Node.js v10.0.0-test201804064cc80f32f9 Documentation

Table of Contents

About this Documentation#

The goal of this documentation is to comprehensively explain the Node.js API, both from a reference as well as a conceptual point of view. Each section describes a built-in module or high-level concept.

Where appropriate, property types, method arguments, and the arguments provided to event handlers are detailed in a list underneath the topic heading.

Contributing#

If errors are found in this documentation, please submit an issue or see the contributing guide for directions on how to submit a patch.

Every file is generated based on the corresponding .md file in the doc/api/ folder in Node.js's source tree. The documentation is generated using the tools/doc/generate.js program. An HTML template is located at doc/template.html.

Stability Index#

Throughout the documentation are indications of a section's stability. The Node.js API is still somewhat changing, and as it matures, certain parts are more reliable than others. Some 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 and in the process of being redesigned.

The stability indices are as follows:

Stability: 0 - Deprecated This feature is known to be problematic, and changes may be planned. Do not rely on it. Use of the feature may cause warnings to be emitted. Backwards compatibility across major versions should not be expected.
Stability: 1 - Experimental This feature is still under active development and subject to non-backwards compatible changes, or even removal, in any future version. Use of the feature is not recommended in production environments. Experimental features are not subject to the Node.js Semantic Versioning model.
Stability: 2 - Stable The API has proven satisfactory. Compatibility with the npm ecosystem is a high priority, and will not be broken unless absolutely necessary.

Caution must be used when making use of Experimental features, particularly within modules that may be used as dependencies (or dependencies of dependencies) within a Node.js application. End users may not be aware that experimental features are being used, and therefore may experience unexpected failures or behavior changes when API modifications occur. To help avoid such surprises, Experimental features may require a command-line flag to explicitly enable them, or may cause a process warning to be emitted. By default, such warnings are printed to stderr and may be handled by attaching a listener to the process.on('warning') event.

JSON Output#

Added in: v0.6.12
Stability: 1 - Experimental

Every .html document has a corresponding .json document presenting the same information in a structured manner. This feature is experimental, and added for the benefit of IDEs and other utilities that wish to do programmatic things with the documentation.

Syscalls and man pages#

System calls like open(2) and read(2) define the interface between user programs and the underlying operating system. Node.js functions which simply wrap a syscall, like fs.open(), will document that. The docs link to the corresponding man pages (short for manual pages) which describe how the syscalls work.

Some syscalls, like lchown(2), are BSD-specific. That means, for example, that fs.lchown() only works on macOS and other BSD-derived systems, and is not available on Linux.

Most Unix syscalls have Windows equivalents, but behavior may differ on Windows relative to Linux and macOS. For an example of the subtle ways in which it's sometimes impossible to replace Unix syscall semantics on Windows, see Node issue 4760.

Usage#

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

Please see the Command Line Options document for information about different options and ways to run scripts with Node.js.

Example#

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

Commands displayed in this document are shown starting with $ or > to replicate how they would appear in a user's terminal. Do not include the $ and > character they are there to indicate the start of each command.

There are many tutorials and examples that follow this convention: $ or > for commands run as a regular user, and # for commands that should be executed as an administrator.

Lines that don’t start with $ or > character are typically showing the output of the previous command.

Firstly, make sure to have downloaded and installed Node.js. See this guide for further install information.

Now, create an empty project folder called projects, navigate into it: Project folder can be named base on user's current project title but this example will use projects as the project folder.

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.

In Node.js it is considered good style to use hyphens (-) or underscores (_) to separate multiple words in filenames.

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

const http = require('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 enter the following command:

$ node hello-world.js

An output like this should appear in the terminal to indicate Node.js server is running:

 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.

Many of the examples in the documentation can be run similarly.

Assert#

Stability: 2 - Stable

The assert module provides a simple set of assertion tests that can be used to test invariants.

A strict and a legacy mode exist, while it is recommended to only use strict mode.

For more information about the used equality comparisons see MDN's guide on equality comparisons and sameness.

Class: assert.AssertionError#

A subclass of Error that indicates the failure of an assertion. All errors thrown by the 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 going to be set to this value.
    • actual <any> The actual property on the error instance is going to contain this value. Internally used for the actual error input in case e.g., assert.strictEqual() is used.
    • expected <any> The expected property on the error instance is going to contain this value. Internally used for the expected error input in case e.g., assert.strictEqual() is used.
    • operator <string> The operator property on the error instance is going to contain this value. Internally used to indicate what operation was used for comparison (or what assertion function triggered the error).
    • stackStartFn <Function> If provided, the generated stack trace is going to remove all frames up to the provided 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 value in case e.g., assert.strictEqual() is used.
  • expected <any> Set to the expected value in case e.g., assert.strictEqual() is used.
  • generatedMessage <boolean> Indicates if the message was auto-generated (true) or not.
  • code <string> This is always set to the string ERR_ASSERTION to indicate that the error is actually an assertion error.
  • operator <string> Set to the passed in operator value.
const assert = require('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 [ERR_ASSERTION]');
  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);
}

Strict mode#

History
VersionChanges
v9.9.0

Added error diffs to the strict mode

v9.9.0

Added strict mode to the assert module.

v9.9.0

Added in: v9.9.0

When using the strict mode, any assert function will use the equality used in the strict function mode. So assert.deepEqual() will, for example, work the same as assert.deepStrictEqual().

On top of that, error messages which involve objects produce an error diff instead of displaying both objects. That is not the case for the legacy mode.

It can be accessed using:

const assert = require('assert').strict;

Example error diff (the expected, actual, and Lines skipped will be on a single row):

const assert = require('assert').strict;

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);

AssertionError [ERR_ASSERTION]: Input A expected to deepStrictEqual input B:
+ expected
- actual
... Lines skipped

  [
    [
...
      2,
-     3
+     '3'
    ],
...
    5
  ]

To deactivate the colors, use the NODE_DISABLE_COLORS environment variable. Please note that this will also deactivate the colors in the REPL.

Legacy mode#

Stability: 0 - Deprecated: Use strict mode instead.

When accessing assert directly instead of using the strict property, the Abstract Equality Comparison will be used for any function without "strict" in its name, such as assert.deepEqual().

It can be accessed using:

const assert = require('assert');

It is recommended to use the strict mode instead as the Abstract Equality Comparison can often have surprising results. This is especially true for assert.deepEqual(), where the comparison rules are lax:

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

assert(value[, message])#

Added in: v0.5.9
  • value <any>
  • message <any>

An alias of assert.ok().

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

History
VersionChanges
v9.0.0

Error names and messages are now properly compared

v8.0.0

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

  • actual <any>
  • expected <any>
  • message <any>

Strict mode

An alias of assert.deepStrictEqual().

Legacy mode

Stability: 0 - Deprecated: Use assert.deepStrictEqual() instead.

Tests for deep equality between the actual and expected parameters. Primitive values are compared with the Abstract Equality Comparison ( == ).

Only enumerable "own" properties are considered. The assert.deepEqual() implementation does not test the [[Prototype]] of objects or enumerable own Symbol properties. For such checks, consider using assert.deepStrictEqual() instead. assert.deepEqual() can have potentially surprising results. The following example does not throw an AssertionError because the properties on the RegExp object are not enumerable:

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

An exception is made for Map and Set. Maps and Sets have their contained items compared too, as expected.

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

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

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

assert.deepEqual(obj1, obj3);
// OK, objects are equal

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

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
v9.0.0

Enumerable symbol properties are now compared.

v9.0.0

NaN is now compared using the SameValueZero comparison.

v8.5.0

Error names and messages are now properly compared

v8.0.0

Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

  • actual <any>
  • expected <any>
  • message <any>

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 the SameValue Comparison, used by Object.is().
  • Type tags of objects should be the same.
  • [[Prototype]] of objects are compared using the Strict Equality Comparison.
  • 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.
const assert = require('assert').strict;

assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using SameValue comparison

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

assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]

assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags

assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison

assert.deepStrictEqual(new Number(1), new Number(2));
// Fails because the wrapped number is unwrapped and compared as well.
assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK
assert.deepStrictEqual(0, -0);
// AssertionError: 0 deepStrictEqual -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 });
// Fails because symbol1 !== symbol2!

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
assert.deepStrictEqual(weakMap1, weakMap3);
// Fails because weakMap3 has a property that weakMap1 does not contain!

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.doesNotReject(block[, error][, message])#

Added in: REPLACEME

Awaits for the promise returned by function block to complete and not be rejected.

Please note: Using assert.doesNotReject() is actually not useful because there is little benefit by 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.

When assert.doesNotReject() is called, it will immediately call the block function, and awaits for completion. See assert.rejects() for more details.

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

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

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

assert.doesNotThrow(block[, 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 block does not throw an error. See assert.throws() for more details.

Please note: Using assert.doesNotThrow() is actually not useful because there is no benefit by 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 block 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.

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

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

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

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:

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

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

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Strict mode

An alias of assert.strictEqual().

Legacy mode

Stability: 0 - Deprecated: Use assert.strictEqual() instead.

Tests shallow, coercive equality between the actual and expected parameters using the Abstract Equality Comparison ( == ).

const assert = require('assert');

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

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
  • message <any> Default: 'Failed'

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.

const assert = require('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[, stackStartFunction]]])#

History
VersionChanges
REPLACEME

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

v0.1.21

Added in: v0.1.21

  • actual <any>
  • expected <any>
  • message <any>
  • operator <string> Default: '!='
  • stackStartFunction <Function> Default: assert.fail
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 stackStartFunction 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.

const assert = require('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 stackStartFunction for truncating the exception's stacktrace:

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
REPLACEME

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

REPLACEME

Value may now only be undefined or null. Before any truthy input was accepted.

v0.1.97

Added in: v0.1.97

  • value <any>

Throws value if value is not undefined or null. This is useful when testing the error argument in callbacks.

const assert = require('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

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

History
VersionChanges
v9.0.0

Error names and messages are now properly compared

v8.0.0

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

  • actual <any>
  • expected <any>
  • message <any>

Strict mode

An alias of assert.notDeepStrictEqual().

Legacy mode

Stability: 0 - Deprecated: Use assert.notDeepStrictEqual() instead.

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

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

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

assert.notDeepEqual(obj1, obj2);
// OK: obj1 and obj2 are not deeply equal

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

assert.notDeepEqual(obj1, obj4);
// OK: obj1 and obj4 are not deeply equal

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

-0 and +0 are not considered equal anymore.

v9.0.0

NaN is now compared using the SameValueZero comparison.

v9.0.0

Error names and messages are now properly compared

v8.0.0

Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

  • actual <any>
  • expected <any>
  • message <any>

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

const assert = require('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])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Strict mode

An alias of assert.notStrictEqual().

Legacy mode

Stability: 0 - Deprecated: Use assert.notStrictEqual() instead.

Tests shallow, coercive inequality with the Abstract Equality Comparison ( != ).

const assert = require('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
REPLACEME

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

v0.1.21

Added in: v0.1.21

  • actual <any>
  • expected <any>
  • message <any>

Tests strict inequality between the actual and expected parameters as determined by the SameValue Comparison.

const assert = require('assert').strict;

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

assert.notStrictEqual(1, 1);
// AssertionError: 1 notStrictEqual 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
REPLACEME

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

v0.1.21

Added in: v0.1.21

  • value <any>
  • message <any>

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.

const assert = require('assert').strict;

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

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

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

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

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

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

assert.ok(0);
// throws:
// "AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)

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

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

Added in: REPLACEME

Awaits for promise returned by function block to be rejected.

When assert.rejects() is called, it will immediately call the block function, and awaits for completion.

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

If specified, error can be a constructor, RegExp, a validation function, or an object where each property will be tested for.

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

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

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

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

History
VersionChanges
REPLACEME

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

v0.1.21

Added in: v0.1.21

  • actual <any>
  • expected <any>
  • message <any>

Tests strict equality between the actual and expected parameters as determined by the SameValue Comparison.

const assert = require('assert').strict;

assert.strictEqual(1, 2);
// AssertionError: 1 strictEqual 2

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

assert.strictEqual(1, '1');
// AssertionError: 1 strictEqual '1'

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(block[, error][, message])#

History
VersionChanges
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 block to throw an error.

If specified, error can be a constructor, RegExp, a validation function, or an object where each property will be tested for.

If specified, message will be the message provided by the AssertionError if the block fails to throw.

Validate instanceof using constructor:

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.

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

Custom error validation:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  function(err) {
    if ((err instanceof Error) && /value/.test(err)) {
      return true;
    }
  },
  'unexpected error'
);

Custom error object / error instance:

assert.throws(
  () => {
    const err = new TypeError('Wrong value');
    err.code = 404;
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value'
    // Note that only properties on the error object will be tested!
  }
);

Note that error can not 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 below carefully if using a string as the second argument gets considered:

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.
// In that case both cases do not throw as neither is going to try to
// match for the error message thrown by the input function!
assert.throws(throwingFirst, 'Second');
assert.throws(throwingSecond, 'Second');

// 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:
assert.throws(throwingSecond, /Second$/);
// Does not throw because the error messages match.
assert.throws(throwingFirst, /Second$/);
// Throws a error:
// Error: First
//     at throwingFirst (repl:2:9)

Due to the confusing notation, it is recommended not to use a string as the second argument. This might lead to difficult-to-spot errors.

Async Hooks#

Stability: 1 - Experimental

The async_hooks module provides an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application. It can be accessed using:

const async_hooks = require('async_hooks');

Terminology#

An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, for example, 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.

Public API#

Overview#

Following is a simple overview of the public API.

const async_hooks = require('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 (e.g. TCPWrap), and will be called exactly 1
// time for requests (e.g. FSReqWrap).
function before(asyncId) { }

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

// destroy is called when an AsyncWrap instance 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.

const async_hooks = require('async_hooks');

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

Note that 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());
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 AsyncHooks callbacks#

Because printing to the console is an asynchronous operation, console.log() will cause the AsyncHooks callbacks to be called. Using console.log() or similar asynchronous operations inside an AsyncHooks callback function will thus cause an infinite recursion. An easy solution to this when debugging is to use a synchronous logging operation such as fs.writeSync(1, msg). This will print to stdout because 1 is the file descriptor for stdout and will not invoke AsyncHooks recursively because it is synchronous.

const fs = require('fs');
const util = require('util');

function debug(...args) {
  // use a function like this one when debugging inside an AsyncHooks callback
  fs.writeSync(1, `${util.format(...args)}\n`);
}

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 AsyncHooks itself. The logging should then be skipped when it was the logging itself that caused AsyncHooks callback to call. By doing this the otherwise infinite recursion is broken.

asyncHook.enable()#

Enable the callbacks for a given AsyncHook instance. If no callbacks are provided enabling is a noop.

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

const async_hooks = require('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.

require('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 process.

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.

FSEVENTWRAP, FSREQWRAP, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPPARSER,
JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, SHUTDOWNWRAP,
SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVER, TCPWRAP, TIMERWRAP, TTYWRAP,
UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,
RANDOMBYTESREQUEST, TLSWRAP, Timeout, Immediate, TickObject

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.

triggerId#

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:

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

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

Output when hitting the server with nc localhost 8080:

TCPSERVERWRAP(2): trigger: 1 execution: 1
TCPWRAP(4): trigger: 2 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. This can contain useful information that can vary based on the value of type. For instance, for the GETADDRINFOREQWRAP resource type, resource provides the hostname used when looking up the IP address for the hostname in net.Server.listen(). The API for accessing this information is currently not considered public, but using the Embedder API, users can provide and document their own resource objects. For example, such a resource object could contain the SQL query being executed.

In the case of Promises, the resource object will have promise property that refers to the Promise that is being initialized, and a isChainedPromise property, set to true if the promise has a parent promise, and false otherwise. For example, in the case of b = a.then(handler), a is considered a parent Promise of b. Here, b is considered a chained promise.

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 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.

let indent = 0;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      1,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}before:  ${asyncId}\n`);
    indent += 2;
  },
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}after:   ${asyncId}\n`);
  },
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}destroy: ${asyncId}\n`);
  },
}).enable();

require('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(2): trigger: 1 execution: 1
TickObject(3): trigger: 2 execution: 1
before:  3
  Timeout(4): trigger: 3 execution: 3
  TIMERWRAP(5): trigger: 3 execution: 3
after:   3
destroy: 3
before:  5
  before:  4
    TTYWRAP(6): trigger: 4 execution: 4
    SIGNALWRAP(7): trigger: 4 execution: 4
    TTYWRAP(8): trigger: 4 execution: 4
>>> 4
    TickObject(9): trigger: 4 execution: 4
  after:   4
after:   5
before:  9
after:   9
destroy: 4
destroy: 9
destroy: 5

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:

TTYWRAP(6) -> Timeout(4) -> TIMERWRAP(5) -> TickObject(3) -> root(1)

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 hostname is a synchronous operation, but to maintain a completely asynchronous API the user's callback is placed in a process.nextTick().

The graph only shows when a resource was created, not why, so to track the why use triggerAsyncId.

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.

promiseResolve(asyncId)#

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

Note that 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.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.
const async_hooks = require('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(function onConnection(conn) {
  // Returns the ID of the server, not of the new connection, because the
  // onConnection callback runs in the execution scope of the server's
  // MakeCallback().
  async_hooks.executionAsyncId();

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

Note that 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();
});

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

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.

Here's an example:

const ah = require('async_hooks');
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.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 note that 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. Example:

const ah = require('async_hooks');
ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.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 AsyncWrap JavaScript API so that all the appropriate callbacks are called.

class AsyncResource()#

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.

const { AsyncResource, executionAsyncId } = require('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();

// Call AsyncHooks before callbacks.
// Deprecated: Use asyncResource.runInAsyncScope instead.
asyncResource.emitBefore();

// Call AsyncHooks after callbacks.
// Deprecated: Use asyncResource.runInAsyncScope instead.
asyncResource.emitAfter();

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> Disables automatic 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. 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();
  }
}

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.emitBefore()#

Deprecated since: v9.6.0
Stability: 0 - Deprecated: Use asyncResource.runInAsyncScope() instead.

Call all before callbacks to notify that a new asynchronous execution context is being entered. If nested calls to emitBefore() are made, the stack of asyncIds will be tracked and properly unwound.

before and after calls must be unwound in the same order that they are called. Otherwise, an unrecoverable exception will occur and the process will abort. For this reason, the emitBefore and emitAfter APIs are considered deprecated. Please use runInAsyncScope, as it provides a much safer alternative.

asyncResource.emitAfter()#

Deprecated since: v9.6.0
Stability: 0 - Deprecated: Use asyncResource.runInAsyncScope() instead.

Call all after callbacks. If nested calls to emitBefore() were made, then make sure the stack is unwound properly. Otherwise an error will be thrown.

If the user's callback throws an exception, emitAfter() will automatically be called for all asyncIds on the stack if the error is handled by a domain or 'uncaughtException' handler.

before and after calls must be unwound in the same order that they are called. Otherwise, an unrecoverable exception will occur and the process will abort. For this reason, the emitBefore and emitAfter APIs are considered deprecated. Please use runInAsyncScope, as it provides a much safer alternative.

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.

Buffer#

Stability: 2 - Stable

Prior to the introduction of TypedArray, the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to make it possible to interact with octet streams in the context of things like TCP streams and file system operations.

With TypedArray now available, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js.

Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be changed.

The Buffer class is within the global scope, making it unlikely that one would need to ever use require('buffer').Buffer.

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

// Creates a Buffer of length 10, filled with 0x1.
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 either fill() or write().
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing [0x1, 0x2, 0x3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74].
const buf5 = Buffer.from('tést');

// Creates a Buffer containing Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf6 = Buffer.from('tést', 'latin1');

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. 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. Starting in Node.js 8.0.0, Buffer(num) and new Buffer(num) will 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.

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() 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, including buffers returned by new Buffer(size), Buffer.allocUnsafe(), Buffer.allocUnsafeSlow(), and new SlowBuffer(size). Use of this flag can have a significant negative impact on performance. Use of the --zero-fill-buffers option is recommended 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.

Buffers and Character Encodings#

History
VersionChanges
v6.4.0

Introduced latin1 as an alias for binary.

v5.0.0

Removed the deprecated raw and raws encodings.

When string data is stored in or extracted out of a Buffer instance, a character encoding may be specified.

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

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

console.log(Buffer.from('fhqwhgads', 'ascii'));
// 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>

The character encodings currently supported by Node.js include:

  • 'ascii' - For 7-bit ASCII data only. This encoding is fast and will strip the high bit if set.

  • 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.

  • 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.

  • 'ucs2' - Alias of 'utf16le'.

  • 'base64' - Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5.

  • 'latin1' - A way of encoding the Buffer into a one-byte encoded string (as defined by the IANA in RFC1345, page 63, to be the Latin-1 supplement block and C0/C1 control codes).

  • 'binary' - Alias for 'latin1'.

  • 'hex' - Encode each byte as two hexadecimal characters.

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 TypedArray#

History
VersionChanges
v3.0.0

The Buffers class now inherits from Uint8Array.

Buffer instances are also Uint8Array instances. However, there are subtle incompatibilities with TypedArray. For example, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient.

It is also possible to create new TypedArray instances from a Buffer with the following caveats:

  1. The Buffer object's memory is copied to the TypedArray, not shared.

  2. The Buffer object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], not a Uint32Array with a single element [0x1020304] or [0x4030201].

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypeArray object's .buffer property.

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>

Note that 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.

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:

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

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

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

Class: Buffer#

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

new Buffer(array)#

History
VersionChanges
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.

Allocates a new Buffer using an array of octets.

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

new Buffer(arrayBuffer[, byteOffset [, length]])#

History
VersionChanges
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.

This creates a view of the ArrayBuffer or SharedArrayBuffer 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.

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

const arr = new Uint16Array(2);

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

// Shares memory with `arr`
const buf = new Buffer(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>

new Buffer(buffer)#

History
VersionChanges
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.
  • buffer <Buffer> An existing Buffer to copy data from.

Copies the passed buffer data onto a new Buffer instance.

const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);

buf1[0] = 0x61;

console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer

new Buffer(size)#

History
VersionChanges
v8.0.0

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.

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

Prior to Node.js 8.0.0, the underlying memory for Buffer instances created in this way is not initialized. The contents of a newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc(size) instead to initialize a Buffer with zeroes.

const buf = new Buffer(10);

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

new Buffer(string[, encoding])#

History
VersionChanges
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'.

Creates a new Buffer containing string. The encoding parameter identifies the character encoding of string.

const buf1 = new Buffer('this is a tést');
const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex');

console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('ascii'));
// Prints: this is a tC)st

Class Method: Buffer.alloc(size[, fill[, encoding]])#

History
VersionChanges
REPLACEME

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

REPLACEME

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

  • size <integer> The desired length of the new Buffer.
  • fill <string> | <Buffer> | <integer> A value to pre-fill the new Buffer with. Default: 0.
  • encoding <string> If fill is a string, this is its encoding. Default: 'utf8'.

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

const buf = Buffer.alloc(5);

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

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

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

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).

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 significantly slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data.

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

Class Method: Buffer.allocUnsafe(size)#

History
VersionChanges
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 the size is larger than buffer.constants.MAX_LENGTH or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be 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 Buffer.alloc() instead to initialize Buffer instances with zeroes.

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.

Note that 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() 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.

Class Method: Buffer.allocUnsafeSlow(size)#

Added in: v5.12.0
  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If the size is larger than buffer.constants.MAX_LENGTH or smaller than 0, a RangeError will be thrown. A zero-length Buffer will be 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 4KB 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 persistent 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.

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

socket.on('readable', () => {
  const data = socket.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);
});

Buffer.allocUnsafeSlow() should be used only as a last resort after a developer has observed undue memory retention in their applications.

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

Class 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 actual byte length of a string. This is not the same as String.prototype.length since that returns the number of characters in a string.

For 'base64' 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.

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 actual byte length is returned.

Class 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).

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])

Class 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. This however causes an additional loop to be executed in order to calculate the totalLength, so it is faster to provide the length explicitly if it is already known.

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.

// 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

Class Method: Buffer.from(array)#

Added in: v5.10.0

Allocates a new Buffer using an array of octets.

// Creates a new Buffer containing 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.

Class 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.

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.

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.

Class Method: Buffer.from(buffer)#

Added in: v5.10.0
  • buffer <Buffer> An existing Buffer to copy data from.

Copies the passed buffer data onto a new Buffer instance.

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.

Class 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 of string.

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('ascii'));
// Prints: this is a tC)st

A TypeError will be thrown if string is not a string.

Class Method: Buffer.from(object[, offsetOrEncoding[, length]])#

Added in: v8.2.0
  • object <Object> An object supporting Symbol.toPrimitive or valueOf()
  • offsetOrEncoding <number> | <string> A byte-offset or encoding, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]().
  • length <number> A length, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]().

For objects whose valueOf() function returns a value not strictly equal to object, returns Buffer.from(object.valueOf(), offsetOrEncoding, length).

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](), offsetOrEncoding, length).

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>

Class Method: Buffer.isBuffer(obj)#

Added in: v0.1.101

Returns true if obj is a Buffer, false otherwise.

Class Method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if encoding contains a supported character encoding, or false otherwise.

Class Property: Buffer.poolSize#

Added in: v0.11.3

This is the number of bytes used to determine the size 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 - that is, getting returns undefined and setting does nothing.

// Copy an ASCII string into a `Buffer` one byte at a time.

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('ascii'));
// Prints: Node.js

buf.buffer#

The buffer property references the underlying ArrayBuffer object based on which this Buffer object is created.

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: true

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 to compare to.
  • targetStart <integer> The offset within target at which to begin comparison. Default: 0.
  • targetEnd <integer> The offset with 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.
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.

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

A RangeError will be 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 copying to. Default: 0.
  • sourceStart <integer> The offset within buf at which to begin copying from. 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.

// 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);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!

// 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.

// 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.

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
REPLACEME

Negative end values throw an ERR_INDEX_OUT_OF_RANGE error.

REPLACEME

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

REPLACEME

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

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled. This is meant to be a small simplification to allow the creation and filling of a Buffer to be done on a single line.

// Fill a `Buffer` with the ASCII character 'h'.

const b = Buffer.allocUnsafe(50).fill('h');

console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

value is coerced to a uint32 value if it is not a String or Integer.

If the final write of a fill() operation falls on a multi-byte character, then only the first bytes of that character that fit into buf are written.

// Fill a `Buffer` with a two-byte character.

console.log(Buffer.allocUnsafe(3).fill('\u0222'));
// Prints: <Buffer c8 a2 c8>

If value contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown:

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> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in 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.

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. Default: 0.
  • encoding <string> If value is a string, this is its encoding. 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.slice().
  • a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.
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. Any arguments that coerce to NaN or 0, like {}, [], null or undefined, will search the whole buffer. This behavior matches String#indexOf().

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).

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

Identical to buf.indexOf(), except buf is searched from back to front instead of front to back.

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#lastIndexOf().

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 amount of memory allocated for buf in bytes. Note that this does not necessarily reflect the amount of "usable" data within buf.

// Create a `Buffer` and write a shorter ASCII string to it.

const buf = Buffer.alloc(1234);

console.log(buf.length);
// Prints: 1234

buf.write('some string', 0, 'ascii');

console.log(buf.length);
// Prints: 1234

While the length property is not immutable, changing the value of length can result in undefined and inconsistent behavior. Applications that wish to modify the length of a Buffer should therefore treat length as read-only and use buf.slice() to create a new Buffer.

let buf = Buffer.allocUnsafe(10);

buf.write('abcdefghj', 0, 'ascii');

console.log(buf.length);
// Prints: 10

buf = buf.slice(0, 5);

console.log(buf.length);
// Prints: 5

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.readDoubleBE(offset)#

buf.readDoubleLE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <number>

Reads a 64-bit double from buf at the specified offset with specified endian format (readDoubleBE() returns big endian, readDoubleLE() returns little endian).

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304
console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws an exception: RangeError: Index out of range
console.log(buf.readDoubleLE(1, true));
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!

buf.readFloatBE(offset)#

buf.readFloatLE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <number>

Reads a 32-bit float from buf at the specified offset with specified endian format (readFloatBE() returns big endian, readFloatLE() returns little endian).

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

console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38
console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws an exception: RangeError: Index out of range
console.log(buf.readFloatLE(1, true));
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!

buf.readInt8(offset)#

History
VersionChanges
REPLACEME

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.
  • 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.

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 an exception: RangeError: Index out of range

buf.readInt16BE(offset)#

buf.readInt16LE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer>

Reads a signed 16-bit integer from buf at the specified offset with the specified endian format (readInt16BE() returns big endian, readInt16LE() returns little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16BE(0));
// Prints: 5
console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws an exception: RangeError: Index out of range

buf.readInt32BE(offset)#

buf.readInt32LE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer>

Reads a signed 32-bit integer from buf at the specified offset with the specified endian format (readInt32BE() returns big endian, readInt32LE() returns little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32BE(0));
// Prints: 5
console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws an exception: RangeError: Index out of range

buf.readIntBE(offset, byteLength)#

buf.readIntLE(offset, byteLength)#

History
VersionChanges
REPLACEME

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 two's complement signed value. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee
console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_INDEX_OUT_OF_RANGE:
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE:

buf.readUInt8(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

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 an exception: RangeError: Index out of range

buf.readUInt16BE(offset)#

buf.readUInt16LE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer>

Reads an unsigned 16-bit integer from buf at the specified offset with specified endian format (readUInt16BE() returns big endian, readUInt16LE() returns little endian).

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws an exception: RangeError: Index out of range

buf.readUInt32BE(offset)#

buf.readUInt32LE(offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer>

Reads an unsigned 32-bit integer from buf at the specified offset with specified endian format (readUInt32BE() returns big endian, readUInt32LE() returns little endian).

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678
console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws an exception: RangeError: Index out of range

buf.readUIntBE(offset, byteLength)#

buf.readUIntLE(offset, byteLength)#

History
VersionChanges
REPLACEME

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 integer. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws an exception: RangeError: Index out of range

buf.slice([start[, end]])#

History
VersionChanges
v7.1.0, v6.9.2

Coercing the offsets to integers now handles values outside the 32-bit integer range properly.

v7.0.0

All offsets are now coerced to integers before doing any calculations with them.

v0.3.0

Added in: v0.3.0

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.

Modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

// 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.slice(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.

const buf = Buffer.from('buffer');

console.log(buf.slice(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.slice(0, 5))

console.log(buf.slice(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.slice(0, 4))

console.log(buf.slice(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.slice(1, 4))

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 a RangeError if buf.length is not a multiple of 2.

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 an exception: RangeError: Buffer size must be a multiple of 16-bits

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 a RangeError if buf.length is not a multiple of 4.

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 an exception: RangeError: Buffer size must be a multiple of 32-bits

buf.swap64()#

Added in: v6.3.0

Interprets buf as an array of 64-bit numbers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 8.

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 an exception: RangeError: Buffer size must be a multiple of 64-bits

Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-bit floats.

buf.toJSON()#

Added in: v0.9.2

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

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.data) :
    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 decode to. 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.

The maximum length of a string instance (in UTF-16 code units) is available as buffer.constants.MAX_STRING_LENGTH.

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('ascii'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('ascii', 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.

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 be written to buf.
  • offset <integer> Number of bytes to skip before starting to write string. Default: 0.
  • length <integer> Number of bytes to write. 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 a partial amount of string will be written. However, partially encoded characters will not be written.

const buf = Buffer.allocUnsafe(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾

buf.writeDoubleBE(value, offset)#

buf.writeDoubleLE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeDoubleBE() writes big endian, writeDoubleLE() writes little endian). value should be a valid 64-bit double. Behavior is undefined when value is anything other than a 64-bit double.

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(0xdeadbeefcafebabe, 0);

console.log(buf);
// Prints: <Buffer 43 eb d5 b7 dd f9 5f d7>

buf.writeDoubleLE(0xdeadbeefcafebabe, 0);

console.log(buf);
// Prints: <Buffer d7 5f f9 dd b7 d5 eb 43>

buf.writeFloatBE(value, offset)#

buf.writeFloatLE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeFloatBE() writes big endian, writeFloatLE() writes little endian). value should be a valid 32-bit float. Behavior is undefined when value is anything other than a 32-bit float.

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer 4f 4a fe bb>

buf.writeFloatLE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer bb fe 4a 4f>

buf.writeInt8(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value should 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.

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

console.log(buf);
// Prints: <Buffer 02 fe>

buf.writeInt16BE(value, offset)#

buf.writeInt16LE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeInt16BE() writes big endian, writeInt16LE() writes little endian). value should be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(4);

buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);

console.log(buf);
// Prints: <Buffer 01 02 04 03>

buf.writeInt32BE(value, offset)#

buf.writeInt32LE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeInt32BE() writes big endian, writeInt32LE() writes little endian). value should be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(8);

buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);

console.log(buf);
// Prints: <Buffer 01 02 03 04 08 07 06 05>

buf.writeIntBE(value, offset, byteLength)#

buf.writeIntLE(value, offset, byteLength)#

History
VersionChanges
REPLACEME

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. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

buf.writeUInt8(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value should be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

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)#

buf.writeUInt16LE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeUInt16BE() writes big endian, writeUInt16LE() writes little endian). value should be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer de ad be ef>

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer ad de ef be>

buf.writeUInt32BE(value, offset)#

buf.writeUInt32LE(value, offset)#

History
VersionChanges
REPLACEME

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.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeUInt32BE() writes big endian, writeUInt32LE() writes little endian). value should be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer fe ed fa ce>

buf.writeUInt32LE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer ce fa ed fe>

buf.writeUIntBE(value, offset, byteLength)#

buf.writeUIntLE(value, offset, byteLength)#

History
VersionChanges
REPLACEME

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. Default: false.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeUIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

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.

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.kMaxLength#

Added in: v3.0.0
  • <integer> The largest size allowed for a single Buffer instance.

An alias for buffer.constants.MAX_LENGTH

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

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.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

const buffer = require('buffer');

const newBuf = buffer.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.

Note that this is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

Class: SlowBuffer#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

Returns an un-pooled Buffer.

In order to avoid the garbage collection overhead of creating many individually allocated Buffer instances, by default allocations under 4KB are sliced from a single larger allocated object.

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 SlowBuffer then copy out the relevant bits.

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

socket.on('readable', () => {
  const data = socket.read();

  // Allocate for retained data
  const sb = SlowBuffer(10);

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

  store.push(sb);
});

Use of SlowBuffer should be used only as a last resort after a developer has observed undue memory retention in their applications.

new SlowBuffer(size)#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.
  • size <integer> The desired length of the new SlowBuffer.

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

The underlying memory for SlowBuffer instances is not initialized. The contents of a newly created SlowBuffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize a SlowBuffer with zeroes.

const { SlowBuffer } = require('buffer');

const buf = new SlowBuffer(5);

console.log(buf);
// Prints: (contents may vary): <Buffer 78 e0 82 02 01>

buf.fill(0);

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

Buffer Constants#

Added in: v8.2.0

Note that buffer.constants is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.constants.MAX_LENGTH#

Added in: v8.2.0
  • <integer> The largest size allowed for a single Buffer instance.

On 32-bit architectures, this value is (2^30)-1 (~1GB). On 64-bit architectures, this value is (2^31)-1 (~2GB).

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.

C++ Addons#

Node.js Addons are dynamically-linked shared objects, written in C++, that can be loaded into Node.js using the require() function, and used just as if they were an ordinary Node.js module. They are used primarily to provide an interface between JavaScript running in Node.js and C/C++ libraries.

At the moment, the method for implementing Addons is rather complicated, involving knowledge of several components and APIs :

  • V8: the C++ library Node.js currently 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 filesystem, sockets, timers, and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads.

  • Internal Node.js libraries. Node.js itself exports a number of C++ APIs that Addons can use — the most important of which is the node::ObjectWrap class.

  • Node.js includes a number of 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 Node.js' own dependencies 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"));
}

void init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "hello", Method);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, init)

}  // namespace demo

Note that 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 init and the Addon module name is 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'

Please see the examples below for further information or https://github.com/arturadib/node-qt for an example in production.

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.

Note that 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 Node.js' own dependencies#

Node.js uses a number of 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 make direct use of the Node.js and V8 APIs for implementing Addons. It is important to understand that 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 currently 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.

N-API#

Stability: 1 - Experimental

N-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 version 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 N-API are used.

To use N-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 N-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, "hello", 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 the section titled C/C++ Addons - N-API.

Addon examples#

Following are some example Addons intended to help developers get started. The examples make use of 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")));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong arguments")));
    return;
  }

  // Perform the operation
  double value = args[0]->NumberValue() + args[1]->NumberValue();
  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::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<Function> cb = Local<Function>::Cast(args[0]);
  const unsigned argc = 1;
  Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") };
  cb->Call(Null(isolate), argc, argv);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", RunCallback);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

Note that 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'
});

Note that, 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::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<Object> obj = Object::New(isolate);
  obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString());

  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::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"));
}

void CreateFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
  Local<Function> fn = tpl->GetFunction();

  // omit this to make it anonymous
  fn->SetName(String::NewFromUtf8(isolate, "theFunction"));

  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);
  static v8::Persistent<v8::Function> constructor;
  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::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Local<Object> exports) {
  Isolate* isolate = exports->GetIsolate();

  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
  exports->Set(String::NewFromUtf8(isolate, "MyObject"),
               tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    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<Context> context = isolate->GetCurrentContext();
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    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

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::Persistent<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 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::Persistent;
using v8::String;
using v8::Value;

Persistent<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"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    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<Context> context = isolate->GetCurrentContext();
    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::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();

  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
      args[0]->ToObject());
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
      args[1]->ToObject());

  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::Persistent<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 v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<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"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    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<Context> context = isolate->GetCurrentContext();
    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

AtExit hooks#

An "AtExit" hook is a function that is invoked after the Node.js event loop has ended but before the JavaScript VM is terminated and Node.js shuts down. "AtExit" hooks are registered using the node::AtExit API.

void AtExit(callback, args)#

  • callback <void (*)(void*)> A pointer to the function to call at exit.
  • args <void*> A pointer to pass to the callback at exit.

Registers exit hooks that run after the event loop has ended but before the VM is killed.

AtExit takes two parameters: a pointer to a callback function to run at exit, and a pointer to untyped context data to be passed to that callback.

Callbacks are run in last-in first-out order.

The following addon.cc implements AtExit:

// addon.cc
#include <assert.h>
#include <stdlib.h>
#include <node.h>

namespace demo {

using node::AtExit;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;

static char cookie[] = "yum yum";
static int at_exit_cb1_called = 0;
static int at_exit_cb2_called = 0;

static void at_exit_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());
  at_exit_cb1_called++;
}

static void at_exit_cb2(void* arg) {
  assert(arg == static_cast<void*>(cookie));
  at_exit_cb2_called++;
}

static void sanity_check(void*) {
  assert(at_exit_cb1_called == 1);
  assert(at_exit_cb2_called == 2);
}

void init(Local<Object> exports) {
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb1, exports->GetIsolate());
  AtExit(sanity_check);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, init)

}  // namespace demo

Test in JavaScript by running:

// test.js
require('./build/Release/addon');

N-API#

Stability: 2 - Stable

N-API (pronounced N as in the letter, followed by API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (ex 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 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 the N-API are used.

APIs exposed by N-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA262 Language Specification. The APIs have the following properties:

  • All N-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.

The documentation for N-API is structured as follows:

The N-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. However, we also understand that a C++ API can be easier to use in many cases. To support these cases we expect there to be one or more C++ wrapper modules that provide an inlineable C++ API. Binaries built with these wrapper modules will depend on the symbols for the N-API C based functions exported by Node.js. These wrappers are not part of N-API, nor will they be maintained as part of Node.js. One such example is: node-api.

In order to use the N-API functions, include the file node_api.h which is located in the src directory in the node development tree:

#include <node_api.h>

Basic N-API Data Types#

N-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 N-API calls.

napi_status#

Integral status code indicating the success or failure of a N-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_status_last
} 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#

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 N-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 N-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 N-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 N-API calls. Caching the napi_env for the purpose of general reuse is not allowed.

napi_value#

This is an opaque pointer that is used to represent a JavaScript value.

N-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, N-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, N-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#

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#

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.

N-API Callback types#

napi_callback_info#

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#

Function pointer type for user-provided native functions which are to be exposed to JavaScript via N-API. Callback functions should satisfy the following signature:

typedef napi_value (*napi_callback)(napi_env, napi_callback_info);

napi_finalize#

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);

napi_async_execute_callback#

Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:

typedef void (*napi_async_execute_callback)(napi_env env, void* data);

napi_async_complete_callback#

Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:

typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

Error Handling#

N-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 N-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:

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: n-api status code for the last error.

napi_get_last_error_info returns the information for the last N-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 
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 an n-api function is called on the same env.

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 N-API function call may result in a pending JavaScript exception. This is obviously the case for any function that may cause the execution of JavaScript, but N-API specifies that an exception may be pending on return from any of the API functions.

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.

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 N-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception, so it is important to 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 Error object 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 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, and napi_create_range_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 N-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 
NODE_EXTERN napi_status napi_throw(napi_env env, napi_value error);
  • [in] env: The environment that the API is invoked under.
  • [in] error: The napi_value for the Error to be thrown.

Returns napi_ok if the API succeeded.

This API throws the JavaScript Error provided.

napi_throw_error#

Added in: v8.0.0 
NODE_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 
NODE_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 
NODE_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.

napi_is_error#

Added in: v8.0.0 
NODE_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] msg: 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 
NODE_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 
NODE_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 
NODE_EXTERN napi_status napi_create_range_error(napi_env env,
                                                napi_value code,
                                                const char* 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.

napi_get_and_clear_last_exception#

Added in: v8.0.0 
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 returns true if an exception is pending.

This API can be called even if there is a pending JavaScript exception.

napi_is_exception_pending#

Added in: v8.0.0 
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 returns true if an exception is pending.

This API can be called even if there is a pending JavaScript exception.

napi_fatal_exception#

Added in: v9.10.0 
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 you want to pass 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 module, a fatal error can be thrown to immediately terminate the process.

napi_fatal_error#

Added in: v8.2.0 
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 N-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 N-API functions than 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(e, 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, N-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.

N-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(e, 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. N-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 
NODE_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 open a new scope.

napi_close_handle_scope#

Added in: v8.0.0 
NODE_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 
NODE_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 open a new scope from which one object can be promoted to the outer scope.

napi_close_escapable_handle_scope#

Added in: v8.0.0 
NODE_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 
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.

N-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 will result 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 will result 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.

napi_create_reference#

Added in: v8.0.0 
NODE_EXTERN napi_status napi_create_reference(napi_env env,
                                              napi_value value,
                                              int 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 create a new reference with the specified reference count to the Object passed in.

napi_delete_reference#

Added in: v8.0.0 
NODE_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 
NODE_EXTERN napi_status napi_reference_ref(napi_env env,
                                           napi_ref ref,
                                           int* 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 
NODE_EXTERN napi_status napi_reference_unref(napi_env env,
                                             napi_ref ref,
                                             int* 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 
NODE_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.

Module registration#

N-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 N-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. N-API modules cannot modify the module object but can specify anything as the exports property of the module.

For example, 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", Method, 0, 0, 0, napi_default, 0};
  if (status != napi_ok) return NULL;
  status = napi_define_properties(env, exports, 1, &desc);
  if (status != napi_ok) return NULL;
  return exports;
}

For example, 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;
}

For example, 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, GetValue, SetValue, 0, napi_default, 0 },
    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;
}

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#

N-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 an N-API value
  3. Convert from N-API value to a primitive C type
  4. Get global instances including undefined and null

N-API values are represented by the type napi_value. Any N-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_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_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.

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_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 
napi_status napi_create_array(napi_env env, napi_value* result)
  • [in] env: The environment that the N-API call is invoked under.
  • [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns an N-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 
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 an N-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 
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.
  • [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns an N-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 
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.
  • [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 
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.
  • [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_external#

Added in: v8.0.0 
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.
  • [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. The API allows the caller to pass in a finalize callback, in case the underlying native resource needs to be cleaned up when the external JavaScript value gets 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 
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.
  • [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.

This API returns an N-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.

JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_external_buffer#

Added in: v8.0.0 
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 copy from.
  • [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected.
  • [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.

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.

For Node.js >=4 Buffers are Uint8Arrays.

napi_create_function#

Added in: v8.0.0 
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: A string representing the name of the function encoded as UTF8.
  • [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] cb: A function pointer to the native function to be invoked when the created function is invoked from JavaScript.
  • [in] data: Optional arbitrary context data to be passed into the native function when it is invoked.
  • [out] result: A napi_value representing a JavaScript function.

Returns napi_ok if the API succeeded.

This API returns an N-API value corresponding to a JavaScript Function object. It's used to wrap native functions so that they can be invoked from JavaScript.

JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.

napi_create_object#

Added in: v8.0.0 
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 
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 object from a UTF8-encoded C string

The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification.

napi_create_typedarray#

Added in: v8.0.0 
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 
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 N-API#

napi_create_int32#

Added in: v8.4.0 
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 
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 
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 
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_string_latin1#

Added in: v8.0.0 
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 a 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 object from a ISO-8859-1-encoded C string.

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 
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 object from a UTF16-LE-encoded C string

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 
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 object from a UTF8-encoded C string

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

Functions to convert from N-API to C types#

napi_get_array_length#

Added in: v8.0.0 
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 
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.
  • [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 
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.
  • [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 it's 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 
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 
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: Number of elements in the TypedArray.
  • [out] data: The data buffer underlying the typed array.
  • [out] byte_offset: The byte offset within the data buffer from which to start projecting the TypedArray.

Returns napi_ok if the API succeeded.

This API returns various properties of a typed array.

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 
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.
  • [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.

This API returns various properties of a DataView.

napi_get_value_bool#

Added in: v8.0.0 
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 
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_external#

Added in: v8.0.0 
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 
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 > 2^31 -1.

napi_get_value_int64#

Added in: v8.0.0 
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.

napi_get_value_string_latin1#

Added in: v8.0.0 
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) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [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 
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) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [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 
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) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [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 
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 
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 
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 
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 
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 - Abstract Operations#

N-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 
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. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_number#

Added in: v8.0.0 
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 API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_object#

Added in: v8.0.0 
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. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_string#

Added in: v8.0.0 
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 API can be re-entrant if getters are defined on the passed-in Object.

napi_typeof#

Added in: v8.0.0 
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, it has support for detecting an External value. If value has a type that is invalid, an error is returned.

napi_instanceof#

Added in: v8.0.0 
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 
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 
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 
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_error#

Added in: v8.0.0 
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 
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 
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 
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.

Working with JavaScript Properties#

N-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 N-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 N-API by napi_value. This can be a napi_value representing a String, Number, or Symbol.

N-API values are represented by the type napi_value. Any N-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 N-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 N-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 N-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 N-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, 0, 0, 0, fooValue, napi_default, 0 },
  { "bar", NULL, 0, 0, 0, barValue, napi_default, 0 }
}
status = napi_define_properties(env,
                                obj,
                                sizeof(descriptors) / sizeof(descriptors[0]),
                                descriptors);
if (status != napi_ok) return status;

Structures#

napi_property_attributes#

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,
} 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 - Used to indicate that no explicit attributes are set on the given property. By default, a property is read only, not enumerable and not configurable.
  • napi_writable - Used to indicate that a given property is writable.
  • napi_enumerable - Used to indicate that a given property is enumerable.
  • napi_configurable - Used to indicate that a given property is configurable, as defined in Section 6.1.7.1 of the ECMAScript Language Specification.
  • napi_static - Used to indicate that 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_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 N-API call).
  • 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 N-API call).
  • 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).
  • data: The callback data passed into method, getter and setter if this function is invoked.
  • attributes: The attributes associated with the particular property. See napi_property_attributes.

Functions#

napi_get_property_names#

Added in: v8.0.0 
napi_status napi_get_property_names(napi_env env,
                                    napi_value object,
                                    napi_value* result);
  • [in] env: The environment that the N-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 array of properties for the Object passed in

napi_set_property#

Added in: v8.0.0 
napi_status napi_set_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value value);
  • [in] env: The environment that the N-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 
napi_status napi_get_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value* result);
  • [in] env: The environment that the N-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 
napi_status napi_has_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              bool* result);
  • [in] env: The environment that the N-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 
napi_status napi_delete_property(napi_env env,
                                 napi_value object,
                                 napi_value key,
                                 bool* result);
  • [in] env: The environment that the N-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 
napi_status napi_has_own_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  bool* result);
  • [in] env: The environment that the N-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. N-API will not perform any conversion between data types.

napi_set_named_property#

Added in: v8.0.0 
napi_status napi_set_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value value);
  • [in] env: The environment that the N-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 
napi_status napi_get_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value* result);
  • [in] env: The environment that the N-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 
napi_status napi_has_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    bool* result);
  • [in] env: The environment that the N-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 
napi_status napi_set_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value value);
  • [in] env: The environment that the N-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 and element on the Object passed in.

napi_get_element#

Added in: v8.0.0 
napi_status napi_get_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value* result);
  • [in] env: The environment that the N-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 
napi_status napi_has_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             bool* result);
  • [in] env: The environment that the N-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 
napi_status napi_delete_element(napi_env env,
                                napi_value object,
                                uint32_t index,
                                bool* result);
  • [in] env: The environment that the N-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 
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 N-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 ECMA262 specification).

Working with JavaScript Functions#

N-API provides a set of APIs that allow JavaScript code to call back into native code. N-API 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, N-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.

napi_call_function#

Added in: v8.0.0 
napi_status napi_call_function(napi_env env,
                               napi_value recv,
                               napi_value func,
                               int argc,
                               const napi_value* argv,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] recv: The this object 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;
}

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 
napi_status napi_create_function(napi_env env,
                                 const char* utf8name,
                                 napi_callback cb,
                                 void* data,
                                 napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] utf8Name: The name of the function encoded as UTF8. This is visible within JavaScript as the new function object's name property.
  • [in] cb: The native function which should be called when this function object is invoked.
  • [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 not necessarily the name passed into NAPI_MODULE in the earlier snippet but the name of the target in binding.gyp responsible for creating the .node file.

napi_get_cb_info#

Added in: v8.0.0 
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 size of the provided argv array and receives the actual count of arguments.
  • [out] argv: Buffer to which the napi_value representing the arguments are 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.
  • [out] this: Receives the JavaScript this argument for the call.
  • [out] data: Receives the data pointer for the callback.

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 
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 
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.
  • [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 N-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#

N-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.

As an example:

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.

napi_define_class#

Added in: v8.0.0 
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; this is not required to be the same as the C++ class name, though it is recommended for clarity.
  • [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. (This should be a static method on the class, not an actual C++ constructor function.)
  • [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 that corresponds to a C++ class, including:

  • A JavaScript constructor function that has the class name and invokes the provided C++ constructor callback.
  • Properties on the constructor function corresponding to 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 corresponding to non-static data properties, accessors, and methods of the C++ class (defined by property descriptors without the napi_static attribute).

The C++ constructor callback 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 check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, create a persistent reference to it using napi_create_reference and ensure the reference count is kept >= 1.

napi_wrap#

Added in: v8.0.0 
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. This object must have been created from the prototype of a constructor that was created using napi_define_class().
  • [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 is ready for garbage-collection.
  • [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 proper of the reference.

This API may modify the prototype chain of the wrapper object. Afterward, additional manipulation of the wrapper's prototype chain may cause napi_unwrap() to fail.

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 
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 
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, thereby restoring the JavaScript object's prototype chain. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected.

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 is important in order to allow them to avoid blocking overall execution of the Node.js application.

N-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.

N-API defines the napi_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. 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

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.
  • [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 
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 
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.

napi_cancel_async_work#

Added in: v8.0.0 
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 
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: 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.
  • [out] result: The initialized async context.

Returns napi_ok if the API succeeded.

napi_async_destroy#

Added in: v8.6.0 
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

napi_status napi_make_callback(napi_env env,
                               napi_async_context async_context,
                               napi_value recv,
                               napi_value func,
                               int 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. However NULL is also allowed, which indicates the current async context (if any) is to be used for the callback.
  • [in] recv: The this object 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.
  • [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.

napi_open_callback_scope#

Added in: v9.6.0 
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 optional object associated with the async work that will be passed to possible async_hooks init hooks.
  • [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 N-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 
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 
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 
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 N-API supported.

Returns napi_ok if the API succeeded.

This API returns the highest N-API version supported by the Node.js runtime. N-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 
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 module). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise.

Promises#

N-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 
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 
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 
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 
napi_status napi_is_promise(napi_env env,
                            napi_value promise,
                            bool* is_promise);
  • [in] env: The environment that the API is invoked under.
  • [in] promise: The promise 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#

N-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.

napi_run_script#

Added in: v8.5.0 
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.

libuv event loop#

N-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 
NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               uv_loop_t** loop);
  • [in] env: The environment that the API is invoked under.
  • [out] loop: The current libuv loop instance.

Child Process#

Stability: 2 - Stable

The child_process module provides the ability to spawn child processes 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('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`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 child. These pipes have limited (and platform-specific) capacity. If the child process writes to stdout in excess of that limit without the output being captured, the child process will block 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 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 child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Note that 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('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.log(data.toString());
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

// OR...
const { exec } = require('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
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:

exec('"/path/to/test file/test.sh" arg1 arg2');
// Double quotes are used so that the space in the path is not interpreted as
// 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.

const { exec } = require('child_process');
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

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 child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error.

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 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. 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 an additional two properties stdout and stderr.

const util = require('util');
const exec = util.promisify(require('child_process').exec);

async function lsExample() {
  const { stdout, stderr } = await exec('ls');
  console.log('stdout:', stdout);
  console.log('stderr:', stderr);
}
lsExample();

child_process.execFile(file[, args][, options][, callback])#

History
VersionChanges
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('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. 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 an additional two properties stdout and stderr.

const util = require('util');
const execFile = util.promisify(require('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.

child_process.fork(modulePath[, args][, options])#

History
VersionChanges
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> The module to run in the child.
  • args <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs.
    • execPath <string> Executable used to create the child process.
    • execArgv <Array> List of string arguments passed to the executable. Default: process.execArgv.
    • 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'].
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
  • 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.

It is important to 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.

child_process.spawn(command[, args][, options])#

History
VersionChanges
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 <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs.
    • 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)).
    • 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. Default: false.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • 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.

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('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`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('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.log(`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.log(`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('child_process');
const subprocess = spawn('bad_command');

subprocess.on('error', (err) => {
  console.log('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 currently 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.

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. Note that 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, 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 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('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('fs');
const { spawn } = require('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
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)
  • 'ignore' - equivalent to ['ignore', 'ignore', 'ignore']
  • 'inherit' - equivalent to [process.stdin, process.stdout, process.stderr] 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 - 2 are also available as subprocess.stdin, subprocess.stdout and subprocess.stderr, respectively.

  2. '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(), process.disconnect(), process.on('disconnect'), and process.on('message') 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.

  3. 'ignore' - Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 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.
  4. <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. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).
  5. Positive integer - The integer value is interpreted as a file descriptor that is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared.
  6. 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'.

Example:

const { spawn } = require('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 process.on('disconnect') event or the process.on('message') event. This allows the child to exit normally without the process being held open by the open IPC channel.

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
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> Current working directory of the child process.
    • input <string> | <Buffer> | <Uint8Array> 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.
    • 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: 200 * 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
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> Current working directory of the child process.
    • input <string> | <Buffer> | <Uint8Array> 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.
    • 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. See caveat at maxBuffer and Unicode. Default: 200 * 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. Note that 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
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.

v5.7.0

The shell option is supported now.

v0.11.12

Added in: v0.11.12

  • command <string> The command to run.
  • args <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> | <Uint8Array> 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.
    • env <Object> Environment key-value pairs.
    • 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: 200 * 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. 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> The exit code of the child process.
    • signal <string> The signal used to kill the child process.
    • 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. Note that 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 class are EventEmitters that 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 when 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.

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:

  1. The process could not be spawned, or
  2. The process could not be killed, or
  3. Sending a message to the child process failed.

The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, it is important to 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.

Note that when the 'exit' event is triggered, child process stdio streams might still be open.

Also, note that 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.

subprocess.channel#

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 currently exists, this property is undefined.

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().

Note that 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.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.

const { spawn } = require('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.

Note that while the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

Also note: 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 use of the shell option of ChildProcess, such as in this example:

'use strict';
const { spawn } = require('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 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.

Example:

const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

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 - A Boolean 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 process.on('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('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 process.on('message') event. Rather, such messages are emitted using the process.on('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 process.on('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('child_process').fork('subprocess.js');

// Open up the server object and send the handle.
const server = require('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 net module, 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, currently 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('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('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`);
    }
  }
});

Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null. It is recommended not to use .maxConnections when this occurs.

It is also recommended that any 'message' handlers in the child process verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

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.

subprocess.stdin#

Added in: v0.1.90

A Writable Stream that represents the child process's stdin.

Note that 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.

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'. Note that 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('assert');
const fs = require('fs');
const child_process = require('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);

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.

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 on UNIX or /d /s /c on Windows. On Windows, command line parsing should be compatible with 'cmd.exe'.

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.

Cluster#

Stability: 2 - Stable

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows easy creation of child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${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
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

Please note that 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 master 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 master 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 master 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 master, 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 master 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 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 master 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.

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
});

It is not emitted in the worker.

Event: 'message'#

Added in: v0.7.0

Similar to the cluster.on('message') event, but specific to this worker.

Within a worker, process.on('message') may also be used.

See process event: 'message'.

As an example, here is a cluster that keeps count of the number of requests in the master process using the message system:

const cluster = require('cluster');
const http = require('http');

if (cluster.isMaster) {

  // 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('os').cpus().length;
  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 master 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 master, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

Note that 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.

Note that 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.isMaster) {
  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('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

Set by calling .kill() or .disconnect(). Until then, it is undefined.

The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the master 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 master via its IPC channel, false otherwise. A worker is connected to its master 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.

worker.kill([signal='SIGTERM'])#

Added in: v0.9.12
  • signal <string> Name of the kill signal to send to the worker process.

This function will kill the worker. In the master, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by disconnecting the channel, and then exiting with code 0.

Causes .exitedAfterDisconnect to be set.

This method is aliased as worker.destroy() for backwards compatibility.

Note that 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

Note that 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][, callback])#

History
VersionChanges
v4.0.0

The callback parameter is supported now.

v0.7.0

Added in: v0.7.0

Send a message to a worker or master, optionally with a handle.

In the master this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker this sends a message to the master. It is identical to process.send().

This example will echo back all messages from the master:

if (cluster.isMaster) {
  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 master.

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' (UDP v4 or v6)

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 master receives a message from any worker.

See child_process event: 'message'.

Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated.

If support for older versions is required but a worker object is not required, it is possible to work around the discrepancy by checking the number of arguments:

cluster.on('message', (worker, message, handle) => {
  if (arguments.length === 2) {
    handle = message;
    message = worker;
    worker = undefined;
  }
  // ...
});

Event: 'online'#

Added in: v0.7.0

After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master 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 .setupMaster() is called.

The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() 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 master 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 master process.

cluster.fork([env])#

Added in: v0.6.0

Spawn a new worker process.

This can only be called from the master process.

cluster.isMaster#

Added in: v0.8.1

True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true.

cluster.isWorker#

Added in: v0.6.0

True if the process is not a master (it is the negation of cluster.isMaster).

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 cluster.setupMaster() 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
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 <Array> 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 <Array> 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).
    • 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 master's process.debugPort.
    • windowsHide <boolean> Hide the forked processes console window that would normally be created on Windows systems. Default: false.

After calling .setupMaster() (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
v6.4.0

The stdio option is supported now.

v0.7.1

Added in: v0.7.1

setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Note that:

  • 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 .setupMaster() is the env passed to .fork().
  • The defaults above apply to the first call only, the defaults for later calls is the current value at the time of cluster.setupMaster() is called.

Example:

const cluster = require('cluster');
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'http']
});
cluster.fork(); // http worker

This can only be called from the master process.

cluster.worker#

Added in: v0.7.0

A reference to the current worker object. Not available in the master process.

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('I am master');
  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. Makes it easy to loop through all the workers. It is only available in the master 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 last 'disconnect' or 'exit' event is emitted.

// Go through all workers
function eachWorker(callback) {
  for (const id in cluster.workers) {
    callback(cluster.workers[id]);
  }
}
eachWorker((worker) => {
  worker.send('big announcement to all workers');
});

Using the worker's unique id is the easiest way to locate the worker.

socket.on('data', (id) => {
  const worker = cluster.workers[id];
});

Command Line Options#

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] [script.js | -e "script" | -] [--] [arguments]

node debug [script.js | -e "script" | <host>:<port>] …

node --v8-options

Execute without arguments to start the REPL.

For more info about node debug, please see the debugger documentation.

Options#

-v, --version#

Added in: v0.1.3

Print node's version.

-h, --help#

Added in: v0.1.3

Print node command line options. The output of this option is less detailed than this document.

-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.

-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.

-c, --check#

Added in: v5.0.0, v4.2.0

Syntax check the script without executing.

-i, --interactive#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

-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.

--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 Debugging Protocol.

--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.

--no-deprecation#

Added in: v0.8.0

Silence deprecation warnings.

--trace-deprecation#

Added in: v0.8.0

Print stack traces for deprecations.

--throw-deprecation#

Added in: v0.11.14

Throw errors for deprecations.

--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.

--no-warnings#

Added in: v6.0.0

Silence all process warnings (including deprecations).

--abort-on-uncaught-exception#

Added in: v0.10

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 domain module that uses it).

--trace-warnings#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

--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.

--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.

--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.

--trace-events-enabled#

Added in: v7.7.0

Enables the collection of trace event tracing information.

--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}.

--zero-fill-buffers#

Added in: v6.0.0

Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.

--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).

--track-heap-objects#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

--prof-process#

Added in: v5.2.0

Process V8 profiler output generated using the V8 option --prof.

--v8-options#

Added in: v0.1.3

Print V8 command line options.

V8 options allow words to be separated by both dashes (-) or underscores (_).

For example, --stack-trace-limit is equivalent to --stack_trace_limit.

--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))

--enable-fips#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --openssl-fips)

--force-fips#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips)

--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 with ./configure --openssl-fips.

--use-openssl-ca, --use-bundled-ca#

Added in: v6.11.0

Use OpenSSL's default CA store or use bundled Mozilla CA store as supplied by current Node.js version. The default store is selectable at build-time.

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.

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.

See SSL_CERT_DIR and SSL_CERT_FILE.

--icu-data-dir=file#

Added in: v0.11.15

Specify ICU data load path. (overrides NODE_ICU_DATA)

-#

Added in: v8.0.0

Alias for stdin, analogous to the use of - in other command line utilities, meaning that the script will be 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 will be used as a script filename.

Environment Variables#

NODE_DEBUG=module[,…]#

Added in: v0.1.32

','-separated list of core modules that should print debug information.

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_DISABLE_COLORS=1#

Added in: v0.3.0

When set to 1 colors will not be used in the REPL.

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 as if they had been specified on the command line before the actual command line (so they can be overridden). 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.

Node options that are allowed are:

  • --enable-fips
  • --force-fips
  • --icu-data-dir
  • --inspect-brk
  • --inspect-port
  • --inspect
  • --no-deprecation
  • --no-warnings
  • --openssl-config
  • --redirect-warnings
  • --require, -r
  • --throw-deprecation
  • --tls-cipher-list
  • --trace-deprecation
  • --trace-events-categories
  • --trace-events-enabled
  • --trace-event-file-pattern
  • --trace-sync-io
  • --trace-warnings
  • --track-heap-objects
  • --use-bundled-ca
  • --use-openssl-ca
  • --v8-pool-size
  • --zero-fill-buffers

V8 options that are allowed are:

  • --abort-on-uncaught-exception
  • --max-old-space-size
  • --perf-basic-prof
  • --perf-prof
  • --stack-trace-limit

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_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_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_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.

Note that 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.

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.

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.

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
  • crypto.pbkdf2()
  • crypto.randomBytes(), unless it is used without a callback
  • crypto.randomFill()
  • 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.

Console#

Stability: 2 - Stable

The 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('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: Whoops, something bad happened], to stderr

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.

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console (or their destructured counterparts):

const { Console } = require('console');

const { Console } = console;

new Console(stdout[, stderr])#

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(output, 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(process.stdout, process.stderr);

console.assert(value[, ...message])#

History
VersionChanges
REPLACEME

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.

A simple assertion test that verifies whether value is truthy. If it is not, Assertion failed is logged. If provided, the error message is formatted using util.format() by passing along all message arguments. The output is used as the error message.

console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work

Calling console.assert() with a falsy assertion will only cause the message to be printed to the console without interrupting execution of subsequent code.

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='default'])#

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
v9.3.0

console.debug is now an alias for console.log.

v8.0.0

Added in: v8.0.0

  • data <any>
  • ...args <any>

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

  • ...data <any>

This method calls console.log() passing it the arguments received. Please note that this method does not produce any XML formatting.

console.error([data][, ...args])#

Added in: v0.1.100
  • data <any>
  • ...args <any>

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
  • ...label <any>

Increases indentation of subsequent lines by two spaces.

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 two spaces.

console.info([data][, ...args])#

Added in: v0.1.100
  • data <any>
  • ...args <any>

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100
  • data <any>
  • ...args <any>

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: REPLACEME
  • 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 logit. 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 milliseconds to stdout. Timer durations are accurate to the sub-millisecond.

console.timeEnd(label)#

History
VersionChanges
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('100-elements');
for (let i = 0; i < 100; i++) {}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

console.trace([message][, ...args])#

Added in: v0.1.104
  • message <any>
  • ...args <any>

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
  • data <any>
  • ...args <any>

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.markTimeline(label)#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.markTimeline() method is the deprecated form of console.timeStamp().

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();
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.

console.profileEnd()#

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.

console.table(array[, columns])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Prints to stdout the array array formatted as a table.

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.

console.timeline([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.timeline() method is the deprecated form of console.time().

console.timelineEnd([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.timelineEnd() method is the deprecated form of console.timeEnd().

Crypto#

Stability: 2 - Stable

The crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions.

Use require('crypto') to access this module.

const crypto = require('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 crypto module. In such cases, calling require('crypto') will result in an error being thrown.

let crypto;
try {
  crypto = require('crypto');
} catch (err) {
  console.log('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.

Note that <keygen> is deprecated since HTML 5.2 and new projects should not use this element anymore.

The 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.

Certificate.exportChallenge(spkac)#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

Certificate.exportPublicKey(spkac[, encoding])#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

Certificate.verifySpkac(spkac)#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Legacy API#

As a still supported legacy interface, it is possible (but not recommended) 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 crypto = require('crypto');

const cert1 = new crypto.Certificate();
const cert2 = crypto.Certificate();

certificate.exportChallenge(spkac)#

Added in: v0.11.8 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

certificate.exportPublicKey(spkac)#

Added in: v0.11.8 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

certificate.verifySpkac(spkac)#

Added in: v0.11.8 
const cert = require('crypto').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 crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = '';
cipher.on('readable', () => {
  const data = cipher.read();
  if (data)
    encrypted += data.toString('hex');
});
cipher.on('end', () => {
  console.log(encrypted);
  // Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
});

cipher.write('some clear text data');
cipher.end();

Example: Using Cipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const cipher = crypto.createCipher('aes192', 'a password');

const input = fs.createReadStream('test.js');
const output = fs.createWriteStream('test.enc');

input.pipe(cipher).pipe(output);

Example: Using the cipher.update() and cipher.final() methods:

const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504

cipher.final([outputEncoding])#

Added in: v0.1.94

Returns any remaining enciphered contents. If outputEncoding parameter is one of 'latin1', 'base64' or 'hex', 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.setAAD(buffer)#

Added in: v1.0.0
  • buffer <Buffer>
  • Returns the <Cipher> for method chaining.

When using an authenticated encryption mode (only GCM is currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The cipher.setAAD() method must be called before cipher.update().

cipher.getAuthTag()#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is 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.

cipher.setAutoPadding([autoPadding])#

Added in: v0.7.1
  • autoPadding <boolean> Default: true
  • Returns the <Cipher> for method chaining.

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, its value must be one of 'utf8', 'ascii', or 'latin1' and 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, and can be 'latin1', 'base64' or 'hex'. 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:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

let decrypted = '';
decipher.on('readable', () => {
  const data = decipher.read();
  if (data)
    decrypted += data.toString('utf8');
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

const encrypted =
    'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
decipher.write(encrypted, 'hex');
decipher.end();

Example: Using Decipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const decipher = crypto.createDecipher('aes192', 'a password');

const input = fs.createReadStream('test.enc');
const output = fs.createWriteStream('test.js');

input.pipe(decipher).pipe(output);

Example: Using the decipher.update() and decipher.final() methods:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

const encrypted =
    'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
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

Returns any remaining deciphered contents. If outputEncoding parameter is one of 'latin1', 'ascii' or 'utf8', 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)#

History
VersionChanges
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 (only GCM is currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The decipher.setAAD() method must be called before decipher.update().

decipher.setAuthTag(buffer)#

History
VersionChanges
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 (only GCM is 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.

Note that this Node.js version does not verify the length of GCM authentication tags. Such a check must be implemented by applications and is crucial to the authenticity of the encrypted data, otherwise, an attacker can use an arbitrarily short authentication tag to increase the chances of successfully passing authentication (up to 0.39%). It is highly recommended to associate one of the values 16, 15, 14, 13, 12, 8 or 4 bytes with each key, and to only permit authentication tags of that length, see NIST SP 800-38D.

The decipher.setAuthTag() method must be called before decipher.final().

decipher.setAutoPadding([autoPadding])#

Added in: v0.7.1
  • autoPadding <boolean> Default: true
  • Returns the <Cipher> for method chaining.

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, its value must be one of 'latin1', 'base64', or 'hex' and 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, and can be 'latin1', 'ascii' or 'utf8'. 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.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.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. Encodings can be 'latin1', 'hex', or 'base64'. 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. Encoding can be 'latin1', 'hex', or 'base64'. 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, which can be 'latin1', 'hex', or 'base64'. 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, which can be 'latin1', 'hex', or 'base64'. 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, which can be 'latin1', 'hex', or 'base64'. 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, which can be 'latin1', 'hex', or 'base64'. 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 and is either 'latin1', 'hex', or 'base64', 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 and is either 'latin1', 'hex' or 'base64', 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 constants module):

  • DH_CHECK_P_NOT_SAFE_PRIME
  • DH_CHECK_P_NOT_PRIME
  • DH_UNABLE_TO_CHECK_GENERATOR
  • DH_NOT_SUITABLE_GENERATOR

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.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.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

ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])#

Added in: REPLACEME

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. Encodings can be 'latin1', 'hex', or 'base64'.

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 { ECDH } = require('crypto');

const ecdh = ECDH('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
v6.0.0

The default inputEncoding changed from binary to utf8

REPLACEME

Changed error format to better support invalid public key error

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. Encodings can be 'latin1', 'hex', or 'base64'. 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, its recommended for developers 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.

The encoding argument can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPrivateKey([encoding])#

Added in: v0.11.14

Returns the EC Diffie-Hellman private key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPublicKey([encoding][, format])#

Added in: v0.11.14

Returns the EC Diffie-Hellman public key in the specified encoding and format.

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.

The encoding argument can be 'latin1', 'hex', or 'base64'. 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. The encoding can be 'latin1', 'hex' or 'base64'. 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. Key encoding can be 'latin1', 'hex' or 'base64'. If encoding is provided publicKey is expected to be a string; otherwise a Buffer, TypedArray, or DataView is expected.

Note that 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 crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');

// Note: This is a shortcut way to specify one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  crypto.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 crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.on('readable', () => {
  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:

const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);

Example: Using the hash.update() and hash.digest() methods:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50

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). The encoding can be 'hex', 'latin1' or 'base64'. 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 and can be 'utf8', 'ascii' or 'latin1'. 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 crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  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:

const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream('test.js');
input.pipe(hmac).pipe(process.stdout);

Example: Using the hmac.update() and hmac.digest() methods:

const crypto = require('crypto');
const hmac = crypto.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(). The encoding can be 'hex', 'latin1' or 'base64'. 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 and can be 'utf8', 'ascii' or 'latin1'. 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: 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 objects as streams:

const crypto = require('crypto');
const sign = crypto.createSign('SHA256');

sign.write('some data to sign');
sign.end();

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature using the specified private key and
// SHA-256. For RSA keys, the algorithm is RSASSA-PKCS1-v1_5 (see padding
// parameter below for RSASSA-PSS). For EC keys, the algorithm is ECDSA.

Example: Using the sign.update() and sign.sign() methods:

const crypto = require('crypto');
const sign = crypto.createSign('SHA256');

sign.update('some data to sign');

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

In some cases, a Sign instance can also be created by passing in a signature algorithm name, such as 'RSA-SHA256'. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256'. Use digest names instead.

Example: signing using legacy signature algorithm name

const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');

sign.update('some data to sign');

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

sign.sign(privateKey[, outputFormat])#

History
VersionChanges
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().

The privateKey argument can be an object or a string. If privateKey is a string, it is treated as a raw key with no passphrase. If privateKey is an object, it must contain one or more of the following properties:

  • key: <string> - PEM encoded private key (required)
  • passphrase: <string> - passphrase for the private key

  • padding: <integer> - Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    Note that 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 outputFormat can specify one of 'latin1', 'hex' or 'base64'. If outputFormat 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 and can be 'utf8', 'ascii' or 'latin1'. 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.

Example: Using Verify objects as streams:

const crypto = require('crypto');
const verify = crypto.createVerify('SHA256');

verify.write('some data to sign');
verify.end();

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

Example: Using the verify.update() and verify.verify() methods:

const crypto = require('crypto');
const verify = crypto.createVerify('SHA256');

verify.update('some data to sign');

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

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 and can be 'utf8', 'ascii' or 'latin1'. 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.

verify.verify(object, signature[, signatureFormat])#

History
VersionChanges
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. The object argument can be either a string containing a PEM encoded object, which can be an RSA public key, a DSA public key, or an X.509 certificate, or an object with one or more of the following properties:

  • key: <string> - PEM encoded public key (required)

  • padding: <integer> - Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    Note that 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.

  • 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 signatureFormat which can be 'latin1', 'hex' or 'base64'. If a signatureFormat is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer, TypedArray, or DataView.

Returns true or false depending on the validity of the signature for the data and public key.

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.

crypto module methods and properties#

crypto.constants#

Added in: v6.3.0

Returns 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: REPLACEME

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 backwards 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: REPLACEME

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.createCipher(algorithm, password[, options])#

Added in: v0.1.94Deprecated since: REPLACEME
Stability: 0 - Deprecated: Use crypto.createCipheriv() instead.

Creates and returns a Cipher object that uses the given algorithm and password. Optional options argument controls stream behavior.

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.

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 PBKDF2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() 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
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). Optional options argument controls stream behavior.

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. If the cipher does not need an initialization vector, iv may be null.

crypto.createCredentials(details)#

Added in: v0.1.92Deprecated since: v0.11.13
Stability: 0 - Deprecated: Use tls.createSecureContext() instead.

The crypto.createCredentials() method is a deprecated function for creating and returning a tls.SecureContext. It should not be used. Replace it with tls.createSecureContext() which has the exact same arguments and return value.

Returns a tls.SecureContext, as-if tls.createSecureContext() had been called.

crypto.createDecipher(algorithm, password[, options])#

Added in: v0.1.94Deprecated since: REPLACEME
Stability: 0 - Deprecated: Use crypto.createDecipheriv() instead.

Creates and returns a Decipher object that uses the given algorithm and password (key). Optional options argument controls stream behavior.

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 PBKDF2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() and to use crypto.createDecipheriv() to create the Decipher object.

crypto.createDecipheriv(algorithm, key, iv[, options])#

History
VersionChanges
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). Optional options argument controls stream behavior.

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. If the cipher does not need an initialization vector, iv may be null.

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.

The primeEncoding and generatorEncoding arguments can be 'latin1', 'hex', or 'base64'.

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.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])#

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.

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-message-digest-algorithms will display the available digest algorithms.

Example: generating the sha256 sum of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hash = crypto.createHash('sha256');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});

crypto.createHmac(algorithm, key[, options])#

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-message-digest-algorithms will display the available digest algorithms.

The key is the HMAC key used to generate the cryptographic HMAC hash.

Example: generating the sha256 HMAC of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});

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 an array of names of the available signing algorithms. Optional options argument controls the stream.Writable behavior.

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.

crypto.getCiphers()#

Added in: v0.9.3

Returns an array with the names of the supported cipher algorithms.

Example:

const ciphers = crypto.getCiphers();
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]

crypto.getCurves()#

Added in: v2.3.0

Returns an array with the names of the supported elliptic curves.

Example:

const curves = crypto.getCurves();
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]

crypto.getDiffieHellman(groupName)#

Added in: v0.7.5

Creates a predefined DiffieHellman key exchange object. The supported groups are: 'modp1', 'modp2', 'modp5' (defined in RFC 2412, but see Caveats) and 'modp14', 'modp15', 'modp16', 'modp17', 'modp18' (defined in RFC 3526). 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 crypto = require('crypto');
const alice = crypto.getDiffieHellman('modp14');
const bob = crypto.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: REPLACEME

Returns true if and only if a FIPS compliant crypto provider is currently in use.

crypto.getHashes()#

Added in: v0.9.3

Returns an array of the names of the supported hash algorithms, such as RSA-SHA256.

Example:

const hashes = crypto.getHashes();
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]

crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#

History
VersionChanges
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 also be as unique as possible. It is recommended that the salts are random and their lengths are at least 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
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.

const crypto = require('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().

Note that 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
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 also be as unique as possible. It is recommended that the salts are random and their lengths are at least 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
const key = crypto.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, has been deprecated and use should be avoided.

const crypto = require('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)#

Added in: v0.11.14

Decrypts buffer with privateKey.

privateKey can be an object or a string. If privateKey is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING.

crypto.privateEncrypt(privateKey, buffer)#

Added in: v1.1.0

Encrypts buffer with privateKey.

privateKey can be an object or a string. If privateKey is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING.

crypto.publicDecrypt(key, buffer)#

Added in: v1.1.0

Decrypts buffer with key.

key can be an object or a string. If key is a string, it is treated as the key with no passphrase and will use 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)#

Added in: v0.11.14

Encrypts the content of buffer with key and returns a new Buffer with encrypted content.

key can be an object or a string. If key is a string, it is treated as the key with no passphrase and will use 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
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 pseudo-random 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 crypto = require('crypto');
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 buf = crypto.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.

Note that 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

Added in: v7.10.0

Synchronous version of crypto.randomFill().

Returns buffer

const buf = Buffer.alloc(10);
console.log(crypto.randomFillSync(buf).toString('hex'));

crypto.randomFillSync(buf, 5);
console.log(buf.toString('hex'));

// The above is equivalent to the following:
crypto.randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));

Any TypedArray or DataView instance may be passed as buffer.

const a = new Uint32Array(10);
console.log(crypto.randomFillSync(a).toString('hex'));

const b = new Float64Array(10);
console.log(crypto.randomFillSync(a).toString('hex'));

const c = new DataView(new ArrayBuffer(10));
console.log(crypto.randomFillSync(a).toString('hex'));

crypto.randomFill(buffer[, offset][, size], callback)#

History
VersionChanges
v9.0.0

The buffer argument may be any TypedArray or DataView.

v7.10.0

Added in: v7.10.0

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.

const buf = Buffer.alloc(10);
crypto.randomFill(buf, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

crypto.randomFill(buf, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

// The above is equivalent to the following:
crypto.randomFill(buf, 5, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

Any TypedArray or DataView instance may be passed as buffer.

const a = new Uint32Array(10);
crypto.randomFill(a, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

const b = new Float64Array(10);
crypto.randomFill(b, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

const c = new DataView(new ArrayBuffer(10));
crypto.randomFill(c, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

Note that 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.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_ECDH
  • crypto.constants.ENGINE_METHOD_ECDSA
  • crypto.constants.ENGINE_METHOD_CIPHERS
  • crypto.constants.ENGINE_METHOD_DIGESTS
  • crypto.constants.ENGINE_METHOD_STORE
  • 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: REPLACEME

Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available.

crypto.timingSafeEqual(a, b)#

Added in: v6.6.0

This function is based on a constant-time algorithm. Returns true if a is equal to b, without leaking 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 length.

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.

Notes#

Legacy Streams API (pre Node.js v0.10)#

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.

Recent ECDH Changes#

Usage of ECDH with non-dynamically generated key pairs has been simplified. Now, ecdh.setPrivateKey() can be called with a preselected private key and the associated public point (key) will be computed and stored in the object. This allows code to only store and provide the private part of the EC key pair. ecdh.setPrivateKey() now also validates that the private key is valid for the selected curve.

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Either a previously stored private key should be set, which automatically generates the associated public key, or ecdh.generateKeys() should be called. The main drawback of using ecdh.setPublicKey() is that it can be used to put the ECDH key pair into an inconsistent state.

Support for weak or compromised algorithms#

The 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 considered to be 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.

Crypto Constants#

The following constants exported by crypto.constants apply to various uses of the crypto, tls, and https modules and are generally specific to OpenSSL.

OpenSSL Options#

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_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_QUERY_MTU
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_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
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_ECDH Limit engine usage to ECDH
ENGINE_METHOD_ECDSA Limit engine usage to ECDSA
ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS
ENGINE_METHOD_STORE Limit engine usage to STORE
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 an out-of-process debugging utility accessible via a V8 Inspector and built-in debugging client. To use it, start Node.js with the inspect argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
  3   console.log('world');
debug>

Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code:

// myscript.js
global.x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

Once the debugger is run, a breakpoint will occur at line 3:

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
  3   debugger;
debug> cont
< hello
break in myscript.js:3
  1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
> 3   debugger;
  4   console.log('world');
  5 }, 1000);
debug> next
break in myscript.js:4
  2 setTimeout(() => {
  3   debugger;
> 4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2 + 2
4
debug> next
< world
break in myscript.js:5
  3   debugger;
  4   console.log('world');
> 5 }, 1000);
  6 console.log('hello');
  7
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 functions body
  • setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
  • 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://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd
< For help see https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in main.js:1
> 1 (function (exports, require, module, __filename, __dirname) { 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>

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 - Execute an expression in debugging script's context

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 Debugging 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 127.0.0.1:9229.
To start debugging, open the following URL in Chrome:
    chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29

(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.)

Deprecated APIs#

Node.js may deprecate APIs when either: (a) use of the API is considered to be unsafe, (b) an improved alternative API has been made available, or (c) breaking changes to the API are expected in a future major release.

Node.js utilizes 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 to identify code that either has been removed or will soon be removed from Node.js.

Un-deprecation#

From time-to-time the deprecation of an API may be reversed. Such action may happen in either a semver-minor or semver-major release. 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#

Type: Runtime

The OutgoingMessage.prototype.flush() method is deprecated. Use OutgoingMessage.prototype.flushHeaders() instead.

DEP0002: require('_linklist')#

Type: End-of-Life

The _linklist module is deprecated. Please use a userland alternative.

DEP0003: _writableState.buffer#

Type: Runtime

The _writableState.buffer property is deprecated. Use the _writableState.getBuffer() method instead.

DEP0004: CryptoStream.prototype.readyState#

Type: Documentation-only

The CryptoStream.prototype.readyState property is deprecated and should not be used.

DEP0005: Buffer() constructor#

Type: Documentation-only (supports --pending-deprecation)

The Buffer() function and new Buffer() constructor are deprecated due to API usability issues that can potentially lead to accidental security issues.

As an alternative, use of the following methods of constructing Buffer objects is strongly recommended:

DEP0006: child_process options.customFds#

Type: Runtime

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#

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('constants')#

Type: Documentation-only

The constants module has been 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('fs').constants and require('os').constants.

DEP0009: crypto.pbkdf2 without digest#

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 an undefined digest will throw a TypeError.

DEP0010: crypto.createCredentials#

Type: Runtime

The crypto.createCredentials() API is deprecated. Please use tls.createSecureContext() instead.

DEP0011: crypto.Credentials#

Type: Runtime

The crypto.Credentials class is deprecated. Please use tls.SecureContext instead.

DEP0012: Domain.dispose#

Type: End-of-Life

Domain.dispose() is removed. Recover from failed I/O actions explicitly via error event handlers set on the domain instead.

DEP0013: fs asynchronous function without callback#

Type: End-of-Life

Calling an asynchronous function without a callback throws a TypeError REPLACEME onwards. Refer: PR 12562

DEP0014: fs.read legacy String interface#

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#

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#

Type: Runtime

The GLOBAL and root aliases for the global property have been deprecated and should no longer be used.

DEP0017: Intl.v8BreakIterator#

Type: End-of-Life

Intl.v8BreakIterator was a non-standard extension and has been removed. See Intl.Segmenter.

DEP0018: Unhandled promise rejections#

Type: Runtime

Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

DEP0019: require('.') resolved outside directory#

Type: Runtime

In certain cases, require('.') may resolve outside the package directory. This behavior is deprecated and will be removed in a future major Node.js release.

DEP0020: Server.connections#

Type: Runtime

The Server.connections property is deprecated. Please use the Server.getConnections() method instead.

DEP0021: Server.listenFD#

Type: Runtime

The Server.listenFD() method is deprecated. Please use Server.listen({fd: <number>}) instead.

DEP0022: os.tmpDir()#

Type: Runtime

The os.tmpDir() API is deprecated. Please use os.tmpdir() instead.

DEP0023: os.getNetworkInterfaces()#

Type: Runtime

The os.getNetworkInterfaces() method is deprecated. Please use the os.networkInterfaces property instead.

DEP0024: REPLServer.prototype.convertToContext()#

Type: End-of-Life

The REPLServer.prototype.convertToContext() API is deprecated and should not be used.

DEP0025: require('sys')#

Type: Runtime

The sys module is deprecated. Please use the util module instead.

DEP0026: util.print()#

Type: Runtime

The util.print() API is deprecated. Please use console.log() instead.

DEP0027: util.puts()#

Type: Runtime

The util.puts() API is deprecated. Please use console.log() instead.

DEP0028: util.debug()#

Type: Runtime

The util.debug() API is deprecated. Please use console.error() instead.

DEP0029: util.error()#

Type: Runtime

The util.error() API is deprecated. Please use console.error() instead.

DEP0030: SlowBuffer#

Type: Documentation-only

The SlowBuffer class has been deprecated. Please use Buffer.allocUnsafeSlow(size) instead.

DEP0031: ecdh.setPublicKey()#

Type: Documentation-only

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful.

DEP0032: domain module#

Type: Documentation-only

The domain module is deprecated and should not be used.

DEP0033: EventEmitter.listenerCount()#

Type: Documentation-only

The EventEmitter.listenerCount(emitter, eventName) API has been deprecated. Please use emitter.listenerCount(eventName) instead.

DEP0034: fs.exists(path, callback)#

Type: Documentation-only

The fs.exists(path, callback) API has been deprecated. Please use fs.stat() or fs.access() instead.

DEP0035: fs.lchmod(path, mode, callback)#

Type: Documentation-only

The fs.lchmod(path, mode, callback) API has been deprecated.

DEP0036: fs.lchmodSync(path, mode)#

Type: Documentation-only

The fs.lchmodSync(path, mode) API has been deprecated.

DEP0037: fs.lchown(path, uid, gid, callback)#

Type: Documentation-only

The fs.lchown(path, uid, gid, callback) API has been deprecated.

DEP0038: fs.lchownSync(path, uid, gid)#

Type: Documentation-only

The fs.lchownSync(path, uid, gid) API has been deprecated.

DEP0039: require.extensions#

Type: Documentation-only

The require.extensions property has been deprecated.

DEP0040: punycode module#

Type: Documentation-only

The punycode module has been deprecated. Please use a userland alternative instead.

DEP0041: NODE_REPL_HISTORY_FILE environment variable#

Type: End-of-life

The NODE_REPL_HISTORY_FILE environment variable was removed. Please use NODE_REPL_HISTORY instead.

DEP0042: tls.CryptoStream#

Type: Documentation-only

The tls.CryptoStream class has been deprecated. Please use tls.TLSSocket instead.

DEP0043: tls.SecurePair#

Type: Documentation-only

The tls.SecurePair class has been deprecated. Please use tls.TLSSocket instead.

DEP0044: util.isArray()#

Type: Documentation-only

The util.isArray() API has been deprecated. Please use Array.isArray() instead.

DEP0045: util.isBoolean()#

Type: Documentation-only

The util.isBoolean() API has been deprecated.

DEP0046: util.isBuffer()#

Type: Documentation-only

The util.isBuffer() API has been deprecated. Please use Buffer.isBuffer() instead.

DEP0047: util.isDate()#

Type: Documentation-only

The util.isDate() API has been deprecated.

DEP0048: util.isError()#

Type: Documentation-only

The util.isError() API has been deprecated.

DEP0049: util.isFunction()#

Type: Documentation-only

The util.isFunction() API has been deprecated.

DEP0050: util.isNull()#

Type: Documentation-only

The util.isNull() API has been deprecated.

DEP0051: util.isNullOrUndefined()#

Type: Documentation-only

The util.isNullOrUndefined() API has been deprecated.

DEP0052: util.isNumber()#

Type: Documentation-only

The util.isNumber() API has been deprecated.

DEP0053 util.isObject()#

Type: Documentation-only

The util.isObject() API has been deprecated.

DEP0054: util.isPrimitive()#

Type: Documentation-only

The util.isPrimitive() API has been deprecated.

DEP0055: util.isRegExp()#

Type: Documentation-only

The util.isRegExp() API has been deprecated.

DEP0056: util.isString()#

Type: Documentation-only

The util.isString() API has been deprecated.

DEP0057: util.isSymbol()#

Type: Documentation-only

The util.isSymbol() API has been deprecated.

DEP0058: util.isUndefined()#

Type: Documentation-only

The util.isUndefined() API has been deprecated.

DEP0059: util.log()#

Type: Documentation-only

The util.log() API has been deprecated.

DEP0060: util._extend()#

Type: Documentation-only

The util._extend() API has been deprecated.

DEP0061: fs.SyncWriteStream#

Type: Runtime

The fs.SyncWriteStream class was never intended to be a publicly accessible API. No alternative API is available. Please use a userland alternative.

DEP0062: node --debug#

Type: Runtime

--debug activates the legacy V8 debugger interface, which has been removed as of V8 5.8. It is replaced by Inspector which is activated with --inspect instead.

DEP0063: ServerResponse.prototype.writeHeader()#

Type: Documentation-only

The http module ServerResponse.prototype.writeHeader() API has been deprecated. Please use ServerResponse.prototype.writeHead() instead.

The ServerResponse.prototype.writeHeader() method was never documented as an officially supported API.

DEP0064: tls.createSecurePair()#

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#

Type: End-of-Life

The 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 v6.0.0, 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._headers, outgoingMessage._headerNames#

Type: Documentation-only

The http module outgoingMessage._headers and outgoingMessage._headerNames properties have been deprecated. Please instead use one of the public methods (e.g. outgoingMessage.getHeader(), outgoingMessage.getHeaders(), outgoingMessage.getHeaderNames(), outgoingMessage.hasHeader(), outgoingMessage.removeHeader(), outgoingMessage.setHeader()) for working with outgoing headers.

The outgoingMessage._headers and outgoingMessage._headerNames properties were never documented as officially supported properties.

DEP0067: OutgoingMessage.prototype._renderHeaders#

Type: Documentation-only

The http module OutgoingMessage.prototype._renderHeaders() API has been deprecated.

The OutgoingMessage.prototype._renderHeaders property was never documented as an officially supported API.

DEP0068: node debug#

Type: Runtime

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)#

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()#

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()#

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()#

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#

Type: End-of-Life

Accessing several internal, undocumented properties of net.Server instances with inappropriate names has been deprecated.

As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided.

DEP0074: REPLServer.bufferedCommand#

Type: Runtime

The REPLServer.bufferedCommand property was deprecated in favor of REPLServer.clearBufferedCommand().

DEP0075: REPLServer.parseREPLKeyword()#

Type: Runtime

REPLServer.parseREPLKeyword() was removed from userland visibility.

DEP0076: tls.parseCertString()#

Type: Runtime

tls.parseCertString() is a trivial parsing helper that was made public by mistake. This function can usually be replaced with:

const querystring = require('querystring');
querystring.parse(str, '\n', '=');

This function is not completely equivalent to querystring.parse(). One difference is that querystring.parse() does url decoding:

> querystring.parse('%E5%A5%BD=1', '\n', '=');
{ '好': '1' }
> tls.parseCertString('%E5%A5%BD=1');
{ '%E5%A5%BD': '1' }

DEP0077: Module._debug()#

Type: Runtime

Module._debug() has been deprecated.

The Module._debug() function was never documented as an officially supported API.

DEP0078: REPLServer.turnOffEditorMode()#

Type: Runtime

REPLServer.turnOffEditorMode() was removed from userland visibility.

DEP0079: Custom inspection function on Objects via .inspect()#

Type: Runtime

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 backwards compatibility with Node.js prior to version 6.4.0, both may be specified.

DEP0080: path._makeLong()#

Type: Documentation-only

The internal path._makeLong() was not intended for public use. However, userland modules have found it useful. The internal API has been deprecated and replaced with an identical, public path.toNamespacedPath() method.

DEP0081: fs.truncate() using a file descriptor#

Type: Runtime

fs.truncate() fs.truncateSync() usage with a file descriptor has been deprecated. Please use fs.ftruncate() or fs.ftruncateSync() to work with file descriptors.

DEP0082: REPLServer.prototype.memory()#

Type: Runtime

REPLServer.prototype.memory() is a function only necessary for the internal mechanics of the REPLServer itself, and is therefore not necessary in user space.

DEP0083: Disabling ECDH by setting ecdhCurve to false#

Type: Runtime

The ecdhCurve option to tls.createSecureContext() and tls.TLSSocket could be set to false to disable ECDH entirely on the server only. This mode is deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with the client. Use the ciphers parameter instead.

DEP0084: requiring bundled internal dependencies#

Type: Runtime

Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for internal usage are mistakenly exposed to user code through require(). These modules are:

  • 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 may 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#

Type: End-of-Life

The AsyncHooks Sensitive API was never documented and had various of minor issues, see https://github.com/nodejs/node/issues/15572. Use the AsyncResource API instead.

DEP0086: Remove runInAsyncIdScope#

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 for more details.

DEP0089: require('assert')#

Type: Documentation-only

Importing assert directly is not recommended as the exposed functions will use loose equality checks. Use require('assert').strict instead. The API is the same as the legacy assert but it will always use strict equality checks.

DEP0090: Invalid GCM authentication tag lengths#

Type: Runtime

Node.js supports all GCM authentication tag lengths which are accepted by OpenSSL when calling decipher.setAuthTag(). This behavior will change in a future version at which point only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32 bits will be allowed. Authentication tags whose length is not included in this list will be considered invalid in compliance with NIST SP 800-38D.

DEP0091: crypto.DEFAULT_ENCODING#

Type: Runtime

The crypto.DEFAULT_ENCODING property is deprecated.

DEP0092: Top-level this bound to module.exports#

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.#

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.#

Type: Runtime

Using assert.fail() with more than one argument has no benefit over writing an individual error message. Either use assert.fail() with one argument or switch to one of the other assert methods.

DEP0095: timers.enroll()#

Type: Runtime

timers.enroll() is deprecated. Please use the publicly documented setTimeout() or setInterval() instead.

DEP0096: timers.unenroll()#

Type: Runtime

timers.unenroll() is deprecated. Please use the publicly documented clearTimeout() or clearInterval() instead.

DEP0097: MakeCallback with domain property#

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.emit<Before,After> APIs#

Type: Runtime

The embedded API provided by AsyncHooks exposes emit<Before,After> 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 for more details.

DEP0099: async context-unaware node::MakeCallback C++ APIs#

Type: Compile-time

Certain versions of node::MakeCallback APIs available to native modules are deprecated. Please use the versions of the API that accept an async_context parameter.

DEP0100: process.assert()#

Type: Runtime

process.assert() is deprecated. Please use the assert module instead.

This was never a documented feature.

DEP0101: --with-lttng#

Type: End-of-Life

The --with-lttng compile time option is removed.

DEP0102: Using noAssert in Buffer#(read|write) operations.#

Type: End-of-Life

Using the noAssert argument has no functionality anymore. All input is going to be verified, no matter if it is set to true or not. Skipping the verification could lead to hard to find errors and crashes.

DEP0103: process.binding('util').is[...] typechecks#

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.

DEP0104: process.env string coercion#

Type: Documentation-only (supports --pending-deprecation)

Currently when assigning a property to process.env, the assigned value is implicitly converted to a string if it is not a string. This behavior is deprecated if the assigned value is not a string, boolean, or number. In the future, such assignment may result in a thrown error. Please convert the property to a string before assigning it to process.env.

DEP0105: decipher.finaltol#

Type: Runtime

decipher.finaltol() has never been documented and is currently an alias for decipher.final(). In the future, this API will likely be removed, and it is recommended to use decipher.final() instead.

DEP0106: crypto.createCipher and crypto.createDecipher#

Type: Documentation-only

Using crypto.createCipher() and crypto.createDecipher() should 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() and to use crypto.createCipheriv() and crypto.createDecipheriv() to obtain the Cipher and Decipher objects respectively.

DEP0107: tls.convertNPNProtocols()#

Type: Runtime

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.

DNS#

Stability: 2 - Stable

The dns module contains functions belonging to two different categories:

1) Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily perform any network communication. This category contains only one function: dns.lookup(). Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use dns.lookup().

For example, looking up iana.org.

const dns = require('dns');

dns.lookup('iana.org', (err, address, family) => {
  console.log('address: %j family: IPv%s', address, family);
});
// address: "192.0.43.8" family: IPv4

2) Functions that connect to an actual DNS server to perform name resolution, and that always use the network to perform DNS queries. This category contains all functions in the dns module except dns.lookup(). These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries.

Below is an example that resolves 'archive.org' then reverse resolves the IP addresses that are returned.

const dns = require('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)}`);
    });
  });
});

There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information.

Class dns.Resolver#

Added in: v8.3.0

An independent resolver for DNS requests.

Note that creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolver:

const { Resolver } = require('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 dns module are available:

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.

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings, formatted according to rfc5952, 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
v1.2.0

The all option is supported now.

v0.1.90

Added in: v0.1.90

  • hostname <string>
  • options <integer> | <Object>
    • family <integer> The record family. Must be 4 or 6. IPv4 and IPv6 addresses are both returned by default.
    • 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: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true }.
  • 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.

Resolves a hostname (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 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 'ENOENT' not only when the hostname 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('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#

The following flags can be passed as hints to dns.lookup().

  • dns.ADDRCONFIG: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.
  • dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1).

dns.lookupService(address, port, callback)#

Added in: v0.11.14

Resolves the given address and port into a hostname 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('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)#

Added in: v0.1.27

Uses the DNS protocol to resolve a hostname (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:

rrtype records contains Result type Shorthand method
'A' IPv4 addresses (default) <string> dns.resolve4()
'AAAA' IPv6 addresses <string> dns.resolve6()
'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()
'ANY' any records <Object> dns.resolveAny()

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)#

History
VersionChanges
v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Hostname 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: '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
v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Hostname 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 a IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

dns.resolveCname(hostname, callback)#

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.resolveMx(hostname, callback)#

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)#

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)#

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)#

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)#

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)#

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)#

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.resolveAny(hostname, 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:

Type Properties
'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(), eg. { 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.reverse(ip, callback)#

Added in: v0.1.16

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.

On error, err is an Error object, where err.code is one of the DNS error codes.

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 rfc5952 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.

Error codes#

Each DNS query can return one of the following error codes:

  • dns.NODATA: DNS server returned 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 requested operation.
  • dns.REFUSED: DNS server refused query.
  • dns.BADQUERY: Misformatted DNS query.
  • dns.BADNAME: Misformatted hostname.
  • 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 hostname 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.

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 note that 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.

Note that various networking APIs will call dns.lookup() internally to resolve host names. If that is an issue, consider resolving the hostname to and 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 than what 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.

Stability: 0 - Deprecated

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users 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 you 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 master 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('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
  // resources like crazy if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
  require('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('cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isMaster) {
  // A more realistic scenario would have more than 2 workers,
  // and perhaps not put the master 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 master 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('domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests. How it works, caveats, etc.

  const server = require('http').createServer((req, res) => {
    const d = domain.create();
    d.on('error', (er) => {
      console.error(`error ${er.stack}`);

      // Note: 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 master know we're dead. This will trigger a
        // 'disconnect' in the cluster master, 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('domain');
const http = require('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()#

Returns a new Domain object.

Class: Domain#

The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.

Domain is a child class of EventEmitter. 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.

Example#

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.

Example#

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.

Example:

const domain = require('domain');
const fs = require('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
  }));
});

Note that domains will not interfere with the error handling mechanisms for Promises, i.e. no error event will be emitted for unhandled Promise rejections.

ECMAScript Modules#

Stability: 1 - Experimental

Node.js contains support for ES Modules based upon the Node.js EP for ES Modules.

Not all features of the EP are complete and will be landing as both VM support and implementation is ready. Error messages are still being polished.

Enabling#

The --experimental-modules flag can be used to enable features for loading ESM modules.

Once this has been set, files ending with .mjs will be able to be loaded as ES Modules.

node --experimental-modules my-app.mjs

Features#

Supported#

Only the CLI argument for the main entry point to the program can be an entry point into an ESM graph. Dynamic import can also be used to create entry points into ESM graphs at runtime.

import.meta#

The import.meta metaproperty is an Object that contains the following property:

  • url <string> The absolute file: URL of the module

Unsupported#

Feature Reason
require('./foo.mjs') ES Modules have differing resolution and timing, use dynamic import

Notable differences between import and require#

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. It has a separate cache.

URL based paths#

ESM are resolved and cached based upon URL semantics. This means that files containing special characters such as # and ? need to be escaped.

Modules will be loaded multiple times if the import specifier used to resolve them have a different query or fragment.

import './foo?query=1'; // loads ./foo with query of "?query=1"
import './foo?query=2'; // loads ./foo with query of "?query=2"

For now, only modules using the file: protocol can be loaded.

Interop with existing modules#

All CommonJS, JSON, and C++ modules can be used with import.

Modules loaded this way will only be loaded once, even if their query or fragment string differs between import statements.

When loaded via import these modules will provide a single default export representing the value of module.exports at the time they finished evaluating.

import fs from 'fs';
fs.readFile('./foo.txt', (err, body) => {
  if (err) {
    console.error(err);
  } else {
    console.log(body);
  }
});

Loader hooks#

To customize the default module resolution, loader hooks can optionally be provided via a --loader ./loader-name.mjs argument to Node.js.

When hooks are used they only apply to ES module loading and not to any CommonJS modules loaded.

Resolve hook#

The resolve hook returns the resolved file URL and module format for a given module specifier and parent file URL:

const baseURL = new URL('file://');
baseURL.pathname = `${process.cwd()}/`;

export async function resolve(specifier,
                              parentModuleURL = baseURL,
                              defaultResolver) {
  return {
    url: new URL(specifier, parentModuleURL).href,
    format: 'esm'
  };
}

The parentURL is provided as undefined when performing main Node.js load itself.

The default Node.js ES module resolution function is provided as a third argument to the resolver for easy compatibility workflows.

In addition to returning the resolved file URL value, the resolve hook also returns a format property specifying the module format of the resolved module. This can be one of the following:

format Description
'esm' Load a standard JavaScript module
'cjs' Load a node-style CommonJS module
'builtin' Load a node builtin CommonJS module
'json' Load a JSON file
'addon' Load a C++ Addon
'dynamic' Use a dynamic instantiate hook

For example, a dummy loader to load JavaScript restricted to browser resolution rules with only JS file extension and Node.js builtin modules support could be written:

import path from 'path';
import process from 'process';
import Module from 'module';

const builtins = Module.builtinModules;
const JS_EXTENSIONS = new Set(['.js', '.mjs']);

const baseURL = new URL('file://');
baseURL.pathname = `${process.cwd()}/`;

export function resolve(specifier, parentModuleURL = baseURL, defaultResolve) {
  if (builtins.includes(specifier)) {
    return {
      url: specifier,
      format: 'builtin'
    };
  }
  if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) {
    // For node_modules support:
    // return defaultResolve(specifier, parentModuleURL);
    throw new Error(
      `imports must begin with '/', './', or '../'; '${specifier}' does not`);
  }
  const resolved = new URL(specifier, parentModuleURL);
  const ext = path.extname(resolved.pathname);
  if (!JS_EXTENSIONS.has(ext)) {
    throw new Error(
      `Cannot load file with non-JavaScript file extension ${ext}.`);
  }
  return {
    url: resolved.href,
    format: 'esm'
  };
}

With this loader, running:

NODE_OPTIONS='--experimental-modules --loader ./custom-loader.mjs' node x.js

would load the module x.js as an ES module with relative resolution support (with node_modules loading skipped in this example).

Dynamic instantiate hook#

To create a custom dynamic module that doesn't correspond to one of the existing format interpretations, the dynamicInstantiate hook can be used. This hook is called only for modules that return format: 'dynamic' from the resolve hook.

export async function dynamicInstantiate(url) {
  return {
    exports: ['customExportName'],
    execute: (exports) => {
      // get and set functions provided for pre-allocated export names
      exports.customExportName.set('value');
    }
  };
}

With the list of module exports provided upfront, the execute function will then be called at the exact point of module evaluation order for that module in the import tree.

Errors#

Applications running in Node.js will generally experience four categories of errors:

  • Standard JavaScript errors such as:
    • <EvalError> : thrown when a call to eval() fails.
    • <SyntaxError> : thrown in response to improper JavaScript language syntax.
    • <RangeError> : thrown when a value is not within an expected range
    • <ReferenceError> : thrown when using undefined variables
    • <TypeError> : thrown when passing arguments of the wrong type
    • <URIError> : thrown when a global URI handling function is misused.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist, attempting to send data over a closed socket, etc;
  • And User-specified errors triggered by application code.
  • Assertion Errors are a special class of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the 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 undefined
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('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('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 unhandled exception and crash unless either: The domain module is used appropriately or a handler has been registered for the process.on('uncaughtException') event.

const EventEmitter = require('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 (sometimes referred to as a Node.js style 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('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('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 (err) { } 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.

For crypto only, Error objects will include the OpenSSL error stack in a separate property called opensslErrorStack if it is available when the error is thrown.

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)#

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 message.toString(). 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 an end 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.code#

The error.code property is a string label that identifies the kind of error. 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, 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.

System-level errors are generated as augmented Error instances, which are detailed here.

Class: AssertionError#

A subclass of Error that indicates the failure of an assertion. For details, see Class: assert.AssertionError.

Class: RangeError#

A subclass of Error that 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('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#

A subclass of Error that 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 should always be considered a bug in the code or its dependencies.

Class: SyntaxError#

A subclass of Error that 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('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: TypeError#

A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.

require('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.

System Errors#

System errors are generated when exceptions occur within the Node.js runtime environment. Typically, these are operational errors that occur when an application violates an operating system constraint such as attempting to read a file that does not exist or when the user does not have sufficient permissions.

System errors are typically generated at the syscall level: an exhaustive list of error codes and their meanings is available by running man 2 intro or man 3 errno on most Unices; or online.

In Node.js, system errors are represented as augmented Error objects with added properties.

Class: SystemError#

error.info#

SystemError instances may have an additional info property whose value is an object with additional details about the error conditions.

The following properties are provided:

  • code <string> The string error code
  • errno <number> The system-provided error number
  • message <string> A system-provided human readable description of the error
  • syscall <string> The name of the system call that triggered the error
  • path <Buffer> When reporting a file system error, the path will identify the file path.
  • dest <Buffer> When reporting a file system error, the dest will identify the file path destination (if any).

error.code#

The error.code property is a string representing the error code, which is typically E followed by a sequence of capital letters.

error.errno#

The error.errno property is a number or a string. The number is a negative value which corresponds to the error code defined in libuv Error handling. See uv-errno.h header file (deps/uv/include/uv-errno.h in the Node.js source tree) for details. In case of a string, it is the same as error.code.

error.syscall#

The error.syscall property is a string describing the syscall that failed.

error.path#

When present (e.g. in fs or child_process), the error.path property is a string containing a relevant invalid pathname.

error.address#

When present (e.g. in net or dgram), the error.address property is a string describing the address to which the connection failed.

error.port#

When present (e.g. in net or dgram), the error.port property is a number representing the connection's port that is not available.

Common System Errors#

This list is not exhaustive, but enumerates many of the common system errors encountered when writing a Node.js program. An exhaustive list may be found here.

  • 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.

  • 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.

Node.js Error Codes#

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 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. Note that users are also able to define their own types if using the public embedder API.

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_CONSOLE_WRITABLE_STREAM##

Console was instantiated without stdout stream, or Console has a non-writable stdout or stderr stream.

ERR_CPU_USAGE##

The native call from process.cpuUsage could not be processed.

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('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 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_DIGEST_NO_UTF16##

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_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_INVALID_DIGEST##

An invalid crypto digest algorithm was specified.

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_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_DNS_SET_SERVERS_FAILED##

c-ares failed to set the DNS server.

ERR_DOMAIN_CALLBACK_NOT_AVAILABLE##

The 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 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 domain module had been loaded.

ERR_ENCODING_INVALID_ENCODED_DATA##

Data provided to util.TextDecoder() API was invalid according to the encoding provided.

ERR_ENCODING_NOT_SUPPORTED##

Encoding provided to util.TextDecoder() API was not one of the WHATWG Supported Encodings.

ERR_FALSY_VALUE_REJECTION##

A Promise that was callbackified via util.callbackify() was rejected with a falsy value.

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_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_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_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_NO_SOCKET_MANIPULATION##

An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an Http2Session.

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_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_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_UNSUPPORTED_PROTOCOL##

http2.connect() was passed a URL that uses any protocol other than http: or https:.

ERR_INDEX_OUT_OF_RANGE##

A given index was out of the accepted range (e.g. negative offsets).

ERR_INSPECTOR_ALREADY_CONNECTED##

While using the inspector module, an attempt was made to connect when the inspector was already connected.

ERR_INSPECTOR_CLOSED##

While using the inspector module, an attempt was made to use the inspector after the session had already closed.

ERR_INSPECTOR_NOT_AVAILABLE##

The inspector module is not available for use.

ERR_INSPECTOR_NOT_CONNECTED##

While using the inspector module, an attempt was made to use the inspector before it was connected.

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_ARRAY_LENGTH##

An Array was not of the expected length or in a valid range.

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_CALLBACK##

A callback function was required but was not been provided to a Node.js API.

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_DOMAIN_NAME##

hostname can not be parsed from a provided URL.

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_OPT_VALUE##

An invalid or unexpected value was passed in an options object.

ERR_INVALID_OPT_VALUE_ENCODING##

An invalid or unknown file encoding was passed.

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.

ERR_INVALID_REPL_EVAL_CONFIG##

Both breakEvalOnSigint and eval options were set in the REPL config, which is not supported.

ERR_INVALID_SYNC_FORK_INPUT##

A Buffer, Uint8Array or string was provided as stdio input to a synchronous 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.

Example:

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_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 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_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_MODULE##

Stability: 1 - Experimental

An ES6 module could not be resolved.

ERR_MODULE_RESOLUTION_LEGACY##

Stability: 1 - Experimental

A failure occurred resolving imports in an ES6 module.

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 N-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_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_NO_LONGER_SUPPORTED##

A Node.js API was called in an unsupported manner, such as Buffer.write(string, encoding, offset[, length]).

ERR_OUT_OF_RANGE##

A given value is out of the accepted range.

ERR_REQUIRE_ESM##

Stability: 1 - Experimental

An attempt was made to require() an ES6 module.

ERR_SCRIPT_EXECUTION_INTERRUPTED##

Script execution was interrupted by SIGINT (For example, when Ctrl+C was pressed).

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_CANNOT_SEND##

Data could be sent on a socket.

ERR_SOCKET_CLOSED##

An attempt was made to operate on an already closed socket.

ERR_SOCKET_DGRAM_NOT_RUNNING##

A call was made and the UDP subsystem was not running.

ERR_STDERR_CLOSE##

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##

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_CANNOT_PIPE##

An attempt was made to call stream.pipe() on a Writable stream.

ERR_STREAM_NULL_VALUES##

An attempt was made to call stream.write() with a null chunk.

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_READ_NOT_IMPLEMENTED##

An attempt was made to use a readable stream that did not implement readable._read().

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.

Example

const Socket = require('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_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_STREAM_DESTROYED##

A stream method was called that cannot complete because the stream was destroyed using stream.destroy().

ERR_TLS_CERT_ALTNAME_INVALID##

While using TLS, the hostname/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_REQUIRED_SERVER_NAME##

While using TLS, the server.addContext() method was called without providing a hostname 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_TLS_RENEGOTIATION_DISABLED##

An attempt was made to renegotiate TLS on a socket instance with TLS disabled.

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_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_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_UNKNOWN_STDIN_TYPE##

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##

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_VALID_PERFORMANCE_ENTRY_TYPE##

While using the Performance Timing API (perf_hooks), no valid performance entry types were found.

ERR_VALUE_OUT_OF_RANGE##

Superseded by ERR_OUT_OF_RANGE

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_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_LINKING_ERRORED##

The linker function returned a module for which linking has failed.

ERR_VM_MODULE_NOT_LINKED##

The module must be successfully linked before instantiation.

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_ZLIB_INITIALIZATION_FAILED##

Creation of a zlib object failed due to incorrect configuration.

Events#

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") periodically 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 will be 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.

const EventEmitter = require('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. It is important to keep in mind that when an ordinary listener function is called by the EventEmitter, the standard this keyword is intentionally set to reference the EventEmitter to which the listener is attached.

const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this);
  // Prints:
  //   a b MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined }
});
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:

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 is important to ensure the proper sequencing of events and to avoid race conditions or logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

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 will be invoked every time the named event is emitted.

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.

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.

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 domain module has been deprecated.)

As a best practice, listeners should always be added for the 'error' events.

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

Class: EventEmitter#

Added in: v0.1.26

The EventEmitter class is defined and exposed by the events module:

const EventEmitter = require('events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

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 will be 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 will be inserted before the listener that is in the process of being added.

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.

EventEmitter.listenerCount(emitter, eventName)#

Added in: v0.9.12Deprecated since: v4.0.0
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.

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2

EventEmitter.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 EventEmitter.defaultMaxListeners property can be used. If this value is not a positive number, a TypeError will be thrown.

Take caution when setting the EventEmitter.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 EventEmitter.defaultMaxListeners.

Note that 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:

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'.

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.

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 will be strings or Symbols.

const EventEmitter = require('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 EventEmitter.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.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.

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.

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.

Note that 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.

Note that once an event has been emitted, all listeners attached to it at the time of emitting will be 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 will behave as expected.

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.

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. Obviously, not all events should be limited to just 10 listeners. 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).

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');

File System#

Stability: 2 - Stable

The fs module provides an API for interacting with the file system in a manner closely modeled around standard POSIX functions.

To use this module:

const fs = require('fs');

All file system operations have synchronous and asynchronous forms.

The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined.

const fs = require('fs');

fs.unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Exceptions that occur using synchronous operations are thrown immediately and may be handled using try/catch, or may be allowed to bubble up.

const fs = require('fs');

try {
  fs.unlinkSync('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (err) {
  // handle the error
}

Note that there is no guaranteed ordering when using asynchronous methods. So the following is prone to error because the fs.stat() operation may 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)}`);
});

To correctly order the operations, move the fs.stat() call into the callback of the fs.rename() operation:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  fs.stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete — halting all connections.

While it is not recommended, most fs functions allow the callback argument to be omitted, in which case a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable:

Omitting the callback function on asynchronous fs functions is deprecated and may result in an error being thrown in the future.

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env NODE_DEBUG=fs node script.js
fs.js:88
        throw backtrace;
        ^
Error: EISDIR: illegal operation on a directory, read
    <stack trace.>

File paths#

Most fs operations accept filepaths that may be specified in the form of a string, a Buffer, or a URL object using the file: protocol.

String form 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 specified by process.cwd().

Example using an absolute path on POSIX:

const fs = require('fs');

fs.open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

Example using a relative path on POSIX (relative to process.cwd()):

fs.open('file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

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:

fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

Note: 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.

URL object support#

Added in: v7.6.0

For most fs module functions, the path or filename argument may be passed as a WHATWG URL object. Only URL objects using the file: protocol are supported.

const fs = require('fs');
const fileUrl = new URL('file:///tmp/hello');

fs.readFileSync(fileUrl);

file: URLs are always absolute paths.

Using WHATWG URL objects might introduce platform-specific behaviors.

On Windows, file: URLs with a hostname convert to UNC paths, while file: URLs with drive letters convert to local absolute paths. file: URLs without a hostname nor a drive letter will result in a throw :

// On Windows :

// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
fs.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
fs.readFileSync(new URL('file:///C:/tmp/hello'));

// - WHATWG file URLs without hostname must have a drive letters
fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
fs.readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute

file: URLs with drive letters must use : as a separator just after the drive letter. Using another separator will result in a throw.

On all other platforms, file: URLs with a hostname are unsupported and will result in a throw:

// On other platforms:

// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
fs.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
fs.readFileSync(new URL('file:///tmp/hello'));

A file: URL having encoded slash characters will result in a throw on all platforms:

// On Windows
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.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
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.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: URLs having encoded backslash will result in a throw:

// On Windows
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

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 specific differences between operating systems and assigns all open files a numeric file descriptor.

The fs.open() method is used to 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.

fs.open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.fstat(fd, (err, stat) => {
    if (err) throw err;
    // use stat

    // always close the file descriptor!
    fs.close(fd, (err) => {
      if (err) throw err;
    });
  });
});

Most 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.

Threadpool Usage#

Note that all file system APIs except fs.FSWatcher() and those that are explicitly synchronous use libuv's threadpool, which can have surprising and negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information.

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 are EventEmitter's that will 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.

// Example when handled through fs.watch listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
  if (filename) {
    console.log(filename);
    // Prints: <Buffer ...>
  }
});

Event: 'error'#

Added in: v0.5.8

Emitted when an error occurs while watching the file.

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.

Class: fs.ReadStream#

Added in: v0.1.93

A successful call to fs.createReadStream() will return a new fs.ReadStream object.

All fs.ReadStream objects are Readable Streams.

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
  • fd <integer> Integer file descriptor used by the ReadStream.

Emitted when the fs.ReadStream's file descriptor has been opened.

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.

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() and fs.fstat() and their synchronous counterparts are of this type.

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 }

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.

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 is considered "special".

stats.size#

The size of the file in bytes.

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.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 numbers that hold the corresponding times in milliseconds. 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 filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, unix epoch timestamp 0). Note that 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 v0.12, the ctime held the birthtime on Windows systems. Note that as of v0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream#

Added in: v0.1.93

WriteStream is a Writable Stream.

Event: 'close'#

Added in: v0.1.93

Emitted when the WriteStream's underlying file descriptor has been closed.

Event: 'open'#

Added in: v0.1.93
  • fd <integer> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's file is opened.

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.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.

fs.access(path[, mode], callback)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

  • fs.constants.F_OK - path 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.
  • fs.constants.R_OK - path can be read by the calling process.
  • fs.constants.W_OK - path can be written by the calling process.
  • fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

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 example checks if the file /etc/passwd can be read and written by the current process.

fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
  console.log(err ? 'no access!' : 'can read/write');
});

Using fs.access() to check for the accessibility 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 is not accessible.

write (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  fs.open('myfile', 'wx', (err, fd) => {
    if (err) throw err;
    writeMyData(fd);
  });
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  fs.open('myfile', 'r', (err, fd) => {
    if (err) throw err;
    readMyData(fd);
  });
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

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.

fs.accessSync(path[, mode])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

  • fs.constants.F_OK - path 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.
  • fs.constants.R_OK - path can be read by the calling process.
  • fs.constants.W_OK - path can be written by the calling process.
  • fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

If any of the accessibility checks fail, an Error will be thrown. Otherwise, the method will return undefined.

try {
  fs.accessSync('etc/passwd', fs.constants.R_OK | fs.constants.W_OK);
  console.log('can read/write');
} catch (err) {
  console.error('no access!');
}

fs.appendFile(file, data[, options], callback)#

History
VersionChanges
REPLACEME

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.

Example:

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. Example:

fs.appendFile('message.txt', 'data to append', 'utf8', callback);

The file 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.

fs.open('message.txt', 'a', (err, fd) => {
  if (err) throw err;
  fs.appendFile(fd, 'data to append', 'utf8', (err) => {
    fs.close(fd, (err) => {
      if (err) throw err;
    });
    if (err) throw err;
  });
});

fs.appendFileSync(file, 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.

Example:

try {
  fs.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. Example:

fs.appendFileSync('message.txt', 'data to append', 'utf8');

The file 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.

let fd;

try {
  fd = fs.openSync('message.txt', 'a');
  fs.appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
  /* Handle the error */
} finally {
  if (fd !== undefined)
    fs.closeSync(fd);
}

fs.chmod(path, mode, callback)#

History
VersionChanges
REPLACEME

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. 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.

v0.1.30

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 also: chmod(2)

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:

Constant Octal Description
fs.constants.S_IRUSR 0o400 read by owner
fs.constants.S_IWUSR 0o200 write by owner
fs.constants.S_IXUSR 0o100 execute/search by owner
fs.constants.S_IRGRP 0o40 read by group
fs.constants.S_IWGRP 0o20 write by group
fs.constants.S_IXGRP 0o10 execute/search by group
fs.constants.S_IROTH 0o4 read by others
fs.constants.S_IWOTH 0o2 write by others
fs.constants.S_IXOTH 0o1 execute/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.

Number Description
7 read, write, and execute
6 read and write
5 read and execute
4 read only
3 write and execute
2 write only
1 execute only
0 no 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.

fs.chmodSync(path, mode)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.6.7

Added in: v0.6.7

Synchronously changes the permissions of a file. Returns undefined. This is the synchronous version of fs.chmod().

See also: chmod(2)

fs.chown(path, uid, gid, callback)#

History
VersionChanges
REPLACEME

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. 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.

v0.1.97

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 also: chown(2)

fs.chownSync(path, uid, gid)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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 also: chown(2)

fs.close(fd, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.

fs.closeSync(fd)#

Added in: v0.1.21

Synchronous close(2). Returns undefined.

fs.constants#

Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.

fs.copyFile(src, dest[, flags], callback)#

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.

flags is an optional integer that specifies the behavior of the copy operation. The only supported flag is fs.constants.COPYFILE_EXCL, which causes the copy operation to fail if dest already exists.

Example:

const fs = require('fs');

// destination.txt will be created or overwritten by default.
fs.copyFile('source.txt', 'destination.txt', (err) => {
  if (err) throw err;
  console.log('source.txt was copied to destination.txt');
});

If the third argument is a number, then it specifies flags, as shown in the following example.

const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL, callback);

fs.copyFileSync(src, dest[, flags])#

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.

flags is an optional integer that specifies the behavior of the copy operation. The only supported flag is fs.constants.COPYFILE_EXCL, which causes the copy operation to fail if dest already exists.

Example:

const fs = require('fs');

// destination.txt will be created or overwritten by default.
fs.copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');

If the third argument is a number, then it specifies flags, as shown in the following example.

const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFileSync('source.txt', 'destination.txt', COPYFILE_EXCL);

fs.createReadStream(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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

Returns a new ReadStream object. (See Readable Streams).

Be aware that, unlike the default value set for highWaterMark on a readable stream (16 kb), the stream returned by this method has a default value of 64 kb for the same parameter.

options is an object or string with the following defaults:

const defaults = {
  flags: 'r',
  encoding: null,
  fd: null,
  mode: 0o666,
  autoClose: true,
  highWaterMark: 64 * 1024
};

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. 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. Note that fd should be blocking; non-blocking fds should be passed to net.Socket.

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:

fs.createReadStream('sample.txt', { start: 90, end: 99 });

If options is a string, then it specifies the encoding.

fs.createWriteStream(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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

Returns a new WriteStream object. (See Writable Stream).

options is an object or string with the following defaults:

const defaults = {
  flags: 'w',
  encoding: 'utf8',
  fd: null,
  mode: 0o666,
  autoClose: true
};

options may also include a start option to allow writing data at some position past the beginning of the file. Modifying a file rather than replacing it may require a flags mode of r+ rather than the default mode w. The encoding can be any one of those accepted by Buffer.

If autoClose is set to true (default behavior) on error or end 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.

Like ReadStream, if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. Note that 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
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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. Example:

fs.exists('/etc/passwd', (exists) => {
  console.log(exists ? 'it\'s there' : 'no passwd!');
});

Note that the parameter to this callback is 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)

fs.exists('myfile', (exists) => {
  if (exists) {
    console.error('myfile already exists');
  } else {
    fs.open('myfile', 'wx', (err, fd) => {
      if (err) throw err;
      writeMyData(fd);
    });
  }
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    fs.open('myfile', 'r', (err, fd) => {
      if (err) throw err;
      readMyData(fd);
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

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.existsSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous version of fs.exists(). Returns true if the path exists, false otherwise.

Note that 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.)

fs.fchmod(fd, mode, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous fchmod(2). No arguments other than a possible exception are given to the completion callback.

fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Synchronous fchmod(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.

fs.fdatasyncSync(fd)#

Added in: v0.1.96

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous fstat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object. fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor fd.

fs.fstatSync(fd)#

Added in: v0.1.95

Synchronous fstat(2). Returns an instance of fs.Stats.

fs.fsync(fd, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.

fs.fsyncSync(fd)#

Added in: v0.1.96

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd[, len], callback)#

History
VersionChanges
REPLACEME

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

Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.

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

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

fs.ftruncateSync(fd[, len])#

Added in: v0.8.6

Synchronous ftruncate(2). Returns undefined.

fs.futimes(fd, atime, mtime, callback)#

History
VersionChanges
REPLACEME

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().

This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS.

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.lchmod(path, mode, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.

Only available on macOS.

fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Synchronous lchmod(2). Returns undefined.

fs.lchown(path, uid, gid, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous lchown(2). No arguments other than a possible exception are given to the completion callback.

fs.lchownSync(path, uid, gid)#

Deprecated since: v0.4.7

Synchronous lchown(2). Returns undefined.

fs.link(existingPath, newPath, callback)#

History
VersionChanges
REPLACEME

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

Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.

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

Synchronous link(2). Returns undefined.

fs.lstat(path, callback)#

History
VersionChanges
REPLACEME

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. 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.

v0.1.30

Added in: v0.1.30

Asynchronous lstat(2). 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.

fs.lstatSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.30

Added in: v0.1.30

Synchronous lstat(2). Returns an instance of fs.Stats.

fs.mkdir(path[, mode], callback)#

History
VersionChanges
REPLACEME

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. 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.

v0.1.8

Added in: v0.1.8

Asynchronously creates a directory. No arguments other than a possible exception are given to the completion callback.

See also: mkdir(2)

fs.mkdirSync(path[, mode])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronously creates a directory. Returns undefined. This is the synchronous version of fs.mkdir().

See also: mkdir(2)

fs.mkdtemp(prefix[, options], callback)#

History
VersionChanges
REPLACEME

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.

The created folder 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.

Example:

fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // 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('path').sep).

// The parent directory for the new temporary directory
const tmpDir = os.tmpdir();

// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmpabc123`.
  // Note that a new temporary directory is created
  // at the file system root rather than *within*
  // the /tmp directory.
});

// This method is *CORRECT*:
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

fs.mkdtempSync(prefix[, options])#

Added in: v5.10.0

The synchronous version of fs.mkdtemp(). Returns the created folder path.

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.open(path, flags[, mode], callback)#

History
VersionChanges
v9.9.0

The as and as+ modes are supported now.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.0.2

Added in: v0.0.2

Asynchronous file open. See open(2). flags can be:

  • '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.

    Note that this doesn't turn fs.open() into a synchronous blocking call. If synchronous operation is desired 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 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 path exists.

  • 'a' - Open file for appending. The file is created if it does not exist.

  • 'ax' - Like 'a' but fails if path exists.

  • 'as' - Open file for appending in synchronous mode. The file is created if it does not exist.

  • 'a+' - Open file for reading and appending. The file is created if it does not exist.

  • 'ax+' - Like 'a+' but fails if path exists.

  • 'as+' - Open file for reading and appending in synchronous mode. The file is created if it does not exist.

mode sets the file mode (permission and sticky bits), but only if the file was created.

The callback gets two arguments (err, fd).

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

flags 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.

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.

The behavior of fs.open() is platform-specific for some flags. As such, opening a directory on macOS and Linux with the 'a+' flag - see example below - will return an error. In contrast, on Windows and FreeBSD, a file descriptor 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>
});

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. eg. fs.writeFile(), fs.readFile(), etc.

Note: On Windows, opening an existing hidden file using the w flag (either through fs.open() or fs.writeFile()) will fail with EPERM. Existing hidden files can be opened for writing with the r+ flag. A call to fs.ftruncate() can be used to reset the file contents.

fs.openSync(path, flags[, mode])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous version of fs.open(). Returns an integer representing the file descriptor.

fs.read(fd, buffer, offset, length, position, callback)#

History
VersionChanges
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.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

The callback is given the three arguments, (err, bytesRead, buffer).

If this method is invoked as its util.promisify()ed version, it returns a Promise for an object with bytesRead and buffer properties.

fs.readdir(path[, options], callback)#

History
VersionChanges
REPLACEME

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. 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.

v6.0.0

The options parameter was added.

v0.1.8

Added in: v0.1.8

Asynchronous readdir(3). 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 '..'.

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.

fs.readdirSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous readdir(3). Returns an array of filenames excluding '.' and '..'.

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.

fs.readFile(path[, options], callback)#

History
VersionChanges
REPLACEME

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. 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.

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. Example:

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. Example:

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.

// macOS, Linux, and Windows
fs.readFile('<directory>', (err, data) => {
  // => [Error: EISDIR: illegal operation on a directory, read <directory>]
});

//  FreeBSD
fs.readFile('<directory>', (err, data) => {
  // => null, <data>
});

Any specified file descriptor has to support reading.

If a file descriptor is specified as the path, it will not be closed automatically.

The fs.readFile() function buffers the entire file. To minimize memory costs, when possible prefer streaming via fs.createReadStream().

fs.readFileSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v5.0.0

The path parameter can be a file descriptor now.

v0.1.8

Added in: v0.1.8

Synchronous version of fs.readFile(). Returns the contents of the path.

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.

// macOS, Linux, and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]

//  FreeBSD
fs.readFileSync('<directory>'); // => null, <data>

fs.readlink(path[, options], callback)#

History
VersionChanges
REPLACEME

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. 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

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

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.readlinkSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Synchronous readlink(2). Returns the symbolic link's string value.

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.readSync(fd, buffer, offset, length, position)#

History
VersionChanges
v6.0.0

The length parameter can now be 0.

v0.1.21

Added in: v0.1.21

Synchronous version of fs.read(). Returns the number of bytesRead.

fs.realpath(path[, options], callback)#

History
VersionChanges
REPLACEME

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. 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.

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.

Note that "canonical" does not mean "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)#

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.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. Support is currently still experimental.

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

Synchronously computes the canonical pathname by resolving ., .. and symbolic links.

Note that "canonical" does not mean "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 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 returned value. 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.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 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
REPLACEME

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. No arguments other than a possible exception are given to the completion callback.

See also: rename(2).

fs.rename('oldFile.txt', 'newFile.txt', (err) => {
  if (err) throw err;
  console.log('Rename complete!');
});

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

Synchronous rename(2). Returns undefined.

fs.rmdir(path, callback)#

History
VersionChanges
REPLACEME

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. 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

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.

fs.rmdirSync(path)#

History
VersionChanges
v7.6.0

The path parameters can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

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.

fs.stat(path, callback)#

History
VersionChanges
REPLACEME

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. 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

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.

fs.statSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous stat(2). Returns an instance of fs.Stats.

fs.symlink(target, path[, type], callback)#

History
VersionChanges
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

Asynchronous symlink(2). No arguments other than a possible exception are given to the completion callback. The type argument can be set to 'dir', 'file', or 'junction' and is only available on Windows (ignored on other platforms). Note that Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Here is an example below:

fs.symlink('./foo', './new-port', callback);

It creates a symbolic link named "new-port" that points to "foo".

fs.symlinkSync(target, path[, type])#

History
VersionChanges
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

Synchronous symlink(2). Returns undefined.

fs.truncate(path[, len], callback)#

History
VersionChanges
REPLACEME

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

Asynchronous truncate(2). 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.

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

fs.truncateSync(path[, len])#

Added in: v0.8.6

Synchronous truncate(2). 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.unlink(path, callback)#

History
VersionChanges
REPLACEME

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. 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 removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.

// Assuming that 'path/file.txt' is a regular file.
fs.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 also: unlink(2)

fs.unlinkSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous unlink(2). Returns undefined.

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
REPLACEME

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. 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.

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, Dates, or a numeric string like '123456789.0'.
  • If the value can not be converted to a number, or is NaN, Infinity or -Infinity, a Error will be thrown.

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. Support is currently still experimental.

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.utimes(). Returns undefined.

fs.watch(filename[, options][, listener])#

History
VersionChanges
v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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'.
  • listener <Function> | <undefined> Default: undefined

Watch for changes on filename, where filename is either a file or a directory. The returned object is a fs.FSWatcher.

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.

Note that on most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

Also note 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.

Caveats#

The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations.

The recursive option is only supported on macOS and Windows.

Availability#

This feature depends on the underlying operating system providing a way to be notified of filesystem changes.

  • On Linux systems, this uses inotify
  • On BSD systems, this uses kqueue
  • On macOS, this uses kqueue 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.

If the underlying functionality is not available for some reason, then fs.watch will not be able to function. 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, Docker, etc.

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.

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
v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

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:

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.

To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtime and prev.mtime.

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). In Windows, blksize and blocks fields will be undefined, instead of zero. 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 previousStat reported in the second callback event (the file's reappearance) will be the same as the previousStat of the first callback event (its disappearance).

This happens when:

  • the file is deleted, followed by a restore
  • the file is renamed twice - the second time back to its original name

fs.write(fd, buffer[, offset[, length[, position]]], callback)#

History
VersionChanges
REPLACEME

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.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly 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, string[, position[, encoding]], callback)#

History
VersionChanges
REPLACEME

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, then the value will be coerced to one.

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. Note that bytes written is not the same as string characters. See Buffer.byteLength.

Unlike when writing buffer, the entire string must be written. No substring may be specified. This is because the byte offset of the resulting data may not be the same as the string offset.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly 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.writeFile(file, data[, options], callback)#

History
VersionChanges
REPLACEME

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

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer.

The encoding option is ignored if data is a buffer.

Example:

fs.writeFile('message.txt', 'Hello Node.js', (err) => {
  if (err) throw err;
  console.log('The file has been saved!');
});

If options is a string, then it specifies the encoding. Example:

fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

Any specified file descriptor has to support writing.

Note that it is unsafe to use fs.writeFile multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

If a file descriptor is specified as the file, it will not be closed automatically.

fs.writeFileSync(file, data[, options])#

History
VersionChanges
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

The synchronous version of fs.writeFile(). Returns undefined.

fs.writeSync(fd, buffer[, offset[, length[, position]]])#

History
VersionChanges
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

fs.writeSync(fd, string[, position[, encoding]])#

History
VersionChanges
v7.2.0

The position parameter is optional now.

v0.11.5

Added in: v0.11.5

Synchronous versions of fs.write(). Returns the number of bytes written.

fs Promises API#

Stability: 1 - Experimental

The fs/promises API provides an alternative set of asynchronous file system methods that return Promise objects rather than using callbacks. The API is accessible via require('fs/promises').

class: FileHandle#

Added in: REPLACEME

A FileHandle object is a wrapper for a numeric file descriptor. Instances of FileHandle are distinct from numeric file descriptors in that, if the FileHandle is not explicitly closed using the filehandle.close() method, they will automatically close the file descriptor and will emit a process warning, thereby helping to prevent memory leaks.

Instances of the FileHandle object are created internally by the fsPromises.open() method.

Unlike callback-based such as fs.fstat(), fs.fchown(), fs.fchmod(), fs.ftruncate(), fs.read(), and fs.write(), operations — all of which use a simple numeric file descriptor, all fsPromises.* variations use the FileHandle class in order to help protect against accidental leaking of unclosed file descriptors after a Promise is resolved or rejected.

filehandle.fd#

Added in: REPLACEME

Value: <number> The numeric file descriptor managed by the FileHandle object.

filehandle.appendFile(data, options)#

Added in: REPLACEME

Asynchronously append data to this file, creating the file if it does not yet exist. data can be a string or a Buffer. The Promise will be resolved with no arguments upon success.

If options is a string, then it specifies the encoding.

The FileHandle must have been opened for appending.

filehandle.chmod(mode)#

Added in: REPLACEME

Modifies the permissions on the file. The Promise is resolved with no arguments upon success.

filehandle.chown(uid, gid)#

Added in: REPLACEME

Changes the ownership of the file then resolves the Promise with no arguments upon success.

filehandle.close()#

Added in: REPLACEME
  • Returns: <Promise> A Promise that will be resolved once the underlying file descriptor is closed, or will be rejected if an error occurs while closing.

Closes the file descriptor.

async function openAndClose() {
  let filehandle;
  try {
    filehandle = await fsPromises.open('thefile.txt', 'r');
  } finally {
    if (filehandle !== undefined)
      await filehandle.close();
  }
}

filehandle.datasync()#

Added in: REPLACEME

Asynchronous fdatasync(2). The Promise is resolved with no arguments upon success.

filehandle.read(buffer, offset, length, position)#

Added in: REPLACEME

Read data from the file.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

Following successful read, the Promise is resolved with an object with a bytesRead property specifying the number of bytes read, and a buffer property that is a reference to the passed in buffer argument.

filehandle.readFile(options)#

Added in: REPLACEME

Asynchronously reads the entire contents of a file.

The Promise is resolved 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.

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.

The FileHandle has to support reading.

filehandle.stat()#

Added in: REPLACEME

Retrieves the fs.Stats for the file.

filehandle.sync()#

Added in: REPLACEME

Asynchronous fsync(2). The Promise is resolved with no arguments upon success.

filehandle.truncate(len)#

Added in: REPLACEME

Truncates the file then resolves the Promise with no arguments upon success.

If the file 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:

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  const fd = await fsPromises.open('temp.txt', 'r+');
  await fsPromises.ftruncate(fd, 4);
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints: Node
}

doTruncate().catch(console.error);

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  const fd = await fsPromises.open('temp.txt', 'r+');
  await fsPromises.ftruncate(fd, 10);
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints Node.js\0\0\0
}

doTruncate().catch(console.error);

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

filehandle.utimes(atime, mtime)#

Added in: REPLACEME

Change the file system timestamps of the object referenced by the FileHandle then resolves the Promise with no arguments upon success.

This function does not work on AIX versions before 7.1, it will resolve the Promise with an error using code UV_ENOSYS.

filehandle.write(buffer, offset, length, position)#

Added in: REPLACEME

Write buffer to the file.

The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the buffer written.

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).

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, fs.createWriteStream is strongly recommended.

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)#

Added in: REPLACEME

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success.

The encoding option is ignored if data is a buffer.

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).

fsPromises.access(path[, mode])#

Added in: REPLACEME

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. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

  • fs.constants.F_OK - path 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.
  • fs.constants.R_OK - path can be read by the calling process.
  • fs.constants.W_OK - path can be written by the calling process.
  • fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

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.

fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
  .then(() => 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(file, data[, options])#

Added in: REPLACEME

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer. The Promise will be resolved with no arguments upon success.

If options is a string, then it specifies the encoding.

The file may be specified as a FileHandle that has been opened for appending (using fsPromises.open()).

fsPromises.chmod(path, mode)#

Added in: REPLACEME

Changes the permissions of a file then resolves the Promise with no arguments upon succces.

fsPromises.chown(path, uid, gid)#

Added in: REPLACEME

Changes the ownership of a file then resolves the Promise with no arguments upon success.

fsPromises.copyFile(src, dest[, flags])#

Added in: REPLACEME

Asynchronously copies src to dest. By default, dest is overwritten if it already exists. The Promise will be resolved with no arguments upon success.

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.

flags is an optional integer that specifies the behavior of the copy operation. The only supported flag is fs.constants.COPYFILE_EXCL, which causes the copy operation to fail if dest already exists.

Example:

const fs = require('fs');

// destination.txt will be created or overwritten by default.
fsPromises.copyFile('source.txt', 'destination.txt')
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));

If the third argument is a number, then it specifies flags, as shown in the following example.

const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fsPromises.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL)
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));

fsPromises.fchmod(filehandle, mode)#

Added in: REPLACEME

Asynchronous fchmod(2). The Promise is resolved with no arguments upon success.

fsPromises.fchown(filehandle, uid, gid)#

Added in: REPLACEME

Changes the ownership of the file represented by filehandle then resolves the Promise with no arguments upon success.

fsPromises.fdatasync(filehandle)#

Added in: REPLACEME

Asynchronous fdatasync(2). The Promise is resolved with no arguments upon success.

fsPromises.fstat(filehandle)#

Added in: REPLACEME

Retrieves the fs.Stats for the given filehandle.

fsPromises.fsync(filehandle)#

Added in: REPLACEME

Asynchronous fsync(2). The Promise is resolved with no arguments upon success.

fsPromises.ftruncate(filehandle[, len])#

Added in: REPLACEME

Truncates the file represented by filehandle then resolves the Promise with no arguments upon success.

If the file referred to by the FileHandle 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:

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  const fd = await fsPromises.open('temp.txt', 'r+');
  await fsPromises.ftruncate(fd, 4);
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints: Node
}

doTruncate().catch(console.error);

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  const fd = await fsPromises.open('temp.txt', 'r+');
  await fsPromises.ftruncate(fd, 10);
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints Node.js\0\0\0
}

doTruncate().catch(console.error);

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

fsPromises.futimes(filehandle, atime, mtime)#

Added in: REPLACEME

Change the file system timestamps of the object referenced by the supplied FileHandle then resolves the Promise with no arguments upon success.

This function does not work on AIX versions before 7.1, it will resolve the Promise with an error using code UV_ENOSYS.

fsPromises.lchmod(path, mode)#

Deprecated since: REPLACEME

Changes the permissions on a symbolic link then resolves the Promise with no arguments upon success. This method is only implemented on macOS.

fsPromises.lchown(path, uid, gid)#

Deprecated since: REPLACEME

Changes the ownership on a symbolic link then resolves the Promise with no arguments upon success. This method is only implemented on macOS.

fsPromises.link(existingPath, newPath)#

Added in: REPLACEME

Asynchronous link(2). The Promise is resolved with no arguments upon success.

fsPromises.lstat(path)#

Added in: REPLACEME

Asynchronous lstat(2). The Promise is resolved with the fs.Stats object for the given symbolic link path.

fsPromises.mkdir(path[, mode])#

Added in: REPLACEME

Asynchronously creates a directory then resolves the Promise with no arguments upon success.

fsPromises.mkdtemp(prefix[, options])#

Added in: REPLACEME

Creates a unique temporary directory then resolves the Promise with the created folder path. A unique directory name is generated by appending six random characters to the end of the provided prefix.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

Example:

fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
  .catch(console.error);

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('path').sep).

fsPromises.open(path, flags[, mode])#

Added in: REPLACEME

Asynchronous file open that returns a Promise that, when resolved, yields a FileHandle object. See open(2).

The flags argument can be:

  • '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.

    Note that this does not turn fsPromises.open() into a synchronous blocking call.

  • '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 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 path exists.

  • 'a' - Open file for appending. The file is created if it does not exist.

  • 'ax' - Like 'a' but fails if path exists.

  • 'as' - Open file for appending in synchronous mode. The file is created if it does not exist.

  • 'a+' - Open file for reading and appending. The file is created if it does not exist.

  • 'ax+' - Like 'a+' but fails if path exists.

  • 'as+' - Open file for reading and appending in synchronous mode. The file is created if it does not exist.

mode sets the file mode (permission and sticky bits), but only if the file was created.

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

flags 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.

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.

The behavior of fsPromises.open() is platform-specific for some flags. As such, opening a directory on macOS and Linux with the 'a+' flag will return an error. In contrast, on Windows and FreeBSD, a FileHandle will be returned.

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.

Note: On Windows, opening an existing hidden file using the w flag (e.g. using fsPromises.open()) will fail with EPERM. Existing hidden files can be opened for writing with the r+ flag. A call to fsPromises.ftruncate() can be used to reset the file contents.

fsPromises.read(filehandle, buffer, offset, length, position)#

Added in: REPLACEME

Read data from the file specified by filehandle.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

Following successful read, the Promise is resolved with an object with a bytesRead property specifying the number of bytes read, and a buffer property that is a reference to the passed in buffer argument.

fsPromises.readdir(path[, options])#

Added in: REPLACEME

Reads the contents of a directory then resolves the Promise with an array of the names of the files in the directory excludiing '.' and '..'.

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.

fsPromises.readFile(path[, options])#

Added in: REPLACEME

Asynchronously reads the entire contents of a file.

The Promise is resolved 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.

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.

Any specified FileHandle has to support reading.

fsPromises.readlink(path[, options])#

Added in: REPLACEME

Asynchronous readlink(2). 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: REPLACEME

Determines the actual location of path using the same semantics as the fs.realpath.native() function then resolves the Promise with the resolved path.

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: REPLACEME

Renames oldPath to newPath and resolves the Promise with no arguments upon success.

fsPromises.rmdir(path)#

Added in: REPLACEME

Removes the directory identified by path then resolves the Promise with no arguments upon success.

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.

fsPromises.stat(path)#

Added in: REPLACEME

The Promise is resolved with the fs.Stats object for the given path.

fsPromises.symlink(target, path[, type])#

Added in: REPLACEME

Creates a symbolic link then resolves the Promise with no arguments upon success.

The type argument is only used on Windows platforms and can be one of 'dir', 'file', or 'junction'. Note that 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: REPLACEME

Truncates the path then resolves the Promise with no arguments upon success. The path must be a string or Buffer.

fsPromises.unlink(path)#

Added in: REPLACEME

Asynchronous unlink(2). The Promise is resolved with no arguments upon success.

fsPromises.utimes(path, atime, mtime)#

Added in: REPLACEME

Change the file system timestamps of the object referenced by path then resolves the Promise with no arguments upon success.

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.write(filehandle, buffer[, offset[, length[, position]]])#

Added in: REPLACEME

Write buffer to the file specified by filehandle.

The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the buffer written.

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).

It is unsafe to use fsPromises.write() multiple times on the same file without waiting for the Promise to be resolved (or rejected). For this scenario, fs.createWriteStream is strongly recommended.

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.

fsPromises.writeFile(file, data[, options])#

Added in: REPLACEME

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success.

The encoding option is ignored if data is a buffer.

If options is a string, then it specifies the encoding.

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 resolved (or rejected).

FS Constants#

The following constants are exported by fs.constants.

Not every constant will be available on every operating system.

File Access Constants#

The following constants are meant for use with fs.access().

Constant Description
F_OK Flag indicating that the file is visible to the calling process.
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.

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.

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.

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.

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 a number of built-in objects that are part of the JavaScript language itself, which are also globally accessible.

Class: Buffer#

Added in: v0.1.103

Used to handle binary data. See the buffer section.

__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.

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.

console#

Added in: v0.1.100

Used to print to stdout and stderr. See the console section.

exports#

This variable may appear to be global but is not. See exports.

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.

module#

This variable may appear to be global but is not. See module.

process#

Added in: v0.1.7

The process object. See the process object section.

require()#

This variable may appear to be global but is not. See require().

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.

URL#

Added in: REPLACEME

The WHATWG URL class. See the URL section.

URLSearchParams#

Added in: REPLACEME

The WHATWG URLSearchParams class. See the URLSearchParams section.

HTTP#

Stability: 2 - Stable

To use the HTTP server and client one must require('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 — 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': 'mysite.com',
  'accept': '*/*' }

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, Node.js's 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', 'mysite.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)#

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. 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. Default: Infinity.
    • maxFreeSockets <number> Maximum number of sockets to leave open in a free state. Only relevant if keepAlive is set to true. Default: 256.

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('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.

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.

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.

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 will no longer be used. Otherwise, sockets may hang open for quite a long time before the server terminates them.

agent.freeSockets#

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.

agent.getName(options)#

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.requests#

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#

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.

Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

The request implements the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'abort'#

Added in: v1.4.1

Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort().

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.

A client and server pair demonstrating how to listen for the 'connect' event:

const http = require('http');
const net = require('net');
const url = require('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, cltSocket, head) => {
  // connect to an origin server
  const srvUrl = url.parse(`http://${req.url}`);
  const srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => {
    cltSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    srvSocket.write(head);
    srvSocket.pipe(cltSocket);
    cltSocket.pipe(srvSocket);
  });
});

// now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // make a request to a tunneling proxy
  const options = {
    port: 1337,
    hostname: '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: 'information'#

Added in: REPLACEME

Emitted when the server sends a 1xx response (excluding 101 Upgrade). This event is emitted with a callback containing an object with a status code.

const http = require('http');

const options = {
  hostname: '127.0.0.1',
  port: 8080,
  path: '/length_request'
};

// Make a request
const req = http.request(options);
req.end();

req.on('information', (res) => {
  console.log(`Got information prior to main response: ${res.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

Emitted after a socket is assigned to this request.

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 aborted 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, clients receiving an upgrade header will have their connections closed.

A client server pair demonstrating how to listen for the 'upgrade' event.

const http = require('http');

// Create an HTTP server
const srv = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('okay');
});
srv.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
srv.listen(1337, '127.0.0.1', () => {

  // make a request
  const options = {
    port: 1337,
    hostname: '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: v0.3.8

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#

Added in: v0.11.14

If a request has been aborted, this value is the time when the request was aborted, in milliseconds since 1 January 1970 00:00:00 UTC.

request.connection#

Added in: v0.3.0

See request.socket

request.end([data[, encoding]][, callback])#

History
VersionChanges
REPLACEME

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.flushHeaders()#

Added in: v1.6.0

Flush 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. Note that the name is case insensitive.

Example:

const contentType = request.getHeader('Content-Type');

request.removeHeader(name)#

Added in: v1.6.0

Removes a header that's already defined into headers object.

Example:

request.removeHeader('Content-Type');

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.

Example:

request.setHeader('Content-Type', 'application/json');

or

request.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

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])#

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.

Once a socket is assigned to this request and is connected socket.setTimeout() will be called.

Returns request.

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. After response.end(), the property is nulled. The socket may also be accessed via request.connection.

Example:

const http = require('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
});

request.write(chunk[, encoding][, callback])#

Added in: v0.1.29

Sends a chunk of the body. By calling this method many times, a request body can be sent to a server — in that case it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating 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.

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.

Class: http.Server#

Added in: v0.1.17

This class inherits from net.Server and has the following additional events:

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.

Note that 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.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'clientError'#

History
VersionChanges
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.

Default behavior is to close the socket with an HTTP '400 Bad Request' response if possible, otherwise the socket is immediately destroyed.

socket is the net.Socket object that the error originated from.

const http = require('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.

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.

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.connection.

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: v0.1.0

Emitted each time there is a request. Note that there may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a client requests an HTTP upgrade. If this event is not listened for, then clients requesting an upgrade will have their connections closed.

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.

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections. See net.Server.close().

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

A Boolean indicating 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.setTimeout([msecs][, callback])#

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's timeout value is 2 minutes, and sockets are destroyed automatically if they time out. However, if a callback is assigned to the Server's 'timeout' event, timeouts must be handled explicitly.

Returns server.

server.timeout#

Added in: v0.9.12
  • <number> Timeout in milliseconds. Default: 120000 (2 minutes).

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.

The response implements, but does not inherit from, the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.6.7

Indicates that the underlying connection was terminated before response.end() was called or able to flush.

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.

After this event, no more events will be emitted on the response object.

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.

Note that 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: v0.3.0

See response.socket.

response.end([data][, encoding][, callback])#

History
VersionChanges
REPLACEME

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 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: v0.0.2

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: v0.4.0

Reads out a header that's already been queued but not sent to the client. Note that the name is case insensitive.

Example:

const contentType = response.getHeader('content-type');

response.getHeaderNames()#

Added in: v7.7.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

Example:

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.

Example:

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. Note that the header name matching is case-insensitive.

Example:

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.

Example:

response.removeHeader('Content-Encoding');

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

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.

Example:

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');
});

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.

Returns response.

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. The socket may also be accessed via response.connection.

Example:

const http = require('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);

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.

Example:

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.

Example:

response.statusMessage = 'Not found';

After response header was sent to the client, this property indicates the status message which was sent out.

response.write(chunk[, encoding][, callback])#

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.

Note that in the 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 a HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server.

response.writeHead(statusCode[, statusMessage][, headers])#

History
VersionChanges
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.

Example:

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

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.

// 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');
});

Note that Content-Length is given in bytes not characters. The above example works because the string 'hello world' contains only single byte characters. If the body contains higher coded characters then Buffer.byteLength() should be used to determine the number of bytes in a given encoding. And Node.js does not 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 TypeError being thrown.

response.writeProcessing()#

Added in: REPLACEME

Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.

Class: http.IncomingMessage#

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.

It implements the Readable Stream interface, as well as the following additional events, methods, and properties.

Event: 'aborted'#

Added in: v0.3.8

Emitted when the request has been aborted and the network socket has closed.

Event: 'close'#

Added in: v0.4.2

Indicates that the underlying connection was closed. Just like 'end', this event occurs only once per response.

message.destroy([error])#

Added in: v0.3.0

Calls destroy() on the socket that received the IncomingMessage. If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event.

message.headers#

Added in: v0.1.5

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased. Example:

// 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, or user-agent are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For all other headers, the values are joined together with ', '.

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. Example: 'GET', 'DELETE'.

message.rawHeaders#

Added in: v0.11.6

The raw request/response headers list exactly as they were received.

Note that 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.connection.setTimeout(msecs, callback).

Returns message.

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.

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.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. If the request is:

GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n

Then request.url will be:

'/status?name=ryan'

To parse the url into its parts require('url').parse(request.url) can be used. Example:

$ node
> require('url').parse('/status?name=ryan')
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

To extract the parameters from the query string, the require('querystring').parse function can be used, or true can be passed as the second argument to require('url').parse. Example:

$ node
> require('url').parse('/status?name=ryan', true)
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: { name: 'ryan' },
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

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
v9.6.0

The options argument is supported now.

v0.1.13

Added in: v0.1.13

  • options <Object>
    • IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to be used. Useful for extending the original IncomingMessage. Default: IncomingMessage.
    • ServerResponse <http.ServerResponse> Specifies the ServerResponse class to be used. Useful for extending the original ServerResponse. Default: ServerResponse.

  • 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.

http.get(options[, callback])#

History
VersionChanges
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. Note that 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://nodejs.org/dist/index.json', (res) => {
  const { statusCode } = res;
  const contentType = res.headers['content-type'];

  let error;
  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}`);
});

http.globalAgent#

Added in: v0.5.9

Global instance of Agent which is used as the default for all HTTP client requests.

http.request(options[, callback])#

History
VersionChanges
v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

  • options <Object> | <string> | <URL>
    • protocol <string> Protocol to use. Default: http:.
    • 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 is preferred over host.
    • family <number> IP address family to use when resolving host and hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
    • port <number> Port of remote server. Default: 80.
    • localAddress <string> Local interface to bind for network connections.
    • socketPath <string> Unix Domain Socket (use one of host:port or socketPath).
    • 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: '/'.
    • headers <Object> An object containing request headers.
    • auth <string> Basic authentication i.e. 'user:password' to compute an Authorization header.
    • 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.
    • 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.
    • timeout <number>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
    • setHost <boolean>: Specifies whether or not to automatically add the Host header. Defaults to true.
  • callback <Function>
  • Returns: <http.ClientRequest>

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with url.parse(). If it is a URL object, it will be automatically converted to an ordinary options object.

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.

Example:

const postData = querystring.stringify({
  'msg': 'Hello World!'
});

const options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
    '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();

Note that 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 RFC2616 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

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
  • close
  • error with an error with message Error: socket hang up and code ECONNRESET

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
  • close
    • aborted on the res object
    • end on the res object
    • close on the res object

Note that setting the timeout option or using the setTimeout function will not abort the request or do anything besides add a timeout event.

HTTP/2#

Stability: 1 - Experimental

The http2 module provides an implementation of the HTTP/2 protocol. It can be accessed using:

const http2 = require('http2');

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('http2');
const fs = require('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',
    ':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('http2');
const fs = require('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.

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 the 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.

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

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 'frameError' event on the Http2Stream is made.

When invoked, the handler function will receive three arguments:

  • An integer identifying the frame type.
  • An integer identifying the error code.
  • An integer identifying the stream (or 0 if the frame is not associated with a stream).

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

The 'goaway' event is emitted when a GOAWAY frame is received. When invoked, the handler function will receive three arguments:

  • 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 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 invoked, the handler function will receive a copy of the local settings.

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: 'remoteSettings'#

Added in: v8.4.0

The 'remoteSettings' event is emitted when a new SETTINGS frame is received from the connected peer. When invoked, the handler function will receive a copy of the remote settings.

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. When invoked, the handler function will receive a reference to the Http2Stream object, a HTTP/2 Headers Object, and numeric flags associated with the creation of the stream.

const http2 = require('http2');
session.on('stream', (stream, headers, flags) => {
  const method = headers[':method'];
  const path = headers[':path'];
  // ...
  stream.respond({
    ':status': 200,
    'content-type': 'text/plain'
  });
  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('http2');

// Create an unencrypted HTTP/2 server
const server = http2.createServer();

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

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.

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.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.

http2session.pendingSettingsAck#

Added in: v8.4.0

Indicates whether or not the Http2Session is currently waiting for an acknowledgment for 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)#

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.setTimeout(msecs, callback)#

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.

  • Value: <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)#

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('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.

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('http2');
const client = http2.connect('https://example.org');

client.on('altsvc', (alt, origin, streamId) => {
  console.log(alt);
  console.log(origin);
  console.log(streamId);
});

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).
    • getTrailers <Function> Callback function invoked to collect trailer headers.

  • 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.

This method is only available if http2session.type is equal to http2.constants.NGHTTP2_SESSION_CLIENT.

const http2 = require('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 set, the options.getTrailers() function is called immediately after queuing the last chunk of payload data to be sent. The callback is passed a single object (with a null prototype) that the listener may use to specify the trailing header fields to send to the peer.

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':method', ':path', etc). An 'error' event will be emitted if the getTrailers callback attempts to set such header fields.

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.

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.
  • The http2stream.close() method is called.
  • 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 will be sent 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.

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

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: '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().

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.

stream.on('trailers', (headers, flags) => {
  console.log(headers);
});

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.close(code[, callback])#

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.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 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)#

Added in: v8.4.0 
const http2 = require('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.

  • Value: <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> true if this Http2Stream has been closed locally.
    • remoteClose <number> true 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.

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('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

Boolean (read-only). True if headers were sent, false otherwise.

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)#

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('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.

http2stream.respond([headers[, options]])#

Added in: v8.4.0 
const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 });
  stream.end('some data');
});

When set, the options.getTrailers() function is called immediately after queuing the last chunk of payload data to be sent. The callback is passed a single object (with a null prototype) that the listener may use to specify the trailing header fields to send to the peer.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 }, {
    getTrailers(trailers) {
      trailers.ABC = 'some value to send';
    }
  });
  stream.end('some data');
});

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':status', ':path', etc). An 'error' event will be emitted if the getTrailers callback attempts to set such header fields.

http2stream.respondWithFD(fd[, headers[, options]])#

History
VersionChanges
REPLACEME

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('http2');
const fs = require('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'
  };
  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 is not closed when the stream is closed, so it will need to be closed manually once it is no longer needed. Note that 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 set, the options.getTrailers() function is called immediately after queuing the last chunk of payload data to be sent. The callback is passed a single object (with a null prototype) that the listener may use to specify the trailing header fields to send to the peer.

const http2 = require('http2');
const fs = require('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'
  };
  stream.respondWithFD(fd, headers, {
    getTrailers(trailers) {
      trailers.ABC = 'some value to send';
    }
  });

  stream.on('close', () => fs.closeSync(fd));
});

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':status', ':path', etc). An 'error' event will be emitted if the getTrailers callback attempts to set such header fields.

http2stream.respondWithFile(path[, headers[, options]])#

History
VersionChanges
REPLACEME

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('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function statCheck(stat, headers) {
    headers['last-modified'] = stat.mtime.toUTCString();
  }

  function onError(err) {
    if (err.code === 'ENOENT') {
      stream.respond({ ':status': 404 });
    } else {
      stream.respond({ ':status': 500 });
    }
    stream.end();
  }

  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain' },
                         { 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('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' },
                         { 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 set, the options.getTrailers() function is called immediately after queuing the last chunk of payload data to be sent. The callback is passed a single object (with a null prototype) that the listener may use to specify the trailing header fields to send to the peer.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function getTrailers(trailers) {
    trailers.ABC = 'some value to send';
  }
  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain' },
                         { getTrailers });
});

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':status', ':path', etc). An 'error' event will be emitted if the getTrailers callback attempts to set such header fields.

Class: Http2Server#

Added in: v8.4.0

In Http2Server, there are no 'clientError' events as there are in HTTP1. However, there are 'sessionError', and 'streamError' events for errors emitted on the socket, or from Http2Session or Http2Stream instances.

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.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'request'#

Added in: v8.4.0

Emitted each time there is a request. Note that 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: 'streamError'#

Added in: v8.5.0

If a ServerHttp2Stream emits an 'error' event, it will be forwarded here. The stream will already be destroyed when this event is triggered.

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.

const http2 = require('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'
  });
  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 http2server.setTimeout().

Class: Http2SecureServer#

Added in: v8.4.0

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.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'request'#

Added in: v8.4.0

Emitted each time there is a request. Note that 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.

const http2 = require('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'
  });
  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().

Event: 'unknownProtocol'#

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. See the Compatibility API.

http2.createServer(options[, onRequestHandler])#

History
VersionChanges
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.
    • 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. 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> Identifies 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 - Specifies that no padding is to be applied.
      • http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied.
      • http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding callback is to be used to determine the amount of padding.
      • http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, 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 will be used and the total frame length will not necessarily be 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.
    • selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK, provides the callback function used to determine the padding. See Using options.selectPadding.
    • 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.
  • 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('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',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.createSecureServer(options[, onRequestHandler])#

History
VersionChanges
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.
    • 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. 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> Identifies 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 - Specifies that no padding is to be applied.
      • http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied.
      • http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding callback is to be used to determine the amount of padding.
      • http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, 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 will be used and the total frame length will not necessarily be 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.
    • selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK, provides the callback function used to determine the padding. See Using options.selectPadding.
    • 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.
  • onRequestHandler <Function> See Compatibility API
  • Returns: <Http2SecureServer>

Returns a tls.Server instance that creates and manages Http2Session instances.

const http2 = require('http2');

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',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.connect(authority[, options][, listener])#

History
VersionChanges
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>
  • options <Object>
    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • 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. 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.
    • 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> Identifies 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 - Specifies that no padding is to be applied.
      • http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied.
      • http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding callback is to be used to determine the amount of padding.
      • http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, 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 will be used and the total frame length will not necessarily be 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.
    • selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK, provides the callback function used to determine the padding. See Using options.selectPadding.
    • 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.
  • listener <Function>
  • Returns: <ClientHttp2Session>

Returns a ClientHttp2Session instance.

const http2 = require('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#

Value Name Constant
0x00 No Error http2.constants.NGHTTP2_NO_ERROR
0x01 Protocol Error http2.constants.NGHTTP2_PROTOCOL_ERROR
0x02 Internal Error http2.constants.NGHTTP2_INTERNAL_ERROR
0x03 Flow Control Error http2.constants.NGHTTP2_FLOW_CONTROL_ERROR
0x04 Settings Timeout http2.constants.NGHTTP2_SETTINGS_TIMEOUT
0x05 Stream Closed http2.constants.NGHTTP2_STREAM_CLOSED
0x06 Frame Size Error http2.constants.NGHTTP2_FRAME_SIZE_ERROR
0x07 Refused Stream http2.constants.NGHTTP2_REFUSED_STREAM
0x08 Cancel http2.constants.NGHTTP2_CANCEL
0x09 Compression Error http2.constants.NGHTTP2_COMPRESSION_ERROR
0x0a Connect Error http2.constants.NGHTTP2_CONNECT_ERROR
0x0b Enhance Your Calm http2.constants.NGHTTP2_ENHANCE_YOUR_CALM
0x0c Inadequate Security http2.constants.NGHTTP2_INADEQUATE_SECURITY
0x0d HTTP/1.1 Required http2.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('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().

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.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream, headers) => {
  console.log(headers[':path']);
  console.log(headers.ABC);
});

Settings Object#

History
VersionChanges
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: 4,096 octets.
  • enablePush <boolean> Specifies true if HTTP/2 Push Streams are to be permitted on the Http2Session instances.
  • initialWindowSize <number> Specifies the senders initial window size for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65,535 bytes.
  • maxFrameSize <number> Specifies the size of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default: 16,384 bytes.
  • maxConcurrentStreams <number> Specifies the maximum number of concurrent streams permitted on an Http2Session. There is no default value which implies, at least theoretically, 231-1 streams may be open concurrently at any given time in an Http2Session. The minimum value is 0. The maximum allowed value is 231-1.
  • 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.

All additional properties on the settings object are ignored.

Using options.selectPadding#

When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK, the HTTP/2 implementation will consult the options.selectPadding callback function, if provided, to determine the specific amount of padding to use per HEADERS and DATA frame.

The options.selectPadding function receives two numeric arguments, frameLen and maxFrameLen and must return a number N such that frameLen <= N <= maxFrameLen.

const http2 = require('http2');
const server = http2.createServer({
  paddingStrategy: http2.constants.PADDING_STRATEGY_CALLBACK,
  selectPadding(frameLen, maxFrameLen) {
    return maxFrameLen;
  }
});

The options.selectPadding function is invoked once for every HEADERS and DATA frame. This has a definite noticeable impact on performance.

Error Handling#

There are several types of error conditions that may arise when using the 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('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('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('http2');
const { NGHTTP2_REFUSED_STREAM } = http2.constants;
const net = require('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('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');

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('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' });
  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('http2');
const { readFileSync } = require('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.

It implements the Readable Stream interface, as well as the following additional events, methods, and properties.

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.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. Example:

// 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, hostname, 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. Example: 'GET', 'DELETE'.

request.rawHeaders#

Added in: v8.4.0

The raw request/response headers list exactly as they were received.

Note that 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.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.

Returns request.

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
  • <http2.Http2Stream>

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\r\n
Accept: text/plain\r\n
\r\n

Then request.url will be:

'/status?name=ryan'

To parse the url into its parts require('url').parse(request.url) can be used. Example:

$ node
> require('url').parse('/status?name=ryan')
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

To extract the parameters from the query string, the require('querystring').parse function can be used, or true can be passed as the second argument to require('url').parse. Example:

$ node
> require('url').parse('/status?name=ryan', true)
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: { name: 'ryan' },
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

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.

The response implements, but does not inherit from, the Writable Stream interface. This is an EventEmitter with the following events:

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: v8.4.0

See response.socket.

response.end([data][, encoding][, callback])#

History
VersionChanges
REPLACEME

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: v8.4.0

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. Note that the name is case insensitive.

Example:

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.

Example:

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.

Example:

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. Note that the header name matching is case-insensitive.

Example:

const hasContentType = response.hasHeader('content-type');

response.headersSent#

Added in: v8.4.0

Boolean (read-only). True if headers were sent, false otherwise.

response.removeHeader(name)#

Added in: v8.4.0

Removes a header that has been queued for implicit sending.

Example:

response.removeHeader('Content-Encoding');

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.

Example:

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 = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  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.

Returns response.

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.

Example:

const http2 = require('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.

Example:

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 (RFC7540 8.1.2.4). It returns an empty string.

response.stream#

Added in: v8.4.0
  • <http2.Http2Stream>

The Http2Stream object backing the response.

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.

Note that in the 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.writeHead(statusCode[, statusMessage][, headers])#

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.

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.

Example:

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

Note that 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');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.createPushResponse(headers, callback)#

Added in: v8.4.0

Call http2stream.pushStream() with the given headers, and wraps the given newly created Http2Stream on Http2ServerRespose.

The callback will be called with an error with code ERR_HTTP2_STREAM_CLOSED if the stream is closed.

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('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.

HTTPS#

Stability: 2 - Stable

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

Class: https.Agent#

Added in: v0.4.5

An Agent object for HTTPS similar to http.Agent. See https.request() for more information.

Class: https.Server#

Added in: v0.3.4

This class is a subclass of tls.Server and emits events same as http.Server. See http.Server for more information.

server.close([callback])#

Added in: v0.1.90

See server.close() from the HTTP module for details.

server.listen()#

Starts the HTTPS server listening for encrypted connections. This method is identical to server.listen() from net.Server.

server.setTimeout([msecs][, callback])#

Added in: v0.11.2

See http.Server#setTimeout().

server.timeout#

Added in: v0.11.2

See http.Server#timeout.

server.keepAliveTimeout#

Added in: v8.0.0

See http.Server#keepAliveTimeout.

https.createServer([options][, requestListener])#

Added in: v0.3.4

Example:

// curl -k https://localhost:8000/
const https = require('https');
const fs = require('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('https');
const fs = require('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])#

History
VersionChanges
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 url.parse(). If it is a URL object, it will be automatically converted to an ordinary options object.

Example:

const https = require('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#

Added in: v0.5.9

Global instance of https.Agent for all HTTPS client requests.

https.request(options[, callback])#

History
VersionChanges
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

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with url.parse(). If it is a URL object, it will be automatically converted to an ordinary options object.

Example:

const https = require('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.

Example:

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('tls');
const https = require('https');
const crypto = require('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-sha25 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 then 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: 1 - Experimental

The inspector module provides an API for interacting with the V8 inspector.

It can be accessed using:

const inspector = require('inspector');

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.

inspector.close()#

Deactivate the inspector. Blocks until there are no active connections.

inspector.url()#

Return the URL of the active inspector, or undefined if there is none.

Class: inspector.Session#

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

Constructor: 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.

inspector.Session is an EventEmitter with the following events:

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. An exception will be thrown if there is already a connected session established either through the API or by a front-end connected to the Inspector WebSocket port.

session.post(method[, params][, callback])#

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 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.

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.

Example usage#

CPU Profiler#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol. Here's a simple example showing how to use the CPU profiler:

const inspector = require('inspector');
const fs = require('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));
      }
    });
  });
});

Internationalization Support#

Node.js has many features that make it easier to write internationalized programs. Some of them are:

Node.js (and its underlying V8 engine) uses ICU to implement these features in native C/C++ code. However, some of them require a very large ICU data file in order to support all locales of the world. Because it is expected that most Node.js users will make use of only a small portion of ICU functionality, only a subset of the full ICU data set is provided by Node.js by default. Several options are provided for customizing and expanding 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 (default)
  • --with-intl=full-icu

An overview of available Node.js and JavaScript features for each configure option:

none system-icu small-icu full-icu
String.prototype.normalize() none (function is no-op) full full full
String.prototype.to*Case() full full full full
Intl none (object does not exist) partial/full (depends on OS) partial (English-only) full
String.prototype.localeCompare() partial (not locale-aware) full full full
String.prototype.toLocale*Case() partial (not locale-aware) full full full
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
WHATWG URL Parser partial (no IDN support) full full full
require('buffer').transcode() none (function does not exist) full full full
REPL partial (inaccurate line editing) full full full
require('util').TextDecoder partial (basic encodings support) partial/full (depends on OS) partial (Unicode-only) full
RegExp Unicode Property Escapes none (invalid RegExp error) full full full

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, 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 "M01" on small-icu
// Should print "enero"

This mode provides a good balance between features and binary size, and it is the default behavior if no --with-intl flag is passed. The official binaries are also built in this mode.

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 icudt5X[bl].dat, where 5X 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. See BUILDING.md on how to compile a binary using 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#

Stability: 2 - Stable

In the Node.js module system, 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 module system is implemented in the require('module') module.

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').

Because module provides a filename property (normally equivalent to __filename), the entry point of the current application can be obtained by checking require.main.filename.

Addenda: Package Manager Tips#

The semantics of Node.js's require() function were designed to be general enough to support a number of 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.

Since Node.js looks up the realpath of any modules it loads (that is, resolves symlinks), and then looks for their dependencies in the node_modules folders as described here, this situation is very simple to resolve 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.

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.resolve() 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 filesystem root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
4. LOAD_NODE_MODULES(X, dirname(Y))
5. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  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. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
   d. LOAD_INDEX(M)
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. 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 = DIRS + DIR
   d. let I = I - 1
5. return DIRS

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.

Multiple calls to require('foo') may 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#

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 Node.js's source and are located in the lib/ folder.

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.

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.

.js files are interpreted as JavaScript text files, and .json files are parsed as JSON text files. .node files are interpreted as compiled addon modules loaded with dlopen.

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 an Error with its code property set to 'MODULE_NOT_FOUND'.

Folders as Modules#

It is convenient to organize programs and libraries into self-contained directories, and then provide a single entry point to that library. 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.

This is the extent of Node.js's awareness of package.json files.

If the file specified by the 'main' entry of package.json is missing and can not be resolved, Node.js will report the entire module as missing with the default error:

Error: Cannot find module 'some-library'

If there is no package.json file present in the directory, 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 above example, then require('./some-library') would attempt to load:

  • ./some-library/index.js
  • ./some-library/index.node

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 parent directory of the current module, and adds /node_modules, and attempts to load the module from that location. Node 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 frozen.

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 locations:

  • 1: $HOME/.node_modules
  • 2: $HOME/.node_libraries
  • 3: $PREFIX/lib/node

Where $HOME is the user's home directory, and $PREFIX is Node.js's 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 resolved absolute path of the current module file.

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()#

Added in: v0.1.13

To require modules.

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. Note that this does not apply to native addons, for which reloading will result in an Error.

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.

Since the module system is locked, this feature will probably never go away. However, it may have subtle bugs and complexities that are best left untouched.

Note that the number of file system operations that the module system has to perform in order to resolve a require(...) statement to a filename scales linearly with the number of registered extensions.

In other words, adding extensions slows down the module loader and should be discouraged.

require.main#

Added in: v0.1.17

The Module object representing the entry script loaded when the Node.js process launched. See "Accessing the main module".

In entry.js script:

console.log(require.main);

node entry.js

Module {
  id: '.',
  exports: {},
  parent: null,
  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 <Array> Paths to resolve module location from. If present, these paths are used instead of the default resolution paths. Note that 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.

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 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. Note that 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('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');
});

Note that 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 to the module.

module.id#

Added in: v0.1.16

The identifier for the module. Typically this is the fully resolved filename.

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: v0.1.16

The module that first required this one.

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#

Added in: v0.3.7

Provides general utility methods when interacting with instances of Module — the module variable often seen in file modules. Accessed via require('module').

module.builtinModules#

Added in: v9.3.0

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.

Note that module in this context isn't the same object that's provided by the module wrapper. To access it, require the Module module:

const builtin = require('module').builtinModules;

Net#

Stability: 2 - Stable

The 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('net');

IPC Support#

The 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 filesystem path name. It gets truncated to sizeof(sockaddr_un.sun_path) - 1, which varies on different operating system between 91 and 107 bytes. The typical values are 107 on Linux and 103 on macOS. The path is subject to the same naming conventions and permissions checks as would be done on file creation. It will be visible in the filesystem, 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 appearances, the pipe name space is flat. Pipes will not persist, they are removed when the last reference to them is closed. Do not forget JavaScript string escaping requires paths to be specified with double-backslashes, such as:

net.createServer().listen(
  path.join('\\\\?\\pipe', process.cwd(), 'myctl'));

Class: net.Server#

Added in: v0.1.90

This class is used to create a TCP or IPC server.

new net.Server([options][, connectionListener])#

See net.createServer([options][, connectionListener]).

net.Server is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.5.0

Emitted when the server closes. Note that 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().

server.address()#

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. Returns an object with port, family, and address properties: { 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.

Example:

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());
});

Don't call server.address() until the 'listening' event has been emitted.

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.

Returns server.

server.connections#

Added in: v0.2.0Deprecated since: v0.9.7
Stability: 0 - Deprecated: Use server.getConnections() instead.

The number of concurrent connections on the server.

This becomes null when sending a socket to a child with child_process.fork(). To poll forks and get current number of active connections use asynchronous server.getConnections() instead.

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 a 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.log('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])#

Added in: v0.11.14

If port is specified, it behaves the same as server.listen([port][, hostname][, 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
});

server.listen(path[, backlog][, callback])#

Added in: v0.1.90

Start a 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

A Boolean indicating 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 unrefd server will not let the program exit if it's the only server left (the default behavior). If the server is refd 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 unrefd 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). A net.Socket is also a duplex stream, so it can be both readable and writable, and it is also a 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)#

Added in: v0.3.4

Creates a new socket object.

  • 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> Indicates whether half-opened TCP connections are allowed. 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.
  • Returns: <net.Socket>

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
  • had_error <boolean> true if the socket had a transmission error.

Emitted once the socket is fully closed. The argument had_error 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().

Note that 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 sends a FIN packet, thus ending the readable side of the socket.

By default (allowHalfOpen is false) the socket will send a FIN 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 hostname but before connecting. Not applicable to UNIX sockets.

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()#

Added in: v0.1.90

Returns the bound address, the address family name and port of the socket as reported by the operating system. Returns an object with three properties, e.g. { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.bufferSize#

Added in: v0.3.8

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. (Internally it is polling on the socket's file descriptor for being writable).

The consequence of this internal buffering is that memory may grow. This property shows the number of characters currently buffered to be written. (Number of characters is approximately equal to the number of bytes to be written, but the buffer may contain strings, and the strings are lazily encoded, so the exact number of bytes is not known.)

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.

socket.connect(options[, connectListener])#

History
VersionChanges
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, can be either 4 or 6. Default: 4.
  • hints <number> Optional dns.lookup() hints.
  • lookup <Function> Custom lookup function. Default: dns.lookup().

For IPC connections, available options are:

Returns socket.

socket.connect(path[, connectListener])#

Initiate an IPC connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with { path: path } as options.

Returns socket.

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.

Returns socket.

socket.connecting#

Added in: v6.1.0

If true - socket.connect(options[, connectListener]) was called and haven't yet finished. Will be set to false before emitting connect event and/or calling socket.connect(options[, connectListener])'s callback.

socket.destroy([exception])#

Added in: v0.1.90

Ensures that no more I/O activity happens on this socket. Only necessary in case of errors (parse error or so).

If exception is specified, an 'error' event will be emitted and any listeners for that event will receive exception as an argument.

socket.destroyed#

A Boolean value that indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

socket.end([data][, encoding])#

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.

If data is specified, it is equivalent to calling socket.write(data, encoding) followed by socket.end().

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.pause()#

Pauses the reading of data. That is, 'data' events will not be emitted. Useful to throttle back an upload.

socket.ref()#

Added in: v0.9.1

Opposite of unref, calling ref on a previously unrefd socket will not let the program exit if it's the only socket left (the default behavior). If the socket is refd 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.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])#

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.

socket.setNoDelay([noDelay])#

Added in: v0.1.90

Disables the Nagle algorithm. By default TCP connections use the Nagle algorithm, they buffer data before sending it off. Setting true for noDelay will immediately fire off data each time socket.write() is called.

socket.setTimeout(timeout[, callback])#

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.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 unrefd 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 - this may not be immediately.

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('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 the second line would just be changed to

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.createServer([options][, connectionListener])#

Added in: v0.5.0

Creates a new TCP or IPC server.

  • options <Object>
    • allowHalfOpen <boolean> Indicates whether half-opened TCP connections are allowed. Default: false.
    • pauseOnConnect <boolean> Indicates whether the socket should be paused on incoming connections. Default: false.
  • connectionListener <Function> Automatically set as a listener for the 'connection' event.
  • Returns: <net.Server>

If allowHalfOpen is set to true, when the other end of the socket sends a FIN packet, the server will only send a FIN packet back 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 a IPC server, depending on what it listen() to.

Here is an example of an TCP echo server which listens for connections on port 8124:

const net = require('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 the third line from the last would just be changed to

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

Tests if input is an IP address. Returns 0 for invalid strings, returns 4 for IP version 4 addresses, and returns 6 for IP version 6 addresses.

net.isIPv4(input)#

Added in: v0.3.0

Returns true if input is a version 4 IP address, otherwise returns false.

net.isIPv6(input)#

Added in: v0.3.0

Returns true if input is a version 6 IP address, otherwise returns false.

OS#

Stability: 2 - Stable

The os module provides a number of operating system-related utility methods. It can be accessed using:

const os = require('os');

os.EOL#

Added in: v0.7.8

A string constant defining the operating system-specific end-of-line marker:

  • \n on POSIX
  • \r\n on Windows

os.arch()#

Added in: v0.5.0

The os.arch() method returns a string identifying the operating system CPU architecture for which the Node.js binary was compiled.

The current possible values are: 'arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 's390', 's390x', 'x32', and 'x64'.

Equivalent to process.arch.

os.constants#

Added in: v6.3.0

Returns an object containing commonly used operating system specific constants for error codes, process signals, and so on. The specific constants currently defined are described in OS Constants.

os.cpus()#

Added in: v0.3.3

The os.cpus() method 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
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 511580,
      nice: 20,
      sys: 40900,
      idle: 1070842510,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 291660,
      nice: 0,
      sys: 34360,
      idle: 1070888000,
      irq: 10
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 308260,
      nice: 0,
      sys: 55410,
      idle: 1071129970,
      irq: 880
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 266450,
      nice: 1480,
      sys: 34920,
      idle: 1072572010,
      irq: 30
    }
  }
]

Because nice values are UNIX-specific, on Windows the nice values of all processors are always 0.

os.endianness()#

Added in: v0.9.4

The os.endianness() method returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.

Possible values are:

  • 'BE' for big endian
  • 'LE' for little endian.

os.freemem()#

Added in: v0.3.3

The os.freemem() method returns the amount of free system memory in bytes as an integer.

os.homedir()#

Added in: v2.3.0

The os.homedir() method returns the home directory of the current user as a string.

os.hostname()#

Added in: v0.3.3

The os.hostname() method returns the hostname of the operating system as a string.

os.loadavg()#

Added in: v0.3.3

The os.loadavg() method 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. As a rule of thumb, the load average should ideally be less than the number of logical CPUs in the system.

The load average is a UNIX-specific concept with no real equivalent on Windows platforms. On Windows, the return value is always [0, 0, 0].

os.networkInterfaces()#

Added in: v0.6.0

The os.networkInterfaces() method returns an object containing only 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',
      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',
      internal: false,
      cidr: 'fe80::a00:27ff:fe4e:66a1/64'
    }
  ]
}

os.platform()#

Added in: v0.5.0

The os.platform() method returns a string identifying the operating system platform as set during compile time of Node.js.

Currently possible values are:

  • 'aix'
  • 'darwin'
  • 'freebsd'
  • 'linux'
  • 'openbsd'
  • 'sunos'
  • 'win32'

Equivalent to process.platform.

The value 'android' may also be returned if the Node.js is built on the Android operating system. However, Android support in Node.js is considered to be experimental at this time.

os.release()#

Added in: v0.3.3

The os.release() method returns a string identifying the operating system release.

On POSIX systems, the operating system release is determined by calling uname(3). On Windows, GetVersionExW() is used. Please see https://en.wikipedia.org/wiki/Uname#Examples for more information.

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

The os.tmpdir() method returns a string specifying the operating system's default directory for temporary files.

os.totalmem()#

Added in: v0.3.3

The os.totalmem() method returns the total amount of system memory in bytes as an integer.

os.type()#

Added in: v0.3.3

The os.type() method returns a string identifying the operating system name as returned by uname(3). For example 'Linux' on Linux, 'Darwin' on macOS and 'Windows_NT' on Windows.

Please see https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running uname(3) on various operating systems.

os.uptime()#

Added in: v0.3.3

The os.uptime() method returns the system uptime in number of seconds.

On Windows the returned value includes fractions of a second. Use Math.floor() to get whole 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>

The os.userInfo() method 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 several environment variables for the home directory before falling back to the operating system response.

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 currently 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. Note that while 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.

libuv Constants#

Constant Description
UV_UDP_REUSEADDR

Path#

Stability: 2 - Stable

The path module provides utilities for working with file and directory paths. It can be accessed using:

const path = require('path');

Windows vs. POSIX#

The default operation of the 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 path module will assume that Windows-style paths are being used.

For example, using the path.basename() function with the Windows file path C:\temp\myfile.html, will yield different results when running on POSIX than when run on 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'

Note: 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[, ext])#

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() methods returns the last portion of a path, similar to the Unix basename command. Trailing directory separators are ignored, see path.sep.

path.basename('/foo/bar/baz/asdf/quux.html');
// Returns: 'quux.html'

path.basename('/foo/bar/baz/asdf/quux.html', '.html');
// Returns: 'quux'

A TypeError is thrown if path is not a string or if ext 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 the first character of the basename of path (see path.basename()) is ., then 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: ''

A TypeError is thrown if path is not a string.

path.format(pathObject)#

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'

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#

Added in: v0.11.15

The path.posix property provides access to POSIX specific implementations of the path methods.

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.

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 system. On posix systems, the method is non-operational and always returns path without modifications.

path.win32#

Added in: v0.11.15

The path.win32 property provides access to Windows-specific implementations of the path methods.

Performance Timing API#

Stability: 1 - Experimental

The Performance Timing API provides an implementation of the W3C Performance Timeline specification. The purpose of the API is to support collection of high resolution performance metrics. This is the same Performance API as implemented in modern Web browsers.

const { PerformanceObserver, performance } = require('perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ entryTypes: ['measure'] });

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});

Class: Performance#

Added in: v8.5.0

performance.clearMarks([name])#

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.mark([name])#

Added in: v8.5.0

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.

performance.measure(name, startMark, endMark)#

Added in: v8.5.0

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, then startMark is set to timeOrigin by default.

The endMark argument must identify any existing PerformanceMark in the Performance Timeline or any of the timestamp properties provided by the PerformanceNodeTiming class. If the named endMark does not exist, an error will be thrown.

performance.nodeTiming#

Added in: v8.5.0
  • <PerformanceNodeTiming>

An instance of the PerformanceNodeTiming class that provides performance metrics for specific Node.js operational milestones.

performance.now()#

Added in: v8.5.0

Returns the current high resolution millisecond timestamp, where 0 represents the start of the current node process.

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)#

Added in: v8.5.0

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('perf_hooks');

function someFunction() {
  console.log('hello world');
}

const wrapped = performance.timerify(someFunction);

const obs = new PerformanceObserver((list) => {
  console.log(list.getEntries()[0].duration);
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

// A performance timeline entry will be created
wrapped();

Class: PerformanceEntry#

Added in: v8.5.0

performanceEntry.duration#

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.name#

Added in: v8.5.0

The name of the performance entry.

performanceEntry.startTime#

Added in: v8.5.0

The high resolution millisecond timestamp marking the starting time of the Performance Entry.

performanceEntry.entryType#

Added in: v8.5.0

The type of the performance entry. Currently it may be one of: 'node', 'mark', 'measure', 'gc', 'function', or 'http2'.

performanceEntry.kind#

Added in: v8.5.0

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

Class: PerformanceNodeTiming extends PerformanceEntry#

Added in: v8.5.0

Provides timing details for Node.js itself.

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.clusterSetupEnd#

Added in: v8.5.0

The high resolution millisecond timestamp at which cluster processing ended. If cluster processing has not yet ended, the property has the value of -1.

performanceNodeTiming.clusterSetupStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which cluster processing started. If cluster processing has not yet started, the property has the value of -1.

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.moduleLoadEnd#

Added in: v8.5.0

The high resolution millisecond timestamp at which main module load ended.

performanceNodeTiming.moduleLoadStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which main module load started.

performanceNodeTiming.nodeStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js process was initialized.

performanceNodeTiming.preloadModuleLoadEnd#

Added in: v8.5.0

The high resolution millisecond timestamp at which preload module load ended.

performanceNodeTiming.preloadModuleLoadStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which preload module load started.

performanceNodeTiming.thirdPartyMainEnd#

Added in: v8.5.0

The high resolution millisecond timestamp at which third_party_main processing ended. If third_party_main processing has not yet ended, the property has the value of -1.

performanceNodeTiming.thirdPartyMainStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which third_party_main processing started. If third_party_main processing has not yet started, the property has the value of -1.

performanceNodeTiming.v8Start#

Added in: v8.5.0

The high resolution millisecond timestamp at which the V8 platform was initialized.

Class: PerformanceObserver(callback)#

Added in: v8.5.0
  • callback <Function> A PerformanceObserverCallback callback function.

PerformanceObserver objects provide notifications when new PerformanceEntry instances have been added to the Performance Timeline.

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());
  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.

Callback: PerformanceObserverCallback(list, observer)#

Added in: v8.5.0

The PerformanceObserverCallback is invoked when a PerformanceObserver is notified about new PerformanceEntry instances. The callback receives a PerformanceObserverEntryList instance and a reference to the PerformanceObserver.

Class: PerformanceObserverEntryList#

Added in: v8.5.0

The PerformanceObserverEntryList class is used to provide access to the PerformanceEntry instances passed to a PerformanceObserver.

performanceObserverEntryList.getEntries()#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime.

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.

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.

performanceObserver.disconnect()#

Added in: v8.5.0

Disconnects the PerformanceObserver instance from all notifications.

performanceObserver.observe(options)#

Added in: v8.5.0
  • options <Object>
    • entryTypes <Array> 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 notification callback will be called using setImmediate() and multiple PerformanceEntry instance notifications will be buffered internally. If false, notifications will be immediate and synchronous. Default: false.

Subscribes the PerformanceObserver instance to notifications of new PerformanceEntry instances identified by options.entryTypes.

When options.buffered is false, the callback will be invoked once for every PerformanceEntry instance:

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // called three times synchronously. list contains one item
});
obs.observe({ entryTypes: ['mark'] });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // called once. list contains three items
});
obs.observe({ entryTypes: ['mark'], buffered: true });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

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 to execute the callback).

'use strict';
const async_hooks = require('async_hooks');
const {
  performance,
  PerformanceObserver
} = require('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();
  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('perf_hooks');
const mod = require('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);
  });
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');

Process#

The process object is a global that provides information about, and control over, the current Node.js process. As a global, it is always available to Node.js applications without using require().

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.

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, as the only argument.

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:

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 listener callback is invoked with the following arguments:

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

Event: 'rejectionHandled'#

Added in: v1.4.1

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 listener callback is invoked with a reference to the rejected Promise as the only argument.

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.

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, p) => {
  unhandledRejections.set(p, reason);
});
process.on('rejectionHandled', (p) => {
  unhandledRejections.delete(p);
});

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'#

Added in: v0.1.18

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. Adding a handler for the 'uncaughtException' event overrides this default behavior.

The listener function is called with the Error object passed as the only argument.

process.on('uncaughtException', (err) => {
  fs.writeSync(1, `Caught exception: ${err}\n`);
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

Warning: Using 'uncaughtException' correctly#

Note that '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 of the power cord when upgrading a computer — nine out of ten times nothing happens - but the 10th 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: 'unhandledRejection'#

History
VersionChanges
v7.0.0

Not handling Promise rejections has been deprecated.

v6.6.0

Unhandled Promise rejections will now emit a process warning.

v1.4.1

Added in: v1.4.1

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.

The listener function is called with the following arguments:

  • reason <Error> | <any> The object with which the promise was rejected (typically an Error object).
  • p the Promise that was rejected.
process.on('unhandledRejection', (reason, p) => {
  console.log('Unhandled Rejection at:', p, '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:

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. Alternatively, the 'rejectionHandled' event may be used.

Event: 'warning'#

Added in: v6.0.0

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.

The listener function is called with a single warning argument whose value is an Error object. There are three key properties that describe the warning:

  • name <string> The name of the warning (currently Warning by default).
  • 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.
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.

Emitting custom warnings#

See the process.emitWarning() method for issuing custom or application-specific warnings.

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.

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).

// 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.
  • 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 attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached using process.on() are called asynchronously and therefore unable to correct the underlying problem.

Windows does not support sending signals, but Node.js offers some emulation with process.kill(), and subprocess.kill(). Sending signal 0 can be used to test for the existence of a process. Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional termination of the target 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.

process.arch#

Added in: v0.5.0

The process.arch property returns a string identifying the operating system CPU architecture for which the Node.js binary was compiled.

The current possible values are: 'arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 's390', 's390x', 'x32', and 'x64'.

console.log(`This processor architecture is ${process.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:

// print process.argv
process.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#

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.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).

console.log(`Starting directory: ${process.cwd()}`);
try {
  process.chdir('/tmp');
  console.log(`New directory: ${process.cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}

process.config#

Added in: v0.7.7

The process.config property returns an 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',
     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_dtrace: 'false',
     node_use_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 'true'
   }
}

The process.config property is not read-only and there are existing modules in the ecosystem that are known to extend, modify, or entirely replace the value of process.config.

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.

const startUsage = process.cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(process.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.

console.log(`Current directory: ${process.cwd()}`);

process.debugPort#

Added in: v0.7.2

The port used by Node.js's debugger when enabled.

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 that same as calling the parent process's ChildProcess.disconnect().

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 to dynamically load 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.

The flags argument is an integer that allows to specify dlopen behavior. See the os.constants.dlopen documentation for details.

If there are specific reasons to use process.dlopen() (for instance, to specify dlopen flags), it's often useful to use require.resolve() to look up the module's path.

An important drawback when calling process.dlopen() is that the module instance must be passed. Functions exported by the C++ Addon will be accessible via module.exports.

The example below shows how to load a C++ Addon, named as binding, that exports a foo function. All the symbols will be loaded before the call returns, by passing the RTLD_NOW constant. In this example the constant is assumed to be available.

const os = require('os');
process.dlopen(module, require.resolve('binding'),
               os.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 process.on('warning') event.

// Emit a warning with a code and additional detail.
process.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 process.on('warning') event.

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 process.on('warning') event.

// Emit a warning using a string.
process.emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!

// Emit a warning using a string and a type.
process.emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!

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 process.on('warning') event.

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 process.on('warning') event handler unmodified (and the optional type, code and ctor arguments will be ignored):

// 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';

process.emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

A TypeError is thrown if warning is anything other than a string or Error object.

Note that 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, it is recommended to place the emitWarning() behind a simple boolean flag as illustrated in the example below:

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    process.emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

process.env#

History
VersionChanges
REPLACEME

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. In other words, the following example would not work:

$ node -e 'process.env.foo = "bar"' && echo $foo

While the following will:

process.env.foo = 'bar';
console.log(process.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.

Example:

process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'

Use delete to delete a property from process.env.

Example:

process.env.TEST = 1;
delete process.env.TEST;
console.log(process.env.TEST);
// => undefined

On Windows operating systems, environment variables are case-insensitive.

Example:

process.env.TEST = 1;
console.log(process.env.test);
// => 1

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']

process.execPath#

Added in: v0.1.100

The process.execPath property returns the absolute pathname of the executable that started the Node.js process.

'/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:

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:

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  process.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:

// 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().

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.getegid()#

Added in: v2.0.0

The process.getegid() method returns the numerical effective group identity of the Node.js process. (See getegid(2).)

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).)

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).)

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.

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).)

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
  • time <Array> The result of a previous call to process.hrtime()
  • Returns: <Array>

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:

const NS_PER_SEC = 1e9;
const time = process.hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  const diff = process.hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // benchmark took 1000000552 nanoseconds
}, 1000);

process.initgroups(user, extra_group)#

Added in: v0.9.4

The process.initgroups() method reads the /etc/group file and initializes the group access list, using all groups of which the user is a member. This is a privileged operation that requires that the Node.js process either have root access or the CAP_SETGID capability.

Note that care must be taken when dropping privileges. Example:

console.log(process.getgroups());         // [ 0 ]
process.initgroups('bnoordhuis', 1000);   // switch user
console.log(process.getgroups());         // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000);                     // drop root gid
console.log(process.getgroups());         // [ 27, 30, 46, 1000 ]

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.kill(pid[, signal])#

Added in: v0.0.6
  • pid <number> A process ID
  • signal <string> | <number> The signal to send, either as a string or number. Default: 'SIGTERM'.

The process.kill() method sends the signal to the process identified by pid.

Signal names are strings such as 'SIGINT' or 'SIGHUP'. See Signal Events and kill(2) for more information.

This method will throw an error if the target pid does not exist. As a special case, a signal of 0 can be used to test for the existence of a process. Windows platforms will throw an error if the pid is used to kill a process group.

Even though the name of this function is process.kill(), it is really just a signal sender, like the kill system call. The signal sent may do something other than kill the target process.

process.on('SIGHUP', () => {
  console.log('Got SIGHUP signal.');
});

setTimeout(() => {
  console.log('Exiting.');
  process.exit(0);
}, 100);

process.kill(process.pid, 'SIGHUP');

When SIGUSR1 is received by a Node.js process, Node.js will start the debugger, see Signal Events.

process.mainModule#

Added in: v0.1.17

The process.mainModule property provides an alternative way of retrieving require.main. The difference is that if the main module changes at runtime, require.main may still refer to the original main module in modules that were required before the change occurred. Generally, it's safe to assume that the two refer to the same module.

As with require.main, process.mainModule will be undefined if there is no entry script.

process.memoryUsage()#

History
VersionChanges
v7.2.0

Added external to the returned object.

v0.1.16

Added in: v0.1.16

The process.memoryUsage() method returns an object describing the memory usage of the Node.js process measured in bytes.

For example, the code:

console.log(process.memoryUsage());

Will generate:

{
  rss: 4935680,
  heapTotal: 1826816,
  heapUsed: 650472,
  external: 49879
}

heapTotal and heapUsed refer to V8's memory usage. external refers to the memory usage of C++ objects bound to JavaScript objects managed by V8. rss, Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, which includes the heap, code segment and stack.

The heap is where objects, strings, and closures are stored. Variables are stored in the stack and the actual JavaScript code resides in the code segment.

process.nextTick(callback[, ...args])#

History
VersionChanges
v1.8.1

Additional arguments after callback are now supported.

v0.1.26

Added in: v0.1.26

  • callback <Function>
  • ...args <any> Additional arguments to pass when invoking the callback

The process.nextTick() method adds the callback to the "next tick queue". Once the current turn of the event loop turn runs to completion, all callbacks currently in the next tick queue will be called.

This is not a simple alias to setTimeout(fn, 0). It is much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

console.log('start');
process.nextTick(() => {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:

function MyThing(options) {
  this.setupOptions(options);

  process.nextTick(() => {
    this.startDoingStuff();
  });
}

const thing = new MyThing();
thing.getReadyForStuff();

// thing.startDoingStuff() gets called now, not before.

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

// WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
  if (arg) {
    cb();
    return;
  }

  fs.stat('file', cb);
}

This API is hazardous because in the following case:

const maybeTrue = Math.random() > 0.5;

maybeSync(maybeTrue, () => {
  foo();
});

bar();

It is not clear whether foo() or bar() will be called first.

The following approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

The next tick queue is completely drained on each pass of the event loop before additional I/O is processed. As a result, recursively setting nextTick callbacks will block any I/O from happening, just like a while(true); loop.

process.noDeprecation#

Added in: v0.8.0

The process.noDeprecation property indicates whether the --no-deprecation flag is set on the current Node.js process. See the documentation for the warning event and the emitWarning method for more information about this flag's behavior.

process.pid#

Added in: v0.1.15

The process.pid property returns the PID of the process.

console.log(`This process is pid ${process.pid}`);

process.platform#

Added in: v0.1.16

The process.platform property returns a string identifying the operating system platform on which the Node.js process is running.

Currently possible values are:

  • 'aix'
  • 'darwin'
  • 'freebsd'
  • 'linux'
  • 'openbsd'
  • 'sunos'
  • 'win32'
console.log(`This platform is ${process.platform}`);

The value 'android' may also be returned if the Node.js is built on the Android operating system. However, Android support in Node.js is experimental.

process.ppid#

Added in: v9.2.0

The process.ppid property returns the PID of the current parent process.

console.log(`The parent process is pid ${process.ppid}`);

process.release#

History
VersionChanges
v4.2.0

The lts property is now supported.

v3.0.0

Added in: v3.0.0

The process.release property returns an Object containing metadata related to the current release, including URLs for the source tarball and headers-only tarball.

process.release contains the following properties:

  • name <string> A value that will always be 'node' for Node.js. For legacy io.js releases, this will be 'io.js'.
  • sourceUrl <string> an absolute URL pointing to a .tar.gz file containing the source code of the current release.
  • headersUrl<string> an absolute URL pointing to a .tar.gz file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.
  • libUrl <string> an absolute URL pointing to a node.lib file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.
  • lts <string> a string label identifying the LTS label for this release. This property only exists for LTS releases and is undefined for all other release types, including Current releases. Currently the valid values are:
    • 'Argon' for the 4.x LTS line beginning with 4.2.0.
    • 'Boron' for the 6.x LTS line beginning with 6.9.0.
    • 'Carbon' for the 8.x LTS line beginning with 8.9.1.
{
  name: 'node',
  lts: 'Argon',
  sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
  headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
  libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
}

In custom builds from non-release versions of the source tree, only the name property may be present. The additional properties should not be relied upon to exist.

process.send(message[, sendHandle[, options]][, callback])#

Added in: v0.5.9

If Node.js is spawned with an IPC channel, the process.send() method can be used to send messages to the parent process. Messages will be received as a 'message' event on the parent's ChildProcess object.

If Node.js was not spawned with an IPC channel, process.send() will be undefined.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

process.setegid(id)#

Added in: v2.0.0

The process.setegid() method sets the effective group identity of the process. (See setegid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated a numeric ID.

if (process.getegid && process.setegid) {
  console.log(`Current gid: ${process.getegid()}`);
  try {
    process.setegid(501);
    console.log(`New gid: ${process.getegid()}`);
  } catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.seteuid(id)#

Added in: v2.0.0

The process.seteuid() method sets the effective user identity of the process. (See seteuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.geteuid && process.seteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
  try {
    process.seteuid(501);
    console.log(`New uid: ${process.geteuid()}`);
  } catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.setgid(id)#

Added in: v0.1.31

The process.setgid() method sets the group identity of the process. (See setgid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated numeric ID.

if (process.getgid && process.setgid) {
  console.log(`Current gid: ${process.getgid()}`);
  try {
    process.setgid(501);
    console.log(`New gid: ${process.getgid()}`);
  } catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.setgroups(groups)#

Added in: v0.9.4

The process.setgroups() method sets the supplementary group IDs for the Node.js process. This is a privileged operation that requires the Node.js process to have root or the CAP_SETGID capability.

The groups array can contain numeric group IDs, group names or both.

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.setuid(id)#

Added in: v0.1.28

The process.setuid(id) method sets the user identity of the process. (See setuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.getuid && process.setuid) {
  console.log(`Current uid: ${process.getuid()}`);
  try {
    process.setuid(501);
    console.log(`New uid: ${process.getuid()}`);
  } catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.setUncaughtExceptionCaptureCallback(fn)#

Added in: v9.3.0

The process.setUncaughtExceptionCapture function sets a function that will be invoked when an uncaught exception occurs, which will receive the exception value itself as its first argument.

If such a function is set, the process.on('uncaughtException') event will not be emitted. If --abort-on-uncaught-exception was passed from the command line or set through v8.setFlagsFromString(), the process will not abort.

To unset the capture function, process.setUncaughtExceptionCapture(null) may be used. Calling this method with a non-null argument while another capture function is set will throw an error.

Using this function is mutually exclusive with using the deprecated domain built-in module.

process.stderr#

The process.stderr property returns a stream connected to stderr (fd 2). It is a net.Socket (which is a Duplex stream) unless fd 2 refers to a file, in which case it is a Writable stream.

process.stderr differs from other Node.js streams in important ways, see note on process I/O for more information.

process.stdin#

The process.stdin property returns a stream connected to stdin (fd 0). It is a net.Socket (which is a Duplex stream) unless fd 0 refers to a file, in which case it is a Readable stream.

process.stdin.setEncoding('utf8');

process.stdin.on('readable', () => {
  const chunk = process.stdin.read();
  if (chunk !== null) {
    process.stdout.write(`data: ${chunk}`);
  }
});

process.stdin.on('end', () => {
  process.stdout.write('end');
});

As a Duplex stream, process.stdin can also be used in "old" mode that is compatible with scripts written for Node.js prior to v0.10. For more information see Stream compatibility.

In "old" streams mode the stdin stream is paused by default, so one must call process.stdin.resume() to read from it. Note also that calling process.stdin.resume() itself would switch stream to "old" mode.

process.stdout#

The process.stdout property returns a stream connected to stdout (fd 1). It is a net.Socket (which is a Duplex stream) unless fd 1 refers to a file, in which case it is a Writable stream.

For example, to copy process.stdin to process.stdout:

process.stdin.pipe(process.stdout);

process.stdout differs from other Node.js streams in important ways, see note on process I/O for more information.

A note on process I/O#

process.stdout and process.stderr differ from other Node.js streams in important ways:

  1. They are used internally by console.log() and console.error(), respectively.
  2. They cannot be closed (end() will throw).
  3. They will never emit the 'finish' event.
  4. Writes may be synchronous depending on what the stream is connected to and whether the system is Windows or POSIX:
    • Files: synchronous on Windows and POSIX
    • TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
    • Pipes (and sockets): synchronous on Windows, asynchronous on POSIX

These behaviors are partly for historical reasons, as changing them would create backwards incompatibility, but they are also expected by some users.

Synchronous writes avoid problems such as output written with console.log() or console.error() being unexpectedly interleaved, or not written at all if process.exit() is called before an asynchronous write completes. See process.exit() for more information.

Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, its possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.

To check if a stream is connected to a TTY context, check the isTTY property.

For instance:

$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false
$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false

See the TTY documentation for more information.

process.throwDeprecation#

Added in: v0.9.12

The process.throwDeprecation property indicates whether the --throw-deprecation flag is set on the current Node.js process. See the documentation for the warning event and the emitWarning method for more information about this flag's behavior.

process.title#

Added in: v0.1.104

The process.title property returns the current process title (i.e. returns the current value of ps). Assigning a new value to process.title modifies the current value of ps.

When a new value is assigned, different platforms will impose different maximum length restrictions on the title. Usually such restrictions are quite limited. For instance, on Linux and macOS, process.title is limited to the size of the binary name plus the length of the command line arguments because setting the process.title overwrites the argv memory of the process. Node.js v0.8 allowed for longer process title strings by also overwriting the environ memory but that was potentially insecure and confusing in some (rather obscure) cases.

process.traceDeprecation#

Added in: v0.8.0

The process.traceDeprecation property indicates whether the --trace-deprecation flag is set on the current Node.js process. See the documentation for the warning event and the emitWarning method for more information about this flag's behavior.

process.umask([mask])#

Added in: v0.1.19

The process.umask() method sets or returns the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process. Invoked without an argument, the current mask is returned, otherwise the umask is set to the argument value and the previous mask is returned.

const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
  `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);

process.uptime()#

Added in: v0.5.0

The process.uptime() method returns the number of seconds the current Node.js process has been running.

The return value includes fractions of a second. Use Math.floor() to get whole seconds.

process.version#

Added in: v0.1.3

The process.version property returns the Node.js version string.

console.log(`Version: ${process.version}`);

process.versions#

History
VersionChanges
v9.0.0

The v8 property now includes a Node.js specific suffix.

v4.2.0

The icu property is now supported.

v0.2.0

Added in: v0.2.0

The process.versions property returns an object listing the version strings of Node.js and its dependencies. process.versions.modules indicates the current ABI version, which is increased whenever a C++ API changes. Node.js will refuse to load modules that were compiled against a different module ABI version.

console.log(process.versions);

Will generate an object similar to:

{ http_parser: '2.7.0',
  node: '8.9.0',
  v8: '6.3.292.48-node.6',
  uv: '1.18.0',
  zlib: '1.2.11',
  ares: '1.13.0',
  modules: '60',
  nghttp2: '1.29.0',
  napi: '2',
  openssl: '1.0.2n',
  icu: '60.1',
  unicode: '10.0',
  cldr: '32.0',
  tz: '2016b' }

Exit Codes#

Node.js will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

  • 1 Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an 'uncaughtException' event handler.
  • 2 - Unused (reserved by Bash for builtin misuse)
  • 3 Internal JavaScript Parse Error - The JavaScript source code internal in Node.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 5 Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
  • 6 Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
  • 7 Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a 'uncaughtException' or domain.on('error') handler throws an error.
  • 8 - Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.
  • 9 - Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.
  • 10 Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node.js's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 12 Invalid Debug Argument - The --inspect and/or --inspect-brk options were set, but the port number chosen was invalid or unavailable.
  • >128 Signal Exits - If Node.js receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard POSIX practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code. For example, signal SIGABRT has value 6, so the expected exit code will be 128 + 6, or 134.

Punycode#

History
VersionChanges
v7.0.0

Accessing this module will now emit a deprecation warning.

Stability: 0 - Deprecated

The version of the punycode module bundled in Node.js is being deprecated. In a future major version of Node.js this module will be removed. Users currently depending on the punycode module should switch to using the userland-provided Punycode.js module instead.

The punycode module is a bundled version of the Punycode.js module. It can be accessed using:

const punycode = require('punycode');

Punycode is a character encoding scheme defined by RFC 3492 that is primarily intended for use in Internationalized Domain Names. Because host names in URLs are limited to ASCII characters only, Domain Names that contain non-ASCII characters must be converted into ASCII using the Punycode scheme. For instance, the Japanese character that translates into the English word, 'example' is '例'. The Internationalized Domain Name, '例.com' (equivalent to 'example.com') is represented by Punycode as the ASCII string 'xn--fsq.com'.

The punycode module provides a simple implementation of the Punycode standard.

The punycode module is a third-party dependency used by Node.js and made available to developers as a convenience. Fixes or other modifications to the module must be directed to the Punycode.js project.

punycode.decode(string)#

Added in: v0.5.1

The punycode.decode() method converts a Punycode string of ASCII-only characters to the equivalent string of Unicode codepoints.

punycode.decode('maana-pta'); // 'mañana'
punycode.decode('--dqo34k'); // '☃-⌘'

punycode.encode(string)#

Added in: v0.5.1

The punycode.encode() method converts a string of Unicode codepoints to a Punycode string of ASCII-only characters.

punycode.encode('mañana'); // 'maana-pta'
punycode.encode('☃-⌘'); // '--dqo34k'

punycode.toASCII(domain)#

Added in: v0.6.1

The punycode.toASCII() method converts a Unicode string representing an Internationalized Domain Name to Punycode. Only the non-ASCII parts of the domain name will be converted. Calling punycode.toASCII() on a string that already only contains ASCII characters will have no effect.

// encode domain names
punycode.toASCII('mañana.com');  // 'xn--maana-pta.com'
punycode.toASCII('☃-⌘.com');   // 'xn----dqo34k.com'
punycode.toASCII('example.com'); // 'example.com'

punycode.toUnicode(domain)#

Added in: v0.6.1

The punycode.toUnicode() method converts a string representing a domain name containing Punycode encoded characters into Unicode. Only the Punycode encoded parts of the domain name are be converted.

// decode domain names
punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
punycode.toUnicode('xn----dqo34k.com');  // '☃-⌘.com'
punycode.toUnicode('example.com');       // 'example.com'

punycode.ucs2#

Added in: v0.7.0

punycode.ucs2.decode(string)#

Added in: v0.7.0

The punycode.ucs2.decode() method returns an array containing the numeric codepoint values of each Unicode symbol in the string.

punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
// surrogate pair for U+1D306 tetragram for centre:
punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]

punycode.ucs2.encode(codePoints)#

Added in: v0.7.0

The punycode.ucs2.encode() method returns a string based on an array of numeric code point values.

punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'

punycode.version#

Added in: v0.6.1

Returns a string identifying the current Punycode.js version number.

Query String#

Stability: 2 - Stable

The querystring module provides utilities for parsing and formatting URL query strings. It can be accessed using:

const querystring = require('querystring');

querystring.escape(str)#

Added in: v0.1.25

The querystring.escape() method performs URL percent-encoding on the given str in a manner that is optimized for the specific requirements of URL query strings.

The querystring.escape() method is used by querystring.stringify() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement percent-encoding implementation if necessary by assigning querystring.escape to an alternative function.

querystring.parse(str[, sep[, eq[, options]]])#

History
VersionChanges
v8.0.0

Multiple empty entries are now parsed correctly (e.g. &=&=).

v6.0.0

The returned object no longer inherits from Object.prototype.

v6.0.0, v4.2.4

The eq parameter may now have a length of more than 1.

v0.1.25

Added in: v0.1.25

  • str <string> The URL query string to parse
  • sep <string> The substring used to delimit key and value pairs in the query string. Default: '&'.
  • eq <string>. The substring used to delimit keys and values in the query string. Default: '='.
  • options <Object>
    • decodeURIComponent <Function> The function to use when decoding percent-encoded characters in the query string. Default: querystring.unescape().
    • maxKeys <number> Specifies the maximum number of keys to parse. Specify 0 to remove key counting limitations. Default: 1000.

The querystring.parse() method parses a URL query string (str) into a collection of key and value pairs.

For example, the query string 'foo=bar&abc=xyz&abc=123' is parsed into:

{
  foo: 'bar',
  abc: ['xyz', '123']
}

The object returned by the querystring.parse() 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.

By default, percent-encoded characters within the query string will be assumed to use UTF-8 encoding. If an alternative character encoding is used, then an alternative decodeURIComponent option will need to be specified as illustrated in the following example:

// Assuming gbkDecodeURIComponent function already exists...

querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
                  { decodeURIComponent: gbkDecodeURIComponent });

querystring.stringify(obj[, sep[, eq[, options]]])#

Added in: v0.1.25
  • obj <Object> The object to serialize into a URL query string
  • sep <string> The substring used to delimit key and value pairs in the query string. Default: '&'.
  • eq <string>. The substring used to delimit keys and values in the query string. Default: '='.
  • options
    • encodeURIComponent <Function> The function to use when converting URL-unsafe characters to percent-encoding in the query string. Default: querystring.escape().

The querystring.stringify() method produces a URL query string from a given obj by iterating through the object's "own properties".

It serializes the following types of values passed in obj: <string> | <number> | <boolean> | <string[]> | <number[]> | <boolean[]> Any other input values will be coerced to empty strings.

querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' });
// returns 'foo=bar&baz=qux&baz=quux&corge='

querystring.stringify({ foo: 'bar', baz: 'qux' }, ';', ':');
// returns 'foo:bar;baz:qux'

By default, characters requiring percent-encoding within the query string will be encoded as UTF-8. If an alternative encoding is required, then an alternative encodeURIComponent option will need to be specified as illustrated in the following example:

// Assuming gbkEncodeURIComponent function already exists,

querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
                      { encodeURIComponent: gbkEncodeURIComponent });

querystring.unescape(str)#

Added in: v0.1.25

The querystring.unescape() method performs decoding of URL percent-encoded characters on the given str.

The querystring.unescape() method is used by querystring.parse() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement decoding implementation if necessary by assigning querystring.unescape to an alternative function.

By default, the querystring.unescape() method will attempt to use the JavaScript built-in decodeURIComponent() method to decode. If that fails, a safer equivalent that does not throw on malformed URLs will be used.

Readline#

Stability: 2 - Stable

The readline module provides an interface for reading data from a Readable stream (such as process.stdin) one line at a time. It can be accessed using:

const readline = require('readline');

The following simple example illustrates the basic use of the readline module.

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('What do you think of Node.js? ', (answer) => {
  // TODO: Log the answer in a database
  console.log(`Thank you for your valuable feedback: ${answer}`);

  rl.close();
});

Once this code is invoked, the Node.js application will not terminate until the readline.Interface is closed because the interface waits for data to be received on the input stream.

Class: Interface#

Added in: v0.1.104

Instances of the readline.Interface class are constructed using the readline.createInterface() method. Every instance is associated with a single input Readable stream and a single output Writable stream. The output stream is used to print prompts for user input that arrives on, and is read from, the input stream.

Event: 'close'#

Added in: v0.1.98

The 'close' event is emitted when one of the following occur:

  • The rl.close() method is called and the readline.Interface instance has relinquished control over the input and output streams;
  • The input stream receives its 'end' event;
  • The input stream receives <ctrl>-D to signal end-of-transmission (EOT);
  • The input stream receives <ctrl>-C to signal SIGINT and there is no SIGINT event listener registered on the readline.Interface instance.

The listener function is called without passing any arguments.

The readline.Interface instance is finished once the 'close' event is emitted.

Event: 'line'#

Added in: v0.1.98

The 'line' event is emitted whenever the input stream receives an end-of-line input (\n, \r, or \r\n). This usually occurs when the user presses the <Enter>, or <Return> keys.

The listener function is called with a string containing the single line of received input.

rl.on('line', (input) => {
  console.log(`Received: ${input}`);
});

Event: 'pause'#

Added in: v0.7.5

The 'pause' event is emitted when one of the following occur:

  • The input stream is paused.
  • The input stream is not paused and receives the SIGCONT event. (See events SIGTSTP and SIGCONT)

The listener function is called without passing any arguments.

rl.on('pause', () => {
  console.log('Readline paused.');
});

Event: 'resume'#

Added in: v0.7.5

The 'resume' event is emitted whenever the input stream is resumed.

The listener function is called without passing any arguments.

rl.on('resume', () => {
  console.log('Readline resumed.');
});

Event: 'SIGCONT'#

Added in: v0.7.5

The 'SIGCONT' event is emitted when a Node.js process previously moved into the background using <ctrl>-Z (i.e. SIGTSTP) is then brought back to the foreground using fg(1p).

If the input stream was paused before the SIGTSTP request, this event will not be emitted.

The listener function is invoked without passing any arguments.

rl.on('SIGCONT', () => {
  // `prompt` will automatically resume the stream
  rl.prompt();
});

The 'SIGCONT' event is not supported on Windows.

Event: 'SIGINT'#

Added in: v0.3.0

The 'SIGINT' event is emitted whenever the input stream receives a <ctrl>-C input, known typically as SIGINT. If there are no 'SIGINT' event listeners registered when the input stream receives a SIGINT, the 'pause' event will be emitted.

The listener function is invoked without passing any arguments.

rl.on('SIGINT', () => {
  rl.question('Are you sure you want to exit? ', (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});

Event: 'SIGTSTP'#

Added in: v0.7.5

The 'SIGTSTP' event is emitted when the input stream receives a <ctrl>-Z input, typically known as SIGTSTP. If there are no SIGTSTP event listeners registered when the input stream receives a SIGTSTP, the Node.js process will be sent to the background.

When the program is resumed using fg(1p), the 'pause' and SIGCONT events will be emitted. These can be used to resume the input stream.

The 'pause' and 'SIGCONT' events will not be emitted if the input was paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

rl.on('SIGTSTP', () => {
  // This will override SIGTSTP and prevent the program from going to the
  // background.
  console.log('Caught SIGTSTP.');
});

The 'SIGTSTP' event is not supported on Windows.

rl.close()#

Added in: v0.1.98

The rl.close() method closes the readline.Interface instance and relinquishes control over the input and output streams. When called, the 'close' event will be emitted.

rl.pause()#

Added in: v0.3.4

The rl.pause() method pauses the input stream, allowing it to be resumed later if necessary.

Calling rl.pause() does not immediately pause other events (including 'line') from being emitted by the readline.Interface instance.

rl.prompt([preserveCursor])#

Added in: v0.1.98
  • preserveCursor <boolean> If true, prevents the cursor placement from being reset to 0.

The rl.prompt() method writes the readline.Interface instances configured prompt to a new line in output in order to provide a user with a new location at which to provide input.

When called, rl.prompt() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the prompt is not written.

rl.question(query, callback)#

Added in: v0.3.3
  • query <string> A statement or query to write to output, prepended to the prompt.
  • callback <Function> A callback function that is invoked with the user's input in response to the query.

The rl.question() method displays the query by writing it to the output, waits for user input to be provided on input, then invokes the callback function passing the provided input as the first argument.

When called, rl.question() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the query is not written.

Example usage:

rl.question('What is your favorite food? ', (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
});

The callback function passed to rl.question() does not follow the typical pattern of accepting an Error object or null as the first argument. The callback is called with the provided answer as the only argument.

rl.resume()#

Added in: v0.3.4

The rl.resume() method resumes the input stream if it has been paused.

rl.setPrompt(prompt)#

Added in: v0.1.98

The rl.setPrompt() method sets the prompt that will be written to output whenever rl.prompt() is called.

rl.write(data[, key])#

Added in: v0.1.98

The rl.write() method will write either data or a key sequence identified by key to the output. The key argument is supported only if output is a TTY text terminal.

If key is specified, data is ignored.

When called, rl.write() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the data and key are not written.

rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, { ctrl: true, name: 'u' });

The rl.write() method will write the data to the readline Interface's input as if it were provided by the user.

readline.clearLine(stream, dir)#

Added in: v0.7.7

The readline.clearLine() method clears current line of given TTY stream in a specified direction identified by dir.

readline.clearScreenDown(stream)#

Added in: v0.7.7

The readline.clearScreenDown() method clears the given TTY stream from the current position of the cursor down.

readline.createInterface(options)#

History
VersionChanges
v8.3.0, 6.11.4

Remove max limit of crlfDelay option.

v6.6.0

The crlfDelay option is supported now.

v6.3.0

The prompt option is supported now.

v6.0.0

The historySize option can be 0 now.

v0.1.98

Added in: v0.1.98

  • options <Object>
    • input <stream.Readable> The Readable stream to listen to. This option is required.
    • output <stream.Writable> The Writable stream to write readline data to.
    • completer <Function> An optional function used for Tab autocompletion.
    • terminal <boolean> true if the input and output streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checking isTTY on the output stream upon instantiation.
    • historySize <number> Maximum number of history lines retained. To disable the history set this value to 0. This option makes sense only if terminal is set to true by the user or by an internal output check, otherwise the history caching mechanism is not initialized at all. Default: 30.
    • prompt <string> The prompt string to use. Default: '> '.
    • crlfDelay <number> If the delay between \r and \n exceeds crlfDelay milliseconds, both \r and \n will be treated as separate end-of-line input. crlfDelay will be coerced to a number no less than 100. It can be set to Infinity, in which case \r followed by \n will always be considered a single newline (which may be reasonable for reading files with \r\n line delimiter). Default: 100.
    • removeHistoryDuplicates <boolean> If true, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default: false.

The readline.createInterface() method creates a new readline.Interface instance.

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

Once the readline.Interface instance is created, the most common case is to listen for the 'line' event:

rl.on('line', (line) => {
  console.log(`Received: ${line}`);
});

If terminal is true for this instance then the output stream will get the best compatibility if it defines an output.columns property and emits a 'resize' event on the output if or when the columns ever change (process.stdout does this automatically when it is a TTY).

Use of the completer Function#

The completer function takes the current line entered by the user as an argument, and returns an Array with 2 entries:

  • An Array with matching entries for the completion.
  • The substring that was used for the matching.

For instance: [[substr1, substr2, ...], originalsubstring].

function completer(line) {
  const completions = '.help .error .exit .quit .q'.split(' ');
  const hits = completions.filter((c) => c.startsWith(line));
  // show all completions if none found
  return [hits.length ? hits : completions, line];
}

The completer function can be called asynchronously if it accepts two arguments:

function completer(linePartial, callback) {
  callback(null, [['123'], linePartial]);
}

readline.cursorTo(stream, x, y)#

Added in: v0.7.7

The readline.cursorTo() method moves cursor to the specified position in a given TTY stream.

readline.emitKeypressEvents(stream[, interface])#

Added in: v0.7.7

The readline.emitKeypressEvents() method causes the given Readable stream to begin emitting 'keypress' events corresponding to received input.

Optionally, interface specifies a readline.Interface instance for which autocompletion is disabled when copy-pasted input is detected.

If the stream is a TTY, then it must be in raw mode.

This is automatically called by any readline instance on its input if the input is a terminal. Closing the readline instance does not stop the input from emitting 'keypress' events.

readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
  process.stdin.setRawMode(true);

readline.moveCursor(stream, dx, dy)#

Added in: v0.7.7

The readline.moveCursor() method moves the cursor relative to its current position in a given TTY stream.

Example: Tiny CLI#

The following example illustrates the use of readline.Interface class to implement a small command-line interface:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: 'OHAI> '
});

rl.prompt();

rl.on('line', (line) => {
  switch (line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log(`Say what? I might have heard '${line.trim()}'`);
      break;
  }
  rl.prompt();
}).on('close', () => {
  console.log('Have a great day!');
  process.exit(0);
});

Example: Read File Stream Line-by-Line#

A common use case for readline is to consume input from a filesystem Readable stream one line at a time, as illustrated in the following example:

const readline = require('readline');
const fs = require('fs');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt'),
  crlfDelay: Infinity
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});

REPL#

Stability: 2 - Stable

The repl module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

const repl = require('repl');

Design and Features#

The repl module exports the repl.REPLServer class. While running, instances of repl.REPLServer will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from stdin and stdout, respectively, or may be connected to any Node.js stream.

Instances of repl.REPLServer support automatic completion of inputs, simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions.

Commands and Special Keys#

The following special commands are supported by all REPL instances:

  • .break - When in the process of inputting a multi-line expression, entering the .break command (or pressing the <ctrl>-C key combination) will abort further input or processing of that expression.
  • .clear - Resets the REPL context to an empty object and clears any multi-line expression currently being input.
  • .exit - Close the I/O stream, causing the REPL to exit.
  • .help - Show this list of special commands.
  • .save - Save the current REPL session to a file: > .save ./file/to/save.js
  • .load - Load a file into the current REPL session. > .load ./file/to/load.js
  • .editor - Enter editor mode (<ctrl>-D to finish, <ctrl>-C to cancel)
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
  return `Hello ${name}!`;
}

welcome('Node.js User');

// ^D
'Hello Node.js User!'
>

The following key combinations in the REPL have these special effects:

  • <ctrl>-C - When pressed once, has the same effect as the .break command. When pressed twice on a blank line, has the same effect as the .exit command.
  • <ctrl>-D - Has the same effect as the .exit command.
  • <tab> - When pressed on a blank line, displays global and local(scope) variables. When pressed while entering other input, displays relevant autocompletion options.

Default Evaluation#

By default, all instances of repl.REPLServer use an evaluation function that evaluates JavaScript expressions and provides access to Node.js' built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the repl.REPLServer instance is created.

JavaScript Expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

> 1 + 1
2
> const m = 2
undefined
> m + 1
3

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the const, let, or var keywords are declared at the global scope.

Global and Local Scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the context object associated with each REPLServer:

const repl = require('repl');
const msg = 'message';

repl.start('> ').context.m = msg;

Properties in the context object appear as local within the REPL:

$ node repl_test.js
> m
'message'

Context properties are not read-only by default. To specify read-only globals, context properties must be defined using Object.defineProperty():

const repl = require('repl');
const msg = 'message';

const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
  configurable: false,
  enumerable: true,
  value: msg
});

Accessing Core Node.js Modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input fs will be evaluated on-demand as global.fs = require('fs').

> fs.createReadStream('./some/file');

Assignment of the _ (underscore) variable#

History
VersionChanges
v9.8.0

Added _error support.

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable _ (underscore). Explicitly setting _ to a value will disable this behavior.

> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4

Similarly, _error will refer to the last seen error, if there was any. Explicitly setting _error to a value will disable this behavior.

> throw new Error('foo');
Error: foo
> _error.message
'foo'

Custom Evaluation Functions#

When a new repl.REPLServer is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.

The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:

const repl = require('repl');
const { Translator } = require('translator');

const myTranslator = new Translator('en', 'fr');

function myEval(cmd, context, filename, callback) {
  callback(null, myTranslator.translate(cmd));
}

repl.start({ prompt: '> ', eval: myEval });

Recoverable Errors#

As a user is typing input into the REPL prompt, pressing the <enter> key will send the current line of input to the eval function. In order to support multi-line input, the eval function can return an instance of repl.Recoverable to the provided callback function:

function myEval(cmd, context, filename, callback) {
  let result;
  try {
    result = vm.runInThisContext(cmd);
  } catch (e) {
    if (isRecoverableError(e)) {
      return callback(new repl.Recoverable(e));
    }
  }
  callback(null, result);
}

function isRecoverableError(error) {
  if (error.name === 'SyntaxError') {
    return /^(Unexpected end of input|Unexpected token)/.test(error.message);
  }
  return false;
}

Customizing REPL Output#

By default, repl.REPLServer instances format output using the util.inspect() method before writing the output to the provided Writable stream (process.stdout by default). The useColors boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the util.inspect() method.

It is possible to fully customize the output of a repl.REPLServer instance by passing a new function in using the writer option on construction. The following example, for instance, simply converts any input text to upper case:

const repl = require('repl');

const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });

function myEval(cmd, context, filename, callback) {
  callback(null, cmd);
}

function myWriter(output) {
  return output.toUpperCase();
}

Class: REPLServer#

Added in: v0.1.91

The repl.REPLServer class inherits from the readline.Interface class. Instances of repl.REPLServer are created using the repl.start() method and should not be created directly using the JavaScript new keyword.

Event: 'exit'#

Added in: v0.7.7

The 'exit' event is emitted when the REPL is exited either by receiving the .exit command as input, the user pressing <ctrl>-C twice to signal SIGINT, or by pressing <ctrl>-D to signal 'end' on the input stream. The listener callback is invoked without any arguments.

replServer.on('exit', () => {
  console.log('Received "exit" event from repl!');
  process.exit();
});

Event: 'reset'#

Added in: v0.11.0

The 'reset' event is emitted when the REPL's context is reset. This occurs whenever the .clear command is received as input unless the REPL is using the default evaluator and the repl.REPLServer instance was created with the useGlobal option set to true. The listener callback will be called with a reference to the context object as the only argument.

This can be used primarily to re-initialize REPL context to some pre-defined state as illustrated in the following simple example:

const repl = require('repl');

function initializeContext(context) {
  context.m = 'test';
}

const r = repl.start({ prompt: '> ' });
initializeContext(r.context);

r.on('reset', initializeContext);

When this code is executed, the global 'm' variable can be modified but then reset to its initial value using the .clear command:

$ ./node example.js
> m
'test'
> m = 1
1
> m
1
> .clear
Clearing context...
> m
'test'
>

replServer.defineCommand(keyword, cmd)#

Added in: v0.3.0
  • keyword <string> The command keyword (without a leading . character).
  • cmd <Object> | <Function> The function to invoke when the command is processed.

The replServer.defineCommand() method is used to add new .-prefixed commands to the REPL instance. Such commands are invoked by typing a . followed by the keyword. The cmd is either a Function or an object with the following properties:

  • help <string> Help text to be displayed when .help is entered (Optional).
  • action <Function> The function to execute, optionally accepting a single string argument.

The following example shows two new commands added to the REPL instance:

const repl = require('repl');

const replServer = repl.start({ prompt: '> ' });
replServer.defineCommand('sayhello', {
  help: 'Say hello',
  action(name) {
    this.clearBufferedCommand();
    console.log(`Hello, ${name}!`);
    this.displayPrompt();
  }
});
replServer.defineCommand('saybye', function saybye() {
  console.log('Goodbye!');
  this.close();
});

The new commands can then be used from within the REPL instance:

> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!

replServer.displayPrompt([preserveCursor])#

Added in: v0.1.91

The replServer.displayPrompt() method readies the REPL instance for input from the user, printing the configured prompt to a new line in the output and resuming the input to accept new input.

When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.

When preserveCursor is true, the cursor placement will not be reset to 0.

The replServer.displayPrompt method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

replServer.clearBufferedCommand()#

Added in: v9.0.0

The replServer.clearBufferedCommand() method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

replServer.parseREPLKeyword(keyword, [rest])#

Added in: v0.8.9Deprecated since: v9.0.0
  • keyword <string> the potential keyword to parse and execute
  • rest <any> any parameters to the keyword command
Stability: 0 - Deprecated.

An internal method used to parse and execute REPLServer keywords. Returns true if keyword is a valid keyword, otherwise false.

repl.start(options)#

History
VersionChanges
REPLACEME

The REPL_MAGIC_MODE replMode was removed.

v5.8.0

The options parameter is optional now.

v0.1.91

Added in: v0.1.91

  • options <Object> | <string>
    • prompt <string> The input prompt to display. Default: >. (with a trailing space).
    • input <stream.Readable> The Readable stream from which REPL input will be read. Default: process.stdin.
    • output <stream.Writable> The Writable stream to which REPL output will be written. Default: process.stdout.
    • terminal <boolean> If true, specifies that the output should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. Default: checking the value of the isTTY property on the output stream upon instantiation.
    • eval <Function> The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScript eval() function. An eval function can error with repl.Recoverable to indicate the input was incomplete and prompt for additional lines.
    • useColors <boolean> If true, specifies that the default writer function should include ANSI color styling to REPL output. If a custom writer function is provided then this has no effect. Default: the REPL instances terminal value.
    • useGlobal <boolean> If true, specifies that the default evaluation function will use the JavaScript global as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to true. Default: false.
    • ignoreUndefined <boolean> If true, specifies that the default writer will not output the return value of a command if it evaluates to undefined. Default: false.
    • writer <Function> The function to invoke to format the output of each command before writing to output. Default: util.inspect().
    • completer <Function> An optional function used for custom Tab auto completion. See readline.InterfaceCompleter for an example.
    • replMode <symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:
      • repl.REPL_MODE_SLOPPY - evaluates expressions in sloppy mode.
      • repl.REPL_MODE_STRICT - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.
    • breakEvalOnSigint - Stop evaluating the current piece of code when SIGINT is received, i.e. Ctrl+C is pressed. This cannot be used together with a custom eval function. Default: false.

The repl.start() method creates and starts a repl.REPLServer instance.

If options is a string, then it specifies the input prompt:

const repl = require('repl');

// a Unix style prompt
repl.start('$ ');

The Node.js REPL#

Node.js itself uses the repl module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the -i argument):

$ node
> const a = [1, 2, 3];
undefined
> a
[ 1, 2, 3 ]
> a.forEach((v) => {
...   console.log(v);
...   });
1
2
3

Environment Variable Options#

Various behaviors of the Node.js REPL can be customized using the following environment variables:

  • NODE_REPL_HISTORY - When a valid path is given, persistent REPL history will be saved to the specified file rather than .node_repl_history in the user's home directory. Setting this value to '' will disable persistent REPL history. Whitespace will be trimmed from the value.
  • NODE_REPL_HISTORY_SIZE - Controls how many lines of history will be persisted if history is available. Must be a positive number. Default: 1000.
  • NODE_REPL_MODE - May be either sloppy or strict. Default: sloppy, which will allow non-strict mode code to be run.

Persistent History#

By default, the Node.js REPL will persist history between node REPL sessions by saving inputs to a .node_repl_history file located in the user's home directory. This can be disabled by setting the environment variable NODE_REPL_HISTORY=''.

Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environment variable NODE_NO_READLINE=1. This will start the main and debugger REPL in canonical terminal settings, which will allow use with rlwrap.

For example, the following can be added to a .bashrc file:

alias node="env NODE_NO_READLINE=1 rlwrap node"

Starting multiple REPL instances against a single running instance#

It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single global object but have separate I/O interfaces.

The following example, for instance, provides separate REPLs on stdin, a Unix socket, and a TCP socket:

const net = require('net');
const repl = require('repl');
let connections = 0;

repl.start({
  prompt: 'Node.js via stdin> ',
  input: process.stdin,
  output: process.stdout
});

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via Unix socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen('/tmp/node-repl-sock');

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via TCP socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen(5001);

Running this application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. telnet, for instance, is useful for connecting to TCP sockets, while socat can be used to connect to both Unix and TCP sockets.

By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.

For an example of running a "full-featured" (terminal) REPL over a net.Server and net.Socket instance, see: https://gist.github.com/2209310

For an example of running a REPL instance over curl(1), see: https://gist.github.com/2053342

Stream#

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The stream module provides a base API that makes it easy to build objects that implement the stream interface.

There are many stream objects provided by Node.js. For instance, a request to an HTTP server and process.stdout are both stream instances.

Streams can be readable, writable, or both. All streams are instances of EventEmitter.

The stream module can be accessed using:

const stream = require('stream');

While it is important to understand how streams work, the stream module itself is most useful for developers that are creating new types of stream instances. Developers who are primarily consuming stream objects will rarely need to use the stream module directly.

Organization of this Document#

This document is divided into two primary sections with a third section for additional notes. The first section explains the elements of the stream API that are required to use streams within an application. The second section explains the elements of the API that are required to implement new types of streams.

Types of Streams#

There are four fundamental stream types within Node.js:

Object Mode#

All streams created by Node.js APIs operate exclusively on strings and Buffer (or Uint8Array) objects. It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of null, which serves a special purpose within streams). Such streams are considered to operate in "object mode".

Stream instances are switched into object mode using the objectMode option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

Buffering#

Both Writable and Readable streams will store data in an internal buffer that can be retrieved using writable.writableBuffer or readable.readableBuffer, respectively.

The amount of data potentially buffered depends on the highWaterMark option passed into the streams constructor. For normal streams, the highWaterMark option specifies a total number of bytes. For streams operating in object mode, the highWaterMark specifies a total number of objects.

Data is buffered in Readable streams when the implementation calls stream.push(chunk). If the consumer of the Stream does not call stream.read(), the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by highWaterMark, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal readable._read() method that is used to fill the read buffer).

Data is buffered in Writable streams when the writable.write(chunk) method is called repeatedly. While the total size of the internal write buffer is below the threshold set by highWaterMark, calls to writable.write() will return true. Once the size of the internal buffer reaches or exceeds the highWaterMark, false will be returned.

A key goal of the stream API, particularly the stream.pipe() method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

Because Duplex and Transform streams are both Readable and Writable, each maintain two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, net.Socket instances are Duplex streams whose Readable side allows consumption of data received from the socket and whose Writable side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, it is important for each side to operate (and buffer) independently of the other.

API for Stream Consumers#

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

const http = require('http');

const server = http.createServer((req, res) => {
  // req is an http.IncomingMessage, which is a Readable Stream
  // res is an http.ServerResponse, which is a Writable Stream

  let body = '';
  // Get the data as utf8 strings.
  // If an encoding is not set, Buffer objects will be received.
  req.setEncoding('utf8');

  // Readable streams emit 'data' events once a listener is added
  req.on('data', (chunk) => {
    body += chunk;
  });

  // the end event indicates that the entire body has been received
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // write back something interesting to the user:
      res.write(typeof data);
      res.end();
    } catch (er) {
      // uh oh! bad json!
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token o in JSON at position 1

Writable streams (such as res in the example) expose methods such as write() and end() that are used to write data onto the stream.

Readable streams use the EventEmitter API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both Writable and Readable streams use the EventEmitter API in various ways to communicate the current state of the stream.

Duplex and Transform streams are both Writable and Readable.

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call require('stream').

Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.

Writable Streams#

Writable streams are an abstraction for a destination to which data is written.

Examples of Writable streams include:

Some of these examples are actually Duplex streams that implement the Writable interface.

All Writable streams implement the interface defined by the stream.Writable class.

While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

Class: stream.Writable#

Added in: v0.9.4
Event: 'close'#
Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Writable streams will emit the 'close' event.

Event: 'drain'#
Added in: v0.9.4

If a call to stream.write(chunk) returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // last time!
        writer.write(data, encoding, callback);
      } else {
        // see if we should continue, or wait
        // don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // had to stop early!
      // write some more once it drains
      writer.once('drain', write);
    }
  }
}
Event: 'error'#
Added in: v0.9.4

The 'error' event is emitted if an error occurred while writing or piping data. The listener callback is passed a single Error argument when called.

The stream is not closed when the 'error' event is emitted.

Event: 'finish'#
Added in: v0.9.4

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}
writer.end('This is the end\n');
writer.on('finish', () => {
  console.error('All writes are now complete.');
});
Event: 'pipe'#
Added in: v0.9.4

The 'pipe' event is emitted when the stream.pipe() method is called on a readable stream, adding this writable to its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.error('something is piping into the writer');
  assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'#
Added in: v0.9.4

The 'unpipe' event is emitted when the stream.unpipe() method is called on a Readable stream, removing this Writable from its set of destinations.

This is also emitted in case this Writable stream emits an error when a Readable stream pipes into it.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.error('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()#
Added in: v0.11.2

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the stream.uncork() or stream.end() methods are called.

The primary intent of writable.cork() is to avoid a situation where writing many small chunks of data to a stream do not cause a backup in the internal buffer that would have an adverse impact on performance. In such situations, implementations that implement the writable._writev() method can perform buffered writes in a more optimized manner.

See also: writable.uncork().

writable.destroy([error])#
Added in: v8.0.0

Destroy the stream, and emit the passed error and a close event. After this call, the writable stream has ended and subsequent calls to write / end will give an ERR_STREAM_DESTROYED error. Implementors should not override this method, but instead implement writable._destroy.

writable.end([chunk][, encoding][, callback])#
History
VersionChanges
REPLACEME

This method now returns a reference to writable.

v8.0.0

The chunk argument can now be a Uint8Array instance.

v0.9.4

Added in: v0.9.4

  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding, if chunk is a string
  • callback <Function> Optional callback for when the stream is finished
  • Returns: <this>

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream. If provided, the optional callback function is attached as a listener for the 'finish' event.

Calling the stream.write() method after calling stream.end() will raise an error.

// write 'hello, ' and then end with 'world!'
const fs = require('fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// writing more now is not allowed!
writable.setDefaultEncoding(encoding)#
History
VersionChanges
v6.1.0

This method now returns a reference to writable.

v0.11.15

Added in: v0.11.15

The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

writable.uncork()#
Added in: v0.11.2

The writable.uncork() method flushes all data buffered since stream.cork() was called.

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, it is recommended that calls to writable.uncork() be deferred using process.nextTick(). Doing so allows batching of all writable.write() calls that occur within a given Node.js event loop phase.

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
  stream.uncork();
  // The data will not be flushed until uncork() is called a second time.
  stream.uncork();
});

See also: writable.cork().

writable.writableHighWaterMark#
Added in: v9.3.0

Return the value of highWaterMark passed when constructing this Writable.

writable.writableLength#
Added in: v9.4.0

This property contains the number of bytes (or objects) in the queue ready to be written. The value provides introspection data regarding the status of the highWaterMark.

writable.write(chunk[, encoding][, callback])#
History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

v6.0.0

Passing null as the chunk parameter will always be considered invalid now, even in object mode.

v0.9.4

Added in: v0.9.4

  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding, if chunk is a string
  • callback <Function> Callback for when this chunk of data is flushed
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback may or may not be called with the error as its first argument. To reliably detect write errors, add a listener for the 'error' event.

The return value is true if the internal buffer is less than the highWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. It is recommended that once write() returns false, no more chunks be written until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or an 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use stream.pipe(). However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

function write(data, cb) {
  if (!stream.write(data)) {
    stream.once('drain', cb);
  } else {
    process.nextTick(cb);
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('write completed, do more writes now');
});

A Writable stream in object mode will always ignore the encoding argument.

Readable Streams#

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

All Readable streams implement the interface defined by the stream.Readable class.

Two Modes#

Readable streams effectively operate in one of two modes: flowing and paused.

When in flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the EventEmitter interface.

In paused mode, the stream.read() method must be called explicitly to read chunks of data from the stream.

All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:

The Readable can switch back to paused mode using one of the following:

  • If there are no pipe destinations, by calling the stream.pause() method.
  • If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the stream.unpipe() method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

For backwards compatibility reasons, removing 'data' event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling stream.pause() will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a Readable is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Three States#

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

  • readable.readableFlowing = null
  • readable.readableFlowing = false
  • readable.readableFlowing = true

When readable.readableFlowing is null, no mechanism for consuming the streams data is provided so the stream will not generate its data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving "back pressure" will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event would not cause readable.readableFlowing to switch to true.

const { PassThrough, Writable } = require('stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false

pass.on('data', (chunk) => { console.log(chunk.toString()); });
pass.write('ok'); // will not emit 'data'
pass.resume(); // must be called to make 'data' being emitted

While readable.readableFlowing is false, data may be accumulating within the streams internal buffer.

Choose One#

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream.

Use of the readable.pipe() method is recommended for most users as it has been implemented to provide the easiest way of consuming stream data. Developers that require more fine-grained control over the transfer and generation of data can use the EventEmitter and readable.pause()/readable.resume() APIs.

Class: stream.Readable#

Added in: v0.9.4
Event: 'close'#
Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

Not all Readable streams will emit the 'close' event.

Event: 'data'#
Added in: v0.9.4
  • chunk <Buffer> | <string> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or Buffer. For streams that are in object mode, the chunk can be any JavaScript value other than null.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'#
Added in: v0.9.4

The 'end' event is emitted when there is no more data to be consumed from the stream.

The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling stream.read() repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});
Event: 'error'#
Added in: v0.9.4

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

Event: 'readable'#
History
VersionChanges
REPLACEME

'readable' is always emitted in the next tick after .push() is called

v0.9.4

Added in: v0.9.4

The 'readable' event is emitted when there is data available to be read from the stream. In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

const readable = getReadableStreamSomehow();
readable.on('readable', () => {
  // there is some data to read now
});

The 'readable' event will also be emitted once the end of the stream data has been reached but before the 'end' event is emitted.

Effectively, the 'readable' event indicates that the stream has new information: either new data is available or the end of the stream has been reached. In the former case, stream.read() will return the available data. In the latter case, stream.read() will return null. For instance, in the following example, foo.txt is an empty file:

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

readable.destroy([error])#
Added in: v8.0.0
  • error <Error> Error which will be passed as payload in 'error' event
  • Returns: <this>

Destroy the stream, and emit 'error' and close. After this call, the readable stream will release any internal resources and subsequent calls to push will be ignored. Implementors should not override this method, but instead implement readable._destroy.

readable.isPaused()#
Added in: v0.11.14

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false
readable.pause()#
Added in: v0.9.4

The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
  readable.pause();
  console.log('There will be no additional data for 1 second.');
  setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
  }, 1000);
});
readable.pipe(destination[, options])#
Added in: v0.9.4

The readable.pipe() method attaches a Writable stream to the readable, causing it to switch automatically into flowing mode and push all of its data to the attached Writable. The flow of data will be automatically managed so that the destination Writable stream is not overwhelmed by a faster Readable stream.

The following example pipes all of the data from the readable into a file named file.txt:

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);

It is possible to attach multiple Writable streams to a single Readable stream.

The readable.pipe() method returns a reference to the destination stream making it possible to set up chains of piped streams:

const fs = require('fs');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);

By default, stream.end() is called on the destination Writable stream when the source Readable stream emits 'end', so that the destination is no longer writable. To disable this default behavior, the end option can be passed as false, causing the destination stream to remain open, as illustrated in the following example:

reader.pipe(writer, { end: false });
reader.on('end', () => {
  writer.end('Goodbye\n');
});

One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

The process.stderr and process.stdout Writable streams are never closed until the Node.js process exits, regardless of the specified options.

readable.read([size])#
Added in: v0.9.4

The readable.read() method pulls some data out of the internal buffer and returns it. If no data available to be read, null is returned. By default, the data will be returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

The optional size argument specifies a specific number of bytes to read. If size bytes are not available to be read, null will be returned unless the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

If the size argument is not specified, all of the data contained in the internal buffer will be returned.

The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

const readable = getReadableStreamSomehow();
readable.on('readable', () => {
  let chunk;
  while (null !== (chunk = readable.read())) {
    console.log(`Received ${chunk.length} bytes of data.`);
  }
});

A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of the size argument.

If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

Calling stream.read([size]) after the 'end' event has been emitted will return null. No runtime error will be raised.

readable.readableHighWaterMark#
Added in: v9.3.0

Returns the value of highWaterMark passed when constructing this Readable.

readable.readableLength#
Added in: v9.4.0

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

readable.resume()#
Added in: v0.9.4

The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data as illustrated in the following example:

getReadableStreamSomehow()
  .resume()
  .on('end', () => {
    console.log('Reached the end, but did not read anything.');
  });
readable.setEncoding(encoding)#
Added in: v0.9.4

The readable.setEncoding() method sets the character encoding for data read from the Readable stream.

By default, no encoding is assigned and stream data will be returned as Buffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Buffer objects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Calling readable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('got %d characters of string data', chunk.length);
});
readable.unpipe([destination])#
Added in: v0.9.4

The readable.unpipe() method detaches a Writable stream previously attached using the stream.pipe() method.

If the destination is not specified, then all pipes are detached.

If the destination is specified, but no pipe is set up for it, then the method does nothing.

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second
readable.pipe(writable);
setTimeout(() => {
  console.log('Stop writing to file.txt');
  readable.unpipe(writable);
  console.log('Manually close the file stream');
  writable.end();
}, 1000);
readable.unshift(chunk)#
History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

v0.9.11

Added in: v0.9.11

  • chunk <Buffer> | <Uint8Array> | <string> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for Stream Implementers section for more information.

// Pull off a header delimited by \n\n
// use unshift() if we get too much
// Call the callback with (error, header, stream)
const { StringDecoder } = require('string_decoder');
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  let header = '';
  function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
      const str = decoder.write(chunk);
      if (str.match(/\n\n/)) {
        // found the header boundary
        const split = str.split(/\n\n/);
        header += split.shift();
        const remaining = split.join('\n\n');
        const buf = Buffer.from(remaining, 'utf8');
        stream.removeListener('error', callback);
        // remove the readable listener before unshifting
        stream.removeListener('readable', onReadable);
        if (buf.length)
          stream.unshift(buf);
        // now the body of the message can be read from the stream.
        callback(null, header, stream);
      } else {
        // still reading the header.
        header += str;
      }
    }
  }
}

Unlike stream.push(chunk), stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a stream._read() implementation on a custom stream). Following the call to readable.unshift() with an immediate stream.push('') will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

readable.wrap(stream)#
Added in: v0.9.4

Versions of Node.js prior to v0.10 had streams that did not implement the entire stream module API as it is currently defined. (See Compatibility for more information.)

When using an older Node.js library that emits 'data' events and has a stream.pause() method that is advisory only, the readable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

const { OldReader } = require('./old-api-module.js');
const { Readable } = require('stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);

myReader.on('readable', () => {
  myReader.read(); // etc.
});
readable[@@asyncIterator]#
Added in: REPLACEME
Stability: 1 - Experimental

Returns an AsyncIterator to fully consume the stream.

const fs = require('fs');

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const k of readable) {
    data += k;
  }
  console.log(data);
}

print(fs.createReadStream('file')).catch(console.log);

If the loop terminates with a break or a throw, the stream will be destroyed. In other terms, iterating over a stream will consume the stream fully. The stream will be read in chunks of size equal to the highWaterMark option. In the code example above, data will be in a single chunk if the file has less then 64kb of data because no highWaterMark option is provided to fs.createReadStream().

Duplex and Transform Streams#

Class: stream.Duplex#

History
VersionChanges
v6.8.0

Instances of Duplex now return true when checking instanceof stream.Writable.

v0.9.4

Added in: v0.9.4

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

Class: stream.Transform#

Added in: v0.9.4

Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.

Examples of Transform streams include:

transform.destroy([error])#
Added in: v8.0.0

Destroy the stream, and emit 'error'. After this call, the transform stream would release any internal resources. implementors should not override this method, but instead implement readable._destroy. The default implementation of _destroy for Transform also emit 'close'.

API for Stream Implementers#

The stream module API has been designed to make it possible to easily implement streams using JavaScript's prototypal inheritance model.

First, a stream developer would declare a new JavaScript class that extends one of the four basic stream classes (stream.Writable, stream.Readable, stream.Duplex, or stream.Transform), making sure they call the appropriate parent class constructor:

const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor(options) {
    super(options);
    // ...
  }
}

The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:

Use-case

Class

Method(s) to implement

Reading only

Readable

_read

Writing only

Writable

_write, _writev, _final

Reading and writing

Duplex

_read, _write, _writev, _final

Operate on written data, then read the result

Transform

_transform, _flush, _final

The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.

Simplified Construction#

Added in: v1.2.0

For many simple cases, it is possible to construct a stream without relying on inheritance. This can be accomplished by directly creating instances of the stream.Writable, stream.Readable, stream.Duplex or stream.Transform objects and passing appropriate methods as constructor options.

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  }
});

Implementing a Writable Stream#

The stream.Writable class is extended to implement a Writable stream.

Custom Writable streams must call the new stream.Writable([options]) constructor and implement the writable._write() method. The writable._writev() method may also be implemented.

Constructor: new stream.Writable(options)#

History
VersionChanges
REPLACEME

Add emitClose option to specify if close is emitted on destroy

const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor(options) {
    // Calls the stream.Writable() constructor
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Writable } = require('stream');
const util = require('util');

function MyWritable(options) {
  if (!(this instanceof MyWritable))
    return new MyWritable(options);
  Writable.call(this, options);
}
util.inherits(MyWritable, Writable);

Or, using the Simplified Constructor approach:

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  },
  writev(chunks, callback) {
    // ...
  }
});

writable._write(chunk, encoding, callback)#

  • chunk <Buffer> | <string> | <any> The chunk to be written. Will always be a buffer unless the decodeStrings option was set to false or the stream is operating in object mode.
  • encoding <string> If the chunk is a string, then encoding is the character encoding of that string. If chunk is a Buffer, or if the stream is operating in object mode, encoding may be ignored.
  • callback <Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.

All Writable stream implementations must provide a writable._write() method to send data to the underlying resource.

Transform streams provide their own implementation of the writable._write().

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Writable class methods only.

The callback method must be called to signal either that the write completed successfully or failed with an error. The first argument passed to the callback must be the Error object if the call failed or null if the write succeeded.

All calls to writable.write() that occur between the time writable._write() is called and the callback is called will cause the written data to be buffered. When the callback is invoked, the stream might emit a 'drain' event. If a stream implementation is capable of processing multiple chunks of data at once, the writable._writev() method should be implemented.

If the decodeStrings property is set in the constructor options, then chunk may be a string rather than a Buffer, and encoding will indicate the character encoding of the string. This is to support implementations that have an optimized handling for certain string data encodings. If the decodeStrings property is explicitly set to false, the encoding argument can be safely ignored, and chunk will remain the same object that is passed to .write().

The writable._write() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

writable._writev(chunks, callback)#

  • chunks <Array> The chunks to be written. Each chunk has following format: { chunk: ..., encoding: ... }.
  • callback <Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Writable class methods only.

The writable._writev() method may be implemented in addition to writable._write() in stream implementations that are capable of processing multiple chunks of data at once. If implemented, the method will be called with all chunks of data currently buffered in the write queue.

The writable._writev() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

writable._destroy(err, callback)#

Added in: v8.0.0
  • err <Error> A possible error.
  • callback <Function> A callback function that takes an optional error argument.

The _destroy() method is called by writable.destroy(). It can be overridden by child classes but it must not be called directly.

writable._final(callback)#

Added in: v8.0.0
  • callback <Function> Call this function (optionally with an error argument) when finished writing any remaining data.

The _final() method must not be called directly. It may be implemented by child classes, and if so, will be called by the internal Writable class methods only.

This optional function will be called before the stream closes, delaying the finish event until callback is called. This is useful to close resources or write buffered data before a stream ends.

Errors While Writing#

It is recommended that errors occurring during the processing of the writable._write() and writable._writev() methods are reported by invoking the callback and passing the error as the first argument. This will cause an 'error' event to be emitted by the Writable. Throwing an Error from within writable._write() can result in unexpected and inconsistent behavior depending on how the stream is being used. Using the callback ensures consistent and predictable handling of errors.

If a Readable stream pipes into a Writable stream when Writable emits an error, the Readable stream will be unpiped.

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
});

An Example Writable Stream#

The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:

const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor(options) {
    super(options);
    // ...
  }

  _write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
}

Decoding buffers in a Writable Stream#

Decoding buffers is a common task, for instance, when using transformers whose input is a string. This is not a trivial process when using multi-byte characters encoding, such as UTF-8. The following example shows how to decode multi-byte strings using StringDecoder and Writable.

const { Writable } = require('stream');
const { StringDecoder } = require('string_decoder');

class StringWritable extends Writable {
  constructor(options) {
    super(options);
    const state = this._writableState;
    this._decoder = new StringDecoder(state.defaultEncoding);
    this.data = '';
  }
  _write(chunk, encoding, callback) {
    if (encoding === 'buffer') {
      chunk = this._decoder.write(chunk);
    }
    this.data += chunk;
    callback();
  }
  _final(callback) {
    this.data += this._decoder.end();
    callback();
  }
}

const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
const w = new StringWritable();

w.write('currency: ');
w.write(euro[0]);
w.end(euro[1]);

console.log(w.data); // currency: €

Implementing a Readable Stream#

The stream.Readable class is extended to implement a Readable stream.

Custom Readable streams must call the new stream.Readable([options]) constructor and implement the readable._read() method.

new stream.Readable(options)#

  • options <Object>
    • highWaterMark <number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Default: 16384 (16kb), or 16 for objectMode streams.
    • encoding <string> If specified, then buffers will be decoded to strings using the specified encoding. Default: null.
    • objectMode <boolean> Whether this stream should behave as a stream of objects. Meaning that stream.read(n) returns a single value instead of a Buffer of size n. Default: false.
    • read <Function> Implementation for the stream._read() method.
    • destroy <Function> Implementation for the stream._destroy() method.
const { Readable } = require('stream');

class MyReadable extends Readable {
  constructor(options) {
    // Calls the stream.Readable(options) constructor
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Readable } = require('stream');
const util = require('util');

function MyReadable(options) {
  if (!(this instanceof MyReadable))
    return new MyReadable(options);
  Readable.call(this, options);
}
util.inherits(MyReadable, Readable);

Or, using the Simplified Constructor approach:

const { Readable } = require('stream');

const myReadable = new Readable({
  read(size) {
    // ...
  }
});

readable._read(size)#

History
VersionChanges
REPLACEME

call _read() only once per microtick

v0.9.4

Added in: v0.9.4

  • size <number> Number of bytes to read asynchronously

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Readable stream implementations must provide an implementation of the readable._read() method to fetch data from the underlying resource.

When readable._read() is called, if data is available from the resource, the implementation should begin pushing that data into the read queue using the this.push(dataChunk) method. _read() should continue reading from the resource and pushing data until readable.push() returns false. Only when _read() is called again after it has stopped should it resume pushing additional data onto the queue.

Once the readable._read() method has been called, it will not be called again until the readable.push() method is called. readable._read() is guaranteed to be called only once within a synchronous execution, i.e. a microtick.

The size argument is advisory. For implementations where a "read" is a single operation that returns data can use the size argument to determine how much data to fetch. Other implementations may ignore this argument and simply provide data whenever it becomes available. There is no need to "wait" until size bytes are available before calling stream.push(chunk).

The readable._read() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

readable._destroy(err, callback)#

Added in: v8.0.0
  • err <Error> A possible error.
  • callback <Function> A callback function that takes an optional error argument.

The _destroy() method is called by readable.destroy(). It can be overridden by child classes but it must not be called directly.

readable.push(chunk[, encoding])#

History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

  • chunk <Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to push into the read queue. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value.
  • encoding <string> Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'
  • Returns: <boolean> true if additional chunks of data may continued to be pushed; false otherwise.

When chunk is a Buffer, Uint8Array or string, the chunk of data will be added to the internal queue for users of the stream to consume. Passing chunk as null signals the end of the stream (EOF), after which no more data can be written.

When the Readable is operating in paused mode, the data added with readable.push() can be read out by calling the readable.read() method when the 'readable' event is emitted.

When the Readable is operating in flowing mode, the data added with readable.push() will be delivered by emitting a 'data' event.

The readable.push() method is designed to be as flexible as possible. For example, when wrapping a lower-level source that provides some form of pause/resume mechanism, and a data callback, the low-level source can be wrapped by the custom Readable instance as illustrated in the following example:

// source is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.

class SourceWrapper extends Readable {
  constructor(options) {
    super(options);

    this._source = getLowlevelSourceObject();

    // Every time there's data, push it into the internal buffer.
    this._source.ondata = (chunk) => {
      // if push() returns false, then stop reading from source
      if (!this.push(chunk))
        this._source.readStop();
    };

    // When the source ends, push the EOF-signaling `null` chunk
    this._source.onend = () => {
      this.push(null);
    };
  }
  // _read will be called when the stream wants to pull more data in
  // the advisory size argument is ignored in this case.
  _read(size) {
    this._source.readStart();
  }
}

The readable.push() method is intended be called only by Readable Implementers, and only from within the readable._read() method.

For streams not operating in object mode, if the chunk parameter of readable.push() is undefined, it will be treated as empty string or buffer. See readable.push('') for more information.

Errors While Reading#

It is recommended that errors occurring during the processing of the readable._read() method are emitted using the 'error' event rather than being thrown. Throwing an Error from within readable._read() can result in unexpected and inconsistent behavior depending on whether the stream is operating in flowing or paused mode. Using the 'error' event ensures consistent and predictable handling of errors.

const { Readable } = require('stream');

const myReadable = new Readable({
  read(size) {
    if (checkSomeErrorCondition()) {
      process.nextTick(() => this.emit('error', err));
      return;
    }
    // do some work
  }
});

An Example Counting Stream#

The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.

const { Readable } = require('stream');

class Counter extends Readable {
  constructor(opt) {
    super(opt);
    this._max = 1000000;
    this._index = 1;
  }

  _read() {
    const i = this._index++;
    if (i > this._max)
      this.push(null);
    else {
      const str = String(i);
      const buf = Buffer.from(str, 'ascii');
      this.push(buf);
    }
  }
}

Implementing a Duplex Stream#

A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.

Because JavaScript does not have support for multiple inheritance, the stream.Duplex class is extended to implement a Duplex stream (as opposed to extending the stream.Readable and stream.Writable classes).

The stream.Duplex class prototypically inherits from stream.Readable and parasitically from stream.Writable, but instanceof will work properly for both base classes due to overriding Symbol.hasInstance on stream.Writable.

Custom Duplex streams must call the new stream.Duplex([options]) constructor and implement both the readable._read() and writable._write() methods.

new stream.Duplex(options)#

History
VersionChanges
v8.4.0

The readableHighWaterMark and writableHighWaterMark options are supported now.

  • options <Object> Passed to both Writable and Readable constructors. Also has the following fields:
    • allowHalfOpen <boolean> If set to false, then the stream will automatically end the writable side when the readable side ends. Default: true.
    • readableObjectMode <boolean> Sets objectMode for readable side of the stream. Has no effect if objectMode is true. Default: false.
    • writableObjectMode <boolean> Sets objectMode for writable side of the stream. Has no effect if objectMode is true. Default: false.
    • readableHighWaterMark <number> Sets highWaterMark for the readable side of the stream. Has no effect if highWaterMark is provided.
    • writableHighWaterMark <number> Sets highWaterMark for the writable side of the stream. Has no effect if highWaterMark is provided.
const { Duplex } = require('stream');

class MyDuplex extends Duplex {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Duplex } = require('stream');
const util = require('util');

function MyDuplex(options) {
  if (!(this instanceof MyDuplex))
    return new MyDuplex(options);
  Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);

Or, using the Simplified Constructor approach:

const { Duplex } = require('stream');

const myDuplex = new Duplex({
  read(size) {
    // ...
  },
  write(chunk, encoding, callback) {
    // ...
  }
});

An Example Duplex Stream#

The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.

const { Duplex } = require('stream');
const kSource = Symbol('source');

class MyDuplex extends Duplex {
  constructor(source, options) {
    super(options);
    this[kSource] = source;
  }

  _write(chunk, encoding, callback) {
    // The underlying source only deals with strings
    if (Buffer.isBuffer(chunk))
      chunk = chunk.toString();
    this[kSource].writeSomeData(chunk);
    callback();
  }

  _read(size) {
    this[kSource].fetchSomeData(size, (data, encoding) => {
      this.push(Buffer.from(data, encoding));
    });
  }
}

The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.

Object Mode Duplex Streams#

For Duplex streams, objectMode can be set exclusively for either the Readable or Writable side using the readableObjectMode and writableObjectMode options respectively.

In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.

const { Transform } = require('stream');

// All Transform streams are also Duplex Streams
const myTransform = new Transform({
  writableObjectMode: true,

  transform(chunk, encoding, callback) {
    // Coerce the chunk to a number if necessary
    chunk |= 0;

    // Transform the chunk into something else.
    const data = chunk.toString(16);

    // Push the data onto the readable queue.
    callback(null, '0'.repeat(data.length % 2) + data);
  }
});

myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));

myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64

Implementing a Transform Stream#

A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.

There is no requirement that the output be the same size as the input, the same number of chunks, or arrive at the same time. For example, a Hash stream will only ever have a single chunk of output which is provided when the input is ended. A zlib stream will produce output that is either much smaller or much larger than its input.

The stream.Transform class is extended to implement a Transform stream.

The stream.Transform class prototypically inherits from stream.Duplex and implements its own versions of the writable._write() and readable._read() methods. Custom Transform implementations must implement the transform._transform() method and may also implement the transform._flush() method.

Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.

new stream.Transform(options)#

const { Transform } = require('stream');

class MyTransform extends Transform {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Transform } = require('stream');
const util = require('util');

function MyTransform(options) {
  if (!(this instanceof MyTransform))
    return new MyTransform(options);
  Transform.call(this, options);
}
util.inherits(MyTransform, Transform);

Or, using the Simplified Constructor approach:

const { Transform } = require('stream');

const myTransform = new Transform({
  transform(chunk, encoding, callback) {
    // ...
  }
});

Events: 'finish' and 'end'#

The 'finish' and 'end' events are from the stream.Writable and stream.Readable classes, respectively. The 'finish' event is emitted after stream.end() is called and all chunks have been processed by stream._transform(). The 'end' event is emitted after all data has been output, which occurs after the callback in transform._flush() has been called.

transform._flush(callback)#

  • callback <Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

In some cases, a transform operation may need to emit an additional bit of data at the end of the stream. For example, a zlib compression stream will store an amount of internal state used to optimally compress the output. When the stream ends, however, that additional data needs to be flushed so that the compressed data will be complete.

Custom Transform implementations may implement the transform._flush() method. This will be called when there is no more written data to be consumed, but before the 'end' event is emitted signaling the end of the Readable stream.

Within the transform._flush() implementation, the readable.push() method may be called zero or more times, as appropriate. The callback function must be called when the flush operation is complete.

The transform._flush() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform(chunk, encoding, callback)#

  • chunk <Buffer> | <string> | <any> The chunk to be transformed. Will always be a buffer unless the decodeStrings option was set to false or the stream is operating in object mode.
  • encoding <string> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.
  • callback <Function> A callback function (optionally with an error argument and data) to be called after the supplied chunk has been processed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Transform stream implementations must provide a _transform() method to accept input and produce output. The transform._transform() implementation handles the bytes being written, computes an output, then passes that output off to the readable portion using the readable.push() method.

The transform.push() method may be called zero or more times to generate output from a single input chunk, depending on how much is to be output as a result of the chunk.

It is possible that no output is generated from any given chunk of input data.

The callback function must be called only when the current chunk is completely consumed. The first argument passed to the callback must be an Error object if an error occurred while processing the input or null otherwise. If a second argument is passed to the callback, it will be forwarded on to the readable.push() method. In other words the following are equivalent:

transform.prototype._transform = function(data, encoding, callback) {
  this.push(data);
  callback();
};

transform.prototype._transform = function(data, encoding, callback) {
  callback(null, data);
};

The transform._transform() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform() is never called in parallel; streams implement a queue mechanism, and to receive the next chunk, callback must be called, either synchronously or asynchronously.

Class: stream.PassThrough#

The stream.PassThrough class is a trivial implementation of a Transform stream that simply passes the input bytes across to the output. Its purpose is primarily for examples and testing, but there are some use cases where stream.PassThrough is useful as a building block for novel sorts of streams.

Additional Notes#

Compatibility with Older Node.js Versions#

In versions of Node.js prior to v0.10, the Readable stream interface was simpler, but also less powerful and less useful.

  • Rather than waiting for calls the stream.read() method, 'data' events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost.
  • The stream.pause() method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive 'data' events even when the stream was in a paused state.

In Node.js v0.10, the Readable class was added. For backwards compatibility with older Node.js programs, Readable streams switch into "flowing mode" when a 'data' event handler is added, or when the stream.resume() method is called. The effect is that, even when not using the new stream.read() method and 'readable' event, it is no longer necessary to worry about losing 'data' chunks.

While most applications will continue to function normally, this introduces an edge case in the following conditions:

  • No 'data' event listener is added.
  • The stream.resume() method is never called.
  • The stream is not piped to any writable destination.

For example, consider the following code:

// WARNING!  BROKEN!
net.createServer((socket) => {

  // we add an 'end' method, but never consume the data
  socket.on('end', () => {
    // It will never get here.
    socket.end('The message was received but was not processed.\n');
  });

}).listen(1337);

In versions of Node.js prior to v0.10, the incoming message data would be simply discarded. However, in Node.js v0.10 and beyond, the socket remains paused forever.

The workaround in this situation is to call the stream.resume() method to begin the flow of data:

// Workaround
net.createServer((socket) => {

  socket.on('end', () => {
    socket.end('The message was received but was not processed.\n');
  });

  // start the flow of data, discarding it.
  socket.resume();

}).listen(1337);

In addition to new Readable streams switching into flowing mode, pre-v0.10 style streams can be wrapped in a Readable class using the readable.wrap() method.

readable.read(0)#

There are some cases where it is necessary to trigger a refresh of the underlying readable stream mechanisms, without actually consuming any data. In such cases, it is possible to call readable.read(0), which will always return null.

If the internal read buffer is below the highWaterMark, and the stream is not currently reading, then calling stream.read(0) will trigger a low-level stream._read() call.

While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the Readable stream class internals.

readable.push('')#

Use of readable.push('') is not recommended.

Pushing a zero-byte string, Buffer or Uint8Array to a stream that is not in object mode has an interesting side effect. Because it is a call to readable.push(), the call will end the reading process. However, because the argument is an empty string, no data is added to the readable buffer so there is nothing for a user to consume.

highWaterMark discrepancy after calling readable.setEncoding()#

The use of readable.setEncoding() will change the behavior of how the highWaterMark operates in non-object mode.

Typically, the size of the current buffer is measured against the highWaterMark in bytes. However, after setEncoding() is called, the comparison function will begin to measure the buffer's size in characters.

This is not a problem in common cases with latin1 or ascii. But it is advised to be mindful about this behavior when working with strings that could contain multi-byte characters.

String Decoder#

Stability: 2 - Stable

The string_decoder module provides an API for decoding Buffer objects into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16 characters. It can be accessed using:

const { StringDecoder } = require('string_decoder');

The following example shows the basic use of the StringDecoder class.

const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');

const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent));

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro));

When a Buffer instance is written to the StringDecoder instance, an internal buffer is used to ensure that the decoded string does not contain any incomplete multibyte characters. These are held in the buffer until the next call to stringDecoder.write() or until stringDecoder.end() is called.

In the following example, the three UTF-8 encoded bytes of the European Euro symbol (€) are written over three separate operations:

const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC])));

Class: new StringDecoder([encoding])#

Added in: v0.1.99
  • encoding <string> The character encoding the StringDecoder will use. Default: 'utf8'.

Creates a new StringDecoder instance.

stringDecoder.end(buffer)#

Added in: v0.9.3
  • buffer <Buffer> A Buffer containing the bytes to decode.

Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.

If the buffer argument is provided, one final call to stringDecoder.write() is performed before returning the remaining input.

stringDecoder.write(buffer)#

History
VersionChanges
v8.0.0

Each invalid character is now replaced by a single replacement character instead of one for each individual byte.

v0.1.99

Added in: v0.1.99

  • buffer <Buffer> A Buffer containing the bytes to decode.

Returns a decoded string, ensuring that any incomplete multibyte characters at the end of the Buffer are omitted from the returned string and stored in an internal buffer for the next call to stringDecoder.write() or stringDecoder.end().

Timers#

Stability: 2 - Stable

The timer module exposes a global API for scheduling functions to be called at some future period of time. Because the timer functions are globals, there is no need to call require('timers') to use the API.

The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.

Class: Immediate#

This object is created internally and is returned from setImmediate(). It can be passed to clearImmediate() in order to cancel the scheduled actions.

By default, when an immediate is scheduled, the Node.js event loop will continue running as long as the immediate is active. The Immediate object returned by setImmediate() exports both immediate.ref() and immediate.unref() functions that can be used to control this default behavior.

immediate.ref()#

Added in: v9.7.0

When called, requests that the Node.js event loop not exit so long as the Immediate is active. Calling immediate.ref() multiple times will have no effect.

By default, all Immediate objects are "ref'd", making it normally unnecessary to call immediate.ref() unless immediate.unref() had been called previously.

Returns a reference to the Immediate.

immediate.unref()#

Added in: v9.7.0

When called, the active Immediate 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 Immediate object's callback is invoked. Calling immediate.unref() multiple times will have no effect.

Returns a reference to the Immediate.

Class: Timeout#

This object is created internally and is returned from setTimeout() and setInterval(). It can be passed to clearTimeout() or clearInterval() (respectively) in order to cancel the scheduled actions.

By default, when a timer is scheduled using either setTimeout() or setInterval(), the Node.js event loop will continue running as long as the timer is active. Each of the Timeout objects returned by these functions export both timeout.ref() and timeout.unref() functions that can be used to control this default behavior.

timeout.ref()#

Added in: v0.9.1

When called, requests that the Node.js event loop not exit so long as the Timeout is active. Calling timeout.ref() multiple times will have no effect.

By default, all Timeout objects are "ref'd", making it normally unnecessary to call timeout.ref() unless timeout.unref() had been called previously.

Returns a reference to the Timeout.

timeout.unref()#

Added in: v0.9.1

When called, the active Timeout 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 Timeout object's callback is invoked. Calling timeout.unref() multiple times will have no effect.

Calling timeout.unref() creates an internal timer that will wake the Node.js event loop. Creating too many of these can adversely impact performance of the Node.js application.

Returns a reference to the Timeout.

Scheduling Timers#

A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

setImmediate(callback[, ...args])#

Added in: v0.9.1
  • callback <Function> The function to call at the end of this turn of the Node.js Event Loop
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules the "immediate" execution of the callback after I/O events' callbacks. Returns an Immediate for use with clearImmediate().

When multiple calls to setImmediate() are made, the callback functions are queued for execution in the order in which they are created. The entire callback queue is processed every event loop iteration. If an immediate timer is queued from inside an executing callback, that timer will not be triggered until the next event loop iteration.

If callback is not a function, a TypeError will be thrown.

This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setImmediatePromise = util.promisify(setImmediate);

setImmediatePromise('foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after all I/O callbacks.
});

// or with async function
async function timerExample() {
  console.log('Before I/O callbacks');
  await setImmediatePromise();
  console.log('After I/O callbacks');
}
timerExample();

setInterval(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules repeated execution of callback every delay milliseconds. Returns a Timeout for use with clearInterval().

When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.

Schedules execution of a one-time callback after delay milliseconds. Returns a Timeout for use with clearTimeout().

The callback will likely not be invoked in precisely delay milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

When delay is larger than 2147483647 or less than 1, the delay will be set to 1.

If callback is not a function, a TypeError will be thrown.

This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);

setTimeoutPromise(40, 'foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after about 40 milliseconds.
});

Cancelling Timers#

The setImmediate(), setInterval(), and setTimeout() methods each return objects that represent the scheduled timers. These can be used to cancel the timer and prevent it from triggering.

It is not possible to cancel timers that were created using the promisified variants of setImmediate(), setTimeout().

clearImmediate(immediate)#

Added in: v0.9.1

Cancels an Immediate object created by setImmediate().

clearInterval(timeout)#

Added in: v0.0.1

Cancels a Timeout object created by setInterval().

clearTimeout(timeout)#

Added in: v0.0.1

Cancels a Timeout object created by setTimeout().

TLS (SSL)#

Stability: 2 - Stable

The tls module provides an implementation of the Transport Layer Security (TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL. The module can be accessed using:

const tls = require('tls');

TLS/SSL Concepts#

The TLS/SSL is a public/private key infrastructure (PKI). For most common cases, each client and server must have a private key.

Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:

openssl genrsa -out ryans-key.pem 2048

With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.

The OpenSSL command-line interface can be used to generate a CSR for a private key:

openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem

Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.

Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:

openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem

Once the certificate is generated, it can be used to generate a .pfx or .p12 file:

openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
      -certfile ca-cert.pem -out ryans.pfx

Where:

  • in: is the signed certificate
  • inkey: is the associated private key
  • certfile: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g. cat ca1-cert.pem ca2-cert.pem > ca-cert.pem

Perfect Forward Secrecy#

The term "Forward Secrecy" or "Perfect Forward Secrecy" describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.

Perfect Forward Secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".

Currently two methods are commonly used to achieve Perfect Forward Secrecy (note the character "E" appended to the traditional abbreviations):

  • DHE - An ephemeral version of the Diffie Hellman key-agreement protocol.
  • ECDHE - An ephemeral version of the Elliptic Curve Diffie Hellman key-agreement protocol.

Ephemeral methods may have some performance drawbacks, because key generation is expensive.

To use Perfect Forward Secrecy using DHE with the tls module, it is required to generate Diffie-Hellman parameters and specify them with the dhparam option to tls.createSecureContext(). The following illustrates the use of the OpenSSL command-line interface to generate such parameters:

openssl dhparam -outform PEM -out dhparam.pem 2048

If using Perfect Forward Secrecy using ECDHE, Diffie-Hellman parameters are not required and a default ECDHE curve will be used. The ecdhCurve property can be used when creating a TLS Server to specify the list of names of supported curves to use, see tls.createServer() for more info.

ALPN and SNI#

ALPN (Application-Layer Protocol Negotiation Extension) and SNI (Server Name Indication) are TLS handshake extensions:

  • ALPN - Allows the use of one TLS server for multiple protocols (HTTP, HTTP/2)
  • SNI - Allows the use of one TLS server for multiple hostnames with different SSL certificates.

Client-initiated renegotiation attack mitigation#

The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.

To mitigate the risk, renegotiation is limited to three times every ten minutes. An 'error' event is emitted on the tls.TLSSocket instance when this threshold is exceeded. The limits are configurable:

  • tls.CLIENT_RENEG_LIMIT <number> Specifies the number of renegotiation requests. Default: 3.
  • tls.CLIENT_RENEG_WINDOW <number> Specifies the time renegotiation window in seconds. Default: 600 (10 minutes).

The default renegotiation limits should not be modified without a full understanding of the implications and risks.

To test the renegotiation limits on a server, connect to it using the OpenSSL command-line client (openssl s_client -connect address:port) then input R<CR> (i.e., the letter R followed by a carriage return) multiple times.

Modifying the Default TLS Cipher suite#

Node.js is built with a default suite of enabled and disabled TLS ciphers. Currently, the default cipher suite is:

ECDHE-RSA-AES128-GCM-SHA256:
ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES256-GCM-SHA384:
ECDHE-ECDSA-AES256-GCM-SHA384:
DHE-RSA-AES128-GCM-SHA256:
ECDHE-RSA-AES128-SHA256:
DHE-RSA-AES128-SHA256:
ECDHE-RSA-AES256-SHA384:
DHE-RSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA256:
DHE-RSA-AES256-SHA256:
HIGH:
!aNULL:
!eNULL:
!EXPORT:
!DES:
!RC4:
!MD5:
!PSK:
!SRP:
!CAMELLIA

This default can be replaced entirely using the --tls-cipher-list command line switch. For instance, the following makes ECDHE-RSA-AES128-GCM-SHA256:!RC4 the default TLS cipher suite:

node --tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4"

The default can also be replaced on a per client or server basis using the ciphers option from tls.createSecureContext(), which is also available in tls.createServer(), tls.connect(), and when creating new tls.TLSSockets.

Consult OpenSSL cipher list format documentation for details on the format.

The default cipher suite included within Node.js has been carefully selected to reflect current security best practices and risk mitigation. Changing the default cipher suite can have a significant impact on the security of an application. The --tls-cipher-list switch and ciphers option should by used only if absolutely necessary.

The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for Perfect Forward Secrecy, while offering some backward compatibility.

128 bit AES is preferred over 192 and 256 bit AES in light of specific attacks affecting larger AES key sizes.

Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.

Class: tls.Server#

Added in: v0.3.2

The tls.Server class is a subclass of net.Server that accepts encrypted connections using TLS or SSL.

Event: 'newSession'#

Added in: v0.9.2

The 'newSession' event is emitted upon creation of a new TLS session. This may be used to store sessions in external storage. The listener callback is passed three arguments when called:

  • sessionId - The TLS session identifier
  • sessionData - The TLS session data
  • callback <Function> A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.

Listening for this event will have an effect only on connections established after the addition of the event listener.

Event: 'OCSPRequest'#

Added in: v0.11.13

The 'OCSPRequest' event is emitted when the client sends a certificate status request. The listener callback is passed three arguments when called:

  • certificate <Buffer> The server certificate
  • issuer <Buffer> The issuer's certificate
  • callback <Function> A callback function that must be invoked to provide the results of the OCSP request.

The server's current certificate can be parsed to obtain the OCSP URL and certificate ID; after obtaining an OCSP response, callback(null, resp) is then invoked, where resp is a Buffer instance containing the OCSP response. Both certificate and issuer are Buffer DER-representations of the primary and issuer's certificates. These can be used to obtain the OCSP certificate ID and OCSP endpoint URL.

Alternatively, callback(null, null) may be called, indicating that there was no OCSP response.

Calling callback(err) will result in a socket.destroy(err) call.

The typical flow of an OCSP Request is as follows:

  1. Client connects to the server and sends an 'OCSPRequest' (via the status info extension in ClientHello).
  2. Server receives the request and emits the 'OCSPRequest' event, calling the listener if registered.
  3. Server extracts the OCSP URL from either the certificate or issuer and performs an OCSP request to the CA.
  4. Server receives OCSPResponse from the CA and sends it back to the client via the callback argument
  5. Client validates the response and either destroys the socket or performs a handshake.

The issuer can be null if the certificate is either self-signed or the issuer is not in the root certificates list. (An issuer may be provided via the ca option when establishing the TLS connection.)

Listening for this event will have an effect only on connections established after the addition of the event listener.

An npm module like asn1.js may be used to parse the certificates.

Event: 'resumeSession'#

Added in: v0.9.2

The 'resumeSession' event is emitted when the client requests to resume a previous TLS session. The listener callback is passed two arguments when called:

  • sessionId - The TLS/SSL session identifier
  • callback <Function> A callback function to be called when the prior session has been recovered.

When called, the event listener may perform a lookup in external storage using the given sessionId and invoke callback(null, sessionData) once finished. If the session cannot be resumed (i.e., doesn't exist in storage) the callback may be invoked as callback(null, null). Calling callback(err) will terminate the incoming connection and destroy the socket.

Listening for this event will have an effect only on connections established after the addition of the event listener.

The following illustrates resuming a TLS session:

const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
  tlsSessionStore[id.toString('hex')] = data;
  cb();
});
server.on('resumeSession', (id, cb) => {
  cb(null, tlsSessionStore[id.toString('hex')] || null);
});

Event: 'secureConnection'#

Added in: v0.3.2

The 'secureConnection' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback is passed a single argument when called:

The tlsSocket.authorized property is a boolean indicating whether the client has been verified by one of the supplied Certificate Authorities for the server. If tlsSocket.authorized is false, then socket.authorizationError is set to describe how authorization failed. Note that depending on the settings of the TLS server, unauthorized connections may still be accepted.

The tlsSocket.alpnProtocol property is a string that contains the selected ALPN protocol. When ALPN has no selected protocol, tlsSocket.alpnProtocol equals false.

The tlsSocket.servername property is a string containing the server name requested via SNI.

Event: 'tlsClientError'#

Added in: v6.0.0

The 'tlsClientError' event is emitted when an error occurs before a secure connection is established. The listener callback is passed two arguments when called:

  • exception <Error> The Error object describing the error
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance from which the error originated.

server.addContext(hostname, context)#

Added in: v0.5.3
  • hostname <string> A SNI hostname or wildcard (e.g. '*')
  • context <Object> An object containing any of the possible properties from the tls.createSecureContext() options arguments (e.g. key, cert, ca, etc).

The server.addContext() method adds a secure context that will be used if the client request's SNI hostname matches the supplied hostname (or wildcard).

server.address()#

Added in: v0.6.0

Returns the bound address, the address family name, and port of the server as reported by the operating system. See net.Server.address() for more information.

server.close([callback])#

Added in: v0.3.2
  • callback <Function> An optional listener callback that will be registered to listen for the server instance's 'close' event.

The server.close() method stops the server from accepting new connections.

This function operates asynchronously. The 'close' event will be emitted when the server has no more open connections.

server.connections#

Added in: v0.3.2Deprecated since: v0.9.7
Stability: 0 - Deprecated: Use server.getConnections() instead.

Returns the current number of concurrent connections on the server.

server.getTicketKeys()#

Added in: v3.0.0

Returns a Buffer instance holding the keys currently used for encryption/decryption of the TLS Session Tickets

server.listen()#

Starts the server listening for encrypted connections. This method is identical to server.listen() from net.Server.

server.setTicketKeys(keys)#

Added in: v3.0.0

Updates the keys for encryption/decryption of the TLS Session Tickets.

The key's Buffer should be 48 bytes long. See ticketKeys option in tls.createServer for more information on how it is used.

Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.

Class: tls.TLSSocket#

Added in: v0.11.4

The tls.TLSSocket is a subclass of net.Socket that performs transparent encryption of written data and all required TLS negotiation.

Instances of tls.TLSSocket implement the duplex Stream interface.

Methods that return TLS connection metadata (e.g. tls.TLSSocket.getPeerCertificate() will only return data while the connection is open.

new tls.TLSSocket(socket[, options])#

History
VersionChanges
v5.0.0

ALPN options are supported now.

v0.11.4

Added in: v0.11.4

  • socket <net.Socket> | <stream.Duplex> On the server side, any Duplex stream. On the client side, any instance of net.Socket (for generic Duplex stream support on the client side, tls.connect() must be used).
  • options <Object>
    • isServer: The SSL/TLS protocol is asymmetrical, TLSSockets must know if they are to behave as a server or a client. If true the TLS socket will be instantiated as a server. Default: false.
    • server <net.Server> An optional net.Server instance.
    • requestCert: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (isServer is true) may optionally set requestCert to true to request a client certificate.
    • rejectUnauthorized: Optional, see tls.createServer()
    • ALPNProtocols: Optional, see tls.createServer()
    • SNICallback: Optional, see tls.createServer()
    • session <Buffer> An optional Buffer instance containing a TLS session.
    • requestOCSP <boolean> If true, specifies that the OCSP status request extension will be added to the client hello and an 'OCSPResponse' event will be emitted on the socket before establishing a secure communication
    • secureContext: Optional TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by passing the entire options object to tls.createSecureContext().
    • ...: Optional tls.createSecureContext() options that are used if the secureContext option is missing, otherwise they are ignored.

Construct a new tls.TLSSocket object from an existing TCP socket.

Event: 'OCSPResponse'#

Added in: v0.11.13

The 'OCSPResponse' event is emitted if the requestOCSP option was set when the tls.TLSSocket was created and an OCSP response has been received. The listener callback is passed a single argument when called:

  • response <Buffer> The server's OCSP response

Typically, the response is a digitally signed object from the server's CA that contains information about server's certificate revocation status.

Event: 'secureConnect'#

Added in: v0.11.4

The 'secureConnect' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback will be called regardless of whether or not the server's certificate has been authorized. It is the client's responsibility to check the tlsSocket.authorized property to determine if the server certificate was signed by one of the specified CAs. If tlsSocket.authorized === false, then the error can be found by examining the tlsSocket.authorizationError property. If ALPN was used, the tlsSocket.alpnProtocol property can be checked to determine the negotiated protocol.

tlsSocket.address()#

Added in: v0.11.4

Returns the bound address, the address family name, and port of the underlying socket as reported by the operating system. Returns an object with three properties, e.g. { port: 12346, family: 'IPv4', address: '127.0.0.1' }

tlsSocket.authorizationError#

Added in: v0.11.4

Returns the reason why the peer's certificate was not been verified. This property is set only when tlsSocket.authorized === false.

tlsSocket.authorized#

Added in: v0.11.4

Returns true if the peer certificate was signed by one of the CAs specified when creating the tls.TLSSocket instance, otherwise false.

tlsSocket.disableRenegotiation()#

Added in: v8.4.0

Disables TLS renegotiation for this TLSSocket instance. Once called, attempts to renegotiate will trigger an 'error' event on the TLSSocket.

tlsSocket.encrypted#

Added in: v0.11.4

Always returns true. This may be used to distinguish TLS sockets from regular net.Socket instances.

tlsSocket.getCipher()#

Added in: v0.11.4

Returns an object representing the cipher name. The version key is a legacy field which always contains the value 'TLSv1/SSLv3'.

For example: { name: 'AES256-SHA', version: 'TLSv1/SSLv3' }

See SSL_CIPHER_get_name() in https://www.openssl.org/docs/man1.0.2/ssl/SSL_CIPHER_get_name.html for more information.

tlsSocket.getEphemeralKeyInfo()#

Added in: v5.0.0

Returns an object representing the type, name, and size of parameter of an ephemeral key exchange in Perfect Forward Secrecy on a client connection. It returns an empty object when the key exchange is not ephemeral. As this is only supported on a client socket; null is returned if called on a server socket. The supported types are 'DH' and 'ECDH'. The name property is available only when type is 'ECDH'.

For Example: { type: 'ECDH', name: 'prime256v1', size: 256 }

tlsSocket.getFinished()#

Added in: v9.9.0
  • Returns: <Buffer> | <undefined> The latest Finished message that has been sent to the socket as part of a SSL/TLS handshake, or undefined if no Finished message has been sent yet.

As the Finished messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the SSL_get_finished routine in OpenSSL and may be used to implement the tls-unique channel binding from RFC 5929.

tlsSocket.getPeerCertificate([detailed])#

Added in: v0.11.4
  • detailed <boolean> Include the full certificate chain if true, otherwise include just the peer's certificate.

Returns an object representing the peer's certificate. The returned object has some properties corresponding to the fields of the certificate.

If the full certificate chain was requested, each certificate will include a issuerCertificate property containing an object representing its issuer's certificate.

For example:

{ subject:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuer:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuerCertificate:
   { ... another certificate, possibly with a .issuerCertificate ... },
  raw: < RAW DER buffer >,
  pubkey: < RAW DER buffer >,
  valid_from: 'Nov 11 09:52:22 2009 GMT',
  valid_to: 'Nov 6 09:52:22 2029 GMT',
  fingerprint: '2A:7A:C2:DD:E5:F9:CC:53:72:35:99:7A:02:5A:71:38:52:EC:8A:DF',
  fingerprint256: '2A:7A:C2:DD:E5:F9:CC:53:72:35:99:7A:02:5A:71:38:52:EC:8A:DF:00:11:22:33:44:55:66:77:88:99:AA:BB',
  serialNumber: 'B9B0D332A1AA5635' }

If the peer does not provide a certificate, an empty object will be returned.

tlsSocket.getPeerFinished()#

Added in: v9.9.0
  • Returns: <Buffer> | <undefined> The latest Finished message that is expected or has actually been received from the socket as part of a SSL/TLS handshake, or undefined if there is no Finished message so far.

As the Finished messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the SSL_get_peer_finished routine in OpenSSL and may be used to implement the tls-unique channel binding from RFC 5929.

tlsSocket.getProtocol()#

Added in: v5.7.0

Returns a string containing the negotiated SSL/TLS protocol version of the current connection. The value 'unknown' will be returned for connected sockets that have not completed the handshaking process. The value null will be returned for server sockets or disconnected client sockets.

Example responses include:

  • SSLv3
  • TLSv1
  • TLSv1.1
  • TLSv1.2
  • unknown

See https://www.openssl.org/docs/man1.0.2/ssl/SSL_get_version.html for more information.

tlsSocket.getSession()#

Added in: v0.11.4

Returns the ASN.1 encoded TLS session or undefined if no session was negotiated. Can be used to speed up handshake establishment when reconnecting to the server.

tlsSocket.getTLSTicket()#

Added in: v0.11.4

Returns the TLS session ticket or undefined if no session was negotiated.

This only works with client TLS sockets. Useful only for debugging, for session reuse provide session option to tls.connect().

tlsSocket.localAddress#

Added in: v0.11.4

Returns the string representation of the local IP address.

tlsSocket.localPort#

Added in: v0.11.4

Returns the numeric representation of the local port.

tlsSocket.remoteAddress#

Added in: v0.11.4

Returns the string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'.

tlsSocket.remoteFamily#

Added in: v0.11.4

Returns the string representation of the remote IP family. 'IPv4' or 'IPv6'.

tlsSocket.remotePort#

Added in: v0.11.4

Returns the numeric representation of the remote port. For example, 443.

tlsSocket.renegotiate(options, callback)#

Added in: v0.11.8
  • options <Object>
    • rejectUnauthorized <boolean> If not false, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails; err.code contains the OpenSSL error code. Default: true.
    • requestCert
  • callback <Function> A function that will be called when the renegotiation request has been completed.

The tlsSocket.renegotiate() method initiates a TLS renegotiation process. Upon completion, the callback function will be passed a single argument that is either an Error (if the request failed) or null.

This method can be used to request a peer's certificate after the secure connection has been established.

When running as the server, the socket will be destroyed with an error after handshakeTimeout timeout.

tlsSocket.setMaxSendFragment(size)#

Added in: v0.11.11
  • size <number> The maximum TLS fragment size. The maximum value is 16384. Default: 16384.

The tlsSocket.setMaxSendFragment() method sets the maximum TLS fragment size. Returns true if setting the limit succeeded; false otherwise.

Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.

tls.checkServerIdentity(host, cert)#

Added in: v0.8.4
  • host <string> The hostname to verify the certificate against
  • cert <Object> An object representing the peer's certificate. The returned object has some properties corresponding to the fields of the certificate.

Verifies the certificate cert is issued to host host.

Returns <Error> object, populating it with the reason, host, and cert on failure. On success, returns <undefined>.

This function can be overwritten by providing alternative function as part of the options.checkServerIdentity option passed to tls.connect(). The overwriting function can call tls.checkServerIdentity() of course, to augment the checks done with additional verification.

This function is only called if the certificate passed all other checks, such as being issued by trusted CA (options.ca).

The cert object contains the parsed certificate and will have a structure similar to:

{ subject:
   { OU: [ 'Domain Control Validated', 'PositiveSSL Wildcard' ],
     CN: '*.nodejs.org' },
  issuer:
   { C: 'GB',
     ST: 'Greater Manchester',
     L: 'Salford',
     O: 'COMODO CA Limited',
     CN: 'COMODO RSA Domain Validation Secure Server CA' },
  subjectaltname: 'DNS:*.nodejs.org, DNS:nodejs.org',
  infoAccess:
   { 'CA Issuers - URI':
      [ 'http://crt.comodoca.com/COMODORSADomainValidationSecureServerCA.crt' ],
     'OCSP - URI': [ 'http://ocsp.comodoca.com' ] },
  modulus: 'B56CE45CB740B09A13F64AC543B712FF9EE8E4C284B542A1708A27E82A8D151CA178153E12E6DDA15BF70FFD96CB8A88618641BDFCCA03527E665B70D779C8A349A6F88FD4EF6557180BD4C98192872BCFE3AF56E863C09DDD8BC1EC58DF9D94F914F0369102B2870BECFA1348A0838C9C49BD1C20124B442477572347047506B1FCD658A80D0C44BCC16BC5C5496CFE6E4A8428EF654CD3D8972BF6E5BFAD59C93006830B5EB1056BBB38B53D1464FA6E02BFDF2FF66CD949486F0775EC43034EC2602AEFBF1703AD221DAA2A88353C3B6A688EFE8387811F645CEED7B3FE46E1F8B9F59FAD028F349B9BC14211D5830994D055EEA3D547911E07A0ADDEB8A82B9188E58720D95CD478EEC9AF1F17BE8141BE80906F1A339445A7EB5B285F68039B0F294598A7D1C0005FC22B5271B0752F58CCDEF8C8FD856FB7AE21C80B8A2CE983AE94046E53EDE4CB89F42502D31B5360771C01C80155918637490550E3F555E2EE75CC8C636DDE3633CFEDD62E91BF0F7688273694EEEBA20C2FC9F14A2A435517BC1D7373922463409AB603295CEB0BB53787A334C9CA3CA8B30005C5A62FC0715083462E00719A8FA3ED0A9828C3871360A73F8B04A4FC1E71302844E9BB9940B77E745C9D91F226D71AFCAD4B113AAF68D92B24DDB4A2136B55A1CD1ADF39605B63CB639038ED0F4C987689866743A68769CC55847E4A06D6E2E3F1',
  exponent: '0x10001',
  pubkey: <Buffer ... >,
  valid_from: 'Aug 14 00:00:00 2017 GMT',
  valid_to: 'Nov 20 23:59:59 2019 GMT',
  fingerprint: '01:02:59:D9:C3:D2:0D:08:F7:82:4E:44:A4:B4:53:C5:E2:3A:87:4D',
  fingerprint256: '69:AE:1A:6A:D4:3D:C6:C1:1B:EA:C6:23:DE:BA:2A:14:62:62:93:5C:7A:EA:06:41:9B:0B:BC:87:CE:48:4E:02',
  ext_key_usage: [ '1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2' ],
  serialNumber: '66593D57F20CBC573E433381B5FEC280',
  raw: <Buffer ... > }

tls.connect(options[, callback])#

History
VersionChanges
v8.0.0

The lookup option is supported now.

v8.0.0

The ALPNProtocols option can be a Uint8Array now.

v5.3.0, v4.7.0

The secureContext option is supported now.

v5.0.0

ALPN options are supported now.

v0.11.3

Added in: v0.11.3

  • options <Object>
    • host <string> Host the client should connect to. Default: 'localhost'.
    • port <number> Port the client should connect to.
    • path <string> Creates unix socket connection to path. If this option is specified, host and port are ignored.
    • socket <stream.Duplex> Establish secure connection on a given socket rather than creating a new socket. Typically, this is an instance of net.Socket, but any Duplex stream is allowed. If this option is specified, path, host and port are ignored, except for certificate validation. Usually, a socket is already connected when passed to tls.connect(), but it can be connected later. Note that connection/disconnection/destruction of socket is the user's responsibility, calling tls.connect() will not cause net.connect() to be called.
    • rejectUnauthorized <boolean> If not false, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails; err.code contains the OpenSSL error code. Default: true.
    • ALPNProtocols: <string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings, Buffers or Uint8Arrays, or a single Buffer or Uint8Array containing the supported ALPN protocols. Buffers should have the format [len][name][len][name]... e.g. 0x05hello0x05world, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. ['hello', 'world'].
    • servername: <string> Server name for the SNI (Server Name Indication) TLS extension.
    • checkServerIdentity(servername, cert) <Function> A callback function to be used (instead of the builtin tls.checkServerIdentity() function) when checking the server's hostname (or the provided servername when explicitly set) against the certificate. This should return an <Error> if verification fails. The method should return undefined if the servername and cert are verified.
    • session <Buffer> A Buffer instance, containing TLS session.
    • minDHSize <number> Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less than minDHSize, the TLS connection is destroyed and an error is thrown. Default: 1024.
    • secureContext: Optional TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by passing the entire options object to tls.createSecureContext().
    • lookup: <Function> Custom lookup function. Default: dns.lookup().
    • ...: Optional tls.createSecureContext() options that are used if the secureContext option is missing, otherwise they are ignored.
  • callback <Function>

The callback function, if specified, will be added as a listener for the 'secureConnect' event.

tls.connect() returns a tls.TLSSocket object.

The following implements a simple "echo server" example:

const tls = require('tls');
const fs = require('fs');

const options = {
  // Necessary only if using the client certificate authentication
  key: fs.readFileSync('client-key.pem'),
  cert: fs.readFileSync('client-cert.pem'),

  // Necessary only if the server uses the self-signed certificate
  ca: [ fs.readFileSync('server-cert.pem') ]
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  server.close();
});

Or

const tls = require('tls');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('client.pfx')
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  server.close();
});

tls.connect(path[, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that path can be provided as an argument instead of an option.

A path option, if specified, will take precedence over the path argument.

tls.connect(port[, host][, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that port and host can be provided as arguments instead of options.

A port or host option, if specified, will take precedence over any port or host argument.

tls.createSecureContext(options)#

History
VersionChanges
v9.3.0

The options parameter can now include clientCertEngine.

v7.3.0

If the key option is an array, individual entries do not need a passphrase property anymore. Array entries can also just be strings or Buffers now.

v5.2.0

The ca option can now be a single string containing multiple CA certificates.

v0.11.13

Added in: v0.11.13

  • options <Object>
    • pfx <string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Optional PFX or PKCS12 encoded private key and certificate chain. pfx is an alternative to providing key and cert individually. PFX is usually encrypted, if it is, passphrase will be used to decrypt it. Multiple PFX can be provided either as an array of unencrypted PFX buffers, or an array of objects in the form {buf: <string|buffer>[, passphrase: <string>]}. The object form can only occur in an array. object.passphrase is optional. Encrypted PFX will be decrypted with object.passphrase if provided, or options.passphrase if it is not.
    • key <string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Optional private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted with options.passphrase. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form {pem: <string|buffer>[, passphrase: <string>]}. The object form can only occur in an array. object.passphrase is optional. Encrypted keys will be decrypted with object.passphrase if provided, or options.passphrase if it is not.
    • passphrase <string> Optional shared passphrase used for a single private key and/or a PFX.
    • cert <string> | <string[]> | <Buffer> | <Buffer[]> Optional cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided private key, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, see ca). When providing multiple cert chains, they do not have to be in the same order as their private keys in key. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.
    • ca <string> | <string[]> | <Buffer> | <Buffer[]> Optionally override the trusted CA certificates. Default is to trust the well-known CAs curated by Mozilla. Mozilla's CAs are completely replaced when CAs are explicitly specified using this option. The value can be a string or Buffer, or an Array of strings and/or Buffers. Any string or Buffer can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use the ca option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided.
    • ciphers <string> Optional cipher suite specification, replacing the default. For more information, see modifying the default cipher suite.
    • honorCipherOrder <boolean> Attempt to use the server's cipher suite preferences instead of the client's. When true, causes SSL_OP_CIPHER_SERVER_PREFERENCE to be set in secureOptions, see OpenSSL Options for more information.
    • ecdhCurve <string> A string describing a named curve or a colon separated list of curve NIDs or names, for example P-521:P-384:P-256, to use for ECDH key agreement, or false to disable ECDH. Set to auto to select the curve automatically. Use crypto.getCurves() to obtain a list of available curve names. On recent releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve. Default: tls.DEFAULT_ECDH_CURVE.
    • clientCertEngine <string> Optional name of an OpenSSL engine which can provide the client certificate.
    • crl <string> | <string[]> | <Buffer> | <Buffer[]> Optional PEM formatted CRLs (Certificate Revocation Lists).
    • dhparam <string> | <Buffer> Diffie Hellman parameters, required for Perfect Forward Secrecy. Use openssl dhparam to create the parameters. The key length must be greater than or equal to 1024 bits, otherwise an error will be thrown. It is strongly recommended to use 2048 bits or larger for stronger security. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available.
    • secureOptions <number> Optionally affect the OpenSSL protocol behavior, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of the SSL_OP_* options from OpenSSL Options.
    • secureProtocol <string> Optional SSL method to use. The possible values are listed as SSL_METHODS, use the function names as strings. For example, 'SSLv3_method' to force SSL version 3. Default: 'SSLv23_method'.
    • sessionIdContext <string> Optional opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients.

tls.createServer() sets the default value of the honorCipherOrder option to true, other APIs that create secure contexts leave it unset.

tls.createServer() uses a 128 bit truncated SHA1 hash value generated from process.argv as the default value of the sessionIdContext option, other APIs that create secure contexts have no default value.

The tls.createSecureContext() method creates a credentials object.

A key is required for ciphers that make use of certificates. Either key or pfx can be used to provide it.

If the 'ca' option is not given, then Node.js will use the default publicly trusted list of CAs as given in https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins/certdata.txt.

tls.createServer([options][, secureConnectionListener])#

History
VersionChanges
v9.3.0

The options parameter can now include clientCertEngine.

v8.0.0

The ALPNProtocols option can be a Uint8Array now.

v5.0.0

ALPN options are supported now.

v0.3.2

Added in: v0.3.2

  • options <Object>
    • clientCertEngine <string> Optional name of an OpenSSL engine which can provide the client certificate.
    • handshakeTimeout <number> Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. A 'tlsClientError' is emitted on the tls.Server object whenever a handshake times out. Default: 120000 (120 seconds).
    • requestCert <boolean> If true the server will request a certificate from clients that connect and attempt to verify that certificate. Default: false.
    • rejectUnauthorized <boolean> If not false the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect if requestCert is true. Default: true.
    • ALPNProtocols: <string[]> | <Buffer[]> | <Uint8Array[]> | <Buffer> | <Uint8Array> An array of strings, Buffers or Uint8Arrays, or a single Buffer or Uint8Array containing the supported ALPN protocols. Buffers should have the format [len][name][len][name]... e.g. 0x05hello0x05world, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. ['hello', 'world']. (Protocols should be ordered by their priority.)
    • SNICallback(servername, cb) <Function> A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called: servername and cb. SNICallback should invoke cb(null, ctx), where ctx is a SecureContext instance. (tls.createSecureContext(...) can be used to get a proper SecureContext.) If SNICallback wasn't provided the default callback with high-level API will be used (see below).
    • sessionTimeout <number> An integer specifying the number of seconds after which the TLS session identifiers and TLS session tickets created by the server will time out. See SSL_CTX_set_timeout for more details.
    • ticketKeys: A 48-byte Buffer instance consisting of a 16-byte prefix, a 16-byte HMAC key, and a 16-byte AES key. This can be used to accept TLS session tickets on multiple instances of the TLS server.
    • ...: Any tls.createSecureContext() options can be provided. For servers, the identity options (pfx or key/cert) are usually required.
  • secureConnectionListener <Function>

Creates a new tls.Server. The secureConnectionListener, if provided, is automatically set as a listener for the 'secureConnection' event.

The ticketKeys options is automatically shared between cluster module workers.

The following illustrates a simple echo server:

const tls = require('tls');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),

  // This is necessary only if using the client certificate authentication.
  requestCert: true,

  // This is necessary only if the client uses the self-signed certificate.
  ca: [ fs.readFileSync('client-cert.pem') ]
};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});

Or

const tls = require('tls');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('server.pfx'),

  // This is necessary only if using the client certificate authentication.
  requestCert: true,

};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});

This server can be tested by connecting to it using openssl s_client:

openssl s_client -connect 127.0.0.1:8000

tls.getCiphers()#

Added in: v0.10.2

Returns an array with the names of the supported SSL ciphers.

For example:

console.log(tls.getCiphers()); // ['AES128-SHA', 'AES256-SHA', ...]

tls.DEFAULT_ECDH_CURVE#

Added in: v0.11.13

The default curve name to use for ECDH key agreement in a tls server. The default value is 'auto'. See tls.createSecureContext() for further information.

Deprecated APIs#

Class: CryptoStream#

Added in: v0.3.4Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use tls.TLSSocket instead.

The tls.CryptoStream class represents a stream of encrypted data. This class has been deprecated and should no longer be used.

cryptoStream.bytesWritten#

Added in: v0.3.4Deprecated since: v0.11.3

The cryptoStream.bytesWritten property returns the total number of bytes written to the underlying socket including the bytes required for the implementation of the TLS protocol.

Class: SecurePair#

Added in: v0.3.2Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use tls.TLSSocket instead.

Returned by tls.createSecurePair().

Event: 'secure'#

Added in: v0.3.2Deprecated since: v0.11.3

The 'secure' event is emitted by the SecurePair object once a secure connection has been established.

As with checking for the server secureConnection event, pair.cleartext.authorized should be inspected to confirm whether the certificate used is properly authorized.

tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])#

History
VersionChanges
v5.0.0

ALPN options are supported now.

v0.11.3

Deprecated since: v0.11.3

v0.3.2

Added in: v0.3.2

Stability: 0 - Deprecated: Use tls.TLSSocket instead.
  • context <Object> A secure context object as returned by tls.createSecureContext()
  • isServer <boolean> true to specify that this TLS connection should be opened as a server.
  • requestCert <boolean> true to specify whether a server should request a certificate from a connecting client. Only applies when isServer is true.
  • rejectUnauthorized <boolean> If not false a server automatically reject clients with invalid certificates. Only applies when isServer is true.
  • options
    • secureContext: An optional TLS context object from tls.createSecureContext()
    • isServer: If true the TLS socket will be instantiated in server-mode. Default: false.
    • server <net.Server> An optional net.Server instance
    • requestCert: Optional, see tls.createServer()
    • rejectUnauthorized: Optional, see tls.createServer()
    • ALPNProtocols: Optional, see tls.createServer()
    • SNICallback: Optional, see tls.createServer()
    • session <Buffer> An optional Buffer instance containing a TLS session.
    • requestOCSP <boolean> If true, specifies that the OCSP status request extension will be added to the client hello and an 'OCSPResponse' event will be emitted on the socket before establishing a secure communication

Creates a new secure pair object with two streams, one of which reads and writes the encrypted data and the other of which reads and writes the cleartext data. Generally, the encrypted stream is piped to/from an incoming encrypted data stream and the cleartext one is used as a replacement for the initial encrypted stream.

tls.createSecurePair() returns a tls.SecurePair object with cleartext and encrypted stream properties.

Using cleartext has the same API as tls.TLSSocket.

The tls.createSecurePair() method is now deprecated in favor of tls.TLSSocket(). For example, the code:

pair = tls.createSecurePair(/* ... */);
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);

can be replaced by:

secure_socket = tls.TLSSocket(socket, options);

where secure_socket has the same API as pair.cleartext.

Tracing#

Trace Event provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

Tracing can be enabled by passing the --trace-events-enabled flag when starting a Node.js application.

The set of categories for which traces are recorded can be specified using the --trace-event-categories flag followed by a list of comma separated category names.

The available categories are:

  • node
  • node.async_hooks - Enables capture of detailed async_hooks trace data.
  • node.bootstrap - Enables capture of Node.js bootstrap milestones.
  • node.perf - Enables capture of Performance API measurements.
    • node.perf.usertiming - Enables capture of only Performance API User Timing measures and marks.
    • node.perf.timerify - Enables capture of only Performance API timerify measurements.
  • v8

By default the node, node.async_hooks, and v8 categories are enabled.

node --trace-events-enabled --trace-event-categories v8,node,node.async_hooks server.js

Running Node.js with tracing enabled will produce log files that can be opened in the chrome://tracing tab of Chrome.

The logging file is by default called node_trace.${rotation}.log, where ${rotation} is an incrementing log-rotation id. The filepath pattern can be specified with --trace-event-file-pattern that accepts a template string that supports ${rotation} and ${pid}. For example:

node --trace-events-enabled --trace-event-file-pattern '${pid}-${rotation}.log' server.js

Starting with Node.js 10.0.0, the tracing system uses the same time source as the one used by process.hrtime() however the trace-event timestamps are expressed in microseconds, unlike process.hrtime() which returns nanoseconds.

TTY#

Stability: 2 - Stable

The tty module provides the tty.ReadStream and tty.WriteStream classes. In most cases, it will not be necessary or possible to use this module directly. However, it can be accessed using:

const tty = require('tty');

When Node.js detects that it is being run with a text terminal ("TTY") attached, process.stdin will, by default, be initialized as an instance of tty.ReadStream and both process.stdout and process.stderr will, by default be instances of tty.WriteStream. The preferred method of determining whether Node.js is being run within a TTY context is to check that the value of the process.stdout.isTTY property is true:

$ node -p -e "Boolean(process.stdout.isTTY)"
true
$ node -p -e "Boolean(process.stdout.isTTY)" | cat
false

In most cases, there should be little to no reason for an application to manually create instances of the tty.ReadStream and tty.WriteStream classes.

Class: tty.ReadStream#

Added in: v0.5.8

The tty.ReadStream class is a subclass of net.Socket that represents the readable side of a TTY. In normal circumstances process.stdin will be the only tty.ReadStream instance in a Node.js process and there should be no reason to create additional instances.

readStream.isRaw#

Added in: v0.7.7

A boolean that is true if the TTY is currently configured to operate as a raw device. Defaults to false.

readStream.isTTY#

Added in: v0.5.8

A boolean that is always true for tty.ReadStream instances.

readStream.setRawMode(mode)#

Added in: v0.7.7

Allows configuration of tty.ReadStream so that it operates as a raw device.

When in raw mode, input is always available character-by-character, not including modifiers. Additionally, all special processing of characters by the terminal is disabled, including echoing input characters. Note that CTRL+C will no longer cause a SIGINT when in this mode.

  • mode <boolean> If true, configures the tty.ReadStream to operate as a raw device. If false, configures the tty.ReadStream to operate in its default mode. The readStream.isRaw property will be set to the resulting mode.

Class: tty.WriteStream#

Added in: v0.5.8

The tty.WriteStream class is a subclass of net.Socket that represents the writable side of a TTY. In normal circumstances, process.stdout and process.stderr will be the only tty.WriteStream instances created for a Node.js process and there should be no reason to create additional instances.

Event: 'resize'#

Added in: v0.7.7

The 'resize' event is emitted whenever either of the writeStream.columns or writeStream.rows properties have changed. No arguments are passed to the listener callback when called.

process.stdout.on('resize', () => {
  console.log('screen size has changed!');
  console.log(`${process.stdout.columns}x${process.stdout.rows}`);
});

writeStream.columns#

Added in: v0.7.7

A number specifying the number of columns the TTY currently has. This property is updated whenever the 'resize' event is emitted.

writeStream.isTTY#

Added in: v0.5.8

A boolean that is always true.

writeStream.rows#

Added in: v0.7.7

A number specifying the number of rows the TTY currently has. This property is updated whenever the 'resize' event is emitted.

writeStream.getColorDepth([env])#

Added in: v9.9.0
  • env <Object> A object containing the environment variables to check. Default: process.env.
  • Returns: <number>

Returns:

  • 1 for 2,
  • 4 for 16,
  • 8 for 256,
  • 24 for 16,777,216 colors supported.

Use this to determine what colors the terminal supports. Due to the nature of colors in terminals it is possible to either have false positives or false negatives. It depends on process information and the environment variables that may lie about what terminal is used. To enforce a specific behavior without relying on process.env it is possible to pass in an object with different settings.

Use the NODE_DISABLE_COLORS environment variable to enforce this function to always return 1.

tty.isatty(fd)#

Added in: v0.5.8

The tty.isatty() method returns true if the given fd is associated with a TTY and false if it is not, including whenever fd is not a non-negative integer.

UDP / Datagram Sockets#

Stability: 2 - Stable

The dgram module provides an implementation of UDP Datagram sockets.

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

Class: dgram.Socket#

Added in: v0.1.99

The dgram.Socket object is an EventEmitter that encapsulates the datagram functionality.

New instances of dgram.Socket are created using dgram.createSocket(). The new keyword is not to be used to create dgram.Socket instances.

Event: 'close'#

Added in: v0.1.99

The 'close' event is emitted after a socket is closed with close(). Once triggered, no new 'message' events will be emitted on this socket.

Event: 'error'#

Added in: v0.1.99

The 'error' event is emitted whenever any error occurs. The event handler function is passed a single Error object.

Event: 'listening'#

Added in: v0.1.99

The 'listening' event is emitted whenever a socket begins listening for datagram messages. This occurs as soon as UDP sockets are created.

Event: 'message'#

Added in: v0.1.99

The 'message' event is emitted when a new datagram is available on a socket. The event handler function is passed two arguments: msg and rinfo.

socket.addMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Tells the kernel to join a multicast group at the given multicastAddress and multicastInterface using the IP_ADD_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call addMembership multiple times, once per interface.

socket.address()#

Added in: v0.1.99

Returns an object containing the address information for a socket. For UDP sockets, this object will contain address, family and port properties.

socket.bind([port][, address][, callback])#

Added in: v0.1.99

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

Example of a UDP server listening on port 41234:

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

socket.bind(options[, callback])#

Added in: v0.11.14

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address that are passed as properties of an options object passed as the first argument. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

The options object may contain an additional exclusive property that is use when using dgram.Socket objects with the cluster module. When exclusive is set to false (the default), cluster workers will use the same underlying socket handle allowing connection handling duties to be shared. When exclusive is true, however, the handle is not shared and attempted port sharing results in an error.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

An example socket listening on an exclusive port is shown below.

socket.bind({
  address: 'localhost',
  port: 8000,
  exclusive: true
});

socket.close([callback])#

Added in: v0.1.99

Close the underlying socket and stop listening for data on it. If a callback is provided, it is added as a listener for the 'close' event.

socket.dropMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Instructs the kernel to leave a multicast group at multicastAddress using the IP_DROP_MEMBERSHIP socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If multicastInterface is not specified, the operating system will attempt to drop membership on all valid interfaces.

socket.getRecvBufferSize()#

Added in: v8.7.0
  • Returns: <number> the SO_RCVBUF socket receive buffer size in bytes.

socket.getSendBufferSize()#

Added in: v8.7.0
  • Returns: <number> the SO_SNDBUF socket send buffer size in bytes.

socket.ref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active. The socket.ref() method adds the socket back to the reference counting and restores the default behavior.

Calling socket.ref() multiples times will have no additional effect.

The socket.ref() method returns a reference to the socket so calls can be chained.

socket.send(msg, [offset, length,] port [, address] [, callback])#

History
VersionChanges
v8.0.0

The msg parameter can be an Uint8Array now.

v8.0.0

The address parameter is always optional now.

v6.0.0

On success, callback will now be called with an error argument of null rather than 0.

v5.7.0

The msg parameter can be an array now. Also, the offset and length parameters are optional now.

v0.1.99

Added in: v0.1.99

Broadcasts a datagram on the socket. The destination port and address must be specified.

The msg argument contains the message to be sent. Depending on its type, different behavior can apply. If msg is a Buffer or Uint8Array, the offset and length specify the offset within the Buffer where the message begins and the number of bytes in the message, respectively. If msg is a String, then it is automatically converted to a Buffer with 'utf8' encoding. With messages that contain multi-byte characters, offset and length will be calculated with respect to byte length and not the character position. If msg is an array, offset and length must not be specified.

The address argument is a string. If the value of address is a host name, DNS will be used to resolve the address of the host. If address is not provided or otherwise falsy, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets) will be used by default.

If the socket has not been previously bound with a call to bind, the socket is assigned a random port number and is bound to the "all interfaces" address ('0.0.0.0' for udp4 sockets, '::0' for udp6 sockets.)

An optional callback function may be specified to as a way of reporting DNS errors or for determining when it is safe to reuse the buf object. Note that DNS lookups delay the time to send for at least one tick of the Node.js event loop.

The only way to know for sure that the datagram has been sent is by using a callback. If an error occurs and a callback is given, the error will be passed as the first argument to the callback. If a callback is not given, the error is emitted as an 'error' event on the socket object.

Offset and length are optional but both must be set if either are used. They are supported only when the first argument is a Buffer or Uint8Array.

Example of sending a UDP packet to a port on localhost;

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
  client.close();
});

Example of sending a UDP packet composed of multiple buffers to a port on 127.0.0.1;

const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
  client.close();
});

Sending multiple buffers might be faster or slower depending on the application and operating system. It is important to run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.

A Note about UDP datagram size

The maximum size of an IPv4/v6 datagram depends on the MTU (Maximum Transmission Unit) and on the Payload Length field size.

  • The Payload Length field is 16 bits wide, which means that a normal payload exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.

  • The MTU is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimum MTU of 68 octets, while the recommended MTU for IPv4 is 576 (typically recommended as the MTU for dial-up type applications), whether they arrive whole or in fragments.

    For IPv6, the minimum MTU is 1280 octets, however, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimum MTU of 1500.

It is impossible to know in advance the MTU of each link through which a packet might travel. Sending a datagram greater than the receiver MTU will not work because the packet will get silently dropped without informing the source that the data did not reach its intended recipient.

socket.setBroadcast(flag)#

Added in: v0.6.9

Sets or clears the SO_BROADCAST socket option. When set to true, UDP packets may be sent to a local interface's broadcast address.

socket.setMulticastInterface(multicastInterface)#

Added in: v8.6.0

Note: All references to scope in this section are referring to IPv6 Zone Indices, which are defined by RFC 4007. In string form, an IP with a scope index is written as 'IP%scope' where scope is an interface name or interface number.

Sets the default outgoing multicast interface of the socket to a chosen interface or back to system interface selection. The multicastInterface must be a valid string representation of an IP from the socket's family.

For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.

For IPv6 sockets, multicastInterface should include a scope to indicate the interface as in the examples that follow. In IPv6, individual send calls can also use explicit scope in addresses, so only packets sent to a multicast address without specifying an explicit scope are affected by the most recent successful use of this call.

Examples: IPv6 Outgoing Multicast Interface#

On most systems, where scope format uses the interface name:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%eth1');
});

On Windows, where scope format uses an interface number:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%2');
});

Example: IPv4 Outgoing Multicast Interface#

All systems use an IP of the host on the desired physical interface:

const socket = dgram.createSocket('udp4');

socket.bind(1234, () => {
  socket.setMulticastInterface('10.0.0.2');
});

Call Results#

A call on a socket that is not ready to send or no longer open may throw a Not running Error.

If multicastInterface can not be parsed into an IP then an EINVAL System Error is thrown.

On IPv4, if multicastInterface is a valid address but does not match any interface, or if the address does not match the family then a System Error such as EADDRNOTAVAIL or EPROTONOSUP is thrown.

On IPv6, most errors with specifying or omitting scope will result in the socket continuing to use (or returning to) the system's default interface selection.

A socket's address family's ANY address (IPv4 '0.0.0.0' or IPv6 '::') can be used to return control of the sockets default outgoing interface to the system for future multicast packets.

socket.setMulticastLoopback(flag)#

Added in: v0.3.8

Sets or clears the IP_MULTICAST_LOOP socket option. When set to true, multicast packets will also be received on the local interface.

socket.setMulticastTTL(ttl)#

Added in: v0.3.8

Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through, specifically for multicast traffic. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.

The argument passed to socket.setMulticastTTL() is a number of hops between 0 and 255. The default on most systems is 1 but can vary.

socket.setRecvBufferSize(size)#

Added in: v8.7.0

Sets the SO_RCVBUF socket option. Sets the maximum socket receive buffer in bytes.

socket.setSendBufferSize(size)#

Added in: v8.7.0

Sets the SO_SNDBUF socket option. Sets the maximum socket send buffer in bytes.

socket.setTTL(ttl)#

Added in: v0.1.101

Sets the IP_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded. Changing TTL values is typically done for network probes or when multicasting.

The argument to socket.setTTL() is a number of hops between 1 and 255. The default on most systems is 64 but can vary.

socket.unref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active, allowing the process to exit even if the socket is still listening.

Calling socket.unref() multiple times will have no addition effect.

The socket.unref() method returns a reference to the socket so calls can be chained.

Change to asynchronous socket.bind() behavior#

As of Node.js v0.10, dgram.Socket#bind() changed to an asynchronous execution model. Legacy code that assumes synchronous behavior, as in the following example:

const s = dgram.createSocket('udp4');
s.bind(1234);
s.addMembership('224.0.0.114');

Must be changed to pass a callback function to the dgram.Socket#bind() function:

const s = dgram.createSocket('udp4');
s.bind(1234, () => {
  s.addMembership('224.0.0.114');
});

dgram module functions#

dgram.createSocket(options[, callback])#

History
VersionChanges
v8.7.0

The recvBufferSize and sendBufferSize options are supported now.

v8.6.0

The lookup option is supported.

v0.11.13

Added in: v0.11.13

  • options <Object> Available options are:
    • type <string> The family of socket. Must be either 'udp4' or 'udp6'. Required.
    • reuseAddr <boolean> When true socket.bind() will reuse the address, even if another process has already bound a socket on it. Default: false.
    • recvBufferSize <number> - Sets the SO_RCVBUF socket value.
    • sendBufferSize <number> - Sets the SO_SNDBUF socket value.
    • lookup <Function> Custom lookup function. Default: dns.lookup().
  • callback <Function> Attached as a listener for 'message' events. Optional.
  • Returns: <dgram.Socket>

Creates a dgram.Socket object. Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

dgram.createSocket(type[, callback])#

Added in: v0.1.99

Creates a dgram.Socket object of the specified type. The type argument can be either udp4 or udp6. An optional callback function can be passed which is added as a listener for 'message' events.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

URL#

Stability: 2 - Stable

The url module provides utilities for URL resolution and parsing. It can be accessed using:

const url = require('url');

URL Strings and URL Objects#

A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.

The url module provides two APIs for working with URLs: a legacy API that is Node.js specific, and a newer API that implements the same WHATWG URL Standard used by web browsers.

While the Legacy API has not been deprecated, it is maintained solely for backwards compatibility with existing applications. New application code should use the WHATWG API.

A comparison between the WHATWG and Legacy APIs is provided below. Above the URL 'http://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash', properties of an object returned by the legacy url.parse() are shown. Below it are properties of a WHATWG URL object.

WHATWG URL's origin property includes protocol and host, but not username or password.

┌─────────────────────────────────────────────────────────────────────────────────────────────┐
│                                            href                                             │
├──────────┬──┬─────────────────────┬─────────────────────┬───────────────────────────┬───────┤
│ protocol │  │        auth         │        host         │           path            │ hash  │
│          │  │                     ├──────────────┬──────┼──────────┬────────────────┤       │
│          │  │                     │   hostname   │ port │ pathname │     search     │       │
│          │  │                     │              │      │          ├─┬──────────────┤       │
│          │  │                     │              │      │          │ │    query     │       │
"  https:   //    user   :   pass   @ sub.host.com : 8080   /p/a/t/h  ?  query=string   #hash "
│          │  │          │          │   hostname   │ port │          │                │       │
│          │  │          │          ├──────────────┴──────┤          │                │       │
│ protocol │  │ username │ password │        host         │          │                │       │
├──────────┴──┼──────────┴──────────┼─────────────────────┤          │                │       │
│   origin    │                     │       origin        │ pathname │     search     │ hash  │
├─────────────┴─────────────────────┴─────────────────────┴──────────┴────────────────┴───────┤
│                                            href                                             │
└─────────────────────────────────────────────────────────────────────────────────────────────┘
(all spaces in the "" line should be ignored — they are purely for formatting)

Parsing the URL string using the WHATWG API:

const myURL =
  new URL('https://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash');

Parsing the URL string using the Legacy API:

const url = require('url');
const myURL =
  url.parse('https://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash');

The WHATWG URL API#

Class: URL#

History
VersionChanges
REPLACEME

The class is now available on the global object.

v7.0.0

Added in: v7.0.0

Browser-compatible URL class, implemented by following the WHATWG URL Standard. Examples of parsed URLs may be found in the Standard itself. The URL class is also available on the global object.

In accordance with browser conventions, all properties of URL objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself. Thus, unlike legacy urlObjects, using the delete keyword on any properties of URL objects (e.g. delete myURL.protocol, delete myURL.pathname, etc) has no effect but will still return true.

Constructor: new URL(input[, base])#

  • input <string> The input URL to parse
  • base <string> | <URL> The base URL to resolve against if the input is not absolute.

Creates a new URL object by parsing the input relative to the base. If base is passed as a string, it will be parsed equivalent to new URL(base).

const myURL = new URL('/foo', 'https://example.org/');
// https://example.org/foo

A TypeError will be thrown if the input or base are not valid URLs. Note that an effort will be made to coerce the given values into strings. For instance:

const myURL = new URL({ toString: () => 'https://example.org/' });
// https://example.org/

Unicode characters appearing within the hostname of input will be automatically converted to ASCII using the Punycode algorithm.

const myURL = new URL('https://你好你好');
// https://xn--6qqa088eba/

This feature is only available if the node executable was compiled with ICU enabled. If not, the domain names are passed through unchanged.

url.hash#

Gets and sets the fragment portion of the URL.

const myURL = new URL('https://example.org/foo#bar');
console.log(myURL.hash);
// Prints #bar

myURL.hash = 'baz';
console.log(myURL.href);
// Prints https://example.org/foo#baz

Invalid URL characters included in the value assigned to the hash property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.host#

Gets and sets the host portion of the URL.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.host);
// Prints example.org:81

myURL.host = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:82/foo

Invalid host values assigned to the host property are ignored.

url.hostname#

Gets and sets the hostname portion of the URL. The key difference between url.host and url.hostname is that url.hostname does not include the port.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.hostname);
// Prints example.org

myURL.hostname = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:81/foo

Invalid hostname values assigned to the hostname property are ignored.

url.href#

Gets and sets the serialized URL.

const myURL = new URL('https://example.org/foo');
console.log(myURL.href);
// Prints https://example.org/foo

myURL.href = 'https://example.com/bar';
console.log(myURL.href);
// Prints https://example.com/bar

Getting the value of the href property is equivalent to calling url.toString().

Setting the value of this property to a new value is equivalent to creating a new URL object using new URL(value). Each of the URL object's properties will be modified.

If the value assigned to the href property is not a valid URL, a TypeError will be thrown.

url.origin#

Gets the read-only serialization of the URL's origin.

const myURL = new URL('https://example.org/foo/bar?baz');
console.log(myURL.origin);
// Prints https://example.org

const idnURL = new URL('https://你好你好');
console.log(idnURL.origin);
// Prints https://xn--6qqa088eba

console.log(idnURL.hostname);
// Prints xn--6qqa088eba

url.password#

Gets and sets the password portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.password);
// Prints xyz

myURL.password = '123';
console.log(myURL.href);
// Prints https://abc:123@example.com

Invalid URL characters included in the value assigned to the password property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.pathname#

Gets and sets the path portion of the URL.

const myURL = new URL('https://example.org/abc/xyz?123');
console.log(myURL.pathname);
// Prints /abc/xyz

myURL.pathname = '/abcdef';
console.log(myURL.href);
// Prints https://example.org/abcdef?123

Invalid URL characters included in the value assigned to the pathname property are percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.port#

Gets and sets the port portion of the URL.

const myURL = new URL('https://example.org:8888');
console.log(myURL.port);
// Prints 8888

// Default ports are automatically transformed to the empty string
// (HTTPS protocol's default port is 443)
myURL.port = '443';
console.log(myURL.port);
// Prints the empty string
console.log(myURL.href);
// Prints https://example.org/

myURL.port = 1234;
console.log(myURL.port);
// Prints 1234
console.log(myURL.href);
// Prints https://example.org:1234/

// Completely invalid port strings are ignored
myURL.port = 'abcd';
console.log(myURL.port);
// Prints 1234

// Leading numbers are treated as a port number
myURL.port = '5678abcd';
console.log(myURL.port);
// Prints 5678

// Non-integers are truncated
myURL.port = 1234.5678;
console.log(myURL.port);
// Prints 1234

// Out-of-range numbers are ignored
myURL.port = 1e10;
console.log(myURL.port);
// Prints 1234

The port value may be set as either a number or as a String containing a number in the range 0 to 65535 (inclusive). Setting the value to the default port of the URL objects given protocol will result in the port value becoming the empty string ('').

If an invalid string is assigned to the port property, but it begins with a number, the leading number is assigned to port. Otherwise, or if the number lies outside the range denoted above, it is ignored.

url.protocol#

Gets and sets the protocol portion of the URL.

const myURL = new URL('https://example.org');
console.log(myURL.protocol);
// Prints https:

myURL.protocol = 'ftp';
console.log(myURL.href);
// Prints ftp://example.org/

Invalid URL protocol values assigned to the protocol property are ignored.

url.search#

Gets and sets the serialized query portion of the URL.

const myURL = new URL('https://example.org/abc?123');
console.log(myURL.search);
// Prints ?123

myURL.search = 'abc=xyz';
console.log(myURL.href);
// Prints https://example.org/abc?abc=xyz

Any invalid URL characters appearing in the value assigned the search property will be percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.searchParams#

Gets the URLSearchParams object representing the query parameters of the URL. This property is read-only; to replace the entirety of query parameters of the URL, use the url.search setter. See URLSearchParams documentation for details.

url.username#

Gets and sets the username portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.username);
// Prints abc

myURL.username = '123';
console.log(myURL.href);
// Prints https://123:xyz@example.com/

Any invalid URL characters appearing in the value assigned the username property will be percent-encoded. Note that the selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.toString()#

The toString() method on the URL object returns the serialized URL. The value returned is equivalent to that of url.href and url.toJSON().

Because of the need for standard compliance, this method does not allow users to customize the serialization process of the URL. For more flexibility, require('url').format() method might be of interest.

url.toJSON()#

The toJSON() method on the URL object returns the serialized URL. The value returned is equivalent to that of url.href and url.toString().

This method is automatically called when an URL object is serialized with JSON.stringify().

const myURLs = [
  new URL('https://www.example.com'),
  new URL('https://test.example.org')
];
console.log(JSON.stringify(myURLs));
// Prints ["https://www.example.com/","https://test.example.org/"]

Class: URLSearchParams#

History
VersionChanges
REPLACEME

The class is now available on the global object.

v7.5.0

Added in: v7.5.0

The URLSearchParams API provides read and write access to the query of a URL. The URLSearchParams class can also be used standalone with one of the four following constructors. The URLSearchParams class is also available on the global object.

The WHATWG URLSearchParams interface and the querystring module have similar purpose, but the purpose of the querystring module is more general, as it allows the customization of delimiter characters (& and =). On the other hand, this API is designed purely for URL query strings.

const myURL = new URL('https://example.org/?abc=123');
console.log(myURL.searchParams.get('abc'));
// Prints 123

myURL.searchParams.append('abc', 'xyz');
console.log(myURL.href);
// Prints https://example.org/?abc=123&abc=xyz

myURL.searchParams.delete('abc');
myURL.searchParams.set('a', 'b');
console.log(myURL.href);
// Prints https://example.org/?a=b

const newSearchParams = new URLSearchParams(myURL.searchParams);
// The above is equivalent to
// const newSearchParams = new URLSearchParams(myURL.search);

newSearchParams.append('a', 'c');
console.log(myURL.href);
// Prints https://example.org/?a=b
console.log(newSearchParams.toString());
// Prints a=b&a=c

// newSearchParams.toString() is implicitly called
myURL.search = newSearchParams;
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
newSearchParams.delete('a');
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c

Constructor: new URLSearchParams()#

Instantiate a new empty URLSearchParams object.

Constructor: new URLSearchParams(string)#

Parse the string as a query string, and use it to instantiate a new URLSearchParams object. A leading '?', if present, is ignored.

let params;

params = new URLSearchParams('user=abc&query=xyz');
console.log(params.get('user'));
// Prints 'abc'
console.log(params.toString());
// Prints 'user=abc&query=xyz'

params = new URLSearchParams('?user=abc&query=xyz');
console.log(params.toString());
// Prints 'user=abc&query=xyz'

Constructor: new URLSearchParams(obj)#

Added in: v7.10.0
  • obj <Object> An object representing a collection of key-value pairs

Instantiate a new URLSearchParams object with a query hash map. The key and value of each property of obj are always coerced to strings.

Unlike querystring module, duplicate keys in the form of array values are not allowed. Arrays are stringified using array.toString(), which simply joins all array elements with commas.

const params = new URLSearchParams({
  user: 'abc',
  query: ['first', 'second']
});
console.log(params.getAll('query'));
// Prints [ 'first,second' ]
console.log(params.toString());
// Prints 'user=abc&query=first%2Csecond'

Constructor: new URLSearchParams(iterable)#

Added in: v7.10.0
  • iterable <Iterable> An iterable object whose elements are key-value pairs

Instantiate a new URLSearchParams object with an iterable map in a way that is similar to Map's constructor. iterable can be an Array or any iterable object. That means iterable can be another URLSearchParams, in which case the constructor will simply create a clone of the provided URLSearchParams. Elements of iterable are key-value pairs, and can themselves be any iterable object.

Duplicate keys are allowed.

let params;

// Using an array
params = new URLSearchParams([
  ['user', 'abc'],
  ['query', 'first'],
  ['query', 'second']
]);
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'

// Using a Map object
const map = new Map();
map.set('user', 'abc');
map.set('query', 'xyz');
params = new URLSearchParams(map);
console.log(params.toString());
// Prints 'user=abc&query=xyz'

// Using a generator function
function* getQueryPairs() {
  yield ['user', 'abc'];
  yield ['query', 'first'];
  yield ['query', 'second'];
}
params = new URLSearchParams(getQueryPairs());
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'

// Each key-value pair must have exactly two elements
new URLSearchParams([
  ['user', 'abc', 'error']
]);
// Throws TypeError [ERR_INVALID_TUPLE]:
//        Each query pair must be an iterable [name, value] tuple

urlSearchParams.append(name, value)#

Append a new name-value pair to the query string.

urlSearchParams.delete(name)#

Remove all name-value pairs whose name is name.

urlSearchParams.entries()#

Returns an ES6 Iterator over each of the name-value pairs in the query. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams[@@iterator]().

urlSearchParams.forEach(fn[, thisArg])#

  • fn <Function> Function invoked for each name-value pair in the query.
  • thisArg <Object> Object to be used as this value for when fn is called

Iterates over each name-value pair in the query and invokes the given function.

const myURL = new URL('https://example.org/?a=b&c=d');
myURL.searchParams.forEach((value, name, searchParams) => {
  console.log(name, value, myURL.searchParams === searchParams);
});
// Prints:
//   a b true
//   c d true

urlSearchParams.get(name)#

  • name <string>
  • Returns: <string> or null if there is no name-value pair with the given name.

Returns the value of the first name-value pair whose name is name. If there are no such pairs, null is returned.

urlSearchParams.getAll(name)#

Returns the values of all name-value pairs whose name is name. If there are no such pairs, an empty array is returned.

urlSearchParams.has(name)#

Returns true if there is at least one name-value pair whose name is name.

urlSearchParams.keys()#

Returns an ES6 Iterator over the names of each name-value pair.

const params = new URLSearchParams('foo=bar&foo=baz');
for (const name of params.keys()) {
  console.log(name);
}
// Prints:
//   foo
//   foo

urlSearchParams.set(name, value)#

Sets the value in the URLSearchParams object associated with name to value. If there are any pre-existing name-value pairs whose names are name, set the first such pair's value to value and remove all others. If not, append the name-value pair to the query string.

const params = new URLSearchParams();
params.append('foo', 'bar');
params.append('foo', 'baz');
params.append('abc', 'def');
console.log(params.toString());
// Prints foo=bar&foo=baz&abc=def

params.set('foo', 'def');
params.set('xyz', 'opq');
console.log(params.toString());
// Prints foo=def&abc=def&xyz=opq

urlSearchParams.sort()#

Added in: v7.7.0

Sort all existing name-value pairs in-place by their names. Sorting is done with a stable sorting algorithm, so relative order between name-value pairs with the same name is preserved.

This method can be used, in particular, to increase cache hits.

const params = new URLSearchParams('query[]=abc&type=search&query[]=123');
params.sort();
console.log(params.toString());
// Prints query%5B%5D=abc&query%5B%5D=123&type=search

urlSearchParams.toString()#

Returns the search parameters serialized as a string, with characters percent-encoded where necessary.

urlSearchParams.values()#

Returns an ES6 Iterator over the values of each name-value pair.

urlSearchParams[@@iterator]()#

Returns an ES6 Iterator over each of the name-value pairs in the query string. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams.entries().

const params = new URLSearchParams('foo=bar&xyz=baz');
for (const [name, value] of params) {
  console.log(name, value);
}
// Prints:
//   foo bar
//   xyz baz

url.domainToASCII(domain)#

Added in: v7.4.0

Returns the Punycode ASCII serialization of the domain. If domain is an invalid domain, the empty string is returned.

It performs the inverse operation to url.domainToUnicode().

const url = require('url');
console.log(url.domainToASCII('español.com'));
// Prints xn--espaol-zwa.com
console.log(url.domainToASCII('中文.com'));
// Prints xn--fiq228c.com
console.log(url.domainToASCII('xn--iñvalid.com'));
// Prints an empty string

url.domainToUnicode(domain)#

Added in: v7.4.0

Returns the Unicode serialization of the domain. If domain is an invalid domain, the empty string is returned.

It performs the inverse operation to url.domainToASCII().

const url = require('url');
console.log(url.domainToUnicode('xn--espaol-zwa.com'));
// Prints español.com
console.log(url.domainToUnicode('xn--fiq228c.com'));
// Prints 中文.com
console.log(url.domainToUnicode('xn--iñvalid.com'));
// Prints an empty string

url.format(URL[, options])#

Added in: v7.6.0
  • URL <URL> A WHATWG URL object
  • options <Object>
    • auth <boolean> true if the serialized URL string should include the username and password, false otherwise. Default: true.
    • fragment <boolean> true if the serialized URL string should include the fragment, false otherwise. Default: true.
    • search <boolean> true if the serialized URL string should include the search query, false otherwise. Default: true.
    • unicode <boolean> true if Unicode characters appearing in the host component of the URL string should be encoded directly as opposed to being Punycode encoded. Default: false.

Returns a customizable serialization of a URL String representation of a WHATWG URL object.

The URL object has both a toString() method and href property that return string serializations of the URL. These are not, however, customizable in any way. The url.format(URL[, options]) method allows for basic customization of the output.

For example:

const myURL = new URL('https://a:b@你好你好?abc#foo');

console.log(myURL.href);
// Prints https://a:b@xn--6qqa088eba/?abc#foo

console.log(myURL.toString());
// Prints https://a:b@xn--6qqa088eba/?abc#foo

console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
// Prints 'https://你好你好/?abc'

Legacy URL API#

Legacy urlObject#

The legacy urlObject (require('url').Url) is created and returned by the url.parse() function.

urlObject.auth#

The auth property is the username and password portion of the URL, also referred to as "userinfo". This string subset follows the protocol and double slashes (if present) and precedes the host component, delimited by an ASCII "at sign" (@). The format of the string is {username}[:{password}], with the [:{password}] portion being optional.

For example: 'user:pass'

urlObject.hash#

The hash property consists of the "fragment" portion of the URL including the leading ASCII hash (#) character.

For example: '#hash'

urlObject.host#

The host property is the full lower-cased host portion of the URL, including the port if specified.

For example: 'sub.host.com:8080'

urlObject.hostname#

The hostname property is the lower-cased host name portion of the host component without the port included.

For example: 'sub.host.com'

urlObject.href#

The href property is the full URL string that was parsed with both the protocol and host components converted to lower-case.

For example: 'http://user:pass@sub.host.com:8080/p/a/t/h?query=string#hash'

urlObject.path#

The path property is a concatenation of the pathname and search components.

For example: '/p/a/t/h?query=string'

No decoding of the path is performed.

urlObject.pathname#

The pathname property consists of the entire path section of the URL. This is everything following the host (including the port) and before the start of the query or hash components, delimited by either the ASCII question mark (?) or hash (#) characters.

For example '/p/a/t/h'

No decoding of the path string is performed.

urlObject.port#

The port property is the numeric port portion of the host component.

For example: '8080'

urlObject.protocol#

The protocol property identifies the URL's lower-cased protocol scheme.

For example: 'http:'

urlObject.query#

The query property is either the query string without the leading ASCII question mark (?), or an object returned by the querystring module's parse() method. Whether the query property is a string or object is determined by the parseQueryString argument passed to url.parse().

For example: 'query=string' or {'query': 'string'}

If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.

urlObject.search#

The search property consists of the entire "query string" portion of the URL, including the leading ASCII question mark (?) character.

For example: '?query=string'

No decoding of the query string is performed.

urlObject.slashes#

The slashes property is a boolean with a value of true if two ASCII forward-slash characters (/) are required following the colon in the protocol.

url.format(urlObject)#

History
VersionChanges
v7.0.0

URLs with a file: scheme will now always use the correct number of slashes regardless of slashes option. A false-y slashes option with no protocol is now also respected at all times.

v0.1.25

Added in: v0.1.25

  • urlObject <Object> | <string> A URL object (as returned by url.parse() or constructed otherwise). If a string, it is converted to an object by passing it to url.parse().

The url.format() method returns a formatted URL string derived from urlObject.

url.format({
  protocol: 'https',
  hostname: 'example.com',
  pathname: '/some/path',
  query: {
    page: 1,
    format: 'json'
  }
});

// => 'https://example.com/some/path?page=1&format=json'

If urlObject is not an object or a string, url.format() will throw a TypeError.

The formatting process operates as follows:

  • A new empty string result is created.
  • If urlObject.protocol is a string, it is appended as-is to result.
  • Otherwise, if urlObject.protocol is not undefined and is not a string, an Error is thrown.
  • For all string values of urlObject.protocol that do not end with an ASCII colon (:) character, the literal string : will be appended to result.
  • If either of the following conditions is true, then the literal string // will be appended to result:
    • urlObject.slashes property is true;
    • urlObject.protocol begins with http, https, ftp, gopher, or file;
  • If the value of the urlObject.auth property is truthy, and either urlObject.host or urlObject.hostname are not undefined, the value of urlObject.auth will be coerced into a string and appended to result followed by the literal string @.
  • If the urlObject.host property is undefined then:
    • If the urlObject.hostname is a string, it is appended to result.
    • Otherwise, if urlObject.hostname is not undefined and is not a string, an Error is thrown.
    • If the urlObject.port property value is truthy, and urlObject.hostname is not undefined:
      • The literal string : is appended to result, and
      • The value of urlObject.port is coerced to a string and appended to result.
  • Otherwise, if the urlObject.host property value is truthy, the value of urlObject.host is coerced to a string and appended to result.
  • If the urlObject.pathname property is a string that is not an empty string:
    • If the urlObject.pathname does not start with an ASCII forward slash (/), then the literal string '/' is appended to result.
    • The value of urlObject.pathname is appended to result.
  • Otherwise, if urlObject.pathname is not undefined and is not a string, an Error is thrown.
  • If the urlObject.search property is undefined and if the urlObject.query property is an Object, the literal string ? is appended to result followed by the output of calling the querystring module's stringify() method passing the value of urlObject.query.
  • Otherwise, if urlObject.search is a string:
    • If the value of urlObject.search does not start with the ASCII question mark (?) character, the literal string ? is appended to result.
    • The value of urlObject.search is appended to result.
  • Otherwise, if urlObject.search is not undefined and is not a string, an Error is thrown.
  • If the urlObject.hash property is a string:
    • If the value of urlObject.hash does not start with the ASCII hash (#) character, the literal string # is appended to result.
    • The value of urlObject.hash is appended to result.
  • Otherwise, if the urlObject.hash property is not undefined and is not a string, an Error is thrown.
  • result is returned.

url.parse(urlString[, parseQueryString[, slashesDenoteHost]])#

History
VersionChanges
v9.0.0

The search property on the returned URL object is now null when no query string is present.

v0.1.25

Added in: v0.1.25

  • urlString <string> The URL string to parse.
  • parseQueryString <boolean> If true, the query property will always be set to an object returned by the querystring module's parse() method. If false, the query property on the returned URL object will be an unparsed, undecoded string. Default: false.
  • slashesDenoteHost <boolean> If true, the first token after the literal string // and preceding the next / will be interpreted as the host. For instance, given //foo/bar, the result would be {host: 'foo', pathname: '/bar'} rather than {pathname: '//foo/bar'}. Default: false.

The url.parse() method takes a URL string, parses it, and returns a URL object.

A TypeError is thrown if urlString is not a string.

A URIError is thrown if the auth property is present but cannot be decoded.

url.resolve(from, to)#

History
VersionChanges
v6.6.0

The auth fields are now kept intact when from and to refer to the same host.

v6.5.0, v4.6.2

The port field is copied correctly now.

v6.0.0

The auth fields is cleared now the to parameter contains a hostname.

v0.1.25

Added in: v0.1.25

  • from <string> The Base URL being resolved against.
  • to <string> The HREF URL being resolved.

The url.resolve() method resolves a target URL relative to a base URL in a manner similar to that of a Web browser resolving an anchor tag HREF.

For example:

const url = require('url');
url.resolve('/one/two/three', 'four');         // '/one/two/four'
url.resolve('http://example.com/', '/one');    // 'http://example.com/one'
url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'

Percent-Encoding in URLs#

URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL.

Legacy API#

Within the Legacy API, spaces (' ') and the following characters will be automatically escaped in the properties of URL objects:

< > " ` \r \n \t { } | \ ^ '

For example, the ASCII space character (' ') is encoded as %20. The ASCII forward slash (/) character is encoded as %3C.

WHATWG API#

The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the Legacy API.

The WHATWG algorithm defines four "percent-encode sets" that describe ranges of characters that must be percent-encoded:

  • The C0 control percent-encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E.

  • The fragment percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+003C, U+003E, and U+0060.

  • The path percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+0023, U+003C, U+003E, U+003F, U+0060, U+007B, and U+007D.

  • The userinfo encode set includes the path percent-encode set and code points U+002F, U+003A, U+003B, U+003D, U+0040, U+005B, U+005C, U+005D, U+005E, and U+007C.

The userinfo percent-encode set is used exclusively for username and passwords encoded within the URL. The path percent-encode set is used for the path of most URLs. The fragment percent-encode set is used for URL fragments. The C0 control percent-encode set is used for host and path under certain specific conditions, in addition to all other cases.

When non-ASCII characters appear within a hostname, the hostname is encoded using the Punycode algorithm. Note, however, that a hostname may contain both Punycode encoded and percent-encoded characters. For example:

const myURL = new URL('https://%CF%80.com/foo');
console.log(myURL.href);
// Prints https://xn--1xa.com/foo
console.log(myURL.origin);
// Prints https://π.com

Util#

Stability: 2 - Stable

The util module is primarily designed to support the needs of Node.js' own internal APIs. However, many of the utilities are useful for application and module developers as well. It can be accessed using:

const util = require('util');

util.callbackify(original)#

Added in: v8.2.0

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking a (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

const util = require('util');

async function fn() {
  return 'hello world';
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

  function fn() {
    return Promise.reject(null);
  }
  const callbackFunction = util.callbackify(fn);

  callbackFunction((err, ret) => {
    // When the Promise was rejected with `null` it is wrapped with an Error and
    // the original value is stored in `reason`.
    err && err.hasOwnProperty('reason') && err.reason === null;  // true
  });

util.debuglog(section)#

Added in: v0.11.3
  • section <string> A string identifying the portion of the application for which the debuglog function is being created.
  • Returns: <Function> The logging function

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

const util = require('util');
const debuglog = util.debuglog('foo');

debuglog('hello from foo [%d]', 123);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

The section supports wildcard also:

const util = require('util');
const debuglog = util.debuglog('foo-bar');

debuglog('hi there, it\'s foo-bar [%d]', 2333);

if it is run with NODE_DEBUG=foo* in the environment, then it will output something like:

FOO-BAR 3257: hi there, it's foo-bar [2333]

Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable: NODE_DEBUG=fs,net,tls.

util.deprecate(fn, msg[, code])#

History
VersionChanges
REPLACEME

Deprecation warnings are only emitted once for each code.

v0.8.0

Added in: v0.8.0

  • fn <Function> The function that is being deprecated.
  • msg <string> A warning message to display when the deprecated function is invoked.
  • code <string> A deprecation code. See the list of deprecated APIs for a list of codes.
  • Returns: <Function> The deprecated function wrapped to emit a warning.

The util.deprecate() method wraps fn (which may be a function or class) in such a way that it is marked as deprecated.

const util = require('util');

exports.obsoleteFunction = util.deprecate(() => {
  // Do something here.
}, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');

When called, util.deprecate() will return a function that will emit a DeprecationWarning using the process.on('warning') event. The warning will be emitted and printed to stderr the first time the returned function is called. After the warning is emitted, the wrapped function is called without emitting a warning.

If the same optional code is supplied in multiple calls to util.deprecate(), the warning will be emitted only once for that code.

const util = require('util');

const fn1 = util.deprecate(someFunction, someMessage, 'DEP0001');
const fn2 = util.deprecate(someOtherFunction, someOtherMessage, 'DEP0001');
fn1(); // emits a deprecation warning with code DEP0001
fn2(); // does not emit a deprecation warning because it has the same code

If either the --no-deprecation or --no-warnings command line flags are used, or if the process.noDeprecation property is set to true prior to the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to stderr the first time the deprecated function is called.

If the --throw-deprecation command line flag is set, or the process.throwDeprecation property is set to true, then an exception will be thrown when the deprecated function is called.

The --throw-deprecation command line flag and process.throwDeprecation property take precedence over --trace-deprecation and process.traceDeprecation.

util.format(format[, ...args])#

History
VersionChanges
REPLACEME

The %o specifiers depth option is now set to Infinity.

v8.4.0

The %o and %O specifiers are supported now.

v0.5.3

Added in: v0.5.3

  • format <string> A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format.

The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:

  • %s - String.
  • %d - Number (integer or floating point value).
  • %i - Integer.
  • %f - Floating point value.
  • %j - JSON. Replaced with the string '[Circular]' if the argument contains circular references.
  • %o - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() with options { showHidden: true, showProxy: true }. This will show the full object including non-enumerable properties and proxies.
  • %O - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() without options. This will show the full object not including non-enumerable properties and proxies.
  • %% - single percent sign ('%'). This does not consume an argument.
  • Returns: <string> The formatted string

If the placeholder does not have a corresponding argument, the placeholder is not replaced.

util.format('%s:%s', 'foo');
// Returns: 'foo:%s'

If there are more arguments passed to the util.format() method than the number of placeholders, the extra arguments are coerced into strings then concatenated to the returned string, each delimited by a space. Excessive arguments whose typeof is 'object' or 'symbol' (except null) will be transformed by util.inspect().

util.format('%s:%s', 'foo', 'bar', 'baz'); // 'foo:bar baz'

If the first argument is not a string then util.format() returns a string that is the concatenation of all arguments separated by spaces. Each argument is converted to a string using util.inspect().

util.format(1, 2, 3); // '1 2 3'

If only one argument is passed to util.format(), it is returned as it is without any formatting.

util.format('%% %s'); // '%% %s'

Please note that util.format() is a synchronous method that is mainly intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

util.getSystemErrorName(err)#

Added in: v9.7.0

Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

fs.access('file/that/does/not/exist', (err) => {
  const name = util.getSystemErrorName(err.errno);
  console.error(name);  // ENOENT
});

util.inherits(constructor, superConstructor)#

History
VersionChanges
v5.0.0

The constructor parameter can refer to an ES6 class now.

v0.3.0

Added in: v0.3.0

Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

const util = require('util');
const EventEmitter = require('events');

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"

ES6 example using class and extends

const EventEmitter = require('events');

class MyStream extends EventEmitter {
  write(data) {
    this.emit('data', data);
  }
}

const stream = new MyStream();

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('With ES6');

util.inspect(object[, options])#

History
VersionChanges
REPLACEME

WeakMap and WeakSet entries can now be inspected as well.

REPLACEME

The depth default changed to Infinity.

v9.9.0

The compact option is supported now.

v6.6.0

Custom inspection functions can now return this.

v6.3.0

The breakLength option is supported now.

v6.1.0

The maxArrayLength option is supported now; in particular, long arrays are truncated by default.

v6.1.0

The showProxy option is supported now.

v0.3.0

Added in: v0.3.0

  • object <any> Any JavaScript primitive or Object.
  • options <Object>
    • showHidden <boolean> If true, the object's non-enumerable symbols and properties will be included in the formatted result as well as WeakMap and WeakSet entries. Default: false.
    • colors <boolean> If true, the output will be styled with ANSI color codes. Colors are customizable, see Customizing util.inspect colors. Default: false.
    • customInspect <boolean> If false, then custom inspect(depth, opts) functions will not be called. Default: true.
    • showProxy <boolean> If true, then objects and functions that are Proxy objects will be introspected to show their target and handler objects. Default: false.
    • maxArrayLength <number> Specifies the maximum number of Array, TypedArray, WeakMap and WeakSet elements to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no elements. Default: 100.
    • breakLength <number> The length at which an object's keys are split across multiple lines. Set to Infinity to format an object as a single line. Default: 60 for legacy compatibility.
    • compact <boolean> Setting this to false changes the default indentation to use a line break for each object key instead of lining up multiple properties in one line. It will also break text that is above the breakLength size into smaller and better readable chunks and indents objects the same as arrays. Note that no text will be reduced below 16 characters, no matter the breakLength size. For more information, see the example below. Default: true.
    • depth <number> Specifies the number visible nested Objects in an object. This is useful to minimize the inspection output for large complicated objects. To make it recurse indefinitely pass null or Infinity. Default: Infinity.
  • Returns: <string> The representation of passed object

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter certain aspects of the formatted string. util.inspect() will use the constructor's name and/or @@toStringTag to make an identifiable tag for an inspected value.

class Foo {
  get [Symbol.toStringTag]() {
    return 'bar';
  }
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz);       // '[foo] {}'

The following example limits the inspected output of the paths property:

const util = require('util');

console.log(util.inspect(module, { depth: 0 }));
// Instead of showing all entries in `paths` `[Array]` is used to limit the
// output for readability:

// Module {
//   id: '<repl>',
//   exports: {},
//   parent: undefined,
//   filename: null,
//   loaded: false,
//   children: [],
//   paths: [Array] }

Values may supply their own custom inspect(depth, opts) functions, when called these receive the current depth in the recursive inspection, as well as the options object passed to util.inspect().

The following example highlights the difference with the compact option:

const util = require('util');

const o = {
  a: [1, 2, [[
    'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do ' +
      'eiusmod tempor incididunt ut labore et dolore magna aliqua.',
    'test',
    'foo']], 4],
  b: new Map([['za', 1], ['zb', 'test']])
};
console.log(util.inspect(o, { compact: true, breakLength: 80 }));

// This will print

// { a:
//   [ 1,
//     2,
//     [ [ 'Lorem ipsum dolor sit amet, consectetur [...]', // A long line
//           'test',
//           'foo' ] ],
//     4 ],
//   b: Map { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false changes the output to be more reader friendly.
console.log(util.inspect(o, { compact: false, breakLength: 80 }));

// {
//   a: [
//     1,
//     2,
//     [
//       [
//         'Lorem ipsum dolor sit amet, consectetur ' +
//           'adipiscing elit, sed do eiusmod tempor ' +
//           'incididunt ut labore et dolore magna ' +
//           'aliqua.,
//         'test',
//         'foo'
//       ]
//     ],
//     4
//   ],
//   b: Map {
//     'za' => 1,
//     'zb' => 'test'
//   }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.
// Reducing the `breakLength` will split the "Lorem ipsum" text in smaller
// chunks.

Using the showHidden option allows to inspect WeakMap and WeakSet entries. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same ['WeakSet'][] entries twice might actually result in a different output. Besides this any item might be collected at any point of time by the garbage collector if there is no strong reference left to that object. Therefore there is no guarantee to get a reliable output.

const { inspect } = require('util');

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

Please note that util.inspect() is a synchronous method that is mainly intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

Customizing util.inspect colors#

Color output (if enabled) of util.inspect is customizable globally via the util.inspect.styles and util.inspect.colors properties.

util.inspect.styles is a map associating a style name to a color from util.inspect.colors.

The default styles and associated colors are:

  • number - yellow
  • boolean - yellow
  • string - green
  • date - magenta
  • regexp - red
  • null - bold
  • undefined - grey
  • special - cyan (only applied to functions at this time)
  • name - (no styling)

The predefined color codes are: white, grey, black, blue, cyan, green, magenta, red and yellow. There are also bold, italic, underline and inverse codes.

Color styling uses ANSI control codes that may not be supported on all terminals.

Custom inspection functions on Objects#

Objects may also define their own [util.inspect.custom](depth, opts) (or the equivalent but deprecated inspect(depth, opts)) function that util.inspect() will invoke and use the result of when inspecting the object:

const util = require('util');

class Box {
  constructor(value) {
    this.value = value;
  }

  [util.inspect.custom](depth, options) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = util.inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

util.inspect(box);
// Returns: "Box< true >"

Custom [util.inspect.custom](depth, opts) functions typically return a string but may return a value of any type that will be formatted accordingly by util.inspect().

const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = (depth) => {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"

util.inspect.custom#

Added in: v6.6.0

A Symbol that can be used to declare custom inspect functions, see Custom inspection functions on Objects.

util.inspect.defaultOptions#

Added in: v6.4.0

The defaultOptions value allows customization of the default options used by util.inspect. This is useful for functions like console.log or util.format which implicitly call into util.inspect. It shall be set to an object containing one or more valid util.inspect() options. Setting option properties directly is also supported.

const util = require('util');
const arr = Array(101).fill(0);

console.log(arr); // logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array

util.isDeepStrictEqual(val1, val2)#

Added in: v9.0.0

Returns true if there is deep strict equality between val1 and val2. Otherwise, returns false.

See assert.deepStrictEqual() for more information about deep strict equality.

util.promisify(original)#

Added in: v8.0.0

Takes a function following the common error-first callback style, i.e. taking a (err, value) => ... callback as the last argument, and returns a version that returns promises.

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Handle the error.
});

Or, equivalently using async functions:

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);

async function callStat() {
  const stats = await stat('.');
  console.log(`This directory is owned by ${stats.uid}`);
}

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Custom promisified functions#

Using the util.promisify.custom symbol one can override the return value of util.promisify():

const util = require('util');

function doSomething(foo, callback) {
  // ...
}

doSomething[util.promisify.custom] = (foo) => {
  return getPromiseSomehow();
};

const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'

This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.

For example, with a function that takes in (foo, onSuccessCallback, onErrorCallback):

doSomething[util.promisify.custom] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};

If promisify.custom is defined but is not a function, promisify() will throw an error.

util.promisify.custom#

Added in: v8.0.0

A Symbol that can be used to declare custom promisified variants of functions, see Custom promisified functions.

Class: util.TextDecoder#

Added in: v8.3.0

An implementation of the WHATWG Encoding Standard TextDecoder API.

const decoder = new TextDecoder('shift_jis');
let string = '';
let buffer;
while (buffer = getNextChunkSomehow()) {
  string += decoder.decode(buffer, { stream: true });
}
string += decoder.decode(); // end-of-stream

WHATWG Supported Encodings#

Per the WHATWG Encoding Standard, the encodings supported by the TextDecoder API are outlined in the tables below. For each encoding, one or more aliases may be used.

Different Node.js build configurations support different sets of encodings. While a very basic set of encodings is supported even on Node.js builds without ICU enabled, support for some encodings is provided only when Node.js is built with ICU and using the full ICU data (see Internationalization).

Encodings Supported Without ICU#

Encoding Aliases
'utf-8' 'unicode-1-1-utf-8', 'utf8'
'utf-16le' 'utf-16'

Encodings Supported by Default (With ICU)#

Encoding Aliases
'utf-8' 'unicode-1-1-utf-8', 'utf8'
'utf-16le' 'utf-16'
'utf-16be'

Encodings Requiring Full ICU Data#

Encoding Aliases
'ibm866' '866', 'cp866', 'csibm866'
'iso-8859-2' 'csisolatin2', 'iso-ir-101', 'iso8859-2', 'iso88592', 'iso_8859-2', 'iso_8859-2:1987', 'l2', 'latin2'
'iso-8859-3' 'csisolatin3', 'iso-ir-109', 'iso8859-3', 'iso88593', 'iso_8859-3', 'iso_8859-3:1988', 'l3', 'latin3'
'iso-8859-4' 'csisolatin4', 'iso-ir-110', 'iso8859-4', 'iso88594', 'iso_8859-4', 'iso_8859-4:1988', 'l4', 'latin4'
'iso-8859-5' 'csisolatincyrillic', 'cyrillic', 'iso-ir-144', 'iso8859-5', 'iso88595', 'iso_8859-5', 'iso_8859-5:1988'
'iso-8859-6' 'arabic', 'asmo-708', 'csiso88596e', 'csiso88596i', 'csisolatinarabic', 'ecma-114', 'iso-8859-6-e', 'iso-8859-6-i', 'iso-ir-127', 'iso8859-6', 'iso88596', 'iso_8859-6', 'iso_8859-6:1987'
'iso-8859-7' 'csisolatingreek', 'ecma-118', 'elot_928', 'greek', 'greek8', 'iso-ir-126', 'iso8859-7', 'iso88597', 'iso_8859-7', 'iso_8859-7:1987', 'sun_eu_greek'
'iso-8859-8' 'csiso88598e', 'csisolatinhebrew', 'hebrew', 'iso-8859-8-e', 'iso-ir-138', 'iso8859-8', 'iso88598', 'iso_8859-8', 'iso_8859-8:1988', 'visual'
'iso-8859-8-i' 'csiso88598i', 'logical'
'iso-8859-10' 'csisolatin6', 'iso-ir-157', 'iso8859-10', 'iso885910', 'l6', 'latin6'
'iso-8859-13' 'iso8859-13', 'iso885913'
'iso-8859-14' 'iso8859-14', 'iso885914'
'iso-8859-15' 'csisolatin9', 'iso8859-15', 'iso885915', 'iso_8859-15', 'l9'
'koi8-r' 'cskoi8r', 'koi', 'koi8', 'koi8_r'
'koi8-u' 'koi8-ru'
'macintosh' 'csmacintosh', 'mac', 'x-mac-roman'
'windows-874' 'dos-874', 'iso-8859-11', 'iso8859-11', 'iso885911', 'tis-620'
'windows-1250' 'cp1250', 'x-cp1250'
'windows-1251' 'cp1251', 'x-cp1251'
'windows-1252' 'ansi_x3.4-1968', 'ascii', 'cp1252', 'cp819', 'csisolatin1', 'ibm819', 'iso-8859-1', 'iso-ir-100', 'iso8859-1', 'iso88591', 'iso_8859-1', 'iso_8859-1:1987', 'l1', 'latin1', 'us-ascii', 'x-cp1252'
'windows-1253' 'cp1253', 'x-cp1253'
'windows-1254' 'cp1254', 'csisolatin5', 'iso-8859-9', 'iso-ir-148', 'iso8859-9', 'iso88599', 'iso_8859-9', 'iso_8859-9:1989', 'l5', 'latin5', 'x-cp1254'
'windows-1255' 'cp1255', 'x-cp1255'
'windows-1256' 'cp1256', 'x-cp1256'
'windows-1257' 'cp1257', 'x-cp1257'
'windows-1258' 'cp1258', 'x-cp1258'
'x-mac-cyrillic' 'x-mac-ukrainian'
'gbk' 'chinese', 'csgb2312', 'csiso58gb231280', 'gb2312', 'gb_2312', 'gb_2312-80', 'iso-ir-58', 'x-gbk'
'gb18030'
'big5' 'big5-hkscs', 'cn-big5', 'csbig5', 'x-x-big5'
'euc-jp' 'cseucpkdfmtjapanese', 'x-euc-jp'
'iso-2022-jp' 'csiso2022jp'
'shift_jis' 'csshiftjis', 'ms932', 'ms_kanji', 'shift-jis', 'sjis', 'windows-31j', 'x-sjis'
'euc-kr' 'cseuckr', 'csksc56011987', 'iso-ir-149', 'korean', 'ks_c_5601-1987', 'ks_c_5601-1989', 'ksc5601', 'ksc_5601', 'windows-949'

The 'iso-8859-16' encoding listed in the WHATWG Encoding Standard is not supported.

new TextDecoder([encoding[, options]])#

  • encoding <string> Identifies the encoding that this TextDecoder instance supports. Default: 'utf-8'.
  • options <Object>
    • fatal <boolean> true if decoding failures are fatal. This option is only supported when ICU is enabled (see Internationalization). Default: false.
    • ignoreBOM <boolean> When true, the TextDecoder will include the byte order mark in the decoded result. When false, the byte order mark will be removed from the output. This option is only used when encoding is 'utf-8', 'utf-16be' or 'utf-16le'. Default: false.

Creates an new TextDecoder instance. The encoding may specify one of the supported encodings or an alias.

textDecoder.decode([input[, options]])#

Decodes the input and returns a string. If options.stream is true, any incomplete byte sequences occurring at the end of the input are buffered internally and emitted after the next call to textDecoder.decode().

If textDecoder.fatal is true, decoding errors that occur will result in a TypeError being thrown.

textDecoder.encoding#

The encoding supported by the TextDecoder instance.

textDecoder.fatal#

The value will be true if decoding errors result in a TypeError being thrown.

textDecoder.ignoreBOM#

The value will be true if the decoding result will include the byte order mark.

Class: util.TextEncoder#

Added in: v8.3.0

An implementation of the WHATWG Encoding Standard TextEncoder API. All instances of TextEncoder only support UTF-8 encoding.

const encoder = new TextEncoder();
const uint8array = encoder.encode('this is some data');

textEncoder.encode([input])#

UTF-8 encodes the input string and returns a Uint8Array containing the encoded bytes.

textEncoder.encoding#

The encoding supported by the TextEncoder instance. Always set to 'utf-8'.

util.types#

Added in: REPLACEME

util.types provides a number of type checks for different kinds of built-in objects. Unlike instanceof or Object.prototype.toString.call(value), these checks do not inspect properties of the object that are accessible from JavaScript (like their prototype), and usually have the overhead of calling into C++.

The result generally does not make any guarantees about what kinds of properties or behavior a value exposes in JavaScript. They are primarily useful for addon developers who prefer to do type checking in JavaScript.

util.types.isAnyArrayBuffer(value)#

Added in: REPLACEME

Returns true if the value is a built-in ArrayBuffer or SharedArrayBuffer instance.

See also util.types.isArrayBuffer() and util.types.isSharedArrayBuffer().

For example:

util.types.isAnyArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isAnyArrayBuffer(new SharedArrayBuffer());  // Returns true

util.types.isArgumentsObject(value)#

Added in: REPLACEME

Returns true if the value is an arguments object.

For example:

function foo() {
  util.types.isArgumentsObject(arguments);  // Returns true
}

util.types.isArrayBuffer(value)#

Added in: REPLACEME

Returns true if the value is a built-in ArrayBuffer instance. This does not include SharedArrayBuffer instances. Usually, it is desirable to test for both; See util.types.isAnyArrayBuffer() for that.

For example:

util.types.isArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isArrayBuffer(new SharedArrayBuffer());  // Returns false

util.types.isAsyncFunction(value)#

Added in: REPLACEME

Returns true if the value is an async function. Note that this only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

For example:

util.types.isAsyncFunction(function foo() {});  // Returns false
util.types.isAsyncFunction(async function foo() {});  // Returns true

util.types.isBooleanObject(value)#

Added in: REPLACEME

Returns true if the value is a boolean object, e.g. created by new Boolean().

For example:

util.types.isBooleanObject(false);  // Returns false
util.types.isBooleanObject(true);   // Returns false
util.types.isBooleanObject(new Boolean(false));   // Returns true
util.types.isBooleanObject(new Boolean(true));    // Returns true
util.types.isBooleanObject(Boolean(false)); // Returns false
util.types.isBooleanObject(Boolean(true)); // Returns false

util.types.isDataView(value)#

Added in: REPLACEME

Returns true if the value is a built-in DataView instance.

For example:

const ab = new ArrayBuffer(20);
util.types.isDataView(new DataView(ab));  // Returns true
util.types.isDataView(new Float64Array());  // Returns false

See also ArrayBuffer.isView().

util.types.isDate(value)#

Added in: REPLACEME

Returns true if the value is a built-in Date instance.

For example:

util.types.isDate(new Date());  // Returns true

util.types.isExternal(value)#

Added in: REPLACEME

Returns true if the value is a native External value.

util.types.isFloat32Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Float32Array instance.

For example:

util.types.isFloat32Array(new ArrayBuffer());  // Returns false
util.types.isFloat32Array(new Float32Array());  // Returns true
util.types.isFloat32Array(new Float64Array());  // Returns false

util.types.isFloat64Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Float64Array instance.

For example:

util.types.isFloat64Array(new ArrayBuffer());  // Returns false
util.types.isFloat64Array(new Uint8Array());  // Returns false
util.types.isFloat64Array(new Float64Array());  // Returns true

util.types.isGeneratorFunction(value)#

Added in: REPLACEME

Returns true if the value is a generator function. Note that this only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

For example:

util.types.isGeneratorFunction(function foo() {});  // Returns false
util.types.isGeneratorFunction(function* foo() {});  // Returns true

util.types.isGeneratorObject(value)#

Added in: REPLACEME

Returns true if the value is a generator object as returned from a built-in generator function. Note that this only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

For example:

function* foo() {}
const generator = foo();
util.types.isGeneratorObject(generator);  // Returns true

util.types.isInt8Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Int8Array instance.

For example:

util.types.isInt8Array(new ArrayBuffer());  // Returns false
util.types.isInt8Array(new Int8Array());  // Returns true
util.types.isInt8Array(new Float64Array());  // Returns false

util.types.isInt16Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Int16Array instance.

For example:

util.types.isInt16Array(new ArrayBuffer());  // Returns false
util.types.isInt16Array(new Int16Array());  // Returns true
util.types.isInt16Array(new Float64Array());  // Returns false

util.types.isInt32Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Int32Array instance.

For example:

util.types.isInt32Array(new ArrayBuffer());  // Returns false
util.types.isInt32Array(new Int32Array());  // Returns true
util.types.isInt32Array(new Float64Array());  // Returns false

util.types.isMap(value)#

Added in: REPLACEME

Returns true if the value is a built-in Map instance.

For example:

util.types.isMap(new Map());  // Returns true

util.types.isMapIterator(value)#

Added in: REPLACEME

Returns true if the value is an iterator returned for a built-in Map instance.

For example:

const map = new Map();
util.types.isMapIterator(map.keys());  // Returns true
util.types.isMapIterator(map.values());  // Returns true
util.types.isMapIterator(map.entries());  // Returns true
util.types.isMapIterator(map[Symbol.iterator]());  // Returns true

util.types.isNativeError(value)#

Added in: REPLACEME

Returns true if the value is an instance of a built-in Error type.

For example:

util.types.isNativeError(new Error());  // Returns true
util.types.isNativeError(new TypeError());  // Returns true
util.types.isNativeError(new RangeError());  // Returns true

util.types.isNumberObject(value)#

Added in: REPLACEME

Returns true if the value is a number object, e.g. created by new Number().

For example:

util.types.isNumberObject(0);  // Returns false
util.types.isNumberObject(new Number(0));   // Returns true

util.types.isPromise(value)#

Added in: REPLACEME

Returns true if the value is a built-in Promise.

For example:

util.types.isPromise(Promise.resolve(42));  // Returns true

util.types.isProxy(value)#

Added in: REPLACEME

Returns true if the value is a Proxy instance.

For example:

const target = {};
const proxy = new Proxy(target, {});
util.types.isProxy(target);  // Returns false
util.types.isProxy(proxy);  // Returns true

util.types.isRegExp(value)#

Added in: REPLACEME

Returns true if the value is a regular expression object.

For example:

util.types.isRegExp(/abc/);  // Returns true
util.types.isRegExp(new RegExp('abc'));  // Returns true

util.types.isSet(value)#

Added in: REPLACEME

Returns true if the value is a built-in Set instance.

For example:

util.types.isSet(new Set());  // Returns true

util.types.isSetIterator(value)#

Added in: REPLACEME

Returns true if the value is an iterator returned for a built-in Set instance.

For example:

const set = new Set();
util.types.isSetIterator(set.keys());  // Returns true
util.types.isSetIterator(set.values());  // Returns true
util.types.isSetIterator(set.entries());  // Returns true
util.types.isSetIterator(set[Symbol.iterator]());  // Returns true

util.types.isSharedArrayBuffer(value)#

Added in: REPLACEME

Returns true if the value is a built-in SharedArrayBuffer instance. This does not include ArrayBuffer instances. Usually, it is desirable to test for both; See util.types.isAnyArrayBuffer() for that.

For example:

util.types.isSharedArrayBuffer(new ArrayBuffer());  // Returns false
util.types.isSharedArrayBuffer(new SharedArrayBuffer());  // Returns true

util.types.isStringObject(value)#

Added in: REPLACEME

Returns true if the value is a string object, e.g. created by new String().

For example:

util.types.isStringObject('foo');  // Returns false
util.types.isStringObject(new String('foo'));   // Returns true

util.types.isSymbolObject(value)#

Added in: REPLACEME

Returns true if the value is a symbol object, created by calling Object() on a Symbol primitive.

For example:

const symbol = Symbol('foo');
util.types.isSymbolObject(symbol);  // Returns false
util.types.isSymbolObject(Object(symbol));   // Returns true

util.types.isTypedArray(value)#

Added in: REPLACEME

Returns true if the value is a built-in TypedArray instance.

For example:

util.types.isTypedArray(new ArrayBuffer());  // Returns false
util.types.isTypedArray(new Uint8Array());  // Returns true
util.types.isTypedArray(new Float64Array());  // Returns true

See also ArrayBuffer.isView().

util.types.isUint8Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Uint8Array instance.

For example:

util.types.isUint8Array(new ArrayBuffer());  // Returns false
util.types.isUint8Array(new Uint8Array());  // Returns true
util.types.isUint8Array(new Float64Array());  // Returns false

util.types.isUint8ClampedArray(value)#

Added in: REPLACEME

Returns true if the value is a built-in Uint8ClampedArray instance.

For example:

util.types.isUint8ClampedArray(new ArrayBuffer());  // Returns false
util.types.isUint8ClampedArray(new Uint8ClampedArray());  // Returns true
util.types.isUint8ClampedArray(new Float64Array());  // Returns false

util.types.isUint16Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Uint16Array instance.

For example:

util.types.isUint16Array(new ArrayBuffer());  // Returns false
util.types.isUint16Array(new Uint16Array());  // Returns true
util.types.isUint16Array(new Float64Array());  // Returns false

util.types.isUint32Array(value)#

Added in: REPLACEME

Returns true if the value is a built-in Uint32Array instance.

For example:

util.types.isUint32Array(new ArrayBuffer());  // Returns false
util.types.isUint32Array(new Uint32Array());  // Returns true
util.types.isUint32Array(new Float64Array());  // Returns false

util.types.isWeakMap(value)#

Added in: REPLACEME

Returns true if the value is a built-in WeakMap instance.

For example:

util.types.isWeakMap(new WeakMap());  // Returns true

util.types.isWeakSet(value)#

Added in: REPLACEME

Returns true if the value is a built-in WeakSet instance.

For example:

util.types.isWeakSet(new WeakSet());  // Returns true

util.types.isWebAssemblyCompiledModule(value)#

Added in: REPLACEME

Returns true if the value is a built-in WebAssembly.Module instance.

For example:

const module = new WebAssembly.Module(wasmBuffer);
util.types.isWebAssemblyCompiledModule(module);  // Returns true

Deprecated APIs#

The following APIs have been deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.

util._extend(target, source)#

Added in: v0.7.5Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Object.assign() instead.

The util._extend() method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through Object.assign().

util.debug(string)#

Added in: v0.3.0Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use console.error() instead.
  • string <string> The message to print to stderr

Deprecated predecessor of console.error.

util.error([...strings])#

Added in: v0.3.0Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use console.error() instead.
  • ...strings <string> The message to print to stderr

Deprecated predecessor of console.error.

util.isArray(object)#

Added in: v0.6.0Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use Array.isArray() instead.

Alias for Array.isArray().

Returns true if the given object is an Array. Otherwise, returns false.

const util = require('util');

util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false

util.isBoolean(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use typeof value === 'boolean' instead.

Returns true if the given object is a Boolean. Otherwise, returns false.

const util = require('util');

util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true

util.isBuffer(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use Buffer.isBuffer() instead.

Returns true if the given object is a Buffer. Otherwise, returns false.

const util = require('util');

util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true

util.isDate(object)#

Added in: v0.6.0Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use util.types.isDate() instead.

Returns true if the given object is a Date. Otherwise, returns false.

const util = require('util');

util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false

util.isError(object)#

Added in: v0.6.0Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use util.types.isNativeError() instead.

Returns true if the given object is an Error. Otherwise, returns false.

const util = require('util');

util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false

Note that this method relies on Object.prototype.toString() behavior. It is possible to obtain an incorrect result when the object argument manipulates @@toStringTag.

const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };

util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true

util.isFunction(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use typeof value === 'function' instead.

Returns true if the given object is a Function. Otherwise, returns false.

const util = require('util');

function Foo() {}
const Bar = () => {};

util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true

util.isNull(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use value === null instead.

Returns true if the given object is strictly null. Otherwise, returns false.

const util = require('util');

util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true

util.isNullOrUndefined(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use value === undefined || value === null instead.

Returns true if the given object is null or undefined. Otherwise, returns false.

const util = require('util');

util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true

util.isNumber(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use typeof value === 'number' instead.

Returns true if the given object is a Number. Otherwise, returns false.

const util = require('util');

util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true

util.isObject(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use value !== null && typeof value === 'object' instead.

Returns true if the given object is strictly an Object and not a Function (even though functions are objects in JavaScript). Otherwise, returns false.

const util = require('util');

util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(() => {});
// Returns: false

util.isPrimitive(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use (typeof value !== 'object' && typeof value !== 'function') || value === null instead.

Returns true if the given object is a primitive type. Otherwise, returns false.

const util = require('util');

util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(() => {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false

util.isRegExp(object)#

Added in: v0.6.0Deprecated since: v4.0.0
Stability: 0 - Deprecated

Returns true if the given object is a RegExp. Otherwise, returns false.

const util = require('util');

util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false

util.isString(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use typeof value === 'string' instead.

Returns true if the given object is a string. Otherwise, returns false.

const util = require('util');

util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false

util.isSymbol(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use typeof value === 'symbol' instead.

Returns true if the given object is a Symbol. Otherwise, returns false.

const util = require('util');

util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true

util.isUndefined(object)#

Added in: v0.11.5Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use value === undefined instead.

Returns true if the given object is undefined. Otherwise, returns false.

const util = require('util');

const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false

util.log(string)#

Added in: v0.3.0Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use a third party module instead.

The util.log() method prints the given string to stdout with an included timestamp.

const util = require('util');

util.log('Timestamped message.');

util.print([...strings])#

Added in: v0.3.0Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.

util.puts([...strings])#

Added in: v0.3.0Deprecated since: v0.11.3
Stability: 0 - Deprecated: Use console.log() instead.

Deprecated predecessor of console.log.

V8#

The v8 module exposes APIs that are specific to the version of V8 built into the Node.js binary. It can be accessed using:

const v8 = require('v8');

The APIs and implementation are subject to change at any time.

v8.cachedDataVersionTag()#

Added in: v8.0.0

Returns an integer representing a "version tag" derived from the V8 version, command line flags and detected CPU features. This is useful for determining whether a vm.Script cachedData buffer is compatible with this instance of V8.

v8.getHeapSpaceStatistics()#

History
VersionChanges
v7.5.0

Support values exceeding the 32-bit unsigned integer range.

v6.0.0

Added in: v6.0.0

Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 GetHeapSpaceStatistics function and may change from one V8 version to the next.

The value returned is an array of objects containing the following properties:

[
  {
    "space_name": "new_space",
    "space_size": 2063872,
    "space_used_size": 951112,
    "space_available_size": 80824,
    "physical_space_size": 2063872
  },
  {
    "space_name": "old_space",
    "space_size": 3090560,
    "space_used_size": 2493792,
    "space_available_size": 0,
    "physical_space_size": 3090560
  },
  {
    "space_name": "code_space",
    "space_size": 1260160,
    "space_used_size": 644256,
    "space_available_size": 960,
    "physical_space_size": 1260160
  },
  {
    "space_name": "map_space",
    "space_size": 1094160,
    "space_used_size": 201608,
    "space_available_size": 0,
    "physical_space_size": 1094160
  },
  {
    "space_name": "large_object_space",
    "space_size": 0,
    "space_used_size": 0,
    "space_available_size": 1490980608,
    "physical_space_size": 0
  }
]

v8.getHeapStatistics()#

History
VersionChanges
v7.5.0

Support values exceeding the 32-bit unsigned integer range.

v7.2.0

Added malloced_memory, peak_malloced_memory, and does_zap_garbage.

v1.0.0

Added in: v1.0.0

Returns an object with the following properties:

does_zap_garbage is a 0/1 boolean, which signifies whether the --zap_code_space option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident memory set) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

{
  total_heap_size: 7326976,
  total_heap_size_executable: 4194304,
  total_physical_size: 7326976,
  total_available_size: 1152656,
  used_heap_size: 3476208,
  heap_size_limit: 1535115264,
  malloced_memory: 16384,
  peak_malloced_memory: 1127496,
  does_zap_garbage: 0
}

v8.setFlagsFromString(flags)#

Added in: v1.0.0

The v8.setFlagsFromString() method can be used to programmatically set V8 command line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

The V8 options available for a version of Node.js may be determined by running node --v8-options. An unofficial, community-maintained list of options and their effects is available here.

Usage:

// Print GC events to stdout for one minute.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);

Serialization API#

Stability: 1 - Experimental

The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm. The format is backward-compatible (i.e. safe to store to disk).

This API is under development, and changes (including incompatible changes to the API or wire format) may occur until this warning is removed.

v8.serialize(value)#

Uses a DefaultSerializer to serialize value into a buffer.

v8.deserialize(buffer)#

Uses a DefaultDeserializer with default options to read a JS value from a buffer.

class: v8.Serializer#

new Serializer()#

Creates a new Serializer object.

serializer.writeHeader()#

Writes out a header, which includes the serialization format version.

serializer.writeValue(value)#

Serializes a JavaScript value and adds the serialized representation to the internal buffer.

This throws an error if value cannot be serialized.

serializer.releaseBuffer()#

Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.

serializer.transferArrayBuffer(id, arrayBuffer)#

Marks an ArrayBuffer as havings its contents transferred out of band. Pass the corresponding ArrayBuffer in the deserializing context to deserializer.transferArrayBuffer().

serializer.writeUint32(value)#

Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

serializer.writeUint64(hi, lo)#

Write a raw 64-bit unsigned integer, split into high and low 32-bit parts. For use inside of a custom serializer._writeHostObject().

serializer.writeDouble(value)#

Write a JS number value. For use inside of a custom serializer._writeHostObject().

serializer.writeRawBytes(buffer)#

Write raw bytes into the serializer’s internal buffer. The deserializer will require a way to compute the length of the buffer. For use inside of a custom serializer._writeHostObject().

serializer._writeHostObject(object)#

This method is called to write some kind of host object, i.e. an object created by native C++ bindings. If it is not possible to serialize object, a suitable exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._getDataCloneError(message)#

This method is called to generate error objects that will be thrown when an object can not be cloned.

This method defaults to the Error constructor and can be overridden on subclasses.

serializer._getSharedArrayBufferId(sharedArrayBuffer)#

This method is called when the serializer is going to serialize a SharedArrayBuffer object. It must return an unsigned 32-bit integer ID for the object, using the same ID if this SharedArrayBuffer has already been serialized. When deserializing, this ID will be passed to deserializer.transferArrayBuffer().

If the object cannot be serialized, an exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._setTreatArrayBufferViewsAsHostObjects(flag)#

Indicate whether to treat TypedArray and DataView objects as host objects, i.e. pass them to serializer._writeHostObject().

class: v8.Deserializer#

new Deserializer(buffer)#

Creates a new Deserializer object.

deserializer.readHeader()#

Reads and validates a header (including the format version). May, for example, reject an invalid or unsupported wire format. In that case, an Error is thrown.

deserializer.readValue()#

Deserializes a JavaScript value from the buffer and returns it.

deserializer.transferArrayBuffer(id, arrayBuffer)#

Marks an ArrayBuffer as havings its contents transferred out of band. Pass the corresponding ArrayBuffer in the serializing context to serializer.transferArrayBuffer() (or return the id from serializer._getSharedArrayBufferId() in the case of SharedArrayBuffers).

deserializer.getWireFormatVersion()#

Reads the underlying wire format version. Likely mostly to be useful to legacy code reading old wire format versions. May not be called before .readHeader().

deserializer.readUint32()#

Read a raw 32-bit unsigned integer and return it. For use inside of a custom deserializer._readHostObject().

deserializer.readUint64()#

Read a raw 64-bit unsigned integer and return it as an array [hi, lo] with two 32-bit unsigned integer entries. For use inside of a custom deserializer._readHostObject().

deserializer.readDouble()#

Read a JS number value. For use inside of a custom deserializer._readHostObject().

deserializer.readRawBytes(length)#

Read raw bytes from the deserializer’s internal buffer. The length parameter must correspond to the length of the buffer that was passed to serializer.writeRawBytes(). For use inside of a custom deserializer._readHostObject().

deserializer._readHostObject()#

This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.

This method is not present on the Deserializer class itself but can be provided by subclasses.

class: v8.DefaultSerializer#

A subclass of Serializer that serializes TypedArray (in particular Buffer) and DataView objects as host objects, and only stores the part of their underlying ArrayBuffers that they are referring to.

class: v8.DefaultDeserializer#

A subclass of Deserializer corresponding to the format written by DefaultSerializer.

VM (Executing JavaScript)#

Stability: 2 - Stable

The vm module provides APIs for compiling and running code within V8 Virtual Machine contexts.

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

A common use case is to run the code in a sandboxed environment. The sandboxed code uses a different V8 Context, meaning that it has a different global object than the rest of the code.

One can provide the context by "contextifying" a sandbox object. The sandboxed code treats any property on the sandbox like a global variable. Any changes on global variables caused by the sandboxed code are reflected in the sandbox object.

const vm = require('vm');

const x = 1;

const sandbox = { x: 2 };
vm.createContext(sandbox); // Contextify the sandbox.

const code = 'x += 40; var y = 17;';
// x and y are global variables in the sandboxed environment.
// Initially, x has the value 2 because that is the value of sandbox.x.
vm.runInContext(code, sandbox);

console.log(sandbox.x); // 42
console.log(sandbox.y); // 17

console.log(x); // 1; y is not defined.

The vm module is not a security mechanism. Do not use it to run untrusted code.

Class: vm.Module#

Added in: v9.6.0
Stability: 1 - Experimental

This feature is only available with the --experimental-vm-modules command flag enabled.

The vm.Module class provides a low-level interface for using ECMAScript modules in VM contexts. It is the counterpart of the vm.Script class that closely mirrors Source Text Module Records as defined in the ECMAScript specification.

Unlike vm.Script however, every vm.Module object is bound to a context from its creation. Operations on vm.Module objects are intrinsically asynchronous, in contrast with the synchronous nature of vm.Script objects. With the help of async functions, however, manipulating vm.Module objects is fairly straightforward.

Using a vm.Module object requires four distinct steps: creation/parsing, linking, instantiation, and evaluation. These four steps are illustrated in the following example.

This implementation lies at a lower level than the ECMAScript Module loader. There is also currently no way to interact with the Loader, though support is planned.

const vm = require('vm');

const contextifiedSandbox = vm.createContext({ secret: 42 });

(async () => {
  // Step 1
  //
  // Create a Module by constructing a new `vm.Module` object. This parses the
  // provided source text, throwing a `SyntaxError` if anything goes wrong. By
  // default, a Module is created in the top context. But here, we specify
  // `contextifiedSandbox` as the context this Module belongs to.
  //
  // Here, we attempt to obtain the default export from the module "foo", and
  // put it into local binding "secret".

  const bar = new vm.Module(`
    import s from 'foo';
    s;
  `, { context: contextifiedSandbox });

  // Step 2
  //
  // "Link" the imported dependencies of this Module to it.
  //
  // The provided linking callback (the "linker") accepts two arguments: the
  // parent module (`bar` in this case) and the string that is the specifier of
  // the imported module. The callback is expected to return a Module that
  // corresponds to the provided specifier, with certain requirements documented
  // in `module.link()`.
  //
  // If linking has not started for the returned Module, the same linker
  // callback will be called on the returned Module.
  //
  // Even top-level Modules without dependencies must be explicitly linked. The
  // callback provided would never be called, however.
  //
  // The link() method returns a Promise that will be resolved when all the
  // Promises returned by the linker resolve.
  //
  // Note: This is a contrived example in that the linker function creates a new
  // "foo" module every time it is called. In a full-fledged module system, a
  // cache would probably be used to avoid duplicated modules.

  async function linker(specifier, referencingModule) {
    if (specifier === 'foo') {
      return new vm.Module(`
        // The "secret" variable refers to the global variable we added to
        // "contextifiedSandbox" when creating the context.
        export default secret;
      `, { context: referencingModule.context });

      // Using `contextifiedSandbox` instead of `referencingModule.context`
      // here would work as well.
    }
    throw new Error(`Unable to resolve dependency: ${specifier}`);
  }
  await bar.link(linker);

  // Step 3
  //
  // Instantiate the top-level Module.
  //
  // Only the top-level Module needs to be explicitly instantiated; its
  // dependencies will be recursively instantiated by instantiate().

  bar.instantiate();

  // Step 4
  //
  // Evaluate the Module. The evaluate() method returns a Promise with a single
  // property "result" that contains the result of the very last statement
  // executed in the Module. In the case of `bar`, it is `s;`, which refers to
  // the default export of the `foo` module, the `secret` we set in the
  // beginning to 42.

  const { result } = await bar.evaluate();

  console.log(result);
  // Prints 42.
})();

Constructor: new vm.Module(code[, options])#

  • code <string> JavaScript Module code to parse
  • options
    • url <string> URL used in module resolution and stack traces. Default: 'vm:module(i)' where i is a context-specific ascending index.
    • context <Object> The contextified object as returned by the vm.createContext() method, to compile and evaluate this Module in.
    • lineOffset <integer> Specifies the line number offset that is displayed in stack traces produced by this Module.
    • columnOffset <integer> Spcifies the column number offset that is displayed in stack traces produced by this Module.
    • initalizeImportMeta <Function> Called during evaluation of this Module to initialize the import.meta. This function has the signature (meta, module), where meta is the import.meta object in the Module, and module is this vm.Module object.

Creates a new ES Module object.

Note: Properties assigned to the import.meta object that are objects may allow the Module to access information outside the specified context, if the object is created in the top level context. Use vm.runInContext() to create objects in a specific context.

const vm = require('vm');

const contextifiedSandbox = vm.createContext({ secret: 42 });

(async () => {
  const module = new vm.Module(
    'Object.getPrototypeOf(import.meta.prop).secret = secret;',
    {
      initializeImportMeta(meta) {
        // Note: this object is created in the top context. As such,
        // Object.getPrototypeOf(import.meta.prop) points to the
        // Object.prototype in the top context rather than that in
        // the sandbox.
        meta.prop = {};
      }
    });
  // Since module has no dependencies, the linker function will never be called.
  await module.link(() => {});
  module.initialize();
  await module.evaluate();

  // Now, Object.prototype.secret will be equal to 42.
  //
  // To fix this problem, replace
  //     meta.prop = {};
  // above with
  //     meta.prop = vm.runInContext('{}', contextifiedSandbox);
})();

module.dependencySpecifiers#

The specifiers of all dependencies of this module. The returned array is frozen to disallow any changes to it.

Corresponds to the [[RequestedModules]] field of Source Text Module Records in the ECMAScript specification.

module.error#

  • <any>

If the module.status is 'errored', this property contains the exception thrown by the module during evaluation. If the status is anything else, accessing this property will result in a thrown exception.

The value undefined cannot be used for cases where there is not a thrown exception due to possible ambiguity with throw undefined;.

Corresponds to the [[EvaluationError]] field of Source Text Module Records in the ECMAScript specification.

module.linkingStatus#

The current linking status of module. It will be one of the following values:

  • 'unlinked': module.link() has not yet been called.
  • 'linking': module.link() has been called, but not all Promises returned by the linker function have been resolved yet.
  • 'linked': module.link() has been called, and all its dependencies have been successfully linked.
  • 'errored': module.link() has been called, but at least one of its dependencies failed to link, either because the callback returned a Promise that is rejected, or because the Module the callback returned is invalid.

module.namespace#

The namespace object of the module. This is only available after instantiation (module.instantiate()) has completed.

Corresponds to the GetModuleNamespace abstract operation in the ECMAScript specification.

module.status#

The current status of the module. Will be one of:

  • 'uninstantiated': The module is not instantiated. It may because of any of the following reasons:

    • The module was just created.
    • module.instantiate() has been called on this module, but it failed for some reason.

    This status does not convey any information regarding if module.link() has been called. See module.linkingStatus for that.

  • 'instantiating': The module is currently being instantiated through a module.instantiate() call on itself or a parent module.

  • 'instantiated': The module has been instantiated successfully, but module.evaluate() has not yet been called.

  • 'evaluating': The module is being evaluated through a module.evaluate() on itself or a parent module.

  • 'evaluated': The module has been successfully evaluated.

  • 'errored': The module has been evaluated, but an exception was thrown.

Other than 'errored', this status string corresponds to the specification's Source Text Module Record's [[Status]] field. 'errored' corresponds to 'evaluated' in the specification, but with [[EvaluationError]] set to a value that is not undefined.

module.url#

The URL of the current module, as set in the constructor.

module.evaluate(options)#

  • options <Object>
    • timeout <number> Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, an Error will be thrown.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is interrupted, an Error will be thrown.
  • Returns: <Promise>

Evaluate the module.

This must be called after the module has been instantiated; otherwise it will throw an error. It could be called also when the module has already been evaluated, in which case it will do one of the following two things:

  • return undefined if the initial evaluation ended in success (module.status is 'evaluated')
  • rethrow the same exception the initial evaluation threw if the initial evaluation ended in an error (module.status is 'errored')

This method cannot be called while the module is being evaluated (module.status is 'evaluating') to prevent infinite recursion.

Corresponds to the Evaluate() concrete method field of Source Text Module Records in the ECMAScript specification.

module.instantiate()#

Instantiate the module. This must be called after linking has completed (linkingStatus is 'linked'); otherwise it will throw an error. It may also throw an exception if one of the dependencies does not provide an export the parent module requires.

However, if this function succeeded, further calls to this function after the initial instantiation will be no-ops, to be consistent with the ECMAScript specification.

Unlike other methods operating on Module, this function completes synchronously and returns nothing.

Corresponds to the Instantiate() concrete method field of Source Text Module Records in the ECMAScript specification.

module.link(linker)#

Link module dependencies. This method must be called before instantiation, and can only be called once per module.

Two parameters will be passed to the linker function:

  • specifier The specifier of the requested module: 
    import foo from 'foo';
//              ^^^^^ the module specifier
  • referencingModule The Module object link() is called on.

The function is expected to return a Module object or a Promise that eventually resolves to a Module object. The returned Module must satisfy the following two invariants:

  • It must belong to the same context as the parent Module.
  • Its linkingStatus must not be 'errored'.

If the returned Module's linkingStatus is 'unlinked', this method will be recursively called on the returned Module with the same provided linker function.

link() returns a Promise that will either get resolved when all linking instances resolve to a valid Module, or rejected if the linker function either throws an exception or returns an invalid Module.

The linker function roughly corresponds to the implementation-defined HostResolveImportedModule abstract operation in the ECMAScript specification, with a few key differences:

  • The linker function is allowed to be asynchronous while HostResolveImportedModule is synchronous.
  • The linker function is executed during linking, a Node.js-specific stage before instantiation, while HostResolveImportedModule is called during instantiation.

The actual HostResolveImportedModule implementation used during module instantiation is one that returns the modules linked during linking. Since at that point all modules would have been fully linked already, the HostResolveImportedModule implementation is fully synchronous per specification.

Class: vm.Script#

Added in: v0.3.1

Instances of the vm.Script class contain precompiled scripts that can be executed in specific sandboxes (or "contexts").

new vm.Script(code, options)#

History
VersionChanges
v5.7.0

The cachedData and produceCachedData options are supported now.

v0.3.1

Added in: v0.3.1

  • code <string> The JavaScript code to compile.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • cachedData <Buffer> Provides an optional Buffer with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully.

Creating a new vm.Script object compiles code but does not run it. The compiled vm.Script can be run later multiple times. The code is not bound to any global object; rather, it is bound before each run, just for that run.

script.runInContext(contextifiedSandbox[, options])#

History
VersionChanges
v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • contextifiedSandbox <Object> A contextified object as returned by the vm.createContext() method.
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • breakOnSigint: if true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script object within the given contextifiedSandbox and returns the result. Running code does not have access to local scope.

The following example compiles code that increments a global variable, sets the value of another global variable, then execute the code multiple times. The globals are contained in the sandbox object.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

const script = new vm.Script('count += 1; name = "kitty";');

const context = vm.createContext(sandbox);
for (let i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(util.inspect(sandbox));

// { animal: 'cat', count: 12, name: 'kitty' }

Using the timeout or breakOnSigint options will result in new event loops and corresponding threads being started, which have a non-zero performance overhead.

script.runInNewContext([sandbox[, options]])#

History
VersionChanges
REPLACEME

The contextCodeGeneration option is supported now.

v0.3.1

Added in: v0.3.1

  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.
    • contextCodeGeneration <Object>
      • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
      • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.

First contextifies the given sandbox, runs the compiled code contained by the vm.Script object within the created sandbox, and returns the result. Running code does not have access to local scope.

The following example compiles code that sets a global variable, then executes the code multiple times in different contexts. The globals are set on and contained within each individual sandbox.

const util = require('util');
const vm = require('vm');

const script = new vm.Script('globalVar = "set"');

const sandboxes = [{}, {}, {}];
sandboxes.forEach((sandbox) => {
  script.runInNewContext(sandbox);
});

console.log(util.inspect(sandboxes));

// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

script.runInThisContext(options)#

Added in: v0.3.1
  • options <Object>
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

Runs the compiled code contained by the vm.Script within the context of the current global object. Running code does not have access to local scope, but does have access to the current global object.

The following example compiles code that increments a global variable then executes that code multiple times:

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (let i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

vm.createContext([sandbox[, options]])#

History
VersionChanges
REPLACEME

The sandbox option can no longer be a function.

REPLACEME

The codeGeneration option is supported now.

v0.3.1

Added in: v0.3.1

  • sandbox <Object>
  • options <Object>
    • name <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • origin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.
    • codeGeneration <Object>
      • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
      • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.

If given a sandbox object, the vm.createContext() method will prepare that sandbox so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside such scripts, the sandbox object will be the global object, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

const util = require('util');
const vm = require('vm');

global.globalVar = 3;

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

vm.runInContext('globalVar *= 2;', sandbox);

console.log(util.inspect(sandbox)); // { globalVar: 2 }

console.log(util.inspect(globalVar)); // 3

If sandbox is omitted (or passed explicitly as undefined), a new, empty contextified sandbox object will be returned.

The vm.createContext() method is primarily useful for creating a single sandbox that can be used to run multiple scripts. For instance, if emulating a web browser, the method can be used to create a single sandbox representing a window's global object, then run all <script> tags together within the context of that sandbox.

The provided name and origin of the context are made visible through the Inspector API.

vm.isContext(sandbox)#

Added in: v0.11.7

Returns true if the given sandbox object has been contextified using vm.createContext().

vm.runInContext(code, contextifiedSandbox[, options])#

  • code <string> The JavaScript code to compile and run.
  • contextifiedSandbox <Object> The contextified object that will be used as the global when the code is compiled and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInContext() method compiles code, runs it within the context of the contextifiedSandbox, then returns the result. Running code does not have access to the local scope. The contextifiedSandbox object must have been previously contextified using the vm.createContext() method.

The following example compiles and executes different scripts using a single contextified object:

const util = require('util');
const vm = require('vm');

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

for (let i = 0; i < 10; ++i) {
  vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));

// { globalVar: 1024 }

vm.runInNewContext(code[, sandbox][, options])#

Added in: v0.3.1
  • code <string> The JavaScript code to compile and run.
  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.
    • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.

The vm.runInNewContext() first contextifies the given sandbox object (or creates a new sandbox if passed as undefined), compiles the code, runs it within the context of the created context, then returns the result. Running code does not have access to the local scope.

The following example compiles and executes code that increments a global variable and sets a new one. These globals are contained in the sandbox.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));

// { animal: 'cat', count: 3, name: 'kitty' }

vm.runInThisContext(code[, options])#

Added in: v0.3.1
  • code <string> The JavaScript code to compile and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.

The following example illustrates using both vm.runInThisContext() and the JavaScript eval() function to run the same code:

const vm = require('vm');
let localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);

const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);

// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'

Because vm.runInThisContext() does not have access to the local scope, localVar is unchanged. In contrast, eval() does have access to the local scope, so the value localVar is changed. In this way vm.runInThisContext() is much like an indirect eval() call, e.g. (0,eval)('code').

Example: Running an HTTP Server within a VM#

When using either script.runInThisContext() or vm.runInThisContext(), the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the http module the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

const code = `
((require) => {
  const http = require('http');

  http.createServer((request, response) => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end('Hello World\\n');
  }).listen(8124);

  console.log('Server running at http://127.0.0.1:8124/');
})`;

vm.runInThisContext(code)(require);

The require() in the above case shares the state with the context it is passed from. This may introduce risks when untrusted code is executed, e.g. altering objects in the context in unwanted ways.

What does it mean to "contextify" an object?#

All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method vm.createContext() is called, the sandbox object that is passed in (or a newly created object if sandbox is undefined) is associated internally with a new instance of a V8 Context. This V8 Context provides the code run using the vm module's methods with an isolated global environment within which it can operate. The process of creating the V8 Context and associating it with the sandbox object is what this document refers to as "contextifying" the sandbox.

Zlib#

Stability: 2 - Stable

The zlib module provides compression functionality implemented using Gzip and Deflate/Inflate. It can be accessed using:

const zlib = require('zlib');

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream data through a zlib stream into a destination stream:

const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');

inp.pipe(gzip).pipe(out);

It is also possible to compress or decompress data in a single step:

const input = '.................................';
zlib.deflate(input, (err, buffer) => {
  if (!err) {
    console.log(buffer.toString('base64'));
  } else {
    // handle error
  }
});

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
  if (!err) {
    console.log(buffer.toString());
  } else {
    // handle error
  }
});

Threadpool Usage#

Note that all zlib APIs except those that are explicitly synchronous use libuv's threadpool, which can have surprising and negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information.

Compressing HTTP requests and responses#

The zlib module can be used to implement support for the gzip and deflate content-encoding mechanisms defined by HTTP.

The HTTP Accept-Encoding header is used within an http request to identify the compression encodings accepted by the client. The Content-Encoding header is used to identify the compression encodings actually applied to a message.

The examples given below are drastically simplified to show the basic concept. Using zlib encoding can be expensive, and the results ought to be cached. See Memory Usage Tuning for more information on the speed/memory/compression tradeoffs involved in zlib usage.

// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: 'example.com',
                           path: '/',
                           port: 80,
                           headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
  const output = fs.createWriteStream('example.com_index.html');

  switch (response.headers['content-encoding']) {
    // or, just use zlib.createUnzip() to handle both cases
    case 'gzip':
      response.pipe(zlib.createGunzip()).pipe(output);
      break;
    case 'deflate':
      response.pipe(zlib.createInflate()).pipe(output);
      break;
    default:
      response.pipe(output);
      break;
  }
});

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
http.createServer((request, response) => {
  const raw = fs.createReadStream('index.html');
  let acceptEncoding = request.headers['accept-encoding'];
  if (!acceptEncoding) {
    acceptEncoding = '';
  }

  // Note: This is not a conformant accept-encoding parser.
  // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (/\bdeflate\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    raw.pipe(zlib.createDeflate()).pipe(response);
  } else if (/\bgzip\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    raw.pipe(zlib.createGzip()).pipe(response);
  } else {
    response.writeHead(200, {});
    raw.pipe(response);
  }
}).listen(1337);

By default, the zlib methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to decompress the last chunk of input data:

// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');

zlib.unzip(
  buffer,
  { finishFlush: zlib.constants.Z_SYNC_FLUSH },
  (err, buffer) => {
    if (!err) {
      console.log(buffer.toString());
    } else {
      // handle error
    }
  });

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

Memory Usage Tuning#

From zlib/zconf.h, modified to Node.js's usage:

The memory requirements for deflate are (in bytes):

(1 << (windowBits + 2)) + (1 << (memLevel + 9))

That is: 128K for windowBits = 15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:

const options = { windowBits: 14, memLevel: 7 };

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes) 1 << windowBits. That is, 32K for windowBits = 15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size chunkSize, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to zlib because it will be able to process more data on each write operation. So, this is another factor that affects the speed, at the cost of memory usage.

Flushing#

Calling .flush() on a compression stream will make zlib return as much output as currently possible. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, flush() is used to write a compressed partial HTTP response to the client:

const zlib = require('zlib');
const http = require('http');

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();
  output.pipe(response);

  setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
      output.flush();
    });
  }, 1000);
}).listen(1337);

Constants#

Added in: v0.5.8

All of the constants defined in zlib.h are also defined on require('zlib').constants. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation. See https://zlib.net/manual.html#Constants for more details.

Previously, the constants were available directly from require('zlib'), for instance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is currently still possible but is deprecated.

Allowed flush values.

  • zlib.constants.Z_NO_FLUSH
  • zlib.constants.Z_PARTIAL_FLUSH
  • zlib.constants.Z_SYNC_FLUSH
  • zlib.constants.Z_FULL_FLUSH
  • zlib.constants.Z_FINISH
  • zlib.constants.Z_BLOCK
  • zlib.constants.Z_TREES

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

  • zlib.constants.Z_OK
  • zlib.constants.Z_STREAM_END
  • zlib.constants.Z_NEED_DICT
  • zlib.constants.Z_ERRNO
  • zlib.constants.Z_STREAM_ERROR
  • zlib.constants.Z_DATA_ERROR
  • zlib.constants.Z_MEM_ERROR
  • zlib.constants.Z_BUF_ERROR
  • zlib.constants.Z_VERSION_ERROR

Compression levels.

  • zlib.constants.Z_NO_COMPRESSION
  • zlib.constants.Z_BEST_SPEED
  • zlib.constants.Z_BEST_COMPRESSION
  • zlib.constants.Z_DEFAULT_COMPRESSION

Compression strategy.

  • zlib.constants.Z_FILTERED
  • zlib.constants.Z_HUFFMAN_ONLY
  • zlib.constants.Z_RLE
  • zlib.constants.Z_FIXED
  • zlib.constants.Z_DEFAULT_STRATEGY

Class Options#

History
VersionChanges
v9.4.0

The dictionary option can be an ArrayBuffer.

v8.0.0

The dictionary option can be an Uint8Array now.

v5.11.0

The finishFlush option is supported now.

v0.11.1

Added in: v0.11.1

Each class takes an options object. All options are optional.

Note that some options are only relevant when compressing, and are ignored by the decompression classes.

See the description of deflateInit2 and inflateInit2 at https://zlib.net/manual.html#Advanced for more information on these.

Class: zlib.Deflate#

Added in: v0.5.8

Compress data using deflate.

Class: zlib.DeflateRaw#

Added in: v0.5.8

Compress data using deflate, and do not append a zlib header.

Class: zlib.Gunzip#

History
VersionChanges
v6.0.0

Trailing garbage at the end of the input stream will now result in an error event.

v5.9.0

Multiple concatenated gzip file members are supported now.

v5.0.0

A truncated input stream will now result in an error event.

v0.5.8

Added in: v0.5.8

Decompress a gzip stream.

Class: zlib.Gzip#

Added in: v0.5.8

Compress data using gzip.

Class: zlib.Inflate#

History
VersionChanges
v5.0.0

A truncated input stream will now result in an error event.

v0.5.8

Added in: v0.5.8

Decompress a deflate stream.

Class: zlib.InflateRaw#

History
VersionChanges
v6.8.0

Custom dictionaries are now supported by InflateRaw.

v5.0.0

A truncated input stream will now result in an error event.

v0.5.8

Added in: v0.5.8

Decompress a raw deflate stream.

Class: zlib.Unzip#

Added in: v0.5.8

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

Class: zlib.Zlib#

Added in: v0.5.8

Not exported by the zlib module. It is documented here because it is the base class of the compressor/decompressor classes.

zlib.bytesRead#

Added in: v8.1.0

The zlib.bytesRead property specifies the number of bytes read by the engine before the bytes are processed (compressed or decompressed, as appropriate for the derived class).

zlib.close([callback])#

Added in: v0.9.4

Close the underlying handle.

zlib.flush([kind], callback)#

Added in: v0.5.8
  • kind Default: zlib.constants.Z_FULL_FLUSH

Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal zlib state, and does not perform flushing of any kind on the streams level. Rather, it behaves like a normal call to .write(), i.e. it will be queued up behind other pending writes and will only produce output when data is being read from the stream.

zlib.params(level, strategy, callback)#

Added in: v0.11.4

Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.

zlib.reset()#

Added in: v0.7.0

Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.

zlib.constants#

Added in: v7.0.0

Provides an object enumerating Zlib-related constants.

zlib.createDeflate(options)#

Added in: v0.5.8

Creates and returns a new Deflate object with the given options.

zlib.createDeflateRaw(options)#

Added in: v0.5.8

Creates and returns a new DeflateRaw object with the given options.

An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits is set to 8 for raw deflate streams. zlib would automatically set windowBits to 9 if was initially set to 8. Newer versions of zlib will throw an exception, so Node.js restored the original behavior of upgrading a value of 8 to 9, since passing windowBits = 9 to zlib actually results in a compressed stream that effectively uses an 8-bit window only.

zlib.createGunzip(options)#

Added in: v0.5.8

Creates and returns a new Gunzip object with the given options.

zlib.createGzip(options)#

Added in: v0.5.8

Creates and returns a new Gzip object with the given options.

zlib.createInflate(options)#

Added in: v0.5.8

Creates and returns a new Inflate object with the given options.

zlib.createInflateRaw(options)#

Added in: v0.5.8

Creates and returns a new InflateRaw object with the given options.

zlib.createUnzip(options)#

Added in: v0.5.8

Creates and returns a new Unzip object with the given options.

Convenience Methods#

All of these take a Buffer, TypedArray, DataView, ArrayBuffer or string as the first argument, an optional second argument to supply options to the zlib classes and will call the supplied callback with callback(error, result).

Every method has a *Sync counterpart, which accept the same arguments, but without a callback.

zlib.deflate(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.deflateSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with Deflate.

zlib.deflateRaw(buffer[, options], callback)#

History
VersionChanges
v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.deflateRawSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with DeflateRaw.

zlib.gunzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.gunzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Gunzip.

zlib.gzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.gzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with Gzip.

zlib.inflate(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.inflateSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Inflate.

zlib.inflateRaw(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.inflateRawSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with InflateRaw.

zlib.unzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.unzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView now.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Unzip.