ES6 Module Loader Polyfill

Dynamically loads ES6 modules in NodeJS and current browsers.

• Implemented exactly to the April 27 2014 ES6 specification draft.
• Provides an asynchronous loader (System.import) to dynamically load ES6 modules.
• Uses Traceur for compiling ES6 modules and syntax into ES5 in the browser with source map support.
• Fully supports ES6 circular references and bindings.
• Polyfills ES6 Promises in the browser with a bundled es6-promise implementation.
• Compatible with NodeJS allowing for server-side module loading and tracing extensions.
• Supports ES6 module loading in IE9+, and dynamic module formats in IE8+.
• The complete combined polyfill comes to 7KB minified and gzipped, making it suitable for production use, provided that modules are built into ES5 making them independent of Traceur.

For an overview of build workflows, see the production guide.

See the demo folder in this repo for a working example demonstrating both module loading the module tag in the browser.

For an example of a universal module loader based on this polyfill for loading AMD, CommonJS and globals, see SystemJS.

The current version is tested against Traceur 0.0.41.

Note the ES6 module specification is still in draft, and subject to change.

Basic Use

Download both es6-module-loader.js and traceur.js into the same folder.

If using ES6 syntax (optional), include traceur.js in the page first then include es6-module-loader.js:

  <script src="traceur.js"></script>
  <script src="es6-module-loader.js"></script>

Write an ES6 module:

mymodule.js:

  export class q {
    constructor() {
      console.log('this is an es6 class!');
    }
  }

We can then load the module with the dynamic loader:

<script>
  System.import('mymodule').then(function(m) {
    new m.q();
  }).catch(function(e) {
    setTimeout(function() {
      throw e;
    })
  })
</script>

The dynamic loader returns a Module object, which contains getters for the named exports (in this case, q).

Because the loader is promise-based we need to add a catch handler in order to detect loading errors.

Read the wiki on overview of ES6 modules and syntax.

Module Tag

A simple analog to the module tag is provided with:

<script type="module">
  // loads the 'q' export from 'mymodule.js' in the same path as the page
  import { q } from 'mymodule';

  new q(); // -> 'this is an es6 class!'
</script>

Ideally this should be based on polyfilling the <module> tag, as <script type="module"> is not in the spec.

As such this approach is not really suitable for anything more than experimentation.

See an overview of the specification module tag features here - https://github.com/dherman/web-modules/blob/master/module-tag/explainer.md.

baseURL

All modules are loaded relative to the baseURL, which by default is set to the current page path.

We can alter this with:

  System.baseURL = '/js/lib';
  System.import('module'); // now loads "/js/lib/module.js"

Paths Implementation

Note: This is a specification under discussion and not confirmed. This implementation will likely change.

The System loader provides paths rules used by the standard locate function.

For example, we might want to load jquery from a CDN location. For this we can provide a paths rule:

  System.paths['jquery'] = '//code.jquery.com/jquery-1.10.2.min.js';
  System.import('jquery').then(function($) {
    // ...
  });

Any reference to jquery in other modules will also use this same version.

It is also possible to define wildcard paths rules. The most specific rule will be used:

  System.paths['lodash/*'] = '/js/lodash/*.js'
  System.import('lodash/map').then(function(map) {
    // ...
  });

Circular References & Bindings

Circular references and live bindings are fully supported identically to ES6 in this polyfill.

That is:

• Bindings are set up before module execution.
• Execution is run from depth-first left to right on the module tree stopping at circular references.
• Bindings are live - an adjustment to an export of one module affects all modules importing it.

even.js

  import { odd } from './odd'

  export var counter = 0;

  export function even(n) {
    counter++;
    return n == 0 || odd(n - 1);
  }

odd.js

  import { even } from './even';

  export function odd(n) {
    return n != 0 && even(n - 1);
  }

  System.import('even').then(function(m) {
    m.even(10);
    m.counter;
    m.even(20);
    m.counter;
  });

Moving to Production

When in production, it is not suitable to load ES6 modules and syntax in the browser.

There is a modules=instantiate build output in Traceur that can be used with the ES6 Module Loader, provided it has the System.register extension from SystemJS.

The benefit of this output is that it provides full support for circular references and live module bindings.

Alternatively, Traceur can also output amd or cjs as well.

A basic example of using this extension with a build would be the following:

Building all files into one bundle

1. Build all ES6 modules into ES5 System.register form:

  traceur --out app-build.js app/app.js --modules=instantiate

2. If using additional ES6 features apart from modules syntax, load traceur-runtime.js (included in the bin folder when installing Traceur through Bower or npm). Then include es6-module-loader.js and then apply the register extension before doing the import or loading the bundle as a script:

  <script src="traceur-runtime.js"></script>
  <script src="es6-module-loader.js"></script>
  <script>
    /*
     * This should be a separate external script
     * Register function is included from https://github.com/systemjs/systemjs/blob/master/lib/extension-register.js
     */
    function register(loader) { 
      // ...
    }

    // this needs to be added to apply the extension
    register(System);
  </script>

  <!-- now include the bundle -->
  <script src="app-build.js"></script>

  <!-- now we can import and get modules from the bundle -->
  <script>
    System.import('app/app');
  </script>

Building into separate files

We can also build separate files with:

  traceur --dir app app-build --modules=instantiate

With the above, we can load from the separate files identical to loading ES6.

NodeJS Usage

  npm install es6-module-loader

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

index.js:

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

  System.import('some-module').then(function(m) {
    console.log(m.p);
  });

some-module.js:

  export var p = 'NodeJS test';

Running the application:

> node index.js
NodeJS test

Extending the Loader

The loader in its default state provides only ES6 loading.

We can extend it to load AMD, CommonJS and global scripts as well as various other custom functionality through the loader hooks.

Read the wiki on extending the loader here.

Specification Notes

See the source of https://github.com/ModuleLoader/es6-module-loader/blob/master/lib/es6-module-loader.js, which contains comments detailing the exact specification notes and design decisions.

To follow the current the specification changes, see the marked issues https://github.com/ModuleLoader/es6-module-loader/issues?labels=specification&page=1&state=open.

Contributing

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!

Credit

Copyright (c) 2014 Luke Hoban, Addy Osmani, Guy Bedford

License

Licensed under the MIT license.