This example illustrates a very simple case of Code Splitting with require.ensure.

• a and b are required normally via CommonJS
• c is depended through the require.ensure array.
  • This means: make it available, but don't execute it
  • webpack will load it on demand
• b and d are required via CommonJs in the require.ensure callback
  • webpack detects that these are in the on-demand-callback and
  • will load them on demand
  • webpacks optimizer can optimize b away
    • as it is already available through the parent chunks

You can see that webpack outputs two files/chunks:

• output.js is the entry chunk and contains
  • the module system
  • chunk loading logic
  • the entry point example.js
  • module a
  • module b
• 1.js is an additional chunk (on demand loaded) and contains
  • module c
  • module d

You can see that chunks are loaded via JSONP. The additional chunks are pretty small and minimize well.

example.js

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

js/output.js

`/******/ (function(modules) { /* webpackBootstrap */ })`

``` javascript /******/ (function(modules) { // webpackBootstrap /******/ // install a JSONP callback for chunk loading /******/ var parentJsonpFunction = window["webpackJsonp"]; /******/ window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules, executeModules) { /******/ // add "moreModules" to the modules object, /******/ // then flag all "chunkIds" as loaded and fire callback /******/ var moduleId, chunkId, i = 0, resolves = [], result; /******/ for(;i < chunkIds.length; i++) { /******/ chunkId = chunkIds[i]; /******/ if(installedChunks[chunkId]) /******/ resolves.push(installedChunks[chunkId][0]); /******/ installedChunks[chunkId] = 0; /******/ } /******/ for(moduleId in moreModules) { /******/ if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) { /******/ modules[moduleId] = moreModules[moduleId]; /******/ } /******/ } /******/ if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules, executeModules); /******/ while(resolves.length) /******/ resolves.shift()();

/******/ };

// // The module cache // var installedModules = {};

// // objects to store loaded and loading chunks // var installedChunks = { // 1: 0 // };

// // The require function // function webpack_require(moduleId) {

// // Check if module is in cache // if(installedModules[moduleId]) /******/ return installedModules[moduleId].exports;

// // Create a new module (and put it into the cache) // var module = installedModules[moduleId] = { // i: moduleId, // l: false, // exports: {} // };

// // Execute the module function // modules[moduleId].call(module.exports, module, module.exports, webpack_require);

// // Flag the module as loaded // module.l = true;

// // Return the exports of the module // return module.exports; /******/ }

// // This file contains only the entry chunk. // // The chunk loading function for additional chunks // webpack_require.e = function requireEnsure(chunkId) { // if(installedChunks[chunkId] === 0) /******/ return Promise.resolve();

// // an Promise means "currently loading". // if(installedChunks[chunkId]) { // return installedChunks[chunkId][2]; // } // // start chunk loading // var head = document.getElementsByTagName('head')[0]; // var script = document.createElement('script'); // script.type = 'text/javascript'; // script.charset = 'utf-8'; // script.async = true; /******/ script.timeout = 120000;

// if (webpack_require.nc) { // script.setAttribute("nonce", webpack_require.nc); // } // script.src = webpack_require.p + "" + chunkId + ".output.js"; // var timeout = setTimeout(onScriptComplete, 120000); // script.onerror = script.onload = onScriptComplete; // function onScriptComplete() { // // avoid mem leaks in IE. // script.onerror = script.onload = null; // clearTimeout(timeout); // var chunk = installedChunks[chunkId]; // if(chunk !== 0) { // if(chunk) chunk[1](new Error('Loading chunk ' + chunkId + ' failed.')); // installedChunks[chunkId] = undefined; // } // };

// var promise = new Promise(function(resolve, reject) { // installedChunks[chunkId] = [resolve, reject]; // }); // installedChunks[chunkId][2] = promise;

// head.appendChild(script); // return promise; /******/ };

// // expose the modules object (webpack_modules) // webpack_require.m = modules;

// // expose the module cache // webpack_require.c = installedModules;

// // identity function for calling harmony imports with the correct context // webpack_require.i = function(value) { return value; };

// // define getter function for harmony exports // webpack_require.d = function(exports, name, getter) { // if(!webpack_require.o(exports, name)) { // Object.defineProperty(exports, name, { // configurable: false, // enumerable: true, // get: getter // }); // } // };

// // getDefaultExport function for compatibility with non-harmony modules // webpack_require.n = function(module) { // var getter = module && module.__esModule ? // function getDefault() { return module['default']; } : // function getModuleExports() { return module; }; // webpack_require.d(getter, 'a', getter); // return getter; // };

// // Object.prototype.hasOwnProperty.call // webpack_require.o = function(object, property) { return Object.prototype.hasOwnProperty.call(object, property); };

// // webpack_public_path // webpack_require.p = "js/";

// // on error function for async loading // webpack_require.oe = function(err) { console.error(err); throw err; };

// // Load entry module and return exports // return webpack_require(webpack_require.s = 4); // }) /******************************************************************/

</details>
``` javascript
/******/ ([
/* 0 */
/* unknown exports provided */
/* all exports used */
/*!****************!*\
  !*** ./~/b.js ***!
  \****************/
/***/ function(module, exports) {

// module b

/***/ },
/* 1 */
/* unknown exports provided */
/* all exports used */
/*!****************!*\
  !*** ./~/a.js ***!
  \****************/
/***/ function(module, exports) {

// module a

/***/ },
/* 2 */,
/* 3 */,
/* 4 */
/* unknown exports provided */
/* all exports used */
/*!********************!*\
  !*** ./example.js ***!
  \********************/
/***/ function(module, exports, __webpack_require__) {

var a = __webpack_require__(/*! a */ 1);
var b = __webpack_require__(/*! b */ 0);
__webpack_require__.e/* require.ensure */(0).then((function(require) {
    __webpack_require__(/*! b */ 0).xyz();
    var d = __webpack_require__(/*! d */ 3);
}).bind(null, __webpack_require__)).catch(__webpack_require__.oe);

/***/ }
/******/ ]);

js/0.output.js

webpackJsonp([0],[
/* 0 */,
/* 1 */,
/* 2 */
/* unknown exports provided */
/* all exports used */
/*!****************!*\
  !*** ./~/c.js ***!
  \****************/
/***/ function(module, exports) {

// module c

/***/ },
/* 3 */
/* unknown exports provided */
/* all exports used */
/*!****************!*\
  !*** ./~/d.js ***!
  \****************/
/***/ function(module, exports) {

// module d

/***/ }
]);

Minimized

webpackJsonp([0],[,,function(n,c){},function(n,c){}]);

Info

Uncompressed

Hash: 199e267fd7f9f732d06f
Version: webpack 2.2.0-rc.2
      Asset       Size  Chunks             Chunk Names
0.output.js  407 bytes       0  [emitted]  
  output.js    6.36 kB       1  [emitted]  main
Entrypoint main = output.js
chunk    {0} 0.output.js 22 bytes {1} [rendered]
    > [4] ./example.js 3:0-6:2
    [2] ./~/c.js 11 bytes {0} [built]
        require.ensure item c [4] ./example.js 3:0-6:2
    [3] ./~/d.js 11 bytes {0} [built]
        cjs require d [4] ./example.js 5:12-24
chunk    {1} output.js (main) 166 bytes [entry] [rendered]
    > main [4] ./example.js 
    [0] ./~/b.js 11 bytes {1} [built]
        cjs require b [4] ./example.js 2:8-20
        cjs require b [4] ./example.js 4:4-16
    [1] ./~/a.js 11 bytes {1} [built]
        cjs require a [4] ./example.js 1:8-20
    [4] ./example.js 144 bytes {1} [built]

Minimized (uglify-js, no zip)

Hash: 199e267fd7f9f732d06f
Version: webpack 2.2.0-rc.2
      Asset      Size  Chunks             Chunk Names
0.output.js  54 bytes       0  [emitted]  
  output.js   1.47 kB       1  [emitted]  main
Entrypoint main = output.js
chunk    {0} 0.output.js 22 bytes {1} [rendered]
    > [4] ./example.js 3:0-6:2
    [2] ./~/c.js 11 bytes {0} [built]
        require.ensure item c [4] ./example.js 3:0-6:2
    [3] ./~/d.js 11 bytes {0} [built]
        cjs require d [4] ./example.js 5:12-24
chunk    {1} output.js (main) 166 bytes [entry] [rendered]
    > main [4] ./example.js 
    [0] ./~/b.js 11 bytes {1} [built]
        cjs require b [4] ./example.js 2:8-20
        cjs require b [4] ./example.js 4:4-16
    [1] ./~/a.js 11 bytes {1} [built]
        cjs require a [4] ./example.js 1:8-20
    [4] ./example.js 144 bytes {1} [built]