Announcing a new --experimental-modules

This article was contributed by the Node.js Module Team.

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Back in 2017, Node.js 8.9.0 shipped experimental support for ECMAScript modules, known for their import and export statements. This support was behind the flag --experimental-modules.

A lot has happened since then. All major browsers now support ECMAScript modules (ES modules) via <script type=”module”>. Various projects have sprung up to make npm packages with ES module sources available for use in browsers via <script type=”module”>. Support for import maps, which bring to browsers Node.js-style package names in import statements, is coming to Chrome.

All this progress toward adoption of ES modules has increased the urgency for Node.js to ship its support for ES modules. Now that there are other runtimes and environments where ES modules are in use, it’s more important than ever that Node.js support this JavaScript standard.

Node.js’s initial ES modules support remained in an experimental state in order to allow the community time to provide feedback on that design. The Modules Team was formed to act on this feedback and ship first-class support for ES modules in Node.js. This has led to a new implementation for supporting ES modules, that we are pleased to announce will ship as part of Node.js 12. It will replace the old --experimental-modules implementation, behind the same flag. We hope that this new implementation addresses many of the community’s concerns, and can ship as part of Node.js proper, without a flag, before Node.js 12 reaches LTS status in October 2019.

What’s in --experimental-modules

Like the previous version, this new --experimental-modules adds support to Node.js for the following:

  • ES2015 import statements can reference JavaScript files that are themselves written using ES module syntax. Files can be referenced as relative URLs (‘./file.mjs’), absolute file:// URLs (‘file:///opt/app/file.mjs’), package names (‘es-module-package’) or paths within packages (‘es-module-package/lib/file.mjs’).
  • import statements that reference ES module files can specify both the default export (import _ from ‘es-module-package’), named exports (import { shuffle } from ‘es-module-package’) and namespace exports (import * as fs from ‘fs’). All Node.js built-in packages like fs and path support all three types of exports.
  • import statements that reference CommonJS files (all current JavaScript code written for Node.js, using require and module.exports) can use the CommonJS default export (import _ from ‘commonjs-package’) only. This is a work in progress and may change in the future.
  • export statements in ES module files can specify both default and named exports for import statements to reference.
  • Dynamic import() expressions can be used to import ES modules from either CommonJS or ES module files. Note that import() returns a promise.
  • import.meta.url provides the absolute URL of the current ES module file.
  • Loaders can be written to modify Node.js’s runtime behavior with respect to ES modules. This is still very much a work in progress.
  • Node.js can be run with an ES module file as a program’s initial entry point.
  • Files loaded as ES modules are loaded in strict mode, which in CommonJS requires adding ‘use strict’; to the top of every file.
  • Files ending in .mjs are explicitly treated as ES modules in import statements and when run via the node command.

And the new version of --experimental-modules adds:

import and export syntax in .js files

We heard some very strong feedback that Node.js needs to provide a way to use import and export syntax in .js files.

The new --experimental-modules provides two ways: for files, a “type” field in package.json; and for input via --eval, --print or STDIN, an --input-type flag.

package.json “type” field

Add “type”: “module” to the package.json for your project, and Node.js will treat all .js files in your project as ES modules.

If some of your project’s files use CommonJS and you can’t convert your entire project all at once, you can either rename those files to use the .cjs extension or put them in a subfolder containing a package.json with { “type”: “commonjs” }, under which all .js files are treated as CommonJS.

For any file that Node.js tries to load, it will look for a package.json in that file’s folder, then that file’s parent folder and so on upwards until it reaches the root of the volume. This is similar to how Babel searches for .babelrc files. This new approach allows Node.js to use package.json for package-level metadata and configuration, similar to how it is already used by Babel and other tools.

--input-type flag

Use --input-type=module to run string input (via --eval, --print or STDIN) as an ES module. The --input-type flag can be --input-type=module or --input-type=commonjs.

.cjs extension

Just as the .mjs file extension explicitly signifies that a file should be treated as an ES module, the new .cjs file extension explicitly signifies that a file should be treated as CommonJS. (CommonJS is the other module system that Node.js supports, with require and module.exports.) The .cjs extension provides a way to save CommonJS files in a project where both .mjs and .js files are treated as ES modules.

Explicit filenames

By default in the new --experimental-modules, file extensions are mandatory in import statements: import ‘./file.js’, not import ‘./file’. However, the CommonJS-style automatic extension resolution behavior (‘./file’) can be enabled via a new flag, --es-module-specifier-resolution=node. (Its inverse, the default, is --es-module-specifier-resolution=explicit.) Package names are still just package names, e.g. import fs from ‘fs’.

We’re providing the --es-module-specifier-resolution=node flag to opt in to CommonJS-style extension and index resolution. We’ve turned it off by default to collect feedback on how users feel about using fully specified paths before we unflag the --experimental-modules implementation. You can find our discussion on the topic here.

One of the primary reasons for this design decision was to encourage the authoring of code that could be shared between browser and Node.js environments by giving our specifier resolution algorithm the same baseline functionality as the browser.

module.createRequireFromPath

The “CommonJS globals” (require, exports, module, __filename, __dirname) are not defined in ES modules. However, module.createRequireFromPath() can be used to create a CommonJS require function to be used in an ES module context.

import for JavaScript only

The previous --experimental-modules allowed import statements of JSON and native modules. This has been removed; you may use module.createRequireFromPath() for these.

A separate flag --experimental-json-modules enables experimental support for import of JSON files. There is work in progress for standardizing this feature with browsers, and Node.js hopes to align our support with the eventual standard.

There is also ongoing work to cover WASM and other future potential module types. Node.js will add support for these in spec-compliant ways over time.

ES module code in packages

This is a work in progress and subject to change. You can create a package with ES module sources by using the package.json “main” field to point to an ES module package entry point. Node.js will know to load it as an ES module if the file ends in .mjs or if the package.json also contains “type”: “module”.

Currently, it is not possible to create a package that can be used via both require(‘pkg’) and import ‘pkg’. There are efforts underway to address this, and may involve changes to the above. In particular, Node.js might choose a field other than “main” to define a package’s ES module entry point. While we are aware that the community has embraced the “module” field, it is unlikely that Node.js will adopt that field as many of the packages published using “module” include ES module JavaScript that may not evaluate in Node.js (because extensions are left off of filenames, or the code includes require statements, etc.). Please do not publish any ES module packages intended for use by Node.js until this is resolved.

Works in progress

All of the above is shipping as part of --experimental-modules in Node.js 12. On our road map for potential improvements before the --experimental-modules flag is hopefully dropped in October 2019, when Node.js 12 reaches LTS status:

  • Loaders features. Loaders are being further developed to support process isolation, multiple loaders, and multi-Realm support with lower-level hooks. The --loader API will still change considerably before this is unflagged.
  • Dual CommonJS/ES module packages. We want to provide a standard way for package authors to publish a package that can be both required into CommonJS or imported into an ES module.
  • Easier require. Using Module.createRequireFromPath() involves a lot of boilerplate. We hope to provide a simpler way to use require in ES module code.
  • Package path maps. We would like to support paths to files within packages. This would allow things like import sdk from ‘some-service/sdk’ to have ‘some-service/sdk’ map to something like ./src/sdk/public-api.mjs.
  • Automatic entry point module type detection. This would provide a way for running JavaScript code as either CommonJS or ES modules based on static analysis.

That’s it! We hope you enjoy this new --experimental-modules, and we look forward to hearing your feedback. The modules team’s work is public at https://github.com/nodejs/modules.

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