Table of contents

- Modules: ECMAScript modules
  - Introduction
  - Enabling
  - Packages
  - `import` Specifiers
    - Terminology
    - Mandatory file extensions
    - URLs
      - `file:` URLs
      - `data:` imports
      - `node:` imports
  - Import attributes
  - Builtin modules
  - `import()` expressions
  - `import.meta`
    - `import.meta.dirname`
    - `import.meta.filename`
    - `import.meta.url`
    - `import.meta.resolve(specifier)`
  - Interoperability with CommonJS
    - `import` statements
    - `require`
    - CommonJS Namespaces
    - Differences between ES modules and CommonJS
      - No `require`, `exports`, or `module.exports`
      - No `__filename` or `__dirname`
      - No Addon Loading
      - No `require.resolve`
      - No `NODE_PATH`
      - No `require.extensions`
      - No `require.cache`
  - JSON modules
  - Wasm modules
  - Top-level `await`
  - HTTPS and HTTP imports
    - Imports are limited to HTTP/1
    - HTTP is limited to loopback addresses
    - Authentication is never sent to the destination server.
    - CORS is never checked on the destination server
    - Cannot load non-network dependencies
    - Network-based loading is not enabled by default
  - Loaders
  - Resolution and loading algorithm
    - Features
    - Resolution algorithm
    - Resolution Algorithm Specification
    - Customizing ESM specifier resolution algorithm

## Modules: ECMAScript modules#

History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | Add experimental support for import attributes.
v20.0.0, v18.19.0 | Module customization hooks are executed off the main thread.
v18.6.0, v16.17.0 | Add support for chaining module customization hooks.
v17.1.0, v16.14.0 | Add experimental support for import assertions.
v17.0.0, v16.12.0 | Consolidate customization hooks, removed `getFormat`, `getSource`, `transformSource`, and `getGlobalPreloadCode` hooks added `load` and `globalPreload` hooks allowed returning `format` from either `resolve` or `load` hooks.
v14.8.0 | Unflag Top-Level Await.
v15.3.0, v14.17.0, v12.22.0 | Stabilize modules implementation.
v14.13.0, v12.20.0 | Support for detection of CommonJS named exports.
v14.0.0, v13.14.0, v12.20.0 | Remove experimental modules warning.
v13.2.0, v12.17.0 | Loading ECMAScript modules no longer requires a command-line flag.
v12.0.0 | Add support for ES modules using `.js` file extension via `package.json` `"type"` field.
v8.5.0 | Added in: v8.5.0

Stability: 2 - Stable

### 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.

The following example of an ES module exports a function:

    // addTwo.mjs
    function addTwo(num) {
      return num + 2;
    }

    export { addTwo };

The following example of an ES module imports the function from `addTwo.mjs`:

    // app.mjs
    import { addTwo } from './addTwo.mjs';

    // Prints: 6
    console.log(addTwo(4));

Node.js fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.

### Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

Authors can tell Node.js to interpret JavaScript as an ES module via the `.mjs` file extension, the `package.json` `"type"` field with a value `"module"`, the `--input-type` flag with a value of `"module"`, or the `--experimental-default-type` flag with a value of `"module"`. These are explicit markers of code being intended to run as an ES module.

Inversely, authors can tell Node.js to interpret JavaScript as CommonJS via the `.cjs` file extension, the `package.json` `"type"` field with a value `"commonjs"`, the `--input-type` flag with a value of `"commonjs"`, or the `--experimental-default-type` flag with a value of `"commonjs"`.

When code lacks explicit markers for either module system, Node.js will inspect the source code of a module to look for ES module syntax. If such syntax is found, Node.js will run the code as an ES module; otherwise it will run the module as CommonJS. See Determining module system for more details.

### Packages#

This section was moved to Modules: Packages.

### `import` Specifiers#

#### Terminology#

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

There are three types of specifiers:

- Relative specifiers like `'./startup.js'` or `'../config.mjs'`. They refer to a path relative to the location of the importing file. The file extension is always necessary for these.

- Bare specifiers like `'some-package'` or `'some-package/shuffle'`. They can refer to the main entry point of a package by the package name, or a specific feature module within a package prefixed by the package name as per the examples respectively. Including the file extension is only necessary for packages without an `"exports"` field.

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

Bare specifier resolutions are handled by the Node.js module resolution and loading algorithm. All other specifier resolutions are always only resolved with the standard relative URL resolution semantics.

Like in CommonJS, module 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 can only be accessed via the paths defined in `"exports"`.

For details on these package resolution rules that apply to bare specifiers in the Node.js module resolution, see the packages documentation.

#### Mandatory file extensions#

A file extension must be provided when using the `import` keyword to resolve relative or absolute specifiers. 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.

#### URLs#

ES modules are resolved and cached as URLs. This means that special characters must be percent-encoded, such as `#` with `%23` and `?` with `%3F`.

`file:`, `node:`, and `data:` URL schemes are supported. A specifier like `'https://example.com/app.js'` is not supported natively in Node.js unless using a custom HTTPS loader.

##### `file:` URLs#

Modules are loaded multiple times if the `import` specifier used to resolve them has 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"

The volume root may be referenced via `/`, `//`, or `file:///`. Given the differences between URL and path resolution (such as percent encoding details), it is recommended to use url.pathToFileURL when importing a path.

##### `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

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

`data:` URLs only resolve bare specifiers for builtin modules and absolute specifiers. Resolving relative specifiers does not work because `data:` is not a special scheme. For example, attempting to load `./foo` from `data:text/javascript,import "./foo";` fails to resolve because there is no concept of relative resolution for `data:` URLs.

##### `node:` imports#

History

Version | Changes
--- | ---
v16.0.0, v14.18.0 | Added `node:` import support to `require(...)`.
v14.13.1, v12.20.0 | Added in: v14.13.1, v12.20.0

`node:` URLs are supported as an alternative means to load Node.js builtin modules. This URL scheme allows for builtin modules to be referenced by valid absolute URL strings.

    import fs from 'node:fs/promises';

### Import attributes#

History

Version | Changes
--- | ---
v21.0.0, v20.10.0, v18.19.0 | Switch from Import Assertions to Import Attributes.
v17.1.0, v16.14.0 | Added in: v17.1.0, v16.14.0

Stability: 1.1 - Active development

> This feature was previously named "Import assertions", and using the `assert` keyword instead of `with`. Any uses in code of the prior `assert` keyword should be updated to use `with` instead.

The Import Attributes proposal adds an inline syntax for module import statements to pass on more information alongside the module specifier.

    import fooData from './foo.json' with { type: 'json' };

    const { default: barData } =
      await import('./bar.json', { with: { type: 'json' } });

Node.js supports the following `type` values, for which the attribute is mandatory:

Attribute `type` | Needed for
--- | ---
`'json'` | JSON modules

### Builtin modules#

Core modules 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 'node:events';
    const e = new EventEmitter();

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

    import fs, { readFileSync } from 'node:fs';
    import { syncBuiltinESMExports } from 'node:module';
    import { Buffer } from 'node:buffer';

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

    fs.readFileSync === readFileSync;

### `import()` expressions#

Dynamic `import()` is supported in both CommonJS and ES modules. In CommonJS modules it can be used to load ES modules.

### `import.meta`#

- <Object>

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

#### `import.meta.dirname`#

Added in: v21.2.0

Stability: 1.2 - Release candidate

- <string> The directory name of the current module. This is the same as the `path.dirname()` of the `import.meta.filename`.

> Caveat: only present on `file:` modules.

#### `import.meta.filename`#

Added in: v21.2.0

Stability: 1.2 - Release candidate

- <string> The full absolute path and filename of the current module, with
- symlinks resolved.
- This is the same as the `url.fileURLToPath()` of the
- `import.meta.url`.

> Caveat only local modules support this property. Modules not using the `file:` protocol will not provide it.

#### `import.meta.url`#

- <string> The absolute `file:` URL of the module.

This is defined exactly the same as it is in browsers providing the URL of the current module file.

This enables useful patterns such as relative file loading:

    import { readFileSync } from 'node:fs';
    const buffer = readFileSync(new URL('./data.proto', import.meta.url));

#### `import.meta.resolve(specifier)`#

History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | Unflag `import.meta.resolve``, with` parentURL` parameter still flagged.
v20.6.0, v18.19.0 | This API no longer throws when targeting `file:` URLs that do not map to an existing file on the local FS.
v20.0.0, v18.19.0 | This API now returns a string synchronously instead of a Promise.
v16.2.0, v14.18.0 | Add support for WHATWG `URL` object to `parentURL` parameter.
v13.9.0, v12.16.2 | Added in: v13.9.0, v12.16.2

Stability: 1.2 - Release candidate

- `specifier` <string> The module specifier to resolve relative to the current module.
- Returns: <string> The absolute URL string that the specifier would resolve to.

`import.meta.resolve` is a module-relative resolution function scoped to each module, returning the URL string.

    const dependencyAsset = import.meta.resolve('component-lib/asset.css');
    // file:///app/node_modules/component-lib/asset.css
    import.meta.resolve('./dep.js');
    // file:///app/dep.js

All features of the Node.js module resolution are supported. Dependency resolutions are subject to the permitted exports resolutions within the package.

Caveats:

- This can result in synchronous file-system operations, which can impact performance similarly to `require.resolve`.
- This feature is not available within custom loaders (it would create a deadlock).

Non-standard API:

When using the `--experimental-import-meta-resolve` flag, that function accepts a second argument:

- `parent` <string> | <URL> An optional absolute parent module URL to resolve from. Default: `import.meta.url`

### Interoperability with CommonJS#

#### `import` statements#

An `import` statement can reference an ES module or a CommonJS module. `import` statements are permitted only in ES modules, but dynamic `import()` expressions are supported in CommonJS for loading ES modules.

When importing CommonJS modules, the `module.exports` object is provided as the default export. Named exports may be available, provided by static analysis as a convenience for better ecosystem compatibility.

#### `require`#

The CommonJS module `require` always treats the files it references as CommonJS.

Using `require` to load an ES module is not supported because ES modules have asynchronous execution. Instead, use `import()` to load an ES module from a CommonJS module.

#### CommonJS Namespaces#

CommonJS modules consist of a `module.exports` object which can be of any type.

When importing a CommonJS module, it can be reliably imported using the ES module default import or its corresponding sugar syntax:

    import { default as cjs } from 'cjs';

    // The following import statement is "syntax sugar" (equivalent but sweeter)
    // for `{ default as cjsSugar }` in the above import statement:
    import cjsSugar from 'cjs';

    console.log(cjs);
    console.log(cjs === cjsSugar);
    // Prints:
    //   <module.exports>
    //   true

The ECMAScript Module Namespace representation of a CommonJS module is always a namespace with a `default` export key pointing to the CommonJS `module.exports` value.

This Module Namespace Exotic Object can be directly observed either when using `import * as m from 'cjs'` or a dynamic import:

    import * as m from 'cjs';
    console.log(m);
    console.log(m === await import('cjs'));
    // Prints:
    //   [Module] { default: <module.exports> }
    //   true

For better compatibility with existing usage in the JS ecosystem, Node.js in addition attempts to determine the CommonJS named exports of every imported CommonJS module to provide them as separate ES module exports using a static analysis process.

For example, consider a CommonJS module written:

    // cjs.cjs
    exports.name = 'exported';

The preceding module supports named imports in ES modules:

    import { name } from './cjs.cjs';
    console.log(name);
    // Prints: 'exported'

    import cjs from './cjs.cjs';
    console.log(cjs);
    // Prints: { name: 'exported' }

    import * as m from './cjs.cjs';
    console.log(m);
    // Prints: [Module] { default: { name: 'exported' }, name: 'exported' }

As can be seen from the last example of the Module Namespace Exotic Object being logged, the `name` export is copied off of the `module.exports` object and set directly on the ES module namespace when the module is imported.

Live binding updates or new exports added to `module.exports` are not detected for these named exports.

The detection of named exports is based on common syntax patterns but does not always correctly detect named exports. In these cases, using the default import form described above can be a better option.

Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See cjs-module-lexer for the exact semantics implemented.

#### Differences between ES modules and CommonJS#

##### No `require`, `exports`, or `module.exports`#

In most cases, the ES module `import` can be used to load CommonJS modules.

If needed, a `require` function can be constructed within an ES module using `module.createRequire()`.

##### No `__filename` or `__dirname`#

These CommonJS variables are not available in ES modules.

`__filename` and `__dirname` use cases can be replicated via `import.meta.filename` and `import.meta.dirname`.

##### No Addon Loading#

Addons are not currently supported with ES module imports.

They can instead be loaded with `module.createRequire()` or `process.dlopen`.

##### No `require.resolve`#

Relative resolution can be handled via `new URL('./local', import.meta.url)`.

For a complete `require.resolve` replacement, there is the import.meta.resolve API.

Alternatively `module.createRequire()` can be used.

##### No `NODE_PATH`#

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

##### No `require.extensions`#

`require.extensions` is not used by `import`. Module customization hooks can provide a replacement.

##### No `require.cache`#

`require.cache` is not used by `import` as the ES module loader has its own separate cache.

### JSON modules#

Stability: 1 - Experimental

JSON files can be referenced by `import`:

    import packageConfig from './package.json' with { type: 'json' };

The `with { type: 'json' }` syntax is mandatory; see Import Attributes.

The imported JSON only exposes a `default` export. There is no support for named exports. A cache entry is created in the CommonJS cache to avoid duplication. The same object is returned in CommonJS if the JSON module has already been imported from the same path.

### Wasm modules#

Stability: 1 - Experimental

Importing WebAssembly 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 WebAssembly.

For example, an `index.mjs` containing:

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

executed under:

    node --experimental-wasm-modules index.mjs

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

### Top-level `await`#

Added in: v14.8.0

The `await` keyword may be used in the top level body of an ECMAScript module.

Assuming an `a.mjs` with

    export const five = await Promise.resolve(5);

And a `b.mjs` with

    import { five } from './a.mjs';

    console.log(five); // Logs `5`

    node b.mjs # works

If a top level `await` expression never resolves, the `node` process will exit with a `13` status code.

    import { spawn } from 'node:child_process';
    import { execPath } from 'node:process';

    spawn(execPath, [
      '--input-type=module',
      '--eval',
      // Never-resolving Promise:
      'await new Promise(() => {})',
    ]).once('exit', (code) => {
      console.log(code); // Logs `13`
    });

### HTTPS and HTTP imports#

Stability: 1 - Experimental

Importing network based modules using `https:` and `http:` is supported under the `--experimental-network-imports` flag. This allows web browser-like imports to work in Node.js with a few differences due to application stability and security concerns that are different when running in a privileged environment instead of a browser sandbox.

#### Imports are limited to HTTP/1#

Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.

#### HTTP is limited to loopback addresses#

`http:` is vulnerable to man-in-the-middle attacks and is not allowed to be used for addresses outside of the IPv4 address `127.0.0.0/8` (`127.0.0.1` to `127.255.255.255`) and the IPv6 address `::1`. Support for `http:` is intended to be used for local development.

#### Authentication is never sent to the destination server.#

`Authorization`, `Cookie`, and `Proxy-Authorization` headers are not sent to the server. Avoid including user info in parts of imported URLs. A security model for safely using these on the server is being worked on.

#### CORS is never checked on the destination server#

CORS is designed to allow a server to limit the consumers of an API to a specific set of hosts. This is not supported as it does not make sense for a server-based implementation.

#### Cannot load non-network dependencies#

These modules cannot access other modules that are not over `http:` or `https:`. To still access local modules while avoiding the security concern, pass in references to the local dependencies:

    // file.mjs
    import worker_threads from 'node:worker_threads';
    import { configure, resize } from 'https://example.com/imagelib.mjs';
    configure({ worker_threads });

    // https://example.com/imagelib.mjs
    let worker_threads;
    export function configure(opts) {
      worker_threads = opts.worker_threads;
    }
    export function resize(img, size) {
      // Perform resizing in worker_thread to avoid main thread blocking
    }

#### Network-based loading is not enabled by default#

For now, the `--experimental-network-imports` flag is required to enable loading resources over `http:` or `https:`. In the future, a different mechanism will be used to enforce this. Opt-in is required to prevent transitive dependencies inadvertently using potentially mutable state that could affect reliability of Node.js applications.

### Loaders#

The former Loaders documentation is now at Modules: Customization hooks.

### Resolution and loading algorithm#

#### Features#

The default resolver has the following properties:

- FileURL-based resolution as is used by ES modules
- Relative and absolute URL resolution
- No default extensions
- No folder mains
- Bare specifier package resolution lookup through node_modules
- Does not fail on unknown extensions or protocols
- Can optionally provide a hint of the format to the loading phase

The default loader has the following properties

- Support for builtin module loading via `node:` URLs
- Support for "inline" module loading via `data:` URLs
- Support for `file:` module loading
- Fails on any other URL protocol
- Fails on unknown extensions for `file:` loading (supports only `.cjs`, `.js`, and `.mjs`)

#### Resolution 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.

The resolution algorithm determines the full resolved URL for a module load, along with its suggested module format. The resolution algorithm does not determine whether the resolved URL protocol can be loaded, or whether the file extensions are permitted, instead these validations are applied by Node.js during the load phase (for example, if it was asked to load a URL that has a protocol that is not `file:`, `data:`, `node:`, or if `--experimental-network-imports` is enabled, `https:`).

The algorithm also tries to determine the format of the file based on the extension (see `ESM_FILE_FORMAT` algorithm below). If it does not recognize the file extension (eg if it is not `.mjs`, `.cjs`, or `.json`), then a format of `undefined` is returned, which will throw during the load phase.

The algorithm to determine the module format of a resolved URL is provided by ESM_FILE_FORMAT, which returns the unique module format for any file. 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.

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

The resolver can throw the following errors:

- Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
- Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
- Invalid Package Target: Package exports or imports define a target module for the package that is an invalid type or string target.
- Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
- Package Import Not Defined: Package imports do not define the specifier.
- Module Not Found: The package or module requested does not exist.
- Unsupported Directory Import: The resolved path corresponds to a directory, which is not a supported target for module imports.

#### Resolution Algorithm Specification#

ESM_RESOLVE(specifier, parentURL)

> 1. Let resolved be undefined.
> 2. If specifier is a valid URL, then
>    1. Set resolved to the result of parsing and reserializing specifier as a URL.
> 3. Otherwise, if specifier starts with "/", "./", or "../", then
>    1. Set resolved to the URL resolution of specifier relative to parentURL.
> 4. Otherwise, if specifier starts with "#", then
>    1. Set resolved to the result of PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, defaultConditions).
> 5. Otherwise,
>    1. Note: specifier is now a bare specifier.
>    2. Set resolved the result of PACKAGE_RESOLVE(specifier, parentURL).
> 6. Let format be undefined.
> 7. If resolved is a "file:" URL, then
>    1. If resolved contains any percent encodings of "/" or "\" ("%2F" and "%5C" respectively), then
>       1. Throw an Invalid Module Specifier error.
>    2. If the file at resolved is a directory, then
>       1. Throw an Unsupported Directory Import error.
>    3. If the file at resolved does not exist, then
>       1. Throw a Module Not Found error.
>    4. Set resolved to the real path of resolved, maintaining the same URL querystring and fragment components.
>    5. Set format to the result of ESM_FILE_FORMAT(resolved).
> 8. Otherwise,
>    1. Set format the module format of the content type associated with the URL resolved.
> 9. Return format and resolved to the loading phase

PACKAGE_RESOLVE(packageSpecifier, parentURL)

> 1. Let packageName be undefined.
> 2. If packageSpecifier is an empty string, then
>    1. Throw an Invalid Module Specifier error.
> 3. If packageSpecifier is a Node.js builtin module name, then
>    1. Return the string "node:" concatenated with packageSpecifier.
> 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 Module 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 Module Specifier error.
> 7. Let packageSubpath be "." concatenated with the substring of packageSpecifier from the position at the length of packageName.
> 8. If packageSubpath ends in "/", then
>    1. Throw an Invalid Module Specifier error.
> 9. Let selfUrl be the result of PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL).
> 10. If selfUrl is not undefined, return selfUrl.
> 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. Continue the next loop iteration.
>     4. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
>     5. If pjson is not null and pjson.exports is not null or undefined, then
>        1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
>     6. Otherwise, if packageSubpath is equal to ".", then
>        1. If pjson.main is a string, then
>           1. Return the URL resolution of main in packageURL.
>     7. Otherwise,
>        1. Return the URL resolution of packageSubpath in packageURL.
> 12. Throw a Module Not Found error.

PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL)

> 1. Let packageURL be the result of LOOKUP_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 is null or if pjson.exports is null or undefined, then
>    1. Return undefined.
> 5. If pjson.name is equal to packageName, then
>    1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
> 6. Otherwise, return undefined.

PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, exports, conditions)

> 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 subpath is equal to ".", then
>    1. Let mainExport be undefined.
>    2. If exports is a String or Array, or an Object containing no keys starting with ".", then
>       1. Set mainExport to exports.
>    3. Otherwise if exports is an Object containing a "." property, then
>       1. Set mainExport to exports["."].
>    4. If mainExport is not undefined, then
>       1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, mainExport, null, false, conditions).
>       2. If resolved is not null or undefined, return resolved.
> 3. Otherwise, if exports is an Object and all keys of exports start with ".", then
>    1. Assert: subpath begins with "./".
>    2. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( subpath, exports, packageURL, false, conditions).
>    3. If resolved is not null or undefined, return resolved.
> 4. Throw a Package Path Not Exported error.

PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)

> 1. Assert: specifier begins with "#".
> 2. If specifier is exactly equal to "#" or starts with "#/", then
>    1. Throw an Invalid Module Specifier error.
> 3. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
> 4. If packageURL is not null, then
>    1. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
>    2. If pjson.imports is a non-null Object, then
>       1. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( specifier, pjson.imports, packageURL, true, conditions).
>       2. If resolved is not null or undefined, return resolved.
> 5. Throw a Package Import Not Defined error.

PACKAGE_IMPORTS_EXPORTS_RESOLVE(matchKey, matchObj, packageURL, isImports, conditions)

> 1. If matchKey is a key of matchObj and does not contain "*", then
>    1. Let target be the value of matchObj[matchKey].
>    2. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, null, isImports, conditions).
> 2. Let expansionKeys be the list of keys of matchObj containing only a single "*", sorted by the sorting function PATTERN_KEY_COMPARE which orders in descending order of specificity.
> 3. For each key expansionKey in expansionKeys, do
>    1. Let patternBase be the substring of expansionKey up to but excluding the first "*" character.
>    2. If matchKey starts with but is not equal to patternBase, then
>       1. Let patternTrailer be the substring of expansionKey from the index after the first "*" character.
>       2. If patternTrailer has zero length, or if matchKey ends with patternTrailer and the length of matchKey is greater than or equal to the length of expansionKey, then
>          1. Let target be the value of matchObj[expansionKey].
>          2. Let patternMatch be the substring of matchKey starting at the index of the length of patternBase up to the length of matchKey minus the length of patternTrailer.
>          3. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions).
> 4. Return null.

PATTERN_KEY_COMPARE(keyA, keyB)

> 1. Assert: keyA ends with "/" or contains only a single "*".
> 2. Assert: keyB ends with "/" or contains only a single "*".
> 3. Let baseLengthA be the index of "*" in keyA plus one, if keyA contains "*", or the length of keyA otherwise.
> 4. Let baseLengthB be the index of "*" in keyB plus one, if keyB contains "*", or the length of keyB otherwise.
> 5. If baseLengthA is greater than baseLengthB, return -1.
> 6. If baseLengthB is greater than baseLengthA, return 1.
> 7. If keyA does not contain "*", return 1.
> 8. If keyB does not contain "*", return -1.
> 9. If the length of keyA is greater than the length of keyB, return -1.
> 10. If the length of keyB is greater than the length of keyA, return 1.
> 11. Return 0.

PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions)

> 1. If target is a String, then
>    1. If target does not start with "./", then
>       1. If isImports is false, or if target starts with "../" or "/", or if target is a valid URL, then
>          1. Throw an Invalid Package Target error.
>       2. If patternMatch is a String, then
>          1. Return PACKAGE_RESOLVE(target with every instance of "*" replaced by patternMatch, packageURL + "/").
>       3. Return PACKAGE_RESOLVE(target, packageURL + "/").
>    2. If target split on "/" or "\" contains any "", ".", "..", or "node_modules" segments after the first "." segment, case insensitive and including percent encoded variants, throw an Invalid Package Target error.
>    3. Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
>    4. Assert: packageURL is contained in resolvedTarget.
>    5. If patternMatch is null, then
>       1. Return resolvedTarget.
>    6. If patternMatch split on "/" or "\" contains any "", ".", "..", or "node_modules" segments, case insensitive and including percent encoded variants, throw an Invalid Module Specifier error.
>    7. Return the URL resolution of resolvedTarget with every instance of "*" replaced with patternMatch.
> 2. Otherwise, if target is a non-null Object, then
>    1. If target 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 p equals "default" or conditions 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_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions).
>          3. If resolved is equal to undefined, continue the loop.
>          4. Return resolved.
>    3. Return undefined.
> 3. Otherwise, if target is an Array, then
>    1. If _target.length is zero, return null.
>    2. For each item targetValue in target, do
>       1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions), continuing the loop on any Invalid Package Target error.
>       2. If resolved is undefined, continue the loop.
>       3. Return resolved.
>    3. Return or throw the last fallback resolution null return or error.
> 4. Otherwise, if target is null, return null.
> 5. Otherwise throw an Invalid Package Target error.

ESM_FILE_FORMAT(url)

> 1. Assert: url corresponds to an existing file.
> 2. If url ends in ".mjs", then
>    1. Return "module".
> 3. If url ends in ".cjs", then
>    1. Return "commonjs".
> 4. If url ends in ".json", then
>    1. Return "json".
> 5. If `--experimental-wasm-modules` is enabled and url ends in ".wasm", then
>    1. Return "wasm".
> 6. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(url).
> 7. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
> 8. Let packageType be null.
> 9. If pjson?.type is "module" or "commonjs", then
>    1. Set packageType to pjson.type.
> 10. If url ends in ".js", then
>     1. If packageType is not null, then
>        1. Return packageType.
>     2. If `--experimental-detect-module` is enabled and the source of module contains static import or export syntax, then
>        1. Return "module".
>     3. Return "commonjs".
> 11. If url does not have any extension, then
>     1. If packageType is "module" and `--experimental-wasm-modules` is enabled and the file at url contains the header for a WebAssembly module, then
>        1. Return "wasm".
>     2. If packageType is not null, then
>        1. Return packageType.
>     3. If `--experimental-detect-module` is enabled and the source of module contains static import or export syntax, then
>        1. Return "module".
>     4. Return "commonjs".
> 12. Return undefined (will throw during load phase).

LOOKUP_PACKAGE_SCOPE(url)

> 1. Let scopeURL be url.
> 2. While scopeURL is not the file system root,
>    1. Set scopeURL to the parent URL of scopeURL.
>    2. If scopeURL ends in a "node_modules" path segment, return null.
>    3. Let pjsonURL be the resolution of "package.json" within scopeURL.
>    4. if the file at pjsonURL exists, then
>       1. Return 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#

Module customization hooks provide a mechanism for customizing the ESM specifier resolution algorithm. An example that provides CommonJS-style resolution for ESM specifiers is commonjs-extension-resolution-loader.