Notes of ES Modules and CommonJS Modules

Node

Node version: v8.5.0

Node Modules
Node ECMAScript Modules
Node EP for ES Modules
The --experimental-modules flag can be used to enable features for loading ES modules
ES modules imports will be loaded asynchronously
ES modules files will use .mjs extension
The .mjs file extension will not be loadable via require()

ES consuming CommonJS

cjs.js:

1 2	module.exports = { v: 1 } module.exports.f = () => 2

esm.mjs:

import assert from 'assert'
import cjs from './cjs'

assert.equal(cjs.v, 1)
assert.equal(cjs.f(), 2)

Unsupported Features

import()
import.meta
Loader Hooks

Babel Transform Plugin

Babel version: v6.26.0 Babel plugin transform-es2015-modules-commonjs

Export

In:

1 2	export default { v: 1 } export const f = () => 2

Out:

1
2
3

Object.defineProperty(exports, '__esModule', { value: true })
exports.default = { v: 1 }
const f = exports.f = () => 2

Import

In:

1
2
3

import x from 'x'

x.v

Out:

const __x = require('x')

const _x = _interopRequireDefault(__x)

const x = _x.default

function _interopRequireDefault(o) {
  return o && o.__esModule ? o : { default: o }
}

x.v

In:

1
2
3

import { y } from 'y'

y.v

Out:

const _y = require('y')

const y = _y.y

y.v

In:

1
2
3

import * as z from 'z'

z.v

Out:

const _z = require('z')

const z = _interopRequireWildcard(_z)

function _interopRequireWildcard(o) {
  if (o && o.__esModule) {
    return o
  } else {
    const m = {}
    if (o != null) {
      for (let key in o) {
        if (Object.prototype.hasOwnProperty.call(o, key))
          m[key] = o[key]
      }
    }
    m.default = o
    return m
  }
}

z.v