es6-module-loader

ES6 Module Loader Polyfill

Fully up to date with the latest specification

The new ES6 module specification defines a module system in JavaScript using import and export syntax, along with a module loader factory (new Loader).

A separate browser specification defines the window.System loader, a dynamic browser loader for JavaScript modules, as well as a new <module> tag (<script type="module"> for compatibility with existing browsers).

This polyfill implements the Loader and Module globals, exactly as specified in the 2013-12-02 ES6 Module Specification Draft and the System browser loader exactly as suggested in the sample implementation.

The complete combined polyfill comes to 16KB minified, making it suitable for production use in future, provided that modules are built into ES5 making them independent of Traceur. Build workflows are currently in progress.

• Provides an asynchronous loader (System.import) to dynamically load ES6 modules in all modern browsers including IE8+
• Adds support for the <script type="module"> tag allowing inline module loading.
• Uses Traceur for compiling ES6 modules and syntax into ES5 in the browser with source map support
• Use as a base for creating a custom spec-compliant module loader
• Fully compatible with NodeJS allowing for spec-compliant server-side module loading

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

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

Getting Started

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

Then include the es6-module-loader.js file on its own in the page:

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

Traceur will be downloaded only when needed for ES6 syntax parsing, detected as the existence of module syntax, or as specified by the metadata.es6 property.

Write an ES6 module:

mymodule.js:

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

We can then load this module with a module tag in the page:

<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>

Or we can also use the dynamic loader:

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

The dynamic loader returns an instance of the Module class, which contains getters for the named exports (in this case, q).

Terminology

Modules and Module Loaders

A module is simply a JavaScript file written with module syntax. Modules export values, which can then be imported by other modules.

CommonJS and AMD JavaScript files are modules.

A module loader provides the ability to dynamically load modules, and also keeps track of all loaded modules in a module registry.

Typically, in production, the module registry would be populated by an initial compiled bundle of modules. Later in the page state, it may become necessary to dynamically load a new module. This module can then share dependencies with the initial page bundle without having to reload any dependencies.

The ES6 Module Specification defines the module syntax for ES6 module files, and also defines a module loader factory class for creating ES6-compatible module loaders.

Module code is treated differently to scripts due to the nature of exports and imports. This is why the <script type="module"> tag (which will become the <module> tag in modern browsers) is introduced to distinguish script code from module code. Scripts cannot export or import, but are able to use the dynamic loader System.import(...).

Module Names and baseURL

Module names are just like moduleIDs in RequireJS. Non-relative module names (not starting with .) are converted to a URL with the following rule:

  URL = absolutePath(baseURL, ModuleName + '.js')

Relative module names can be written './local-module' to load relative to their parent module name. .. syntax is also supported allowing easily portable modules.

The baseURL is set to the current page path by default. It is a property of the System loader and can be changed:

  System.baseURL = '/lib/';
  System.baseURL = 'http://mysite.com/js/';

ES6 Module Syntax

Exporting

ES6 module syntax is most similar to the exports.method = function() {} pattern in NodeJS of creating multiple named exports.

In CommonJS one might write:

  exports.someMethod = function() {

  }

  exports.another = {};

In ES6, this same code would be written:

exporter.js:

  export function someMethod() {

  }

  export var another = {};

Notice that the name of the function, class or variable gets used as the export name.

Importing

When importing, we import any exports we need by name, and can also choose to rename them:

importer.js:

  import { someMethod, another as newName } from './exporter';

  someMethod();
  typeof newName == 'object';

Default Import and Export

Sometimes one doesn't want to write an import name at all. For this we can use the default export:

export-default.js:

  export default function foo() {
    console.log('foo');
  }

import-default.js:

  import customName from './export-default';

  customName(); // -> 'foo'

All Supported Syntax

There are a few other variations of module syntax, the full list of supported statements is listed below.

import 'jquery';                        // import a module without any import bindings
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 q = {};                          // export shorthand

export default 42;                      // export the default export
export default function foo() {};       // export the default export as a function

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 from 'crypto';            // import an entire module instance object

Note that any valid declaration can be exported. In ES6, this includes class (as in the example above), const, and let.

Dynamic Module Loading

The dynamic module loader uses promises for resolution. Modules can have both a resolve and reject handler:

  System.import('some-module').then(function(m) {
    // got Module instance m
  }, function(err) {
    // error
  });

Moving to Production

When in production, one wouldn't want to load ES6 modules and syntax in the browser. Rather the modules would be built into ES5 and AMD to be loaded.

Also, suitable bundling would need to be used.

We are actively working on these workflows.

Module Tag

The module tag supports both named and anonymous use.

Anonymous Module

This is just like an anonymous <script> tag, allowing code to be run directly:

  <script type="module">
    import 'some-import';

    class q {

    }

    new q();
  </script>

Named Module

A named module is just like an anonymous module, but defines the module in the registry as well:

  <script type="module" name="my-module">
    export var p = 'named-module';
  </script>
  <script>
    // later on -
    setTimeout(function() {
      System.import('my-module').then(function(m) {
        console.log(m.p); // -> named-module
      });
    }, 100);
  </script>

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);

Traceur support requires npm install traceur, allowing ES6 syntax in NodeJS:

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

  System.import('es6-file', function(module) {
    module.classMethod();
  });

Custom Traceur Location

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

Creating a Custom Loader

The ES6 specification defines a loader through five hooks:

• Normalize: Given the import name, provide the canonical module name.
• Locate: Given a canonical module name, provide the URL for the resource.
• Fetch: Given a URL for a resource, fetch its content.
• Translate: Given module source, make any source modifications.
• Instantiate: Given module source, determine its dependencies, and execute it.

Variations of these hooks can allow creating many different styles of loader.

Each hook can either return a result directly, or a promise (thenable) for the result.

To create a new loader, use the Loader constructor:

var MyLoader = new Loader({
  normalize: function (name, parentName, parentAddress) {
    return resolvedName;
  },
  locate: function (load) {
    // load.name is normalized name
    return this.baseURL + '/' + load.name + '.js';
  },
  fetch: function (load) {
    // return a promise. Alternatively, just use the system fetch
    // promise -return System.fetch(load)
    var defer = MyPromiseLibrary.createDeferred();
    myXhr.get(load.address, defer.resolve, defer.reject);
    return defer.promise;
  },
  translate: function (load) {
    return load.source;
  },
  instantiate: function (load) {
    // use standard es6 linking
    return System.instantiate(load);

    // provide custom linking
    // useful for providing AMD and CJS support
    return {
      deps: ['some', 'dependencies'],
      execute: function(depA, depB) {
        return new Module({
          some: 'export'
        });
      }
    };
  }
});

For a more in-depth overview of creating with custom loaders, some resources are provided below:

• The System Loader implementation
• ES6 Module Specification, latest draft
• Yehuda Katz's essay (outdated)

Specification Notes

Notes on the exact specification implementation differences are included below.

Loader Polyfill

• Implemented exactly to the 2013-12-02 Specification Draft - https://github.com/jorendorff/js-loaders/blob/e60d3651/specs/es6-modules-2013-12-02.pdf with the only exceptions as described here

• Abstract functions have been combined where possible, and their associated functions commented

• Declarative Module Support is entirely disabled, and an error will be thrown if the instantiate loader hook returns undefined

• With this assumption, instead of Link, LinkDynamicModules is run directly

• ES6 support is thus provided through the translate function of the System loader

• EnsureEvaluated is removed, but may in future implement dynamic execution pending issue - https://github.com/jorendorff/js-loaders/issues/63

• Realm implementation is entirely omitted. As such, Loader.global and Loader.realm accessors will throw errors, as well as Loader.eval

• Loader module table iteration currently not yet implemented

System Loader Implementation

• Implemented to https://github.com/jorendorff/js-loaders/blob/master/browser-loader.js, except for Instantiate function

• Instantiate function determines if ES6 module syntax is being used, if so parses with Traceur and returns a dynamic InstantiateResult for loading ES6 module syntax in ES5.

• Custom loaders thus can be implemented by using this System.instantiate function as the fallback loading scenario, after other module format detections.

• Traceur is loaded dynamically when module syntax is detected by a regex (with over- classification), either from require('traceur') on the server, or the 'data-traceur-src' property on the current script in the browser, or if not set, 'traceur.js' in the same URL path as the current script in the browser.

• The <script type="module"> tag is supported, but the <module> tag is not

• ondemand / paths functionality currently not yet implemented

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.

Projects using us

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

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!

Release History

• 0.4.0 Update to revised specification exact algorithm
• 0.3.3 Traceur parser update, detection regex fixes, better error messages
• 0.3.2 Use Traceur for all parsing, module tag support, syntax updates, test workflow
• 0.3.1 IE9 Cross Domain fix, module x from y syntax support, data-init callback support, Traceur fixes
• 0.3.0 Traceur support, better error reporting, source maps support, normalization simplifications
• 0.2.4 NodeJS support, relative normalization fixes, IE8 support

License

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