esm

What is esm?

The esm npm package is a lightweight module loader that allows developers to use ES6 modules in Node.js. It enables the use of import and export syntax for working with modules, which is more standardized and often considered cleaner than the traditional CommonJS require syntax.

What are esm's main functionalities?

Import ES6 Modules

Allows the use of ES6 import syntax in Node.js by requiring the esm module and then using it to load other ES6 modules.

require = require('esm')(module)
const { myFunction } = require('./myModule.mjs')

Export ES6 Modules

Enables developers to define modules using ES6 export syntax, which can then be imported by other modules.

// In myModule.mjs
export const myFunction = () => { console.log('Hello ESM!'); }

Dynamic Import

Supports dynamic import() syntax for loading modules asynchronously, which can be useful for code splitting or on-demand loading.

require = require('esm')(module)
const myModule = require('./myModule.mjs')
myModule.then(module => { module.myFunction() })

Other packages similar to esm

@babel/register

This package allows you to transpile your ES6+ code on the fly by hooking into Node's require. It is more feature-rich than esm, offering a wide range of plugins to transform your code, but it may be heavier and slower for simple use cases.

ts-node

While ts-node is primarily for running TypeScript in Node.js without pre-compiling, it also supports ES6+ features. It is similar to esm in that it allows for the use of modern module syntax, but it is specifically tailored for TypeScript.

esbuild-register

This package uses esbuild to transpile code when requiring modules. It's similar to esm in that it allows you to use newer syntax, but it also includes TypeScript support and is focused on speed, leveraging the fast esbuild bundler.

README

esm

The brilliantly simple, babel-less, bundle-less ECMAScript module loader.

esm is the world’s most advanced ECMAScript module loader. This fast, production ready, zero dependency loader is all you need to support ECMAScript modules in Node 6+. See the release post and video for details!

Install

• New projects

  Run npm init esm or yarn create esm.

  :bulb: Use the -y flag to answer “yes” to all prompts.

• Existing projects

  Run npm i esm or yarn add esm.

Getting started

There are two ways to enable esm.

1. Enable esm for packages:

   Use esm to load the main ES module and export it as CommonJS.

   index.js

   // Set options as a parameter, environment variable, or rc file.
   require = require("esm")(module/*, options*/)
   module.exports = require("./main.js")
   

   main.js

   // ESM syntax is supported.
   export {}
   

   :bulb: These files are automagically created with npm init esm or yarn create esm.

2. Enable esm for local runs:

   node -r esm main.js
   

   :bulb: Omit the filename to enable esm in the REPL.

Features

:clap: By default, :100: percent CJS interoperability is enabled so you can get stuff done.
:lock: .mjs files are limited to basic functionality without support for esm options.

Out of the box esm just works, no configuration necessary, and supports:

• Passing all applicable test262 compliance tests
• import/export
• import.meta
• Dynamic import
• Live bindings
• File URI scheme
• Node stdin, --eval, --print flags
• Node --check flag (Node 10+)

Options

Specify options with one of the following:

• "esm" field in package.json
• CJS/ESM in an .esmrc.js, .esmrc.cjs, or .esmrc.mjs file
• JSON6 in an .esmrc or .esmrc.json file
• JSON6 or file path in the ESM_OPTIONS environment variable
• ESM_DISABLE_CACHE environment variable

{
"cjs":true	A boolean or object for toggling CJS features in ESM. Features { "cache":true A boolean for storing ES modules in require.cache. "esModule":true A boolean for __esModule interoperability. "extensions":true A boolean for respecting require.extensions in ESM. "mutableNamespace":true A boolean for mutable namespace objects. "namedExports":true A boolean for importing named exports of CJS modules. "paths":true A boolean for following CJS path rules in ESM. "vars":true A boolean for __dirname, __filename, and require in ESM. "dedefault":false A boolean for requiring ES modules without the dangling require().default. "topLevelReturn":false A boolean for top-level return support. }
"mainFields":["main"]	An array of fields checked when importing a package.
"mode":"auto"	A string mode: "auto" detect files with import, import.meta, export, "use module", or .mjs as ESM. "all" files besides those with "use script" or .cjs are treated as ESM. "strict" to treat only .mjs files as ESM.
"await":false	A boolean for top-level await in modules without ESM exports. (Node 10+)
"force":false	A boolean to apply these options to all module loads.
"wasm":false	A boolean for WebAssembly module support. (Node 8+)
}

DevOpts

{
"cache":true	A boolean for toggling cache creation or a cache directory path.
"sourceMap":false	A boolean for including inline source maps.
}

Tips

Bundling

• For bundlers like browserify+esmify, parcel-bundler, and webpack add a "module" field to package.json pointing to the main ES module.

  "main": "index.js",
  "module": "main.js"
  

  :bulb: This is automagically done with npm init esm or yarn create esm.

Extensions

• Enable esm for wallaby.js following their integration example.

Loading

• Load esm before loaders/monitors like @babel/register, newrelic, sqreen, and ts-node.

• Load esm for jasmine using the "helpers" field in jasmine.json:

  "helpers": [
    "node_modules/esm"
  ]
  

• Load esm with “node-args" options of:
  

  • pm2: --node-args="-r esm"

• Load esm with “require” options of ava, mocha, nodemon, nyc, qunit, tape, and webpack.

  :bulb: Builtin require cannot sideload .mjs files. However, .js files can be sideloaded or .mjs files may be loaded with dynamic import.