hrtime.bigint() before bigint was introduced in JavaScript.

The process.hrtime() method returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array, where nanoseconds is the remaining part of the real time that can't be represented in second precision.

time is an optional parameter that must be the result of a previous process.hrtime() call to diff with the current time. If the parameter passed in is not a tuple Array, a TypeError will be thrown. Passing in a user-defined array instead of the result of a previous call to process.hrtime() will lead to undefined behavior.

These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:

import { hrtime } from 'node:process';

const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  const diff = hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // Benchmark took 1000000552 nanoseconds
}, 1000);const { hrtime } = require('node:process');

const NS_PER_SEC = 1e9;
const time = hrtime();
// [ 1800216, 25 ]