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# ECMAScript Modules#

Stability: 1 - Experimental

## Introduction#

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

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#

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

Once enabled, 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`, or extensionless files, 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 `--print`, 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`, or extensionless files, 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` 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.

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

    # 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. "Extensionless" refers to file paths which do not contain an extension as opposed to optionally dropping a file extension in a specifier.

`import` statements of `.js` and extensionless 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 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.

    // 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 `--print` (or `-e` or `-p`), or piped to `node` via `STDIN`, will be treated as ES modules when the `--input-type=module` flag is set.

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

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

## Packages#

### Package Entry Points#

There are two fields that 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, part of Package Exports, 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"`. If package entry points are defined in both `"main"` and `"exports"`, the latter takes precedence in versions of Node.js that support `"exports"`. Conditional Exports can also be used within `"exports"` to define different package entry points per environment.

#### `package.json` `"main"`#

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`.

    // ./node_modules/es-module-package/package.json
    {
      "type": "module",
      "main": "./src/index.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.

The `"main"` field can point to exactly one file, regardless of whether the package is referenced via `require` (in a CommonJS context) or `import` (in an ES module context).

#### Package Exports#

By default, all subpaths from a package can be imported (`import 'pkg/x.js'`). Custom subpath aliasing and encapsulation can be provided through the `"exports"` field.

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

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

In addition to defining an alias, subpaths not defined by `"exports"` will throw when an attempt is made to import them:

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

> Note: this is not a strong encapsulation as any private modules can still be loaded by absolute paths.

Folders can also be mapped with package exports:

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

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

If a package has no exports, setting `"exports": false` can be used instead of `"exports": {}` to indicate the package does not intend for submodules to be exposed.

Any invalid exports entries will be ignored. This includes exports not starting with `"./"` or a missing trailing `"/"` for directory exports.

Array fallback support is provided for exports, similarly to import maps in order to be forwards-compatible with possible fallback workflows in future:

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

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

Defining a `"."` export will define the main entry point for the package, and will always take precedence over the `"main"` field in the `package.json`.

This allows defining a different entry point for Node.js versions that support ECMAScript modules and versions that don't, for example:

    {
      "main": "./main-legacy.cjs",
      "exports": {
        ".": "./main-modern.cjs"
      }
    }

#### 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 Node.js and the browser can be written:

    // ./node_modules/pkg/package.json
    {
      "type": "module",
      "main": "./index.js",
      "exports": {
        "./feature": {
          "import": "./feature-default.js",
          "browser": "./feature-browser.js"
        }
      }
    }

When resolving the `"."` export, if no matching target is found, the `"main"` will be used as the final fallback.

The conditions supported in Node.js condition matching:

- `"default"` - the generic fallback that will always match. Can be a CommonJS or ES module file.
- `"import"` - matched when the package is loaded via `import` or `import()`. Can be any module format, this field does not set the type interpretation.
- `"node"` - matched for any Node.js environment. Can be a CommonJS or ES module file.
- `"require"` - matched when the package is loaded via `require()`.

Condition matching is applied in object order from first to last within the `"exports"` object.

Using the `"require"` condition it is possible to define a package that will have a different exported value for CommonJS and ES modules, which can be a hazard in that it can result in having two separate instances of the same package in use in an application, which can cause a number of bugs.

Other conditions such as `"browser"`, `"electron"`, `"deno"`, `"react-native"`, etc. could be defined in other runtimes or tools. Condition names must not start with `"."` or be numbers. Further restrictions, definitions or guidance on condition names may be provided in future.

#### 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"
    }

When using Conditional Exports, the rule is that all keys in the object mapping must not start with a `"."` otherwise they would be indistinguishable from exports subpaths.

    {
      "exports": {
        ".": {
          "import": "./main.js",
          "require": "./main.cjs"
        }
      }
    }

can be written:

    {
      "exports": {
        "import": "./main.js",
        "require": "./main.cjs"
      }
    }

If writing any exports value that mixes up these two forms, an error will be thrown:

    {
      // Throws on resolution!
      "exports": {
        "./feature": "./lib/feature.js",
        "import": "./main.js",
        "require": "./main.cjs"
      }
    }

### 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 obvious 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": {
        "require": "./index.cjs",
        "import": "./wrapper.mjs"
      }
    }

    // ./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 `name`s 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#

The most straightforward `package.json` would be one that defines 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 `require`s 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"
      }
    }

## `import` 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:` and `data:` 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:///`.

#### `data:` Imports#

Added in: v12.10.0

`data:` URLs are supported for importing with the following MIME types:

- `text/javascript` for ES Modules
- `application/json` for JSON
- `application/wasm` for WASM.

`data:` URLs only resolve Bare specifiers for builtin modules and Absolute specifiers. Resolving Relative specifiers will not work because `data:` is not a special scheme. For example, attempting to load `./foo` from `data:text/javascript,import "./foo";` will fail to resolve since there is no concept of relative resolution for `data:` URLs. An example of a `data:` URLs being used is:

    import 'data:text/javascript,console.log("hello!");';
    import _ from 'data:application/json,"world!"';

## `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 `NODE_PATH`#

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

### No `require`, `exports`, `module.exports`, `__filename`, `__dirname`#

These CommonJS variables are not available in ES modules.

`require` can be imported into an ES module using `module.createRequire()`.

Equivalents of `__filename` and `__dirname` can be created inside of each file via `import.meta.url`.

    import { fileURLToPath } from 'url';
    import { dirname } from 'path';

    const __filename = fileURLToPath(import.meta.url);
    const __dirname = dirname(__filename);

### 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#

ES modules 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.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#

### `require`#

`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.createRequire()`.

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

### `import` statements#

An `import` statement can reference an ES module or a CommonJS module. Other file types such as JSON or Native modules are not supported. For those, use `module.createRequire()`.

`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; unless the package’s `package.json` contains an `"exports"` field, in which case files within packages need to be accessed via the path defined in `"exports"`.

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

Only the “default export” is supported for CommonJS files or packages:

    import packageMain from 'commonjs-package'; // Works

    import { method } from 'commonjs-package'; // Errors

### `import()` expressions#

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

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

## CommonJS, JSON, and Native Modules#

CommonJS, JSON, and Native modules can be used with `module.createRequire()`.

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

    // esm.mjs
    import { createRequire } from 'module';

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

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

## Builtin modules#

Builtin modules will provide named exports of their public API. A default export is also provided which is the value of the CommonJS exports. The default export can be used for, among other things, modifying the named exports. Named exports of builtin modules are updated only by calling `module.syncBuiltinESMExports()`.

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

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

    import fs, { readFileSync } from 'fs';
    import { syncBuiltinESMExports } from 'module';

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

    fs.readFileSync === readFileSync;

## Experimental JSON Modules#

Currently importing JSON modules are only supported in the `commonjs` mode and are loaded using the CJS loader. WHATWG JSON modules specification are still 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.mjs` with

    import packageConfig from './package.json';

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

    node --experimental-modules index.mjs # fails
    node --experimental-modules --experimental-json-modules index.mjs # works

## Experimental Wasm Modules#

Importing Web Assembly modules is supported under the `--experimental-wasm-modules` flag, allowing any `.wasm` files to be imported as normal modules while also supporting their module imports.

This integration is in line with the ES Module Integration Proposal for Web Assembly.

For example, an `index.mjs` containing:

    import * as M from './module.wasm';
    console.log(M);

executed under:

    node --experimental-modules --experimental-wasm-modules index.mjs

would provide the exports interface for the instantiation of `module.wasm`.

## Experimental Loaders#

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

To customize the default module resolution, loader hooks can optionally be provided via a `--experimental-loader ./loader-name.mjs` argument to Node.js.

When hooks are used they only apply to ES module loading and not to any CommonJS modules loaded.

### Hooks#

#### `resolve` hook#

> Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The `resolve` hook returns the resolved file URL for a given module specifier and parent URL. The module specifier is the string in an `import` statement or `import()` expression, and the parent URL is the URL of the module that imported this one, or `undefined` if this is the main entry point for the application.

    /**
     * @param {string} specifier
     * @param {object} context
     * @param {string} context.parentURL
     * @param {function} defaultResolve
     * @returns {object} response
     * @returns {string} response.url
     */
    export async function resolve(specifier, context, defaultResolve) {
      const { parentURL = null } = context;
      if (someCondition) {
        // For some or all specifiers, do some custom logic for resolving.
        // Always return an object of the form {url: <string>}
        return {
          url: (parentURL) ?
            new URL(specifier, parentURL).href : new URL(specifier).href
        };
      }
      // Defer to Node.js for all other specifiers.
      return defaultResolve(specifier, context, defaultResolve);
    }

#### `getFormat` hook#

> Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The `getFormat` hook provides a way to define a custom method of determining how a URL should be interpreted. This can be one of the following:

`format` | Description
--- | ---
`'builtin'` | Load a Node.js builtin module
`'commonjs'` | Load a Node.js CommonJS module
`'dynamic'` | Use a dynamic instantiate hook
`'json'` | Load a JSON file
`'module'` | Load a standard JavaScript module (ES module)
`'wasm'` | Load a WebAssembly module

    /**
     * @param {string} url
     * @param {object} context (currently empty)
     * @param {function} defaultGetFormat
     * @returns {object} response
     * @returns {string} response.format
     */
    export async function getFormat(url, context, defaultGetFormat) {
      if (someCondition) {
        // For some or all URLs, do some custom logic for determining format.
        // Always return an object of the form {format: <string>}, where the
        // format is one of the strings in the table above.
        return {
          format: 'module'
        };
      }
      // Defer to Node.js for all other URLs.
      return defaultGetFormat(url, context, defaultGetFormat);
    }

#### `getSource` hook#

> Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The `getSource` hook provides a way to define a custom method for retrieving the source code of an ES module specifier. This would allow a loader to potentially avoid reading files from disk.

    /**
     * @param {string} url
     * @param {object} context
     * @param {string} context.format
     * @param {function} defaultGetSource
     * @returns {object} response
     * @returns {string|buffer} response.source
     */
    export async function getSource(url, context, defaultGetSource) {
      const { format } = context;
      if (someCondition) {
        // For some or all URLs, do some custom logic for retrieving the source.
        // Always return an object of the form {source: <string|buffer>}.
        return {
          source: '...'
        };
      }
      // Defer to Node.js for all other URLs.
      return defaultGetSource(url, context, defaultGetSource);
    }

#### `transformSource` hook#

    NODE_OPTIONS='--experimental-modules --experimental-loader ./custom-loader.mjs' node x.js

> Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The `transformSource` hook provides a way to modify the source code of a loaded ES module file after the source string has been loaded but before Node.js has done anything with it.

If this hook is used to convert unknown-to-Node.js file types into executable JavaScript, a resolve hook is also necessary in order to register any unknown-to-Node.js file extensions. See the transpiler loader example below.

    /**
     * @param {string|buffer} source
     * @param {object} context
     * @param {string} context.url
     * @param {string} context.format
     * @param {function} defaultTransformSource
     * @returns {object} response
     * @returns {string|buffer} response.source
     */
    export async function transformSource(source,
                                          context,
                                          defaultTransformSource) {
      const { url, format } = context;
      if (someCondition) {
        // For some or all URLs, do some custom logic for modifying the source.
        // Always return an object of the form {source: <string|buffer>}.
        return {
          source: '...'
        };
      }
      // Defer to Node.js for all other sources.
      return defaultTransformSource(
        source, context, defaultTransformSource);
    }

#### `dynamicInstantiate` hook#

> Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

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 `getFormat` hook.

    /**
     * @param {string} url
     * @returns {object} response
     * @returns {array} response.exports
     * @returns {function} response.execute
     */
    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.

### Examples#

The various loader hooks can be used together to accomplish wide-ranging customizations of Node.js’ code loading and evaluation behaviors.

#### HTTPS loader#

In current Node.js, specifiers starting with `https://` are unsupported. The loader below registers hooks to enable rudimentary support for such specifiers. While this may seem like a significant improvement to Node.js core functionality, there are substantial downsides to actually using this loader: performance is much slower than loading files from disk, there is no caching, and there is no security.

    // https-loader.mjs
    import { get } from 'https';

    export function resolve(specifier, context, defaultResolve) {
      const { parentURL = null } = context;

      // Normally Node.js would error on specifiers starting with 'https://', so
      // this hook intercepts them and converts them into absolute URLs to be
      // passed along to the later hooks below.
      if (specifier.startsWith('https://')) {
        return {
          url: specifier
        };
      } else if (parentURL && parentURL.startsWith('https://')) {
        return {
          url: new URL(specifier, parentURL).href
        };
      }

      // Let Node.js handle all other specifiers.
      return defaultResolve(specifier, context, defaultResolve);
    }

    export function getFormat(url, context, defaultGetFormat) {
      // This loader assumes all network-provided JavaScript is ES module code.
      if (url.startsWith('https://')) {
        return {
          format: 'module'
        };
      }

      // Let Node.js handle all other URLs.
      return defaultGetFormat(url, context, defaultGetFormat);
    }

    export function getSource(url, context, defaultGetSource) {
      // For JavaScript to be loaded over the network, we need to fetch and
      // return it.
      if (url.startsWith('https://')) {
        return new Promise((resolve, reject) => {
          get(url, (res) => {
            let data = '';
            res.on('data', (chunk) => data += chunk);
            res.on('end', () => resolve({ source: data }));
          }).on('error', (err) => reject(err));
        });
      }

      // Let Node.js handle all other URLs.
      return defaultGetSource(url, context, defaultGetSource);
    }

    // main.mjs
    import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';

    console.log(VERSION);

With this loader, running:

    node --experimental-loader ./https-loader.mjs ./main.js

Will print the current version of CoffeeScript per the module at the URL in `main.mjs`.

#### Transpiler loader#

Sources that are in formats Node.js doesn’t understand can be converted into JavaScript using the `transformSource` hook. Before that hook gets called, however, other hooks need to tell Node.js not to throw an error on unknown file types; and to tell Node.js how to load this new file type.

This is obviously less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.

    // coffeescript-loader.mjs
    import { URL, pathToFileURL } from 'url';
    import CoffeeScript from 'coffeescript';

    const baseURL = pathToFileURL(`${process.cwd()}/`).href;

    // CoffeeScript files end in .coffee, .litcoffee or .coffee.md.
    const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;

    export function resolve(specifier, context, defaultResolve) {
      const { parentURL = baseURL } = context;

      // Node.js normally errors on unknown file extensions, so return a URL for
      // specifiers ending in the CoffeeScript file extensions.
      if (extensionsRegex.test(specifier)) {
        return {
          url: new URL(specifier, parentURL).href
        };
      }

      // Let Node.js handle all other specifiers.
      return defaultResolve(specifier, context, defaultResolve);
    }

    export function getFormat(url, context, defaultGetFormat) {
      // Now that we patched resolve to let CoffeeScript URLs through, we need to
      // tell Node.js what format such URLs should be interpreted as. For the
      // purposes of this loader, all CoffeeScript URLs are ES modules.
      if (extensionsRegex.test(url)) {
        return {
          format: 'module'
        };
      }

      // Let Node.js handle all other URLs.
      return defaultGetFormat(url, context, defaultGetFormat);
    }

    export function transformSource(source, context, defaultTransformSource) {
      const { url, format } = context;

      if (extensionsRegex.test(url)) {
        return {
          source: CoffeeScript.compile(source, { bare: true })
        };
      }

      // Let Node.js handle all other sources.
      return defaultTransformSource(source, context, defaultTransformSource);
    }

    # main.coffee
    import { scream } from './scream.coffee'
    console.log scream 'hello, world'

    import { version } from 'process'
    console.log "Brought to you by Node.js version #{version}"

    # scream.coffee
    export scream = (str) -> str.toUpperCase()

With this loader, running:

    node --experimental-loader ./coffeescript-loader.mjs main.coffee

Will cause `main.coffee` to be turned into JavaScript after its source code is loaded from disk but before Node.js executes it; and so on for any `.coffee`, `.litcoffee` or `.coffee.md` files referenced via `import` statements of any loaded file.

## Resolution Algorithm#

### Features#

The resolver has the following properties:

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

### Resolver Algorithm#

The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL, 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 "addon" can be extended in future updates.

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

defaultEnv is the conditional environment name priority array, `["node", "import"]`.

Resolver algorithm specification

ESM_RESOLVE(specifier, parentURL)

> 1. Let resolvedURL be undefined.

> 2. If specifier is a valid URL, then

>    1. Set resolvedURL to the result of parsing and reserializing specifier as a URL.

> 3. Otherwise, if specifier starts with "/", then

>    1. Throw an Invalid Specifier error.

> 4. Otherwise, if specifier starts with "./" or "../", then

>    1. Set resolvedURL to the URL resolution of specifier relative to parentURL.

> 5. Otherwise,

>    1. Note: specifier is now a bare specifier.
>    2. Set resolvedURL the result of PACKAGE_RESOLVE(specifier, parentURL).

> 6. If resolvedURL contains any percent encodings of "/" or "\" ("%2f" and "%5C" respectively), then

>    1. Throw an Invalid Specifier error.

> 7. If the file at resolvedURL does not exist, then

>    1. Throw a Module Not Found error.
> 8. Set resolvedURL to the real path of resolvedURL.
> 9. Let format be the result of ESM_FORMAT(resolvedURL).
> 10. Load resolvedURL as module format, format.

PACKAGE_RESOLVE(packageSpecifier, parentURL)

> 1. Let packageName be undefined.
> 2. Let packageSubpath be undefined.

> 3. If packageSpecifier is an empty string, then

>    1. Throw an Invalid Specifier error.

> 4. Otherwise,

>    1. If packageSpecifier does not contain a "/" separator, then

>       1. Throw an Invalid Specifier error.
>    2. Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.

> 5. If packageName starts with "." or contains "\" or "%", then

>    1. Throw an Invalid Specifier error.
> 6. Let packageSubpath be undefined.

> 7. If the length of packageSpecifier is greater than the length of packageName, then

>    1. Set packageSubpath to "." concatenated with the substring of packageSpecifier from the position at the length of packageName.

> 8. If packageSubpath contains any "." or ".." segments or percent encoded strings for "/" or "\", then

>    1. Throw an Invalid Specifier error.
> 9. Set selfUrl to the result of SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL).
> 10. If selfUrl isn't empty, return selfUrl.

> 11. If packageSubpath is undefined and packageName is a Node.js builtin module, then

>     1. Return the string "node:" concatenated with packageSpecifier.

> 12. While parentURL is not the file system root,

>     1. Let packageURL be the URL resolution of "node_modules/" concatenated with packageSpecifier, relative to parentURL.
>     2. Set parentURL to the parent folder URL of parentURL.

>     3. If the folder at packageURL does not exist, then

>        1. Set parentURL to the parent URL path of parentURL.
>        2. Continue the next loop iteration.
>     4. Let pjson be the result of READ_PACKAGE_JSON(packageURL).

>     5. If packageSubpath is _undefined__, then

>        1. Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).

>     6. Otherwise,

>        1. If pjson is not null and pjson has an "exports" key, then

>           1. Let exports be pjson.exports.

>           2. If exports is not null or undefined, then

>              1. Return PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports).
>        2. Return the URL resolution of packageSubpath in packageURL.
> 13. Throw a Module Not Found error.

SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL)

> 1. Let packageURL be the result of READ_PACKAGE_SCOPE(parentURL).

> 2. If packageURL is null, then

>    1. Return undefined.
> 3. Let pjson be the result of READ_PACKAGE_JSON(packageURL).

> 4. If pjson does not include an "exports" property, then

>    1. Return undefined.

> 5. If pjson.name is equal to packageName, then

>    1. If packageSubpath is undefined, then

>       1. Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).

>    2. Otherwise,

>       1. If pjson is not null and pjson has an "exports" key, then

>          1. Let exports be pjson.exports.

>          2. If exports is not null or undefined, then

>             1. Return PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, pjson.exports).
>       2. Return the URL resolution of subpath in packageURL.
> 6. Otherwise, return undefined.

PACKAGE_MAIN_RESOLVE(packageURL, pjson)

> 1. If pjson is null, then

>    1. Throw a Module Not Found error.

> 2. If pjson.exports is not null or undefined, then

>    1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an "Invalid Package Configuration" error.

>    2. If pjson.exports is a String or Array, or an Object containing no keys starting with ".", then

>       1. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, pjson.exports, "").

>    3. If pjson.exports is an Object containing a "." property, then

>       1. Let mainExport be the "." property in pjson.exports.
>       2. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, mainExport, "").

> 3. If pjson.main is a String, then

>    1. Let resolvedMain be the URL resolution of packageURL, "/", and pjson.main.

>    2. If the file at resolvedMain exists, then

>       1. Return resolvedMain.

> 4. If pjson.type is equal to "module", then

>    1. Throw a Module Not Found error.
> 5. Let legacyMainURL be the result applying the legacy LOAD_AS_DIRECTORY CommonJS resolver to packageURL, throwing a Module Not Found error for no resolution.
> 6. Return legacyMainURL.

PACKAGE_EXPORTS_RESOLVE(packageURL, packagePath, exports)

> 1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an "Invalid Package Configuration" error.

> 2. If exports is an Object and all keys of exports start with ".", then

>    1. Set packagePath to "./" concatenated with packagePath.

>    2. If packagePath is a key of exports, then

>       1. Let target be the value of exports[packagePath].
>       2. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, "", defaultEnv).
>    3. Let directoryKeys be the list of keys of exports ending in "/", sorted by length descending.

>    4. For each key directory in directoryKeys, do

>       1. If packagePath starts with directory, then

>          1. Let target be the value of exports[directory].
>          2. Let subpath be the substring of target starting at the index of the length of directory.
>          3. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, defaultEnv).
> 3. Throw a Module Not Found error.

PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, env)

> 1. If target is a String, then

>    1. If target does not start with "./", throw a Module Not Found error.
>    2. If subpath has non-zero length and target does not end with "/", throw a Module Not Found error.
>    3. If target or subpath contain any "node_modules" segments including "node_modules" percent-encoding, throw a Module Not Found error.
>    4. Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.

>    5. If resolvedTarget is contained in packageURL, then

>       1. Let resolved be the URL resolution of the concatenation of subpath and resolvedTarget.

>       2. If resolved is contained in resolvedTarget, then

>          1. Return resolved.

> 2. Otherwise, if target is a non-null Object, then

>    1. If exports contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.

>    2. For each property p of target, in object insertion order as,

>       1. If env contains an entry for p, then

>          1. Let targetValue be the value of the p property in target.
>          2. Let resolved be the result of PACKAGE_EXPORTS_TARGET_RESOLVE (packageURL, targetValue, subpath, env).
>          3. Assert: resolved is a String.
>          4. Return resolved.

> 3. Otherwise, if target is an Array, then

>    1. For each item targetValue in target, do

>       1. If targetValue is an Array, continue the loop.
>       2. Let resolved be the result of PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, targetValue, subpath, env), continuing the loop on abrupt completion.
>       3. Assert: resolved is a String.
>       4. Return resolved.
> 4. Throw a Module Not Found error.

ESM_FORMAT(url)

> 1. Assert: url corresponds to an existing file pathname.
> 2. Let pjson be the result of READ_PACKAGE_SCOPE(url).

> 3. If url ends in ".mjs", then

>    1. Return "module".

> 4. If url ends in ".cjs", then

>    1. Return "commonjs".

> 5. If pjson?.type exists and is "module", then

>    1. If url ends in ".js" or lacks a file extension, then

>       1. Return "module".
>    2. Throw an Unsupported File Extension error.

> 6. Otherwise,

>    1. If url lacks a file extension, then

>       1. Return "commonjs".
>    2. Throw an Unsupported File Extension error.

READ_PACKAGE_SCOPE(url)

> 1. Let scopeURL be url.

> 2. While scopeURL is not the file system root,

>    1. If scopeURL ends in a "node_modules" path segment, return null.
>    2. Let pjson be the result of READ_PACKAGE_JSON(scopeURL).

>    3. If pjson is not null, then

>       1. Return pjson.
>    4. Set scopeURL to the parent URL of scopeURL.
> 3. Return null.

READ_PACKAGE_JSON(packageURL)

> 1. Let pjsonURL be the resolution of "package.json" within packageURL.

> 2. If the file at pjsonURL does not exist, then

>    1. Return null.

> 3. If the file at packageURL does not parse as valid JSON, then

>    1. Throw an Invalid Package Configuration error.
> 4. Return the parsed JSON source of the file at pjsonURL.

### Customizing ESM specifier resolution algorithm#

The 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 `--experimental-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.

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