Table of Contents

• Modules: Packages

  • Introduction

  • Determining module system

    • package.json "type" field
    • Package scope and file extensions
    • --input-type flag

  • Package entry points

    • Main entry point export
    • Subpath exports
    • Package exports fallbacks
    • Exports sugar
    • Conditional exports
    • Nested conditions
    • Resolving user conditions
    • Self-referencing a package using its name
  • Internal package imports

  • Dual CommonJS/ES module packages

    • Dual package hazard

    • Writing dual packages while avoiding or minimizing hazards

      • Approach #1: Use an ES module wrapper
      • Approach #2: Isolate state

Modules: Packages#

Introduction#

A package is a folder described by a package.json file.

A folder containing a package.json file, and all subfolders below that folder until the next folder containing another package.json file, are a package scope.

Determining module system#

Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements within ES module code:

• Files ending in .mjs.

• Files ending in .js when the nearest parent package.json file contains a top-level field "type" with a value of "module".

• Strings passed in as an argument to --eval, or piped to node via STDIN, with the flag --input-type=module.

Node.js will treat as CommonJS all other forms of input, such as .js files where the nearest parent package.json file contains no top-level "type" field, or string input without the flag --input-type. This behavior is to preserve backward compatibility. However, now that Node.js supports both CommonJS and ES modules, it is best to be explicit whenever possible. Node.js will treat the following as CommonJS when passed to node as the initial input, or when referenced by import statements within ES module code:

• Files ending in .cjs.

• Files ending in .js when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

• Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=commonjs.

package.json "type" field#

Files ending with .js will be loaded as ES modules when the nearest parent package.json file contains a top-level field "type" with a value of "module".

The nearest parent package.json is defined as the first package.json found when searching in the current folder, that folder’s parent, and so on up until the root of the volume is reached.

// package.json
{
  "type": "module"
}

# In same folder as preceding package.json
node my-app.js # Runs as ES module

If the nearest parent package.json lacks a "type" field, or contains "type": "commonjs", .js files are treated as CommonJS. If the volume root is reached and no package.json is found, Node.js defers to the default, a package.json with no "type" field.

import statements of .js files are treated as ES modules if the nearest parent package.json contains "type": "module".

// my-app.js, part of the same example as above
import './startup.js'; // Loaded as ES module because of package.json

Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

Regardless of the value of the "type" field, .mjs files are always treated as ES modules and .cjs files are always treated as CommonJS.

Package scope and file extensions#

A folder containing a package.json file, and all subfolders below that folder until the next folder containing another package.json, are a package scope. Package scopes do not carry through node_modules folders. The "type" field defines how to treat .js files within the package scope. If a package.json file does not have a "type" field, the default is "commonjs".

The package scope applies not only to initial entry points (node my-app.js) but also to files referenced by import statements and import() expressions.

// my-app.js, in an ES module package scope because there is a package.json
// file in the same folder with "type": "module".

import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the ES module package scope from one level up.

import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

import './node_modules/commonjs-package/index.js';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

Files ending with .mjs are always loaded as ES modules regardless of package scope.

Files ending with .cjs are always loaded as CommonJS regardless of package scope.

import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always loaded as CommonJS.

import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always loaded as ES module.

The .mjs and .cjs extensions may be used to mix types within the same package scope:

• Within a "type": "module" package scope, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a .cjs extension (since both .js and .mjs files are treated as ES modules within a "module" package scope).

• Within a "type": "commonjs" package scope, Node.js can be instructed to interpret a particular file as an ES module by naming it with an .mjs extension (since both .js and .cjs files are treated as CommonJS within a "commonjs" package scope).

--input-type flag#

Strings passed in as an argument to --eval (or -e), or piped to node via STDIN, will be treated as ES modules when the --input-type=module flag is set.

node --input-type=module --eval "import { sep } from 'path'; console.log(sep);"

echo "import { sep } from 'path'; console.log(sep);" | node --input-type=module

For completeness there is also --input-type=commonjs, for explicitly running string input as CommonJS. This is the default behavior if --input-type is unspecified.

Package entry points#

In a package’s package.json file, two fields can define entry points for a package: "main" and "exports". The "main" field is supported in all versions of Node.js, but its capabilities are limited: it only defines the main entry point of the package.

The "exports" field provides an alternative to "main" where the package main entry point can be defined while also encapsulating the package, preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to define a public interface for their package.

If both "exports" and "main" are defined, the "exports" field takes precedence over "main". "exports" are not specific to ES modules or CommonJS; "main" will be overridden by "exports" if it exists. As such "main" cannot be used as a fallback for CommonJS but it can be used as a fallback for legacy versions of Node.js that do not support the "exports" field.

Conditional exports can be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import. For more information about supporting both CommonJS and ES Modules in a single package please consult the dual CommonJS/ES module packages section.

Warning: Introducing the "exports" field prevents consumers of a package from using any entry points that are not defined, including the package.json (e.g. require('your-package/package.json'). This will likely be a breaking change.

To make the introduction of "exports" non-breaking, ensure that every previously supported entry point is exported. It is best to explicitly specify entry points so that the package’s public API is well-defined. For example, a project that previous exported main, lib, feature, and the package.json could use the following package.exports:

{
  "name": "my-mod",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/index": "./lib/index.js",
    "./lib/index.js": "./lib/index.js",
    "./feature": "./feature/index.js",
    "./feature/index.js": "./feature/index.js",
    "./package.json": "./package.json"
  }
}

Alternatively a project could choose to export entire folders:

{
  "name": "my-mod",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/": "./lib/",
    "./feature": "./feature/index.js",
    "./feature/": "./feature/",
    "./package.json": "./package.json"
  }
}

As a last resort, package encapsulation can be disabled entirely by creating an export for the root of the package "./": "./". This will expose every file in the package at the cost of disabling the encapsulation and potential tooling benefits this provides. As the ES Module loader in Node.js enforces the use of the full specifier path, exporting the root rather than being explicit about entry is less expressive than either of the prior examples. Not only will encapsulation be lost but module consumers will be unable to import feature from 'my-mod/feature' as they will need to provide the full path import feature from 'my-mod/feature/index.js.

Main entry point export#

To set the main entry point for a package, it is advisable to define both "exports" and "main" in the package’s package.json file:

{
  "main": "./main.js",
  "exports": "./main.js"
}

The benefit of doing this is that when using the "exports" field all subpaths of the package will no longer be available to importers under require('pkg/subpath.js'), and instead they will get a new error, ERR_PACKAGE_PATH_NOT_EXPORTED.

This encapsulation of exports provides more reliable guarantees about package interfaces for tools and when handling semver upgrades for a package. It is not a strong encapsulation since a direct require of any absolute subpath of the package such as require('/path/to/node_modules/pkg/subpath.js') will still load subpath.js.

Subpath exports#

When using the "exports" field, custom subpaths can be defined along with the main entry point by treating the main entry point as the "." subpath:

{
  "main": "./main.js",
  "exports": {
    ".": "./main.js",
    "./submodule": "./src/submodule.js"
  }
}

Now only the defined subpath in "exports" can be imported by a consumer:

import submodule from 'es-module-package/submodule';
// Loads ./node_modules/es-module-package/src/submodule.js

While other subpaths will error:

import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED

Entire folders can also be mapped with package exports:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/": "./src/features/"
  }
}

With the preceding, all modules within the ./src/features/ folder are exposed deeply to import and require:

import feature from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

When using folder mappings, ensure that you do want to expose every module inside the subfolder. Any modules which are not public should be moved to another folder to retain the encapsulation benefits of exports.

Package exports fallbacks#

For possible new specifier support in future, array fallbacks are supported for all invalid specifiers:

{
  "exports": {
    "./submodule": ["not:valid", "./submodule.js"]
  }
}

Since "not:valid" is not a valid specifier, "./submodule.js" is used instead as the fallback, as if it were the only target.

Exports sugar#

If the "." export is the only export, the "exports" field provides sugar for this case being the direct "exports" field value.

If the "." export has a fallback array or string value, then the "exports" field can be set to this value directly.

{
  "exports": {
    ".": "./main.js"
  }
}

can be written:

{
  "exports": "./main.js"
}

Conditional exports#

Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports for require() and import can be written:

// package.json
{
  "main": "./main-require.cjs",
  "exports": {
    "import": "./main-module.js",
    "require": "./main-require.cjs"
  },
  "type": "module"
}

Node.js supports the following conditions out of the box:

• "import" - matched when the package is loaded via import or import(). Can reference either an ES module or CommonJS file, as both import and import() can load either ES module or CommonJS sources. Always matched when the "require" condition is not matched.
• "require" - matched when the package is loaded via require(). As require() only supports CommonJS, the referenced file must be CommonJS. Always matched when the "import" condition is not matched.
• "node" - matched for any Node.js environment. Can be a CommonJS or ES module file. This condition should always come after "import" or "require".
• "default" - the generic fallback that will always match. Can be a CommonJS or ES module file. This condition should always come last.

Within the "exports" object, key order is significant. During condition matching, earlier entries have higher priority and take precedence over later entries. The general rule is that conditions should be from most specific to least specific in object order.

Other conditions such as "browser", "electron", "deno", "react-native", etc. are unknown to, and thus ignored by Node.js. Runtimes or tools other than Node.js may use them at their discretion. Further restrictions, definitions, or guidance on condition names may occur in the future.

Using the "import" and "require" conditions can lead to some hazards, which are further explained in the dual CommonJS/ES module packages section.

Conditional exports can also be extended to exports subpaths, for example:

{
  "main": "./main.js",
  "exports": {
    ".": "./main.js",
    "./feature": {
      "node": "./feature-node.js",
      "default": "./feature.js"
    }
  }
}

Defines a package where require('pkg/feature') and import 'pkg/feature' could provide different implementations between Node.js and other JS environments.

When using environment branches, always include a "default" condition where possible. Providing a "default" condition ensures that any unknown JS environments are able to use this universal implementation, which helps avoid these JS environments from having to pretend to be existing environments in order to support packages with conditional exports. For this reason, using "node" and "default" condition branches is usually preferable to using "node" and "browser" condition branches.

Nested conditions#

In addition to direct mappings, Node.js also supports nested condition objects.

For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:

{
  "main": "./main.js",
  "exports": {
    "node": {
      "import": "./feature-node.mjs",
      "require": "./feature-node.cjs"
    },
    "default": "./feature.mjs",
  }
}

Conditions continue to be matched in order as with flat conditions. If a nested conditional does not have any mapping it will continue checking the remaining conditions of the parent condition. In this way nested conditions behave analogously to nested JavaScript if statements.

Resolving user conditions#

When running Node.js, custom user conditions can be added with the --conditions flag:

node --conditions=development main.js

which would then resolve the "development" condition in package imports and exports, while resolving the existing "node", "default", "import", and "require" conditions as appropriate.

Any number of custom conditions can be set with repeat flags.

Self-referencing a package using its name#

Within a package, the values defined in the package’s package.json "exports" field can be referenced via the package’s name. For example, assuming the package.json is:

// package.json
{
  "name": "a-package",
  "exports": {
    ".": "./main.mjs",
    "./foo": "./foo.js"
  }
}

Then any module in that package can reference an export in the package itself:

// ./a-module.mjs
import { something } from 'a-package'; // Imports "something" from ./main.mjs.

Self-referencing is available only if package.json has exports, and will allow importing only what that exports (in the package.json) allows. So the code below, given the previous package, will generate a runtime error:

// ./another-module.mjs

// Imports "another" from ./m.mjs. Fails because
// the "package.json" "exports" field
// does not provide an export named "./m.mjs".
import { another } from 'a-package/m.mjs';

Self-referencing is also available when using require, both in an ES module, and in a CommonJS one. For example, this code will also work:

// ./a-module.js
const { something } = require('a-package/foo'); // Loads from ./foo.js.

Internal package imports#

In addition to the "exports" field it is possible to define internal package import maps that only apply to import specifiers from within the package itself.

Entries in the imports field must always start with # to ensure they are clearly disambiguated from package specifiers.

For example, the imports field can be used to gain the benefits of conditional exports for internal modules:

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
}

where import '#dep' would now get the resolution of the external package dep-node-native (including its exports in turn), and instead get the local file ./dep-polyfill.js relative to the package in other environments.

Unlike the exports field, import maps permit mapping to external packages because this provides an important use case for conditional loading and also can be done without the risk of cycles, unlike for exports.

Apart from the above, the resolution rules for the imports field are otherwise analogous to the exports field.

Dual CommonJS/ES module packages#

Prior to the introduction of support for ES modules in Node.js, it was a common pattern for package authors to include both CommonJS and ES module JavaScript sources in their package, with package.json "main" specifying the CommonJS entry point and package.json "module" specifying the ES module entry point. This enabled Node.js to run the CommonJS entry point while build tools such as bundlers used the ES module entry point, since Node.js ignored (and still ignores) the top-level "module" field.

Node.js can now run ES module entry points, and a package can contain both CommonJS and ES module entry points (either via separate specifiers such as 'pkg' and 'pkg/es-module', or both at the same specifier via Conditional exports). Unlike in the scenario where "module" is only used by bundlers, or ES module files are transpiled into CommonJS on the fly before evaluation by Node.js, the files referenced by the ES module entry point are evaluated as ES modules.

Dual package hazard#

When an application is using a package that provides both CommonJS and ES module sources, there is a risk of certain bugs if both versions of the package get loaded. This potential comes from the fact that the pkgInstance created by const pkgInstance = require('pkg') is not the same as the pkgInstance created by import pkgInstance from 'pkg' (or an alternative main path like 'pkg/module'). This is the “dual package hazard,” where two versions of the same package can be loaded within the same runtime environment. While it is unlikely that an application or package would intentionally load both versions directly, it is common for an application to load one version while a dependency of the application loads the other version. This hazard can happen because Node.js supports intermixing CommonJS and ES modules, and can lead to unexpected behavior.

If the package main export is a constructor, an instanceof comparison of instances created by the two versions returns false, and if the export is an object, properties added to one (like pkgInstance.foo = 3) are not present on the other. This differs from how import and require statements work in all-CommonJS or all-ES module environments, respectively, and therefore is surprising to users. It also differs from the behavior users are familiar with when using transpilation via tools like Babel or esm.

Writing dual packages while avoiding or minimizing hazards#

First, the hazard described in the previous section occurs when a package contains both CommonJS and ES module sources and both sources are provided for use in Node.js, either via separate main entry points or exported paths. A package could instead be written where any version of Node.js receives only CommonJS sources, and any separate ES module sources the package may contain could be intended only for other environments such as browsers. Such a package would be usable by any version of Node.js, since import can refer to CommonJS files; but it would not provide any of the advantages of using ES module syntax.

A package could also switch from CommonJS to ES module syntax in a breaking change version bump. This has the disadvantage that the newest version of the package would only be usable in ES module-supporting versions of Node.js.

Every pattern has tradeoffs, but there are two broad approaches that satisfy the following conditions:

1. The package is usable via both require and import.
2. The package is usable in both current Node.js and older versions of Node.js that lack support for ES modules.
3. The package main entry point, e.g. 'pkg' can be used by both require to resolve to a CommonJS file and by import to resolve to an ES module file. (And likewise for exported paths, e.g. 'pkg/feature'.)
4. The package provides named exports, e.g. import { name } from 'pkg' rather than import pkg from 'pkg'; pkg.name.
5. The package is potentially usable in other ES module environments such as browsers.
6. The hazards described in the previous section are avoided or minimized.

Approach #1: Use an ES module wrapper#

Write the package in CommonJS or transpile ES module sources into CommonJS, and create an ES module wrapper file that defines the named exports. Using Conditional exports, the ES module wrapper is used for import and the CommonJS entry point for require.

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    "import": "./wrapper.mjs",
    "require": "./index.cjs"
  }
}

The preceding example uses explicit extensions .mjs and .cjs. If your files use the .js extension, "type": "module" will cause such files to be treated as ES modules, just as "type": "commonjs" would cause them to be treated as CommonJS. See Enabling.

// ./node_modules/pkg/index.cjs
exports.name = 'value';

// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;

In this example, the name from import { name } from 'pkg' is the same singleton as the name from const { name } = require('pkg'). Therefore === returns true when comparing the two names and the divergent specifier hazard is avoided.

If the module is not simply a list of named exports, but rather contains a unique function or object export like module.exports = function () { ... }, or if support in the wrapper for the import pkg from 'pkg' pattern is desired, then the wrapper would instead be written to export the default optionally along with any named exports as well:

import cjsModule from './index.cjs';
export const name = cjsModule.name;
export default cjsModule;

This approach is appropriate for any of the following use cases:

• The package is currently written in CommonJS and the author would prefer not to refactor it into ES module syntax, but wishes to provide named exports for ES module consumers.
• The package has other packages that depend on it, and the end user might install both this package and those other packages. For example a utilities package is used directly in an application, and a utilities-plus package adds a few more functions to utilities. Because the wrapper exports underlying CommonJS files, it doesn’t matter if utilities-plus is written in CommonJS or ES module syntax; it will work either way.
• The package stores internal state, and the package author would prefer not to refactor the package to isolate its state management. See the next section.

A variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package. This could be used via import 'pkg/module' by users who are certain that the CommonJS version will not be loaded anywhere in the application, such as by dependencies; or if the CommonJS version can be loaded but doesn’t affect the ES module version (for example, because the package is stateless):

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    ".": "./index.cjs",
    "./module": "./wrapper.mjs"
  }
}

Approach #2: Isolate state#

A package.json file can define the separate CommonJS and ES module entry points directly:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    "import": "./index.mjs",
    "require": "./index.cjs"
  }
}

This can be done if both the CommonJS and ES module versions of the package are equivalent, for example because one is the transpiled output of the other; and the package’s management of state is carefully isolated (or the package is stateless).

The reason that state is an issue is because both the CommonJS and ES module versions of the package may get used within an application; for example, the user’s application code could import the ES module version while a dependency requires the CommonJS version. If that were to occur, two copies of the package would be loaded in memory and therefore two separate states would be present. This would likely cause hard-to-troubleshoot bugs.

Aside from writing a stateless package (if JavaScript’s Math were a package, for example, it would be stateless as all of its methods are static), there are some ways to isolate state so that it’s shared between the potentially loaded CommonJS and ES module instances of the package:

1. If possible, contain all state within an instantiated object. JavaScript’s Date, for example, needs to be instantiated to contain state; if it were a package, it would be used like this:

   import Date from 'date';
   const someDate = new Date();
   // someDate contains state; Date does not

   The new keyword isn’t required; a package’s function can return a new object, or modify a passed-in object, to keep the state external to the package.

2. Isolate the state in one or more CommonJS files that are shared between the CommonJS and ES module versions of the package. For example, if the CommonJS and ES module entry points are index.cjs and index.mjs, respectively:

   // ./node_modules/pkg/index.cjs
   const state = require('./state.cjs');
   module.exports.state = state;

   // ./node_modules/pkg/index.mjs
   import state from './state.cjs';
   export {
     state
   };

   Even if pkg is used via both require and import in an application (for example, via import in application code and via require by a dependency) each reference of pkg will contain the same state; and modifying that state from either module system will apply to both.

Any plugins that attach to the package’s singleton would need to separately attach to both the CommonJS and ES module singletons.

This approach is appropriate for any of the following use cases:

• The package is currently written in ES module syntax and the package author wants that version to be used wherever such syntax is supported.
• The package is stateless or its state can be isolated without too much difficulty.
• The package is unlikely to have other public packages that depend on it, or if it does, the package is stateless or has state that need not be shared between dependencies or with the overall application.

Even with isolated state, there is still the cost of possible extra code execution between the CommonJS and ES module versions of a package.

As with the previous approach, a variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    ".": "./index.cjs",
    "./module": "./index.mjs"
  }
}