new ESM implementation #26745

MylesBorins · 2019-03-18T15:44:54Z

This PR updates the current --experimental-modules implementation based on the work of the modules team and reflects Phase 2 of our new modules plan.

A longer form description of these changes can be found in our draft blog post.

The largest differences from the current implementation include

packge.type which can be either module or commonjs
- type: "commonjs":
  - .js is parsed as commonjs
  - default for entry point without an extension is commonjs
- type: "module":
  - .js is parsed as esm
  - does not support loading JSON or Native Module by default
  - default for entry point without an extension is esm
--type=[mode] to let you set the type on entry point. Will override package.type for entry point.
A new file extension .cjs.
- this is specifically to support importing commonjs in the module mode.
- this is only in the esm loader, the commonjs loader remains untouched, but the extension will work in the old loader if you use the full file path.
--es-module-specifier-resolution=[type]
- options are explicit (default) and node
- by default our loader will not allow for optional extensions in the import, the path for a module must include the extension if there is one
- by default our loader will not allow for importing directories that have an index file
- developers can use --es-module-specifier-resolution=node to enable the commonjs specifier resolution algorithm
- This is not a “feature” but rather an implementation for experimentation. It is expected to change before the flag is removed
--experimental-json-loader
- the only way to import json when "type": "module"
- when enable all import 'thing.json' will go through the experimental loader independent of mode
- based on JSON modules whatwg/html#4315
You can use package.main to set an entry point for a module
- the file extensions used in main will be resolved based on the type of the module

nodejs-github-bot · 2019-03-18T15:44:58Z

@MylesBorins build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2968/pipeline

MylesBorins · 2019-03-18T15:45:48Z

CI: https://ci.nodejs.org/job/node-test-pull-request/21631/

lib/internal/errors.js

devsnek

I don't think the -m is a wise addition, being the only short flag that doesn't match the long one, and being the only short flag that can disagree with its long counterpart.

antsmartian · 2019-03-18T16:28:18Z

I was trying to build this locally on my mac machine to play around, but got back the following error:

/Users/anto/programs/node/node/out/Release/obj.target/v8_snapshot/geni/embedded.cc:5:10: fatal error: 'src/snapshot/macros.h' file not found
#include "src/snapshot/macros.h"
         ^~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[1]: *** [/Users/abelginrayen/programs/node/node/out/Release/obj.target/v8_snapshot/geni/embedded.o] Error 1
make[1]: *** Waiting for unfinished jobs....
rm f821e0c8fdac2538c3eb950594d8386c58c50c3d.intermediate b79cbb721bec5e5c6395b336bec4afcbcee7dbe9.intermediate
make: *** [node] Error 2

BTW, I'm on mac os sierra 10.12.6.

MylesBorins · 2019-03-18T16:30:17Z

@antsmartian have you built node since the last V8 update? I had the same error as well and had to make clean and attempt to build again. This fixed it for me, (the issue was cache related)

kristoferjoseph · 2019-03-18T16:33:43Z

This PR doesn't mention .mjs what is the status of support for this extension?

MylesBorins · 2019-03-18T16:39:40Z

@kristoferjoseph .mjs is unchanged. In the commonjs mode (which is deafult) .mjs is the only way to import esm. In the module mode .mjs can still be used.

MylesBorins · 2019-03-18T16:52:20Z

@devsnek I've removed -m if you could remove your objection. We can bring the flag back to the modules team to review.

doc/api/esm.md

doc/api/cli.md

doc/api/esm.md

lib/internal/errors.js

doc/api/cli.md

doc/api/esm.md

nodejs-github-bot · 2019-03-27T18:54:49Z

CI: https://ci.nodejs.org/job/node-test-pull-request/21969/

ChALkeR

Ok, rubber stamp LGTM.
I have not reviewed all the code, though, but the main changes here definitely look good.

vsemozhetbyt · 2019-03-27T19:38:37Z

doc/node.1

+Set the top-level module resolution type.
+.
+.It Fl -es-module-specifier-resolution
+Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'


Sorry, still one nit (the period at the end hidden by the scrolling).

Suggested change

Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'

Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'.

This PR updates the current `--experimental-modules` implementation based on the work of the modules team and reflects Phase 2 of our new modules plan. The largest differences from the current implementation include * `packge.type` which can be either `module` or `commonjs` - `type: "commonjs"`: - `.js` is parsed as commonjs - default for entry point without an extension is commonjs - `type: "module"`: - `.js` is parsed as esm - does not support loading JSON or Native Module by default - default for entry point without an extension is esm * `--entry-type=[mode]` - allows you set the type on entry point. * A new file extension `.cjs`. - this is specifically to support importing commonjs in the `module` mode. - this is only in the esm loader, the commonjs loader remains untouched, but the extension will work in the old loader if you use the full file path. * `--es-module-specifier-resolution=[type]` - options are `explicit` (default) and `node` - by default our loader will not allow for optional extensions in the import, the path for a module must include the extension if there is one - by default our loader will not allow for importing directories that have an index file - developers can use `--es-module-specifier-resolution=node` to enable the commonjs specifier resolution algorithm - This is not a “feature” but rather an implementation for experimentation. It is expected to change before the flag is removed * `--experimental-json-loader` - the only way to import json when `"type": "module"` - when enable all `import 'thing.json'` will go through the experimental loader independent of mode - based on whatwg/html#4315 * You can use `package.main` to set an entry point for a module - the file extensions used in main will be resolved based on the `type` of the module Refs: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md Refs: https://github.com/GeoffreyBooth/node-import-file-specifier-resolution-proposal Refs: nodejs/modules#180 Refs: nodejs/ecmascript-modules#6 Refs: nodejs/ecmascript-modules#12 Refs: nodejs/ecmascript-modules#28 Refs: nodejs/modules#255 Refs: whatwg/html#4315 Refs: WICG/webcomponents#770 Co-authored-by: Myles Borins <[email protected]> Co-authored-by: John-David Dalton <[email protected]> Co-authored-by: Evan Plaice <[email protected]> Co-authored-by: Geoffrey Booth <[email protected]> Co-authored-by: Michaël Zasso <[email protected]> PR-URL: nodejs#26745 Reviewed-By: Gus Caplan <[email protected]> Reviewed-By: Guy Bedford <[email protected]> Reviewed-By: Ben Coe <[email protected]> Reviewed-By: James M Snell <[email protected]> Reviewed-By: Joyee Cheung <[email protected]> Reviewed-By: Ruben Bridgewater <[email protected]> Reviewed-By: Сковорода Никита Андреевич <[email protected]>

This PR updates the current `--experimental-modules` implementation based on the work of the modules team and reflects Phase 2 of our new modules plan. The largest differences from the current implementation include * `packge.type` which can be either `module` or `commonjs` - `type: "commonjs"`: - `.js` is parsed as commonjs - default for entry point without an extension is commonjs - `type: "module"`: - `.js` is parsed as esm - does not support loading JSON or Native Module by default - default for entry point without an extension is esm * `--entry-type=[mode]` - allows you set the type on entry point. * A new file extension `.cjs`. - this is specifically to support importing commonjs in the `module` mode. - this is only in the esm loader, the commonjs loader remains untouched, but the extension will work in the old loader if you use the full file path. * `--es-module-specifier-resolution=[type]` - options are `explicit` (default) and `node` - by default our loader will not allow for optional extensions in the import, the path for a module must include the extension if there is one - by default our loader will not allow for importing directories that have an index file - developers can use `--es-module-specifier-resolution=node` to enable the commonjs specifier resolution algorithm - This is not a “feature” but rather an implementation for experimentation. It is expected to change before the flag is removed * `--experimental-json-loader` - the only way to import json when `"type": "module"` - when enable all `import 'thing.json'` will go through the experimental loader independent of mode - based on whatwg/html#4315 * You can use `package.main` to set an entry point for a module - the file extensions used in main will be resolved based on the `type` of the module Refs: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md Refs: https://github.com/GeoffreyBooth/node-import-file-specifier-resolution-proposal Refs: nodejs/modules#180 Refs: nodejs/ecmascript-modules#6 Refs: nodejs/ecmascript-modules#12 Refs: nodejs/ecmascript-modules#28 Refs: nodejs/modules#255 Refs: whatwg/html#4315 Refs: WICG/webcomponents#770 Co-authored-by: Myles Borins <[email protected]> Co-authored-by: John-David Dalton <[email protected]> Co-authored-by: Evan Plaice <[email protected]> Co-authored-by: Geoffrey Booth <[email protected]> Co-authored-by: Michaël Zasso <[email protected]> PR-URL: #26745 Reviewed-By: Gus Caplan <[email protected]> Reviewed-By: Guy Bedford <[email protected]> Reviewed-By: Ben Coe <[email protected]> Reviewed-By: James M Snell <[email protected]> Reviewed-By: Joyee Cheung <[email protected]> Reviewed-By: Ruben Bridgewater <[email protected]> Reviewed-By: Сковорода Никита Андреевич <[email protected]>

jkrems · 2019-04-05T18:50:55Z

@lukaszewczak brought up that this arguably should be included in #26930 but it looks like this just missed the cut of v12.x-staging?

GeoffreyBooth · 2019-04-11T15:47:16Z

Node core folks, please see #27184. It’s a follow-up to this PR that I’d love to see merged in before Node 12 ships 🙏

MylesBorins · 2019-04-16T15:37:17Z

Adding dont-land-on-11.x... I think we should leave 11.x alone for the final release

# Modules: ECMAScript modules    > 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: ```js // addTwo.mjs function addTwo(num) { return num + 2; } export { addTwo }; ``` The following example of an ES module imports the function from `addTwo.mjs`: ```js // 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 use the ECMAScript modules loader via the `.mjs` file extension, the `package.json` [`"type"`][] field, or the [`--input-type`][] flag. Outside of those cases, Node.js will use the CommonJS module loader. See [Determining module system][] for more details.  ## Packages This section was moved to [Modules: Packages](packages.md). ## `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 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 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 packageâ€™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](packages.md). ### 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. ```js import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1" import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2" ``` 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  [`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_][Terminology] for builtin modules and [_Absolute specifiers_][Terminology]. Resolving [_Relative specifiers_][Terminology] 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. An example of a `data:` URLs being used is: ```js import 'data:text/javascript,console.log("hello!");'; import _ from 'data:application/json,"world!"'; ``` #### `node:` Imports  `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. ```js import fs from 'node:fs/promises'; ``` ## Import assertions  > Stability: 1 - Experimental The [Import Assertions proposal][] adds an inline syntax for module import statements to pass on more information alongside the module specifier. ```js import fooData from './foo.json' assert { type: 'json' }; const { default: barData } = await import('./bar.json', { assert: { type: 'json' } }); ``` Node.js supports the following `type` values, for which the assertion is mandatory: | Assertion `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()`][]. ```js import EventEmitter from 'events'; const e = new EventEmitter(); ``` ```js import { readFile } from 'fs'; readFile('./foo.txt', (err, source) => { if (err) { console.error(err); } else { console.log(source); } }); ``` ```js import fs, { readFileSync } from 'fs'; import { syncBuiltinESMExports } from 'module'; import { Buffer } from '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.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: ```js import { readFileSync } from 'fs'; const buffer = readFileSync(new URL('./data.proto', import.meta.url)); ``` ### `import.meta.resolve(specifier[, parent])`  > Stability: 1 - Experimental This feature is only available with the `--experimental-import-meta-resolve` command flag enabled. * `specifier` {string} The module specifier to resolve relative to `parent`. * `parent` {string|URL} The absolute parent module URL to resolve from. If none is specified, the value of `import.meta.url` is used as the default. * Returns: {Promise} Provides a module-relative resolution function scoped to each module, returning the URL string.  ```js const dependencyAsset = await import.meta.resolve('component-lib/asset.css'); ``` `import.meta.resolve` also accepts a second argument which is the parent module from which to resolve from:  ```js await import.meta.resolve('./dep', import.meta.url); ``` This function is asynchronous because the ES module resolver in Node.js is allowed to be asynchronous. ## 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](#commonjs-namespaces), 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:  ```js 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:  ```js 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.cjs exports.name = 'exported'; ``` The preceding module supports named imports in ES modules:  ```js 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.url`][]. #### No Native Module Loading Native modules 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 a flagged experimental [`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`. The expectation is that loader hooks can provide this workflow in the future. #### 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`: ```js import packageConfig from './package.json' assert { type: 'json' }; ``` The `assert { type: 'json' }` syntax is mandatory; see [Import Assertions][]. 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: ```js import * as M from './module.wasm'; console.log(M); ``` executed under: ```bash node --experimental-wasm-modules index.mjs ``` would provide the exports interface for the instantiation of `module.wasm`. ## Top-level `await`  > Stability: 1 - Experimental The `await` keyword may be used in the top level body of an ECMAScript module. Assuming an `a.mjs` with ```js export const five = await Promise.resolve(5); ``` And a `b.mjs` with ```js import { five } from './a.mjs'; console.log(five); // Logs `5` ``` ```bash node b.mjs # works ``` If a top level `await` expression never resolves, the `node` process will exit with a `13` [status code][]. ```js import { spawn } from 'child_process'; import { execPath } from 'process'; spawn(execPath, [ '--input-type=module', '--eval', // Never-resolving Promise: 'await new Promise(() => {})', ]).once('exit', (code) => { console.log(code); // Logs `13` }); ``` ## Loaders > Stability: 1 - Experimental **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 apply to the entry point and all `import` calls. They won't apply to `require` calls; those still follow [CommonJS][] rules. ### Hooks #### `resolve(specifier, context, defaultResolve)`  > Note: The loaders API is being redesigned. This hook may disappear or its > signature may change. Do not rely on the API described below. * `specifier` {string} * `context` {Object} * `conditions` {string\[]} * `importAssertions` {Object} * `parentURL` {string|undefined} * `defaultResolve` {Function} The Node.js default resolver. * Returns: {Object} * `format` {string|null|undefined} `'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'` * `url` {string} The absolute url to the import target (such as `file://â€¦`) The `resolve` hook returns the resolved file URL for a given module specifier and parent URL, and optionally its format (such as `'module'`) as a hint to the `load` hook. If a format is specified, the `load` hook is ultimately responsible for providing the final `format` value (and it is free to ignore the hint provided by `resolve`); if `resolve` provides a `format`, a custom `load` hook is required even if only to pass the value to the Node.js default `load` hook. 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. The `conditions` property in `context` is an array of conditions for [package exports conditions][Conditional Exports] that apply to this resolution request. They can be used for looking up conditional mappings elsewhere or to modify the list when calling the default resolution logic. The current [package exports conditions][Conditional Exports] are always in the `context.conditions` array passed into the hook. To guarantee _default Node.js module specifier resolution behavior_ when calling `defaultResolve`, the `context.conditions` array passed to it _must_ include _all_ elements of the `context.conditions` array originally passed into the `resolve` hook. ```js /** * @param {string} specifier * @param {{ * conditions: string[], * parentURL: string | undefined, * }} context * @param {Function} defaultResolve * @returns {Promise<{ url: string }>} */ export async function resolve(specifier, context, defaultResolve) { const { parentURL = null } = context; if (Math.random() > 0.5) { // Some condition. // 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, }; } if (Math.random() < 0.5) { // Another condition. // When calling `defaultResolve`, the arguments can be modified. In this // case it's adding another value for matching conditional exports. return defaultResolve(specifier, { ...context, conditions: [...context.conditions, 'another-condition'], }); } // Defer to Node.js for all other specifiers. return defaultResolve(specifier, context, defaultResolve); } ``` #### `load(url, context, defaultLoad)` > Note: The loaders API is being redesigned. This hook may disappear or its > signature may change. Do not rely on the API described below. > Note: In a previous version of this API, this was split across 3 separate, now > deprecated, hooks (`getFormat`, `getSource`, and `transformSource`). * `url` {string} * `context` {Object} * `format` {string|null|undefined} The format optionally supplied by the `resolve` hook. * `importAssertions` {Object} * `defaultLoad` {Function} * Returns: {Object} * `format` {string} * `source` {string|ArrayBuffer|TypedArray} The `load` hook provides a way to define a custom method of determining how a URL should be interpreted, retrieved, and parsed. It is also in charge of validating the import assertion. The final value of `format` must be one of the following: | `format` | Description | Acceptable types for `source` returned by `load` | | ------------ | ------------------------------ | ----------------------------------------------------- | | `'builtin'` | Load a Node.js builtin module | Not applicable | | `'commonjs'` | Load a Node.js CommonJS module | Not applicable | | `'json'` | Load a JSON file | { [`string`][], [`ArrayBuffer`][], [`TypedArray`][] } | | `'module'` | Load an ES module | { [`string`][], [`ArrayBuffer`][], [`TypedArray`][] } | | `'wasm'` | Load a WebAssembly module | { [`ArrayBuffer`][], [`TypedArray`][] } | The value of `source` is ignored for type `'builtin'` because currently it is not possible to replace the value of a Node.js builtin (core) module. The value of `source` is ignored for type `'commonjs'` because the CommonJS module loader does not provide a mechanism for the ES module loader to override the [CommonJS module return value](#commonjs-namespaces). This limitation might be overcome in the future. > **Caveat**: The ESM `load` hook and namespaced exports from CommonJS modules > are incompatible. Attempting to use them together will result in an empty > object from the import. This may be addressed in the future. > Note: These types all correspond to classes defined in ECMAScript. * The specific [`ArrayBuffer`][] object is a [`SharedArrayBuffer`][]. * The specific [`TypedArray`][] object is a [`Uint8Array`][]. If the source value of a text-based format (i.e., `'json'`, `'module'`) is not a string, it is converted to a string using [`util.TextDecoder`][]. The `load` 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. It could also be used to map an unrecognized format to a supported one, for example `yaml` to `module`. ```js /** * @param {string} url * @param {{ format: string, }} context If resolve settled with a `format`, that value is included here. * @param {Function} defaultLoad * @returns {Promise<{ format: string, source: string | ArrayBuffer | SharedArrayBuffer | Uint8Array, }>} */ export async function load(url, context, defaultLoad) { const { format } = context; if (Math.random() > 0.5) { // Some condition. /* For some or all URLs, do some custom logic for retrieving the source. Always return an object of the form { format: <string>, source: <string|buffer>, }. */ return { format, source: '...', }; } // Defer to Node.js for all other URLs. return defaultLoad(url, context, defaultLoad); } ``` In a more advanced scenario, this can also be used to transform an unsupported source to a supported one (see [Examples](#examples) below). #### `globalPreload()` > Note: The loaders API is being redesigned. This hook may disappear or its > signature may change. Do not rely on the API described below. > Note: In a previous version of this API, this hook was named > `getGlobalPreloadCode`. * Returns: {string} Sometimes it might be necessary to run some code inside of the same global scope that the application runs in. This hook allows the return of a string that is run as a sloppy-mode script on startup. Similar to how CommonJS wrappers work, the code runs in an implicit function scope. The only argument is a `require`-like function that can be used to load builtins like "fs": `getBuiltin(request: string)`. If the code needs more advanced `require` features, it has to construct its own `require` using `module.createRequire()`. ```js /** * @param {{ port: MessagePort, }} utilities Things that preload code might find useful * @returns {string} Code to run before application startup */ export function globalPreload(utilities) { return `\ globalThis.someInjectedProperty = 42; console.log('I just set some globals!'); const { createRequire } = getBuiltin('module'); const { cwd } = getBuiltin('process'); const require = createRequire(cwd() + '/<preload>'); // [...] `; } ``` In order to allow communication between the application and the loader, another argument is provided to the preload code: `port`. This is available as a parameter to the loader hook and inside of the source text returned by the hook. Some care must be taken in order to properly call [`port.ref()`][] and [`port.unref()`][] to prevent a process from being in a state where it won't close normally. ```js /** * This example has the application context send a message to the loader * and sends the message back to the application context * @param {{ port: MessagePort, }} utilities Things that preload code might find useful * @returns {string} Code to run before application startup */ export function globalPreload({ port }) { port.onmessage = (evt) => { port.postMessage(evt.data); }; return `\ port.postMessage('console.log("I went to the Loader and back");'); port.onmessage = (evt) => { eval(evt.data); }; `; } ``` ### Examples The various loader hooks can be used together to accomplish wide-ranging customizations of Node.jsâ€™ 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. ```js // 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 load(url, context, defaultLoad) { // 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({ // This example assumes all network-provided JavaScript is ES module // code. format: 'module', source: data, })); }).on('error', (err) => reject(err)); }); } // Let Node.js handle all other URLs. return defaultLoad(url, context, defaultLoad); } ``` ```js // main.mjs import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js'; console.log(VERSION); ``` With the preceding loader, running `node --experimental-loader ./https-loader.mjs ./main.mjs` prints the current version of CoffeeScript per the module at the URL in `main.mjs`. #### Transpiler loader Sources that are in formats Node.js doesnâ€™t understand can be converted into JavaScript using the [`load` hook][load hook]. Before that hook gets called, however, a [`resolve` hook][resolve hook] hook needs to tell Node.js not to throw an error on unknown file types. This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes. ```js // coffeescript-loader.mjs import { readFile } from 'node:fs/promises'; import { dirname, extname, resolve as resolvePath } from 'node:path'; import { cwd } from 'node:process'; import { fileURLToPath, pathToFileURL } from 'node:url'; import CoffeeScript from 'coffeescript'; const baseURL = pathToFileURL(`${cwd()}/`).href; // CoffeeScript files end in .coffee, .litcoffee or .coffee.md. const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/; export async 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 async function load(url, context, defaultLoad) { // Now that we patched resolve to let CoffeeScript URLs through, we need to // tell Node.js what format such URLs should be interpreted as. Because // CoffeeScript transpiles into JavaScript, it should be one of the two // JavaScript formats: 'commonjs' or 'module'. if (extensionsRegex.test(url)) { // CoffeeScript files can be either CommonJS or ES modules, so we want any // CoffeeScript file to be treated by Node.js the same as a .js file at the // same location. To determine how Node.js would interpret an arbitrary .js // file, search up the file system for the nearest parent package.json file // and read its "type" field. const format = await getPackageType(url); // When a hook returns a format of 'commonjs', `source` is be ignored. // To handle CommonJS files, a handler needs to be registered with // `require.extensions` in order to process the files with the CommonJS // loader. Avoiding the need for a separate CommonJS handler is a future // enhancement planned for ES module loaders. if (format === 'commonjs') { return { format }; } const { source: rawSource } = await defaultLoad(url, { format }); // This hook converts CoffeeScript source code into JavaScript source code // for all imported CoffeeScript files. const transformedSource = CoffeeScript.compile(rawSource.toString(), { bare: true, filename: url, }); return { format, source: transformedSource, }; } // Let Node.js handle all other URLs. return defaultLoad(url, context, defaultLoad); } async function getPackageType(url) { // `url` is only a file path during the first iteration when passed the // resolved url from the load() hook // an actual file path from load() will contain a file extension as it's // required by the spec // this simple truthy check for whether `url` contains a file extension will // work for most projects but does not cover some edge-cases (such as // extension-less files or a url ending in a trailing space) const isFilePath = !!extname(url); // If it is a file path, get the directory it's in const dir = isFilePath ? dirname(fileURLToPath(url)) : url; // Compose a file path to a package.json in the same directory, // which may or may not exist const packagePath = resolvePath(dir, 'package.json'); // Try to read the possibly nonexistent package.json const type = await readFile(packagePath, { encoding: 'utf8' }) .then((filestring) => JSON.parse(filestring).type) .catch((err) => { if (err?.code !== 'ENOENT') console.error(err); }); // Ff package.json existed and contained a `type` field with a value, voila if (type) return type; // Otherwise, (if not at the root) continue checking the next directory up // If at the root, stop and return false return dir.length > 1 && getPackageType(resolvePath(dir, '..')); } ``` ```coffee # 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}" ``` ```coffee # scream.coffee export scream = (str) -> str.toUpperCase() ``` With the preceding loader, running `node --experimental-loader ./coffeescript-loader.mjs main.coffee` causes `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. The algorithm to determine the module format of a resolved URL is provided by **ESM\_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. ### Resolver 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. Load _resolved_ as module format, _format_. **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_, _""_, **false**, **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. Let _matchKey_ be the string _"./"_ concatenated with _subpath_. > 2. Let _resolved_ be the result of **PACKAGE\_IMPORTS\_EXPORTS\_RESOLVE**( > _matchKey_, _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_, _""_, **false**, _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 _subpath_ 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_, _subpath_, **true**, _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_, _subpath_, _pattern_, _internal_, _conditions_) > 1. If _target_ is a String, then > 1. If _pattern_ is **false**, _subpath_ has non-zero length and _target_ > does not end with _"/"_, throw an _Invalid Module Specifier_ error. > 2. If _target_ does not start with _"./"_, then > 1. If _internal_ is **true** and _target_ does not start with _"../"_ or > _"/"_ and is not a valid URL, then > 1. If _pattern_ is **true**, then > 1. Return **PACKAGE\_RESOLVE**(_target_ with every instance of > _"\*"_ replaced by _subpath_, _packageURL_ + _"/"_). > 2. Return **PACKAGE\_RESOLVE**(_target_ + _subpath_, > _packageURL_ + _"/"_). > 2. Otherwise, throw an _Invalid Package Target_ error. > 3. 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. > 4. Let _resolvedTarget_ be the URL resolution of the concatenation of > _packageURL_ and _target_. > 5. Assert: _resolvedTarget_ is contained in _packageURL_. > 6. If _subpath_ split on _"/"_ or _"\\"_ contains any _"."_, _".."_ or > _"node\_modules"_ segments, case insensitive and including percent > encoded variants, throw an _Invalid Module Specifier_ error. > 7. If _pattern_ is **true**, then > 1. Return the URL resolution of _resolvedTarget_ with every instance of > _"\*"_ replaced with _subpath_. > 8. Otherwise, > 1. Return the URL resolution of the concatenation of _subpath_ and > _resolvedTarget_. > 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 _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_, _subpath_, _pattern_, _internal_, > _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_, _subpath_, _pattern_, _internal_, > _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. Let _packageURL_ be the result of **LOOKUP\_PACKAGE\_SCOPE**(_url_). > 6. Let _pjson_ be the result of **READ\_PACKAGE\_JSON**(_packageURL_). > 7. If _pjson?.type_ exists and is _"module"_, then > 1. If _url_ ends in _".js"_, then > 1. Return _"module"_. > 2. Throw an _Unsupported File Extension_ error. > 8. Otherwise, > 1. Throw an _Unsupported File Extension_ error. **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 > Stability: 1 - Experimental 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. ```console $ node index.mjs success! $ node index # Failure! Error: Cannot find module $ node --experimental-specifier-resolution=node index success! ```  [6.1.7 Array Index]: https://tc39.es/ecma262/#integer-index [CommonJS]: modules.md [Conditional exports]: packages.md#conditional-exports [Core modules]: modules.md#core-modules [Determining module system]: packages.md#determining-module-system [Dynamic `import()`]: https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports [ES Module Integration Proposal for WebAssembly]: https://github.com/webassembly/esm-integration [Import Assertions]: #import-assertions [Import Assertions proposal]: https://github.com/tc39/proposal-import-assertions [JSON modules]: #json-modules [Node.js Module Resolution Algorithm]: #resolver-algorithm-specification [Terminology]: #terminology [URL]: https://url.spec.whatwg.org/ [`"exports"`]: packages.md#exports [`"type"`]: packages.md#type [`--input-type`]: cli.md#--input-typetype [`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer [`SharedArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer [`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray [`Uint8Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array [`data:` URLs]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs [`export`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export [`import()`]: #import-expressions [`import.meta.resolve`]: #importmetaresolvespecifier-parent [`import.meta.url`]: #importmetaurl [`import`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import [`module.createRequire()`]: module.md#modulecreaterequirefilename [`module.syncBuiltinESMExports()`]: module.md#modulesyncbuiltinesmexports [`package.json`]: packages.md#nodejs-packagejson-field-definitions [`port.ref()`]: https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portref [`port.unref()`]: https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portunref [`process.dlopen`]: process.md#processdlopenmodule-filename-flags [`string`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String [`util.TextDecoder`]: util.md#class-utiltextdecoder [cjs-module-lexer]: https://github.com/nodejs/cjs-module-lexer/tree/1.2.2 [custom https loader]: #https-loader [load hook]: #loadurl-context-defaultload [percent-encoded]: url.md#percent-encoding-in-urls [resolve hook]: #resolvespecifier-context-defaultresolve [special scheme]: https://url.spec.whatwg.org/#special-scheme [status code]: process.md#exit-codes [the official standard format]: https://tc39.github.io/ecma262/#sec-modules [url.pathToFileURL]: url.md#urlpathtofileurlpath

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Tracking upstream PR nodejs/ecmascript-modules#64

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MylesBorins force-pushed the modules-lkgr branch from d9cdfd8 to b1094db Compare March 27, 2019 19:52

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Support for ECMAScript Modules (ESM) vercel/ncc#236

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Build: Add type module commonjs to the corresponding packages WordPress/gutenberg#15841

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ESM modules and json importing [feature request] #20494

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ES6 Module/import/export/require 模块源码和使用相关，以及动态导入import() cisen/blog#633

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feat: minify HTML on build nodejs/nodejs.org#2537

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new ESM implementation #26745

new ESM implementation #26745

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	Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'
	Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'.

new ESM implementation #26745

new ESM implementation #26745

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