简要分析webpack打包后代码

开门见山

1.打包单一模块

webpack.config.js

module.exports = {
    entry:"./chunk1.js",
    output: {
        path: __dirname + '/dist',
        filename: '[name].js'
    },
};

chunk1.js

var chunk1=1;
exports.chunk1=chunk1;

打包后，main.js(webpack生成的一些注释已经去掉)

 (function(modules) { // webpackBootstrap
    // The module cache
    var installedModules = {};
    // 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] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.loaded = true;
        // Return the exports of the module
        return module.exports;
    }


    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // __webpack_public_path__
    __webpack_require__.p = "";
    // Load entry module and return exports
    return __webpack_require__(0);
 })([function(module, exports) {
    var chunk1=1;
    exports.chunk1=chunk1;
}]);

这其实就是一个立即执行函数，简化一下就是：

(function(module){})([function(){},function(){}]);

OK,看一下自运行的匿名函数里面干了什么：

function(modules) { // webpackBootstrap
    // modules就是一个数组，元素就是一个个函数体，就是我们声明的模块
    var installedModules = {};
    // The require function
    function __webpack_require__(moduleId) {
        ...
    }
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // __webpack_public_path__
    __webpack_require__.p = "";
    // Load entry module and return exports
    return __webpack_require__(0);
 }

整个函数里就声明了一个变量installedModules 和函数webpack_require，并在函数上添加了一个m,c,p属性，m属性保存的是传入的模块数组，c属性保存的是installedModules变量，P是一个空字符串。最后执行webpack_require函数，参数为零，并将其执行结果返回。下面看一下webpack_require干了什么：

function __webpack_require__(moduleId) {
        //moduleId就是调用是传入的0
        // installedModules[0]是undefined,继续往下
        if(installedModules[moduleId])
            return installedModules[moduleId].exports;
        // module就是{exports: {},id: 0,loaded: false}
        var module = installedModules[moduleId] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // 下面接着分析这个
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // 表明模块已经载入
        module.loaded = true;
        // 返回module.exports(注意modules[moduleId].call的时候module.exports会被修改)
        return module.exports;
    }

接着看一下modules[moduleId].call(module.exports, module, module.exports, webpack_require)，其实就是

modules[moduleId].call({}, module, module.exports, __webpack_require__)

对call不了解当然也可以认为是这样(但是并不是等价，call能确保当模块中使用this的时候，this是指向module.exports的)：

function  a(module, exports) {
    var chunk1=1;
    exports.chunk1=chunk1;
}
a(module, exports,__webpack_require__);

传入的module就是{exports: {},id: 0,loaded: false}，exports就是{}，webpack_require就是声明的webpack_require函数(传入这个函数有什么用呢，第二节将会介绍)；
运行后module.exports就是{chunk1:1}。所以当我们使用chunk1这个模块的时候（比如var chunk1=require("chunk1"),得到的就是一个对象{chunk1:1}）。如果模块里没有exports.chunk1=chunk1或者module.exports=chunk1得到的就是一个空对象{}

2.使用模块

上面我们已经分析了webpack是怎么打包一个模块的（入口文件就是一个模块），现在我们来看一下使用一个模块，然后使用模块的文件作为入口文件
webpack.config.js

module.exports = {
    entry:"./main.js",
    output: {
        path: __dirname + '/dist',
        filename: '[name].js'
    }
};

main.js

var chunk1=require("./chunk1");
console.log(chunk1);

打包后

(function (modules) { // webpackBootstrap
    // The module cache
    var installedModules = {};
    // 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] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.loaded = true;
        // Return the exports of the module
        return module.exports;
    }
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // __webpack_public_path__
    __webpack_require__.p = "";
    // Load entry module and return exports
    return __webpack_require__(0);
})([function (module, exports, __webpack_require__) {
    var chunk1=__webpack_require__(1);
    console.log(chunk1);
}, function (module, exports) {
    var chunk1 = 1;
    exports.chunk1 = chunk1;
}]);

不一样的地方就是自执行函数的参数由

[function(module, exports) { var chunk1=1; exports.chunk1=chunk1;}]

变为

[function (module, exports, __webpack_require__) {
    var chunk1=__webpack_require__(1);
    console.log(chunk1);
}, function (module, exports) {
    var chunk1 = 1;
    exports.chunk1 = chunk1;
}]

其实就是多了一个main模块，不过这个模块没有导出项，而且这个模块依赖于chunk1模块。所以当运行webpack_require(0)的时候，main模块缓存到installedModules[0]上，modules[0].call(也就是调用main模块)的时候，chunk1被缓存到installedModules[1]上，并且导出对象{chunk1：1}给模块main使用

3.重复使用模块

webpack.config.js

module.exports = {
    entry:"./main.js",
    output: {
        path: __dirname + '/dist',
        filename: '[name].js'
    }
};

main.js

var chunk1=require("./chunk1");
var chunk2=require(".chunlk2");
console.log(chunk1);
console.log(chunk2);

chunk1.js

var chunk2=require("./chunk2");
var chunk1=1;
exports.chunk1=chunk1;

chunk2.js

var chunk2=1;
exports.chunk2=chunk2;

打包后

(function (modules) { // webpackBootstrap
    // The module cache
    var installedModules = {};
    // 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] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.loaded = true;
        // Return the exports of the module
        return module.exports;
    }
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // __webpack_public_path__
    __webpack_require__.p = "";
    // Load entry module and return exports
    return __webpack_require__(0);
})([function (module, exports, __webpack_require__) {

    var chunk1 = __webpack_require__(1);
    var chunk2 = __webpack_require__(2);
    console.log(chunk1);
    console.log(chunk2);
}, function (module, exports, __webpack_require__) {

    __webpack_require__(2);
    var chunk1 = 1;
    exports.chunk1 = chunk1;
}, function (module, exports) {

    var chunk2 = 1;
    exports.chunk2 = chunk2;
}]);

不难发现，当需要重复使用模块的时候，缓存变量installedModules 就起作用了

4.多个打包入口

不管是单一模块还是重复模块，和以上两种一样

5.入口参数为数组

webpack.config.js

module.exports = {
    entry:['./main.js','./main1.js'],
    output: {
        path: __dirname + '/dist',
        filename: '[name].js'
    }
};

打包后

[
/* 0 */
/***/ function(module, exports, __webpack_require__) {
    __webpack_require__(1);
    module.exports = __webpack_require__(3);
/***/ },
/* 1 */
/***/ function(module, exports, __webpack_require__) {
    var chunk1=__webpack_require__(2);
    console.log(chunk1);
/***/ },
/* 2 */
/***/ function(module, exports) {
    var chunk1=1;
    exports.chunk1=chunk1;
/***/ },
/* 3 */
/***/ function(module, exports, __webpack_require__) {
    var chunk1=__webpack_require__(2);
/***/ }
/******/ ]

这里只截取自执行匿名函数的参数，因为其他代码与之前一样。可以看到1就是main默模块，2就是chunk1模块，3就是mian1模块，0的作用就是运行模块mian,mian1,然后将main1模块导出（main1中没有导出项，所以到导出{}），总结一下：入口参数是字符串不管是多入口还是单入口，最后都会将入口模块的导出项导出,没有导出项就导出{}，而入口参数是数组，就会将最后一个模块导出（webpackg官网有说明）

6.使用CommonsChunkPlugin插件

webpack.config.js

var CommonsChunkPlugin = require("webpack/lib/optimize/CommonsChunkPlugin");
module.exports = {
    entry: {
        main: './main.js',
        main1: './main1.js',
    },
    output: {
        path: __dirname + '/dist',
        filename: '[name].js'
    },
    plugins: [
        new CommonsChunkPlugin({
        name: "common"
        })
    ]
};

main mian1中都require了chunk1,所以chunk1会被打包到common。
打包后，common.js

(function (modules) { // webpackBootstrap
    // install a JSONP callback for chunk loading
    var parentJsonpFunction = window["webpackJsonp"];
    window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules) {
        // add "moreModules" to the modules object,
        // then flag all "chunkIds" as loaded and fire callback
        var moduleId, chunkId, i = 0, callbacks = [];
        for (; i < chunkIds.length; i++) {
            chunkId = chunkIds[i];
            if (installedChunks[chunkId])
                callbacks.push.apply(callbacks, installedChunks[chunkId]);
            installedChunks[chunkId] = 0;
        }
        for (moduleId in moreModules) {
            modules[moduleId] = moreModules[moduleId];
        }
        if (parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
        while (callbacks.length)
            callbacks.shift().call(null, __webpack_require__);
        if (moreModules[0]) {
            installedModules[0] = 0;
            return __webpack_require__(0);
        }
    };
    // The module cache
    var installedModules = {};
    // object to store loaded and loading chunks
    // "0" means "already loaded"
    // Array means "loading", array contains callbacks
    var installedChunks = {
        2: 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] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.loaded = 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, callback) {
        // "0" is the signal for "already loaded"
        if (installedChunks[chunkId] === 0)
            return callback.call(null, __webpack_require__);
        // an array means "currently loading".
        if (installedChunks[chunkId] !== undefined) {
            installedChunks[chunkId].push(callback);
        } else {
            // start chunk loading
            installedChunks[chunkId] = [callback];
            var head = document.getElementsByTagName('head')[0];
            var script = document.createElement('script');
            script.type = 'text/javascript';
            script.charset = 'utf-8';
            script.async = true;
            script.src = __webpack_require__.p + "" + chunkId + "." + ({ "0": "main", "1": "main1" }[chunkId] || chunkId) + ".js";
            head.appendChild(script);
        }
    };
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // __webpack_public_path__
    __webpack_require__.p = "";
})([, function (module, exports) {

    var chunk1 = 1;
    exports.chunk1 = chunk1;

}]);

main.js

webpackJsonp([0],[function(module, exports, __webpack_require__) {

    var chunk1=__webpack_require__(1);
    console.log(chunk1);
 }]);

main1.js

webpackJsonp([1],[function(module, exports, __webpack_require__) {
    var chunk1=__webpack_require__(1);
    console.log(chunk1);
}]);

与之前相比，多了webpackJsonp函数，立即执行的匿名函数没有立即调用webpack_require(0)。看一下webpackJsonp：

var parentJsonpFunction = window["webpackJsonp"];
    window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules) {
        //moreModules为独立chunk代码，chunkIds标记独立chunk唯一性避免按需加载时重复加载
        //以main.js中代码为例，chunkIds为[0],moreModules为
        //[function(module, exports, __webpack_require__) {
        //  var chunk1=__webpack_require__(1);
        //  console.log(chunk1);
        //}]
        var moduleId, chunkId, i = 0, callbacks = [];
        for (; i < chunkIds.length; i++) {
            chunkId = chunkIds[i];//chunkId=0
            if (installedChunks[chunkId])
                callbacks.push.apply(callbacks,installedChunks[chunkId]);//0 push入callbacks(使用requireEnsure不再是0)
            //赋值为0表明chunk已经loaded
            installedChunks[chunkId] = 0;
        }
        for (moduleId in moreModules) {
            //modules[0]会被覆盖
            modules[moduleId] = moreModules[moduleId];
        }
        //按当前情况parentJsonpFunction一直未undefined
        if (parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
        //按当前情况callbacks=[]
        while (callbacks.length)
            callbacks.shift().call(null, __webpack_require__);
        if (moreModules[0]) {
            installedModules[0] = 0;
            return __webpack_require__(0);
        }
    };
    // 缓存模块，通过闭包引用(window["webpackJsonp"]可以访问到)
    var installedModules = {};
    //2为公共chunck唯一ID，0表示已经loaded
    var installedChunks = {
        2: 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] = {
            exports: {},
            id: moduleId,
            loaded: false
        };
        // Execute the module function
        modules[moduleId].call(module.exports, module, module.exports, __webpack_require__);
        // Flag the module as loaded
        module.loaded = true;
        // Return the exports of the module
        return module.exports;
    }
    //按需加载
    __webpack_require__.e = function requireEnsure(chunkId, callback) {
        // "0" is the signal for "already loaded"
        if (installedChunks[chunkId] === 0)
            return callback.call(null, __webpack_require__);
        // an array means "currently loading".
        if (installedChunks[chunkId] !== undefined) {
            installedChunks[chunkId].push(callback);
        } else {
            // start chunk loading
            installedChunks[chunkId] = [callback];
            var head = document.getElementsByTagName('head')[0];
            var script = document.createElement('script');
            script.type = 'text/javascript';
            script.charset = 'utf-8';
            script.async = true;
            script.src = __webpack_require__.p + "" + chunkId + "." + ({ "0": "main", "1": "main1" }[chunkId] || chunkId) + ".js";
            head.appendChild(script);
        }
    };

好像看不出什么。。。，修改一下
webpack.config.js

var CommonsChunkPlugin = require("webpack/lib/optimize/CommonsChunkPlugin");
module.exports = {
    entry: {
        main: './main.js',
        main1: './main1.js',
        chunk1:["./chunk1"]
    },
    output: {
        path: __dirname + '/dist2',
        filename: '[name].js'
    },
    plugins: [
        new CommonsChunkPlugin({
        name: ["chunk1"],
        filename:"common.js",
        minChunks:3,
        })
    ]
};

main,main1都分别require chunk1,chunk2,然后将chunk1打包到公共模块（minChunks:3，chunk2不会被打包到公共模块）。自运行匿名函数最后多了

   return __webpack_require__(0);

则installedModules[0]为已经loaded,看common.js，installedModules[1]也会loaded。
main.js

webpackJsonp([1], [function (module, exports, __webpack_require__) {

    var chunk1 = __webpack_require__(1);
    var chunk2 = __webpack_require__(2);
    exports.a = 1;
    console.log(chunk1);
}, , function (module, exports) {
    var chunk2 = 1;
    exports.chunk2 = chunk2;

}
]);

main1.js

webpackJsonp([2], [function (module, exports, __webpack_require__) {

    var chunk1 = __webpack_require__(1);
    var chunk2 = __webpack_require__(2);
    exports.a = 1;
    console.log(chunk1);
}, , function (module, exports) {
    var chunk2 = 1;
    exports.chunk2 = chunk2;
}
]);

common.js modules：

[function (module, exports, __webpack_require__) {

    module.exports = __webpack_require__(1);
}, function (module, exports) {

    var chunk1 = 1;
    exports.chunk1 = chunk1;
}]

以main.js的代码为例，调用webpackJsonp，传入的参数chunkIds为[1],moreModules为

[function (module, exports, __webpack_require__) {

    var chunk1 = __webpack_require__(1);
    var chunk2 = __webpack_require__(2);
    exports.a = 1;
    console.log(chunk1);
}, , function (module, exports) {
    var chunk2 = 1;
    exports.chunk2 = chunk2;

}]

var moduleId, chunkId, i = 0, callbacks = [];
        for (; i < chunkIds.length; i++) {
            chunkId = chunkIds[i];//1
            //false,赋值为0后还是false
            if (installedChunks[chunkId])
                callbacks.push.apply(callbacks, installedChunks[chunkId]);
            installedChunks[chunkId] = 0;
        }
        //三个模块
        for (moduleId in moreModules) {
            //moduleId:0,1,2  moreModules[1]为空模块，自执行函数的参数(公共模块)会被覆盖，但是参数中的相应模块已经loaded并且缓存
            modules[moduleId] = moreModules[moduleId];
        }
        if (parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
        while (callbacks.length)
            callbacks.shift().call(null, __webpack_require__);
        if (moreModules[0]) {
            //installedModules[0]会重新load,但是load的是moreModules[0]，因为modules[0]已经被覆盖，moreModules[0]依赖于
            //modules[1]、modules[2],modules[1]已经loaded
            installedModules[0] = 0;
            return __webpack_require__(0);
        }

再看下面的情况：
common.js 自执行函数参数（公共模块）（没有return webpack_require(0)）

[,function(module, exports, __webpack_require__) {

    var chunk1=1;
    var chunk2=__webpack_require__(2);
    exports.chunk1=chunk1;
},function(module, exports) {

    var chunk2=1;
    exports.chunk2=chunk2;
}]

main.js

webpackJsonp([0],[
/* 0 */
/***/ function(module, exports, __webpack_require__) {

    var chunk1=__webpack_require__(1);
    var chunk2=__webpack_require__(2);
    exports.a=1;
    console.log(chunk1);
    //main
/***/ }
]);

以main调用分析

         var moduleId, chunkId, i = 0, callbacks = [];
        for(;i < chunkIds.length; i++) {
            chunkId = chunkIds[i];//0
            if(installedChunks[chunkId])
                callbacks.push.apply(callbacks, installedChunks[chunkId]);
            installedChunks[chunkId] = 0;//表明唯一索引为0的chunk已经loaded
        }
        for(moduleId in moreModules) {
            //moreModules只有一个元素，所以modules[1]、modules[2]不会被覆盖
            modules[moduleId] = moreModules[moduleId];
        }
        if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
        while(callbacks.length)
            callbacks.shift().call(null, __webpack_require__);
        if(moreModules[0]) {
            installedModules[0] = 0;
            //moreModules[0]即modules[0]依赖modules[1]、即modules[2](没有被覆盖很关键)
            return __webpack_require__(0);
        }

还有这种打包情况：
common.js不包含公共模块，即自执行函数参数为[]。
main.js

webpackJsonp([0,1],[
function(module, exports, __webpack_require__) {

    var chunk1=__webpack_require__(1);
    var chunk2=__webpack_require__(2);
    exports.a=1;
    console.log(chunk1);
},function(module, exports) {
    var chunk1=1;
    exports.chunk1=chunk1;
},function(module, exports) {
    var chunk2=1;
    exports.chunk2=chunk2;
}]);

以main调用分析

     var moduleId, chunkId, i = 0, callbacks = [];
        for(;i < chunkIds.length; i++) {
            chunkId = chunkIds[i];//0,1
            if(installedChunks[chunkId])
                callbacks.push.apply(callbacks, installedChunks[chunkId]);
            installedChunks[chunkId] = 0;
        }
        for(moduleId in moreModules) {
            //moreModules全部转移到modules
            modules[moduleId] = moreModules[moduleId];
        }
        if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
        while(callbacks.length)
            callbacks.shift().call(null, __webpack_require__);
        if(moreModules[0]) {
            //modules[0]是chunk文件运行代码
            installedModules[0] = 0;
            return __webpack_require__(0);
        }

7.按需加载

webpack.config.json

module.exports = {
  entry: './main.js',
  output: {
    filename: 'bundle.js'
  }
};

main.js

require.ensure(['./a'], function(require) {
  var content = require('./a');
  document.open();
  document.write('<h1>' + content + '</h1>');
  document.close();
});

a.js

module.exports = 'Hello World';

打包后

bundle.js

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

/******/    };

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

/******/    // object to store loaded and loading chunks
/******/    // "0" means "already loaded"
/******/    // Array means "loading", array contains callbacks
/******/    var installedChunks = {
/******/        0:0
/******/    };

/******/    // 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] = {
/******/            exports: {},
/******/            id: moduleId,
/******/            loaded: false
/******/        };

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

/******/        // Flag the module as loaded
/******/        module.loaded = 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, callback) {
/******/        // "0" is the signal for "already loaded"
/******/        if(installedChunks[chunkId] === 0)
/******/            return callback.call(null, __webpack_require__);

/******/        // an array means "currently loading".
/******/        if(installedChunks[chunkId] !== undefined) {
/******/            installedChunks[chunkId].push(callback);
/******/        } else {
/******/            // start chunk loading
/******/            installedChunks[chunkId] = [callback];
/******/            var head = document.getElementsByTagName('head')[0];
/******/            var script = document.createElement('script');
/******/            script.type = 'text/javascript';
/******/            script.charset = 'utf-8';
/******/            script.async = true;

/******/            script.src = __webpack_require__.p + "" + chunkId + ".bundle.js";
/******/            head.appendChild(script);
/******/        }
/******/    };

/******/    // expose the modules object (__webpack_modules__)
/******/    __webpack_require__.m = modules;

/******/    // expose the module cache
/******/    __webpack_require__.c = installedModules;

/******/    // __webpack_public_path__
/******/    __webpack_require__.p = "";

/******/    // Load entry module and return exports
/******/    return __webpack_require__(0);
/******/ })
/************************************************************************/
/******/ ([
/* 0 */
/***/ function(module, exports, __webpack_require__) {

    __webpack_require__.e/* nsure */(1, function(require) {
      var content = __webpack_require__(1);
      document.open();
      document.write('<h1>' + content + '</h1>');
      document.close();
    });


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

1.bundle.js

webpackJsonp([1],[
/* 0 */,
/* 1 */
/***/ function(module, exports) {

    module.exports = 'Hello World';


/***/ }
]);

和使用CommonsChunkPlugin打包的差异在于

/******/    // This file contains only the entry chunk.
/******/    // The chunk loading function for additional chunks
/******/    __webpack_require__.e = function requireEnsure(chunkId, callback) {
/******/        // "0" is the signal for "already loaded"
/******/        if(installedChunks[chunkId] === 0)
/******/            return callback.call(null, __webpack_require__);

/******/        // an array means "currently loading".
/******/        if(installedChunks[chunkId] !== undefined) {
/******/            installedChunks[chunkId].push(callback);
/******/        } else {
/******/            // start chunk loading
/******/            installedChunks[chunkId] = [callback];
/******/            var head = document.getElementsByTagName('head')[0];
/******/            var script = document.createElement('script');
/******/            script.type = 'text/javascript';
/******/            script.charset = 'utf-8';
/******/            script.async = true;

/******/            script.src = __webpack_require__.p + "" + chunkId + ".bundle.js";
/******/            head.appendChild(script);
/******/        }
/******/    };

模块main的id为0，模块a的id为1。return webpack_require(0)，则加载main模块，
modules[0].call(module.exports, module, module.exports, webpack_require)则调用函数

function(module, exports, __webpack_require__) {

    __webpack_require__.e/* nsure */(1, function(require) {
      var content = __webpack_require__(1);
      document.open();
      document.write('<h1>' + content + '</h1>');
      document.close();
    }

/******/    // This file contains only the entry chunk.
/******/    // The chunk loading function for additional chunks
/******/    __webpack_require__.e = function requireEnsure(chunkId, callback) {
                //installedChunks[1]为undefined
/******/        // "0" is the signal for "already loaded"
/******/        if(installedChunks[chunkId] === 0)
/******/            return callback.call(null, __webpack_require__);

/******/        // an array means "currently loading".
/******/        if(installedChunks[chunkId] !== undefined) {
/******/            installedChunks[chunkId].push(callback);
/******/        } else {
/******/            // start chunk loading
/******/            installedChunks[chunkId] = [callback];//installedChunks[1]为数组，表明currently loading
/******/            var head = document.getElementsByTagName('head')[0];
/******/            var script = document.createElement('script');
/******/            script.type = 'text/javascript';
/******/            script.charset = 'utf-8';
/******/            script.async = true;

/******/            script.src = __webpack_require__.p + "" + chunkId + ".bundle.js";
/******/            head.appendChild(script);
                    //加载完后直接调用
                    /******/webpackJsonp([1],[
                    /******//* 0 */,
                    /******//* 1 */
                    /******//***/ function(module, exports) {
                    /******/
                    /******/    module.exports = 'Hello World';
                    /******/
                    /******/
                    /******//***/ }
                    /******/]);
                    /******/        }
                    /******/    };
                    //installedChunks[1]在webpackJsonp得到调用

installedChunks[1]为数组，元素为main模块的执行代码

/******/    window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules) {
                //moreModules为模块a的代码
/******/        // add "moreModules" to the modules object,
/******/        // then flag all "chunkIds" as loaded and fire callback
/******/        var moduleId, chunkId, i = 0, callbacks = [];
/******/        for(;i < chunkIds.length; i++) {
/******/            chunkId = chunkIds[i];
/******/            if(installedChunks[chunkId])//installedChunks[0]==0,installedChunks[1]为数组
/******/                callbacks.push.apply(callbacks, installedChunks[chunkId]);//callbacks为模块main执行代码，不为数组
/******/            installedChunks[chunkId] = 0;//installedChunks[1]不为数组，表明已经加载
/******/        }
/******/        for(moduleId in moreModules) {
/******/            modules[moduleId] = moreModules[moduleId];
/******/        }
/******/        if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules);
/******/        while(callbacks.length)
/******/            callbacks.shift().call(null, __webpack_require__);

/******/    };