ES6 Module Loader Polyfill

For upgrading to ES6 Module Loader 0.17, read the release notes here.

Dynamically loads ES6 modules in browsers and NodeJS with support for loading existing and custom module formats through loader hooks.

This project implements dynamic module loading through System exactly to the previous ES6-specified loader API at 2014-08-24 ES6 Specification Draft Rev 27, Section 15. The specification for the module loader was removed from the ES6/ES2015 specification in 2014, and a new loader implementing the new draft WhatWG loader spec is pending alpha release on the 1.0 branch.

• Provides an asynchronous loader (System.import) to dynamically load ES6 modules.
• Supports Traceur, Babel and TypeScript for compiling ES6 modules and syntax into ES5 in the browser with source map support.
• Fully supports ES6 circular references and live bindings.
• Includes paths implementation.
• Can be used as a tracing tool for static analysis of modules.
• Supports IE8+, with IE9+ support for ES6 development without pre-compilation.
• The minified production loader is under 5KB minified and gzipped, making it suitable for production use, provided that modules are built into ES5 making them independent of Traceur.
• Supports declaring modules with <script type="module">, the precursor of the proposed <module> tag.

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

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

Documentation

• Configuring the loader
• Production workflows
• Circular References & Bindings
• Extending the loader through loader hooks
• Tracing API

Getting Started

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

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

To use Babel, load Babel's browser.js instead and set the transpiler to babel with the loader configuration:

<script>
  System.transpiler = 'babel';
</script>

To use TypeScript, set the transpiler to typescript in the loader configuration:

<script>
  System.transpiler = 'typescript';
</script>

Then we can write any ES6 module:

mymodule.js:

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

and load the module dynamically in the browser

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

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

See the demo folder in this repo for a working example demonstrating module loading in the browser both with System.import and with the module-type script tag.

Although System.import() does not support the import of multiple modules defined in an array, because System.import() returns a Promise, this can be achieved by creating an array of System.imports and using Promise.all().

Setting transpilation options

If using Traceur, these can be set with:

System.traceurOptions = {...};

With Babel:

System.babelOptions = {...};

With TypeScript:

System.typescriptOptions = {...};

Module Tag

The module tag is specified to provide a new entry point for using module syntax in browsers. This is because normal <script> tags must remain backwards-compatible so won't support this new mechanism.

This polyfill provides simple support for the <script type="module"> tag via:

<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>

Because it is only possible to load ES6 modules with this tag, it is not suitable for production use in this way.

NodeJS Use

  npm install es6-module-loader babel traceur typescript

It is important that Babel, Traceur or TypeScript is installed into the path in order to be found, since these are no longer project dependencies.

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

index.js:

  var System = require('es6-module-loader').System;
  /*  
   *  Include:
   *    System.transpiler = 'babel'; 
   *  to use Babel instead of Traceur or
   *    System.transpiler = 'typescript';
   *  to use TypeScript
   */

  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

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!

Testing

• npm run test:node will use node to to run the tests
• npm run test:browser will run npm run test:browser-babel, npm run test:browser-traceur and npm run test:browser-typescript
• npm run test:browser-[transpiler] use karma to run the tests with Traceur, Babel or TypeScript.
• npm run test:browser:perf will use karma to run benchmarks

npm run test:browser-[transpiler] supports options after a double dash (--) :

• You can use the --polyfill option to test the code with polyfill.

• You can use the --coverage option to test and extract coverage info.

• You can use the --ie8 option to test the code in the ie8 scope only.

• You can use the --saucelabs option to use karma and saucelabs to run the tests in various browsers. Note: you will need to export your username and key to launch it.

  export SAUCE_USERNAME={your user name} && export SAUCE_ACCESS_KEY={the access key that you see once logged in}
  npm run test:browsers -- --saucelabs

Credit

Copyright (c) 2015 Luke Hoban, Addy Osmani, Guy Bedford

License

Licensed under the MIT license.