Closure library dependency loader for Webpack

This is a webpack loader which resolves goog.provide() and goog.require() statements in webpack just like if they were regular CommonJS modules.

Installation

npm install --save-dev closure-loader

Usage

Documentation: Using loaders

NOTE: This loader is mainly meant for building (probably older) closure library projects with webpack and to make a transition to other module systems (CommonJS or ES6) easier.

There are two parts to this loader:

• goog.provide()
  • Basically just creates the given namespace in the local scope of that module
  • Any file containing this statement will be added to a map for require lookups
• goog.require()
  • Like goog.provide() it creates the given namespace in the scope of that module
  • It finds the corresponding file with the goog.provide() statement and loads it (see configuration below)
  • It assigns the value of the namespace from the provide file and assign it to the same namespace in the current module

In the simplest way you can just use those two statements like you usually would with the google closure library.

NOTE: Usually the closure lib simply creates all namespaces on the global scope (i.e. the window object). This is not the case if you use this loader. Every file ("module") has its own scope just like it would have if you used CommonJS syntax.

You can use closure library dependencies in conjunction with CommonJS syntax. You can load any module that uses goog.provide() with require(), but not the other way round.

// module.js
goog.provide('my.app.module');

my.app.module = function () {
    console.log('my module was loaded');
}

// index.js
var module = require('./module.js').my.app.module;

module(); // will output 'my module was loaded' to the console

ES6 Modules

If you use babel you can even use ES6 import syntax. If you have enabled the es6mode in the loader config the first goog.provide() of each file will be exported as "default" in addition to its full namespace.

// module.js
goog.provide('my.app.module');

my.app.module = function () {
    console.log('my module was loaded');
}

// index.js
import module from './module.js';
// is the same as
var module = require('./module.js').default;
// or
var module = require('./module.js').my.app.module;

module(); // will output 'my module was loaded' to the console

Configuration

Here is an example webpack config for this loader:

module.exports = {
    entry: {
        app: './src/index.js'
    },
    output: {
        path: './build',
        filename: '[name].js'
    },
    module: {
        loaders: [
            {
                test: /\/src\/.*\.js$/,
                loaders: [
                    'closure-loader'
                ],
                exclude: [/node_modules/, /test/]
            }
        ]
    },
    closureLoader: {
        paths: [
            __dirname + '/src'
        ],
        es6mode: true,
        watch: true
    }
};

Here are the configuration options specific for this loader:

• paths (array): An array of path strings. The loader will search all *.js files within theses paths for goog.provide() statements when resolving a goog.require(). You can only goog.require() dependencies that can be found under one of these paths.
• es6mode (boolean, default: false): If enabled it will add the value of the first goog.provide() as default export for usage with babel. For this reason it will also export the corresponding flag module.exports.__esModule = true
• watch (boolean, default: true): If true, the loader will intitialise watchers which check for changes in the mapped files. This is neccesary to be able to delete the internal map cache. But it also makes problems with CI sytstems and build scripts, because the watcher will prevent the process from beeing exited.

Examples

In the hopes of clarifying the usage of the loader a bit I have provided a couple of examples which you can find in the examples directory.

To run an example please follow these steps:

• npm install in the closure-loader root directory
• npm install in the directory of the example
• npm start or npm run build in the directory of the example

The following examples are available:

• common-js: This example shows how to load some legacy code that contains goog.provide() and goog.require() via commonJs require() calls.
• common-js-closure-lib: This example shows how to load the closure library via commonJs require() calls.
• es6: This example shows how to load some legacy code that contains goog.provide() and goog.require() via babel and es6 import calls.
• es6-closure-lib: This example shows how to load the closure library via babel and es6 import calls.
• legacy-closure-lib: This example shows how to load the closure library via your own goog.require() calls. This is not advised. If you are using webpack you should think about using a proper module loader, preferably es6 as this is now the standard.

NOTE: This loader does in no way include or wrap the actual google closure library. If you want to use the closure library you will have to include it yourself and ensure correct shimming. See the above examples on how this can be done.

License

MIT (http://www.opensource.org/licenses/mit-license.php)