As developer you want to reuse existing code. As with node.js and web all files are already in the same language, but it is extra work to use your code with the node.js module system and the browser.

The goal of webpack is to bundle CommonJs modules into javascript files which can be loaded by <script>-tags. Simply concatenating all required files has a disadvantage: many code to download (and execute) on page load. Therefore webpack uses the require.ensure function (CommonJs/Modules/Async/A) to split your code automatically into multiple bundles which are loaded on demand. This happens mostly transparent to the developer. Dependencies are resolved for you. The result is a smaller initial code download which results in faster page load.

Another common thing in web development is that some files have to be preprocessed before send to the client (ex. template files). This introduce more complexity to the compile step. webpack supports loaders which process files before including them. You as developer can use such files like any other module.

TL;DR

• bundle CommonJs modules for browser
• reuse server-side code (node.js) on client-side
• create multiple files which are loaded on demand (faster page load in big webapps or on mobile connections)
• dependencies managed for you, on compile time (no resolution on runtime needed)
• loaders can preprocess files

• make node.js and browser development similar
• minimize code size (mobile connection)
• minimize code size on initial download
• download code only on demand
• require minimal configuration, but offer a maximum
• load polyfills for node-specific things if used
• offer replacements for node buildin libraries

See example webapp.

// a.js
var b = require("./b");
b.stuff("It works");

// b.js
exports.stuff = function(text) {
	console.log(text);
}

are compiled to (reformatted)

(/* small webpack header */)
({
0: function(module, exports, require) {

	var b = require(1);
	b.stuff("It works");

},
1: function(module, exports, require) {

	exports.stuff = function(text) {
		console.log(text)
	}

}
})

var a = require("a");
var b = require("b");
require.ensure(["c"], function(require) {
	require("b").xyz();
	var d = require("d");
});

File 1: web.js
- code of that file
- code of module a and dependencies
- code of module b and dependencies

File 2: 1.web.js
- code of module c and dependencies (but code is not used)
- code of module d and dependencies

Initially only web.js is included (and loaded) by your application. 1.web.js is loaded when the call to require.ensure happens. After the second bundle (1.web.js) is loaded, the callback function will be invoked.

See details for exact output.

See more examples.

webpack was built to support most of the code that was coded for node.js environment. For example this works out of the box:

• require("./templates/" + templateName);
• require(condition ? "moduleA" : condition2 ? "moduleB" : "./localStuff");
• function xyz(require) { require("text"); } xyz(function(a) { console.log(a) });
• var r = require; r("./file"); with warning
• function xyz(require) { require("./file"); } xyz(require); with warning
• try { require("missingModule"); } catch(e) { console.log("missing") } with warning
• var require = function(a) { console.log(a) }; require("text");
• if(condition) require("optionalModule") with warning if missing

Somethings it happens that browsers require other code than node.js do. webpack allow module developers to specify replacements which are used in the compile process of webpack.

Modules in web_modules replace modules in node_modules. filename.web.js replaces filename.js when required without file extension.

in options: alias: { "http": "http-browserify" }

in shell: --alias http=http-browserify

If the required module is not known while compile time we get into a problem. A solution is the method require.context which takes a directory as parameter and returns a function which behaves like the require function issued from a file in this directory (but only if used for files in that directory). The whole directory is included while compiling, so you have access to all files in it.

We have a directory full of templates, which are compiled javascript files. A template should be loaded by template name.

var requireTemplate = require.context("./templates");
function getTemplate(templateName) {
	return requireTemplate("./" + templateName);
}

In addition to that webpack uses the require.context function automatically if you use variables or other not parse-able things in the require function. That means the following code behaves like the above:

function getTemplate(templateName) {
	return require("./templates/" + templateName);
}
// is compiled like: return require.context("./templates")("./"+templateName)
// which compiles to: return require(123)("./"+templateName)

See details for complete example.

When try to store the require function in another variable or try to pass it as parameter, webpack convert it to a require.context(".") to be compatible. There is a warning emitted in this case.

Warning: The complete code in the directory are included. So use it carefully.

You can use a syntax for loader plugins to preprocess files before emitting javascript code to the bundle.

The following example loads the raw content of a file with the raw loader:

var content = require("raw!./file.txt");

Multiple loader plugins can be prepended by separating them with !. The loader plugins are resolved like in normal require call but with different default extension.

The raw loader plugin is looked up at modules raw-webpack-web-loader, raw-webpack-loader, raw-web-loader, raw-loader, raw and the following files are looked up: index.webpack-web-loader.js, index.webpack-loader.js, index.web-loader.js, index.loader.js, index, index.js. Note that the web- versions are omitted if loaders are used in node.js.

See example.

The following loaders are included in webpack:

• raw: Loads raw content of a file (as utf-8)
• json (default at .json): Loads file as JSON
• jade (default at .jade): Loads jade template and returns a function
• coffee (default at .coffee): Loads coffee-script like javascript
• css: Loads css file with resolved imports and returns css code
• less: Loads and compiles a less file and returns css code
• val: Excutes code as module and consider exports as javascript code
• bundle: Wraps request in a require.ensure block
• file: Emits the file into the output folder and returns the (relative) url (file/{ext} for some extensions)
• style: Adds result of javascript execution to DOM
• script: Executes a javascript file once in global context (like in script tag), requires are not parsed. Use this to include a library. ex. require("script!./jquery.min.js"). This is synchron, so the $ variable is available after require.
• (.css defaults to style!css loader, so all css rules are added to DOM)
• (.less defaults to style!css!val!less loader, so all less rules are added to DOM)

See docs for loader in github repo of the loader.

var a = require("a"); // require modules
var b = require("./b"); // and files
                          // like in node.js

// polyfill require method to use the new members in node.js too
require = require("webpack/require-polyfill")(require.valueOf());

// create a lazy loaded bundle
require.ensure([], function(require) {
	var c = require("c");

	// require json
	var packageJson = require("../package.json");

	// or jade templates, coffee-script, and many more with own loaders
	var result = require("./template.jade")(require("./dataFrom.coffee"));

	// files are compiled to javascript and packed into the bundle...
});

... and compile from the shell with:

webpack lib/input.js js/output.js

try --min to minimize with uglify-js.

• require should not be overwritten, except from polyfill
• require.ensure should not be overwritten or called indirect
• require.context should not be overwritten or called indirect
• the argument to require.context should be a literal or addition of multiple literals
• An indirect call of require should access a file in current directory: This throws an exception: var r = require; r("../file");

The following cases could result in too much code in result file if used wrong:

• indirect call of require: var r = require; r("./file");. It includes the whole directory.
• require.context. It includes the whole directory.
• expressions in require arguments: require(variable). It includes the whole directory. (except from ?:-operator require(condition ? "a" : "b"))
• the function passed to require.ensure is not inlined in the call. Only requires in inlined function move into the second bundle.

As node.js specific modules like fs will not work in browser they are not included (by default) and cause an exception. You should replace them by own modules if you want to use them. For some simple modules are replacements included in webpack. Expensive replacements are not needed by everyone, so that are not included by default. You need to specify --alias [module]=[replacement] to use them. A warning saying that some module is missing is emitted in the case you use it without providing a replacement.

Some credit goes to the browserify contributors, you can use replacements provided by them.

Included simple replacements:

• assert: copy of node.js' version, small change
• buffer: copy of node-browserify's version
• buffer_ieee754: copy of node-browserify's version
• child_process: disabled
• events: copy of node.js' version
• path: copy of node.js' version
• punycode: copy of node.js' version, one line removed (http://mths.be/punycode by @mathias)
• querystring: copy of node.js' version
• string_decoder: copy of node.js' version
• url: copy of node.js' version
• util: copy of node.js' version

Here is a list of possible useful replacements: (intentionally not by default)

• http=http-browserify
• vm=vm-browserify
• TODO provide some more replacements

webpack offers a command line interface:

after npm install webpack -g you can use the webpack command

if invoked without arguments it prints a usage:

Usage: webpack <options> <input> <output>

Options:
  --single             Disable Code Splitting                                   [boolean]  [default: false]
  --min                Minimize it with uglifyjs                                [boolean]  [default: false]
  --filenames          Output Filenames Into File                               [boolean]  [default: false]
  --options            Options JSON File                                        [string]
  --script-src-prefix  Path Prefix For JavaScript Loading                       [string]
  --libary             Stores the exports into this variable                    [string]
  --colors             Output Stats with colors                                 [boolean]  [default: false]
  --json               Output Stats as JSON                                     [boolean]  [default: false]
  --by-size            Sort modules by size in Stats                            [boolean]  [default: false]
  --verbose            Output dependencies in Stats                             [boolean]  [default: false]
  --alias              Set a alias name for a module. ex. http=http-browserify  [string]
  --debug              Prints debug info to output files                        [boolean]  [default: false]
  --watch              Recompiles on changes (except loaders)                   [boolean]  [default: false]
  --progress           Displays a progress while compiling                      [boolean]  [default: false]

webpack(context, moduleName, [options], callback)
webpack(absoluteModulePath, [options], callback)

You can also save this options object in a JSON file and use it with the shell command.

outputJsonpFunction

JSONP function used to load chunks

scriptSrcPrefix

Path from where chunks are loaded

outputDirectory

write files to this directory (absolute path)

output

write first chunk to this file

outputPostfix

write chunks to files named chunkId plus outputPostfix

libary

exports of input file are stored in this variable

minimize

minimize outputs with uglify-js

debug

prints debug info to output files.

watch

recompiles on changes (except loaders)

includeFilenames

add absolute filenames of input files as comments

resolve.alias (object)

replace a module. ex. {"old-module": "new-module"}

resolve.paths (array)

search paths

resolve.extensions (object)

possible extensions for files

default: ["", ".webpack.js", ".web.js", ".js"]

resolve.loaders (array)

extension to loader mappings. ex. [{test: /\.extension$/, loader: "myloader"}]

loads files that matches the RegExp to the loader if no other loader set

resolve.loaderExtensions (array)

possible extensions for loaders

default: [".webpack-web-loader.js", ".webpack-loader.js", ".web-loader.js", ".loader.js", "", ".js"]

resolve.loaderPostfixes (array)

possible postfixes for loaders

default: ["-webpack-web-loader", "-webpack-loader", "-web-loader", "-loader", ""]

parse.overwrites (object)

free module variables which are replaced with a module. ex. { "$": "jquery" }

function(err, source / stats) source if options.output is not set else stats as json see example

You can use [hash] in scriptSrcPrefix, output, outputDirectory, outputPostfix and in the shell parameters. webpack will replace it with a hash of your files, when writing.

Combine the options --colors --watch --progress to get a pretty shell compilation.

You can run the unit tests with npm test.

You can run the browser tests:

cd test/browsertests
node build

and open test.html in browser. There must be several OKs in the file, no FAIL and no RED boxes.

You are welcome to contribute by writing issues or pull requests. It would be nice if you open source your own loaders or webmodules. :)

You are also welcome to correct any spelling mistakes or any language issues, because my english is not perfect...

• more polyfills for node.js buildin modules, but optional
• require("webpack/require-polyfill.install") to install for all modules
• require from protocol require("http://...")
• write it into the wiki if you have more ideas...

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