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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 `.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.

    // 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.

`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.

### 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#

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).

#### Compatibility with CommonJS-Only Versions of Node.js#

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) `"module"`.

Node.js can now run ES module entry points, but it remains impossible for a package to define separate CommonJS and ES module entry points. This is for good reason: the `pkg` variable created from `import pkg from 'pkg'` is not the same singleton as the `pkg` variable created from `const pkg = require('pkg')`, so if both are referenced within the same app (including dependencies), unexpected behavior might occur.

There are two general approaches to addressing this limitation while still publishing a package that contains both CommonJS and ES module sources:

1. Document a new ES module entry point that’s not the package `"main"`, e.g. `import pkg from 'pkg/module.mjs'` (or `import 'pkg/esm'`, if using package exports). The package `"main"` would still point to a CommonJS file, and thus the package would remain compatible with older versions of Node.js that lack support for ES modules.

2. Switch the package `"main"` entry point to an ES module file as part of a breaking change version bump. This version and above would only be usable on ES module-supporting versions of Node.js. If the package still contains a CommonJS version, it would be accessible via a path within the package, e.g. `require('pkg/commonjs')`; this is essentially the inverse of the previous approach. Package consumers who are using CommonJS-only versions of Node.js would need to update their code from `require('pkg')` to e.g. `require('pkg/commonjs')`.

Of course, a package could also include only CommonJS or only ES module sources. An existing package could make a semver major bump to an ES module-only version, that would only be supported in ES module-supporting versions of Node.js (and other runtimes). New packages could be published containing only ES module sources, and would be compatible only with ES module-supporting runtimes.

### 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.

Exports can also be used to map the main entry point of a package:

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

where the "." indicates loading the package without any subpath. Exports will always override any existing `"main"` value for both CommonJS and ES module packages.

For packages with only a main entry point, an `"exports"` value of just a string is also supported:

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

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 forward-compatible with 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.

## `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, a CommonJS module, or JSON. Other file types such as 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.

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

> Currently only the “default export” is supported for CommonJS files or packages:

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

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

> There are ongoing efforts to make the latter code possible.

### `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 Loader hooks#

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.

### Resolve hook#

The resolve hook returns the resolved file URL and module format for a given module specifier and parent file URL:

    import { URL, pathToFileURL } from 'url';
    const baseURL = pathToFileURL(process.cwd()).href;

    /**
     * @param {string} specifier
     * @param {string} parentModuleURL
     * @param {function} defaultResolver
     */
    export async function resolve(specifier,
                                  parentModuleURL = baseURL,
                                  defaultResolver) {
      return {
        url: new URL(specifier, parentModuleURL).href,
        format: 'module'
      };
    }

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
--- | ---
`'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
`'wasm'` | Load a WebAssembly module

For example, a dummy loader to load JavaScript restricted to browser resolution rules with only JS file extension and Node.js builtin modules support could be written:

    import path from 'path';
    import process from 'process';
    import Module from 'module';
    import { URL, pathToFileURL } from 'url';

    const builtins = Module.builtinModules;
    const JS_EXTENSIONS = new Set(['.js', '.mjs']);

    const baseURL = pathToFileURL(process.cwd()).href;

    /**
     * @param {string} specifier
     * @param {string} parentModuleURL
     * @param {function} defaultResolver
     */
    export async function resolve(specifier,
                                  parentModuleURL = baseURL,
                                  defaultResolver) {
      if (builtins.includes(specifier)) {
        return {
          url: specifier,
          format: 'builtin'
        };
      }
      if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) {
        // For node_modules support:
        // return defaultResolver(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: 'module'
      };
    }

With this loader, running:

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

would load the module `x.js` as an ES module with relative resolution support (with `node_modules` loading skipped in this example).

### Dynamic instantiate hook#

To create a custom dynamic module that doesn't correspond to one of the existing `format` interpretations, the `dynamicInstantiate` hook can be used. This hook is called only for modules that return `format: 'dynamic'` from the `resolve` hook.

    export async function dynamicInstantiate(url) {
      return {
        exports: ['customExportName'],
        execute: (exports) => {
          // Get and set functions provided for pre-allocated export names
          exports.customExportName.set('value');
        }
      };
    }

With the list of module exports provided upfront, the `execute` function will then be called at the exact point of module evaluation order for that module in the import tree.

## 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.

isMain is true when resolving the Node.js application entry point.

Resolver algorithm specification

ESM_RESOLVE(specifier, parentURL, isMain)

> 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, isMain).
> 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. If packageSpecifier does not start with "@", then

>    1. Set packageName to the substring of packageSpecifier until the first "/" separator or the end of the string.

> 5. Otherwise,

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

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

> 6. If packageName starts with "." or contains "\" or "%", then

>    1. Throw an Invalid Specifier error.
> 7. Let packageSubpath be undefined.

> 8. 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.

> 9. If packageSubpath contains any "." or ".." segments or percent encoded strings for "/" or "\" then,

>    1. Throw an Invalid Specifier error.

> 10. If packageSubpath is undefined and packageName is a Node.js builtin module, then

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

> 11. While parentURL is not the file system root,

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

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

>        1. 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.
> 12. Throw a Module Not Found error.

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 pjson.exports is a String or Array, then

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

>    2. If _pjson.exports is an Object, then

>       1. If pjson.exports contains 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, 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, "").
>    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).
> 2. Throw a Module Not Found error.

PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath)

> 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 an Array, then

>    1. For each item targetValue in target, do

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

ESM_FORMAT(url, isMain)

> 1. Assert: url corresponds to an existing file.
> 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 isMain is true or url ends in ".js", then

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

> 6. Otherwise,

>    1. If isMain is true or url ends in ".js", ".json" or ".node", 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 `--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.

    $ 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!