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Polyfill for the ES6 Module Loader
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ES6 Module Loader

An ES6 Module Loader polyfill based on by Luke Hoban, Addy Osmani and Guy Bedford.

Not yet suitable for production use while the specification is still subject to change.

Supports all modern browsers including IE8+.


Getting Started

Check-out the demo sample to see the project in action.

Use the System (pre-configured Loader):

System.baseURL = '/lib';
System.import('js/test1', function (test1) {
  console.log('test1.js loaded', test1);

where, test1 can contain module syntax:


export function tester() {

Load multiple modules:

System.import(['js/test1', 'js/test2'], function(test1, test2) {
  console.log('test1.js loaded', test1);
  console.log('test2.js loaded', test2);
}, function(err) {
  console.log('loading error');

Load a plain JavaScript file from a URL:

System.load('js/libs/jquery-1.7.1.js', function() {
  var $ =;
  console.log('jQuery loaded', $);

Define a new module Loader instance:

var loader = new Loader({
  global: window,
  strict: false,
  normalize: function (name, referer) {
    return normalized(name,;
  resolve: function (normalized, options) {
    return '/' + normalized + '.js';
  fetch: function (url, fulfill, reject, options) {
  translate: function (source, options) {
    return compile(source);
  link: function (source, options) {
    return {
      imports: ['some', 'dependencies'],
      execute: function(depA, depB) {
        return new Module({
          some: 'export'

The above hooks are all optional, using the default System hooks when not present.

For an overview of working with custom loaders, see Yehuda Katz's essay or the ES6 Module Specification.

Define an ES6 module programatically (useful in optimized / production environments):

var module = new Module({ test: 'hello' });
System.set('my-module', module);

Notes and roadmap

Syntax Parsing

The Esprima ES6 Harmony parser is being used to do parsing, loaded only when necessary.

The following module statements are currently supported:

import 'jquery';                        // import a module
import $ from 'jquery';                 // import the default export of a module
import { $ } from 'jquery';             // import a named export of a module
import { $ as jQuery } from 'jquery';   // import a named export to a different name

export var x = 42;                      // export a named variable
export function foo() {};               // export a named function

export default var x = 42;              // export the default export
export default function foo() {};       // export the default export as a function
export default = function foo() {};     // export the default export by assignment

export { encrypt };                     // export an existing variable
export { decrypt as dec };              // export a variable as a new name
export { encrypt as en } from 'crypto'; // export an export from another module
export * from 'crypto';                 // export all exports from another module

module 'crypto' { ... }                 // define a module

NodeJS Support

For use in NodeJS, the Module, Loader and System globals are provided as exports:

  var System = require('es6-module-loader').System;

  System.import('some-module', callback);

Custom Esprima Location

To set a custom path to the Esprima Harmony parser, specify the data-esprima-src attribute on the <script> tag used to include the module loader.

Specification Notes

The polyfill is implemented exactly to the specification as closely as possible.

The only feature which is not possible to fully polyfill is the intrinsics functionality and sandboxing of the loader. Custom builtins and full global encapsulation is still provided.

The System normalization and resolution functions are not fully described by the specification, so some assumptions have been made which are listed here

To follow the current the specification changes, see

Projects using us

  • JSPM Loader is a RequireJS-style loader using our polyfill to load ES6, AMD, CommonJS and global modules


In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using grunt.

Also, please don't edit files in the "dist" subdirectory as they are generated via grunt. You'll find source code in the "lib" subdirectory!

Release History

  • 0.2.4 NodeJS support, relative normalization fixes, IE8 support


Copyright (c) 2012 Luke Hoban, Addy Osmani, Guy Bedford
Licensed under the MIT license.

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