esm2cjs

Easily compile ES modules to CommonJS modules with functionally equivalent interfaces.

Given a file like foo.mjs:

export default function foo () {}

Generates a file like foo.js:

'use strict';
module.exports = function foo () {};

If that is your main module, in your package.json, you can do:

{
  "main": "./foo"
}

Then, if Node supports modules and someone requires or imports your package, then foo.mjs will be imported, and if Node doesn't support modules, then foo.js will be imported instead.

What you're doing is shipping a native ES module (W00T the future is NOW) yet still offering a fallback for not-yet-upgraded environments.

Usage

Install it: npm i -D esm2cjs

Run it: npx esm2cjs

Continuously rebuild: npx esm2cjs --watch (shorthand: npx esm2cjs -w)

Call it in your package's prepare task so it always runs right before you publish a release, ensuring that your ".js" files are up-to-date with your ".mjs" files:

{
  "scripts": {
    "prepare": "esm2cjs"
  }
}

You can specify which files will be compiled by adding a .esm2cjs file to your package root. This file contains a newline-delimited list of file globs, e.g. a .esm2cjs file like:

*.mjs
bin/cli.mjs

Would compile this directory:

directory/
├── index.mjs
└── bin/
    └── cli.mjs

Into this:

directory/
├── index.js
├── index.mjs
└── bin/
    ├── cli.js
    └── cli.mjs

Extending

esm2cjs uses Babel 6 under the hood. This means that if you want to use syntax extensions (e.g. TC39 proposals, JSX, etc) in your code, you can do that by adding a .babelrc file to your project.