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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.


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(Loader, {
  global: window,
  strict: false,
  resolve: function (normalized, options) {
    return '/' + normalized + '.js';
  fetch: function (url, fulfill, reject, options) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
        } else {
    };"GET", url, true);
  translate: function (source, options) {
    return source;

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

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

Notes and roadmap

Specification Notes

The polyfill is implemented exactly to the specification now, except for the following items:

  • The extra metadata property is not yet handled in the resolve.
  • The fetch function is given a different specification between the prototype (Loader.prototype.fetch) and loader instance (options.fetch). Since instance functions are provided on the instance object as in the @wycats essay (System.normalize, System.fetch etc), there seems to be a conflict between these.
  • The intrinsics encapsulation is a tricky one to polyfill, but we have done our best based on a global prototype chain behaviour, where global.__proto__ == intrinsics. And intrinsics.__proto__ == window. All code is evaluated with the window and this properties referencing the global allowing full global encapsulation.

Syntax Parsing

The Esprima ES6 Harmony parser is being used to do parsing, loaded only when necessary. This parser still uses an older syntax, which is currently the major critical issue to sort out for this polyfill.

The issue tracking this is here -

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

(Nothing yet)


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

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