path: root/doc/api/esm.md

# ECMAScript Modules

<!--introduced_in=v8.5.0-->
<!-- type=misc -->

> Stability: 1 - Experimental

## Introduction

<!--name=esm-->

ECMAScript modules are [the official standard format][] to package JavaScript
code for reuse. Modules are defined using a variety of [`import`][] and
[`export`][] statements.

Node.js fully supports ECMAScript modules as they are currently specified and
provides limited interoperability between them and the existing module format,
[CommonJS][].

Node.js contains support for ES Modules based upon the
[Node.js EP for ES Modules][] and the [ECMAScript-modules implementation][].

Expect major changes in the implementation including interoperability support,
specifier resolution, and default behavior.

## Enabling

<!-- type=misc -->

The `--experimental-modules` flag can be used to enable support for
ECMAScript modules (ES modules).

## Running Node.js with an ECMAScript Module

There are a few ways to start Node.js with an ES module as its input.

### Initial entry point with an <code>.mjs</code> extension

A file ending with `.mjs` passed to Node.js as an initial entry point will be
loaded as an ES module.

```sh
node --experimental-modules my-app.mjs
```

### <code>--entry-type=module</code> flag

Files ending with `.js` or `.mjs`, or lacking any extension,
will be loaded as ES modules when the `--entry-type=module` flag is set.

```sh
node --experimental-modules --entry-type=module my-app.js
```

For completeness there is also `--entry-type=commonjs`, for explicitly running
a `.js` file as CommonJS. This is the default behavior if `--entry-type` is
unspecified.

The `--entry-type=module` flag can also be used to configure Node.js to treat
as an ES module input sent in via `--eval` or `--print` (or `-e` or `-p`) or
piped to Node.js via `STDIN`.

```sh
node --experimental-modules --entry-type=module --eval \
  "import { sep } from 'path'; console.log(sep);"

echo "import { sep } from 'path'; console.log(sep);" | \
  node --experimental-modules --entry-type=module
```

### <code>package.json</code> <code>"type"</code> field

Files ending with `.js` or `.mjs`, or lacking any extension,
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.

<!-- eslint-skip -->
```js
// package.json
{
  "type": "module"
}
```

```sh
# In same folder as above package.json
node --experimental-modules my-app.js # Runs as ES module
```

If the nearest parent `package.json` lacks a `"type"` field, or contains
`"type": "commonjs"`, extensionless and `.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.

## Package Scope and File Extensions

A folder containing a `package.json` file, and all subfolders below that
folder down until the next folder containing another `package.json`, is
considered a _package scope_. The `"type"` field defines how `.js` and
extensionless files should be treated within a particular `package.json` file’s
package scope. Every package in a project’s `node_modules` folder contains its
own `package.json` file, so each project’s dependencies have their own package
scopes. A `package.json` lacking a `"type"` field is treated as if it contained
`"type": "commonjs"`.

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

```js
// 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.

```js
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).

## Package Entry Points

The `package.json` `"main"` field defines the entry point for a package,
whether the package is included into CommonJS via `require` or into an ES
module via `import`.

<!-- eslint-skip -->
```js
// ./node_modules/es-module-package/package.json
{
  "type": "module",
  "main": "./src/index.js"
}
```
```js
// ./my-app.mjs

import { something } from 'es-module-package';
// Loads from ./node_modules/es-module-package/src/index.js
```

An attempt to `require` the above `es-module-package` would attempt to load
`./node_modules/es-module-package/src/index.js` as CommonJS, which would throw
an error as Node.js would not be able to parse the `export` statement in
CommonJS.

As with `import` statements, for ES module usage the value of `"main"` must be
a full path including extension: `"./index.mjs"`, not `"./index"`.

If the `package.json` `"type"` field is omitted, a `.js` file in `"main"` will
be interpreted as CommonJS.

> Currently a package can define _either_ a CommonJS entry point **or** an ES
> module entry point; there is no way to specify separate entry points for
> CommonJS and ES module usage. This means that a package entry point can be
> included via `require` or via `import` but not both.
>
> Such a limitation makes it difficult for packages to support both new versions
> of Node.js that understand ES modules and older versions of Node.js that
> understand only CommonJS. There is work ongoing to remove this limitation, and
> it will very likely entail changes to the behavior of `"main"` as defined
> here.

## <code>import</code> Specifiers

### Terminology

The _specifier_ of an `import` statement is the string after the `from` keyword,
e.g. `'path'` in `import { sep } from 'path'`. Specifiers are also used in
`export from` statements, and as the argument to an `import()` expression.

There are four types of specifiers:

- _Bare specifiers_ like `'some-package'`. They refer to an entry point of a
  package by the package name.

- _Deep import specifiers_ like `'some-package/lib/shuffle.mjs'`. They refer to
  a path within a package prefixed by the package name.

- _Relative specifiers_ like `'./startup.js'` or `'../config.mjs'`. They refer
  to a path relative to the location of the importing file.

- _Absolute specifiers_ like `'file:///opt/nodejs/config.js'`. They refer
  directly and explicitly to a full path.

Bare specifiers, and the bare specifier portion of deep import specifiers, are
strings; but everything else in a specifier is a URL.

Only `file://` URLs are supported. A specifier like
`'https://example.com/app.js'` may be supported by browsers but it is not
supported in Node.js.

Specifiers may not begin with `/` or `//`. These are reserved for potential
future use. The root of the current volume may be referenced via `file:///`.

## import.meta

* {Object}

The `import.meta` metaproperty is an `Object` that contains the following
property:

* `url` {string} The absolute `file:` URL of the module.

## Differences Between ES Modules and CommonJS

### Mandatory file extensions

A file extension must be provided when using the `import` keyword. Directory
indexes (e.g. `'./startup/index.js'`) must also be fully specified.

This behavior matches how `import` behaves in browser environments, assuming a
typically configured server.

### No <code>NODE_PATH</code>

`NODE_PATH` is not part of resolving `import` specifiers. Please use symlinks
if this behavior is desired.

### No <code>require</code>, <code>exports</code>, <code>module.exports</code>, <code>\_\_filename</code>, <code>\_\_dirname</code>

These CommonJS variables are not available in ES modules.

`require` can be imported into an ES module using
[`module.createRequireFromPath()`][].

An equivalent for `__filename` and `__dirname` is [`import.meta.url`][].

### No <code>require.extensions</code>

`require.extensions` is not used by `import`. The expectation is that loader
hooks can provide this workflow in the future.

### No <code>require.cache</code>

`require.cache` is not used by `import`. It has a separate cache.

### URL-based paths

ES modules are resolved and cached based upon
[URL](https://url.spec.whatwg.org/) 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.

```js
import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"
```

For now, only modules using the `file:` protocol can be loaded.

## Interoperability with CommonJS

### <code>require</code>

`require` always treats the files it references as CommonJS. This applies
whether `require` is used the traditional way within a CommonJS environment, or
in an ES module environment using [`module.createRequireFromPath()`][].

To include an ES module into CommonJS, use [`import()`][].

### <code>import</code> statements

An `import` statement can reference either ES module or CommonJS JavaScript.
Other file types such as JSON and Native modules are not supported. For those,
use [`module.createRequireFromPath()`][].

`import` statements are permitted only in ES modules. For similar functionality
in CommonJS, see [`import()`][].

The _specifier_ of an `import` statement (the string after the `from` keyword)
can either be an URL-style relative path like `'./file.mjs'` or a package name
like `'fs'`.

Like in CommonJS, files within packages can be accessed by appending a path to
the package name.

```js
import { sin, cos } from 'geometry/trigonometry-functions.mjs';
```

> Currently only the “default export” is supported for CommonJS files or
> packages:
>
> <!-- eslint-disable no-duplicate-imports -->
> ```js
> import packageMain from 'commonjs-package'; // Works
>
> import { method } from 'commonjs-package'; // Errors
> ```
>
> There are ongoing efforts to make the latter code possible.

### <code>import()</code> expressions

Dynamic `import()` is supported in both CommonJS and ES modules. It can be used
to include ES module files from CommonJS code.

```js
(async () => {
  await import('./my-app.mjs');
})();
```

## CommonJS, JSON, and Native Modules

CommonJS, JSON, and Native modules can be used with [`module.createRequireFromPath()`][].

```js
// cjs.js
module.exports = 'cjs';

// esm.mjs
import { createRequireFromPath as createRequire } from 'module';
import { fileURLToPath as fromURL } from 'url';

const require = createRequire(fromURL(import.meta.url));

const cjs = require('./cjs');
cjs === 'cjs'; // true
```

## Builtin modules

Builtin modules will provide named exports of their public API, as well as a
default export which can be used for, among other things, modifying the named
exports. Named exports of builtin modules are updated when the corresponding
exports property is accessed, redefined, or deleted.

```js
import EventEmitter from 'events';
const e = new EventEmitter();
```

```js
import { readFile } from 'fs';
readFile('./foo.txt', (err, source) => {
  if (err) {
    console.error(err);
  } else {
    console.log(source);
  }
});
```

```js
import fs, { readFileSync } from 'fs';

fs.readFileSync = () => Buffer.from('Hello, ESM');

fs.readFileSync === readFileSync;
```

## Experimental JSON Modules

**Note: This API is still being designed and is subject to change.**

Currently importing JSON modules are only supported in the `commonjs` mode
and are loaded using the CJS loader. [WHATWG JSON modules][] are currently
being standardized, and are experimentally supported by including the
additional flag `--experimental-json-modules` when running Node.js.

When the `--experimental-json-modules` flag is included both the
`commonjs` and `module` mode will use the new experimental JSON
loader. The imported JSON only exposes a `default`, there is no
support for named exports. A cache entry is created in the CommonJS
cache, to avoid duplication. The same object will be returned in
CommonJS if the JSON module has already been imported from the
same path.

Assuming an `index.js` with

<!-- eslint-skip -->
```js
import packageConfig from './package.json';
```

The `--experimental-json-modules` flag is needed for the module
to work.

```bash
node --experimental-modules --entry-type=module index.js # fails
node --experimental-modules --entry-type=module --experimental-json-modules index.js # works
```

## Experimental Loader hooks

**Note: This API is currently being redesigned and will still change.**

<!-- type=misc -->

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:

```js
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 `parentModuleURL` 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 |
| --- | --- |
| `'module'` | Load a standard JavaScript module |
| `'commonjs'` | Load a Node.js CommonJS module |
| `'builtin'` | Load a Node.js builtin module |
| `'json'` | Load a JSON file |
| `'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:

```js
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:

```console
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.

```js
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.

## Resolution Algorithm

### Features

The resolver has the following properties:

* FileURL-based resolution as is used by ES modules
* Support for builtin module loading
* Relative and absolute URL resolution
* No default extensions
* No folder mains
* Bare specifier package resolution lookup through node_modules

### Resolver Algorithm

The algorithm to load an ES module specifier is given through the
**ESM_RESOLVE** method below. It returns the resolved URL for a
module specifier relative to a parentURL, in addition to the unique module
format for that resolved URL given by the **ESM_FORMAT** routine.

The _"module"_ format is returned for an ECMAScript Module, while the
_"commonjs"_ format is used to indicate loading through the legacy
CommonJS loader. Additional formats such as _"wasm"_ or _"addon"_ can be
extended in future updates.

In the following algorithms, all subroutine errors are propagated as errors
of these top-level routines.

_isMain_ is **true** when resolving the Node.js application entry point.

When using the `--entry-type` flag, it overrides the ESM_FORMAT result while
providing errors in the case of explicit conflicts.

<details>
<summary>Resolver algorithm specification</summary>

**ESM_RESOLVE(_specifier_, _parentURL_, _isMain_)**
> 1. Let _resolvedURL_ be **undefined**.
> 1. If _specifier_ is a valid URL, then
>    1. Set _resolvedURL_ to the result of parsing and reserializing
>       _specifier_ as a URL.
> 1. Otherwise, if _specifier_ starts with _"/"_, then
>    1. Throw an _Invalid Specifier_ error.
> 1. Otherwise, if _specifier_ starts with _"./"_ or _"../"_, then
>    1. Set _resolvedURL_ to the URL resolution of _specifier_ relative to
>       _parentURL_.
> 1. Otherwise,
>    1. Note: _specifier_ is now a bare specifier.
>    1. Set _resolvedURL_ the result of
>       **PACKAGE_RESOLVE**(_specifier_, _parentURL_).
> 1. If the file at _resolvedURL_ does not exist, then
>    1. Throw a _Module Not Found_ error.
> 1. Set _resolvedURL_ to the real path of _resolvedURL_.
> 1. Let _format_ be the result of **ESM_FORMAT**(_resolvedURL_, _isMain_).
> 1. Load _resolvedURL_ as module format, _format_.

PACKAGE_RESOLVE(_packageSpecifier_, _parentURL_)
> 1. Let _packageName_ be *undefined*.
> 1. Let _packageSubpath_ be *undefined*.
> 1. If _packageSpecifier_ is an empty string, then
>    1. Throw an _Invalid Specifier_ error.
> 1. If _packageSpecifier_ does not start with _"@"_, then
>    1. Set _packageName_ to the substring of _packageSpecifier_ until the
>       first _"/"_ separator or the end of the string.
> 1. Otherwise,
>    1. If _packageSpecifier_ does not contain a _"/"_ separator, then
>       1. Throw an _Invalid Specifier_ error.
>    1. Set _packageName_ to the substring of _packageSpecifier_
>       until the second _"/"_ separator or the end of the string.
> 1. Let _packageSubpath_ be the substring of _packageSpecifier_ from the
>    position at the length of _packageName_ plus one, if any.
> 1. Assert: _packageName_ is a valid package name or scoped package name.
> 1. Assert: _packageSubpath_ is either empty, or a path without a leading
>    separator.
> 1. If _packageSubpath_ contains any _"."_ or _".."_ segments or percent
>    encoded strings for _"/"_ or _"\\"_ then,
>    1. Throw an _Invalid Specifier_ error.
> 1. If _packageSubpath_ is empty and _packageName_ is a Node.js builtin
>    module, then
>    1. Return the string _"node:"_ concatenated with _packageSpecifier_.
> 1. While _parentURL_ is not the file system root,
>    1. Let _packageURL_ be the URL resolution of "node_modules/"
>       concatenated with _packageSpecifier_, relative to _parentURL_.
>    1. Set _parentURL_ to the parent folder URL of _parentURL_.
>    1. If the folder at _packageURL_ does not exist, then
>       1. Set _parentURL_ to the parent URL path of _parentURL_.
>       1. Continue the next loop iteration.
>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_packageURL_).
>    1. If _packageSubpath_ is empty, then
>       1. Return the result of **PACKAGE_MAIN_RESOLVE**(_packageURL_,
>          _pjson_).
>    1. Otherwise,
>       1. Return the URL resolution of _packageSubpath_ in _packageURL_.
> 1. Throw a _Module Not Found_ error.

PACKAGE_MAIN_RESOLVE(_packageURL_, _pjson_)
> 1. If _pjson_ is **null**, then
>    1. Throw a _Module Not Found_ error.
> 1. If _pjson.main_ is a String, then
>    1. Let _resolvedMain_ be the concatenation of _packageURL_, "/", and
>       _pjson.main_.
>    1. If the file at _resolvedMain_ exists, then
>       1. Return _resolvedMain_.
> 1. If _pjson.type_ is equal to _"module"_, then
>    1. Throw a _Module Not Found_ error.
> 1. Let _legacyMainURL_ be the result applying the legacy
>    **LOAD_AS_DIRECTORY** CommonJS resolver to _packageURL_, throwing a
>    _Module Not Found_ error for no resolution.
> 1. If _legacyMainURL_ does not end in _".js"_ then,
>    1. Throw an _Unsupported File Extension_ error.
> 1. Return _legacyMainURL_.

**ESM_FORMAT(_url_, _isMain_)**
> 1. Assert: _url_ corresponds to an existing file.
> 1. Let _pjson_ be the result of **READ_PACKAGE_SCOPE**(_url_).
> 1. If _url_ ends in _".mjs"_, then
>    1. Return _"module"_.
> 1. If _url_ ends in _".cjs"_, then
>    1. Return _"commonjs"_.
> 1. If _pjson?.type_ exists and is _"module"_, then
>    1. If _isMain_ is **true** or _url_ ends in _".js"_, then
>       1. Return _"module"_.
>    1. Throw an _Unsupported File Extension_ error.
> 1. Otherwise,
>    1. If _isMain_ is **true** or _url_ ends in _".js"_, _".json"_ or
>       _".node"_, then
>       1. Return _"commonjs"_.
>    1. Throw an _Unsupported File Extension_ error.

READ_PACKAGE_SCOPE(_url_)
> 1. Let _scopeURL_ be _url_.
> 1. While _scopeURL_ is not the file system root,
>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_scopeURL_).
>    1. If _pjson_ is not **null**, then
>       1. Return _pjson_.
>    1. Set _scopeURL_ to the parent URL of _scopeURL_.
> 1. Return **null**.

READ_PACKAGE_JSON(_packageURL_)
> 1. Let _pjsonURL_ be the resolution of _"package.json"_ within _packageURL_.
> 1. If the file at _pjsonURL_ does not exist, then
>    1. Return **null**.
> 1. If the file at _packageURL_ does not parse as valid JSON, then
>    1. Throw an _Invalid Package Configuration_ error.
> 1. Return the parsed JSON source of the file at _pjsonURL_.

</details>

### Customizing ESM specifier resolution algorithm

The current specifier resolution does not support all default behavior of
the CommonJS loader. One of the behavior differences is automatic resolution
of file extensions and the ability to import directories that have an index
file.

The `--es-module-specifier-resolution=[mode]` flag can be used to customize
the extension resolution algorithm. The default mode is `explicit`, which
requires the full path to a module be provided to the loader. To enable the
automatic extension resolution and importing from directories that include an
index file use the `node` mode.

```bash
$ node --experimental-modules index.mjs
success!
$ node --experimental-modules index #Failure!
Error: Cannot find module
$ node --experimental-modules --es-module-specifier-resolution=node index
success!
```

[`export`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export
[`import`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
[`import()`]: #esm_import-expressions
[`import.meta.url`]: #esm_import_meta
[`module.createRequireFromPath()`]: modules.html#modules_module_createrequirefrompath_filename
[CommonJS]: modules.html
[ECMAScript-modules implementation]: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md
[Node.js EP for ES Modules]: https://github.com/nodejs/node-eps/blob/master/002-es-modules.md
[WHATWG JSON modules]: https://github.com/whatwg/html/issues/4315
[dynamic instantiate hook]: #esm_dynamic_instantiate_hook
[the official standard format]: https://tc39.github.io/ecma262/#sec-modules