摘要：的模塊化不僅支持和，還能通過實(shí)現(xiàn)模塊的動態(tài)加載。根據(jù)官方文檔，實(shí)現(xiàn)動態(tài)加載的方式有兩種和。如果你對如何實(shí)現(xiàn)和感興趣，可以查看我的前兩篇文章模塊化原理和模塊化原理。

webpack的模塊化不僅支持commonjs和es module，還能通過code splitting實(shí)現(xiàn)模塊的動態(tài)加載。根據(jù)wepack官方文檔，實(shí)現(xiàn)動態(tài)加載的方式有兩種：import和require.ensure。

那么，這篇文檔就來分析一下，webpack是如何實(shí)現(xiàn)code splitting的。

PS：如果你對webpack如何實(shí)現(xiàn)commonjs和es module感興趣，可以查看我的前兩篇文章：webpack模塊化原理-commonjs和webpack模塊化原理-ES module。

首先我們依然創(chuàng)建一個簡單入口模塊index.js和兩個依賴模塊foo.js和bar.js：

// index.js
"use strict";
import(/* webpackChunkName: "foo" */ "./foo").then(foo => {
    console.log(foo());
})
import(/* webpackChunkName: "bar" */ "./bar").then(bar => {
    console.log(bar());
})

// foo.js
"use strict";
exports.foo = function () {
    return 2;
}

// bar.js
"use strict";
exports.bar = function () {
    return 1;
}

webpack配置如下：

var path = require("path");

module.exports = {
    entry: path.join(__dirname, "index.js"),
    output: {
        path: path.join(__dirname, "outs"),
        filename: "index.js",
        chunkFilename: "[name].bundle.js"
    },
};

這是一個最簡單的配置，指定了模塊入口和打包文件輸出路徑，值得注意的是，這次還指定了分離模塊的文件名[name].bundle.js（不指定會有默認(rèn)文件名）。

在根目錄下執(zhí)行webpack，得到經(jīng)過webpack打包的代碼如下（去掉了不必要的注釋）：

(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 = {
        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] = {
            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) {
        var installedChunkData = installedChunks[chunkId];
        if(installedChunkData === 0) {
            return new Promise(function(resolve) { resolve(); });
        }
        // a Promise means "currently loading".
        if(installedChunkData) {
            return installedChunkData[2];
        }
        // setup Promise in chunk cache
        var promise = new Promise(function(resolve, reject) {
            installedChunkData = installedChunks[chunkId] = [resolve, reject];
        });
        installedChunkData[2] = promise;
        // 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 + "" + ({"0":"foo","1":"bar"}[chunkId]||chunkId) + ".bundle.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;
            }
        };
        head.appendChild(script);
        return promise;
    };
    // expose the modules object (__webpack_modules__)
    __webpack_require__.m = modules;
    // expose the module cache
    __webpack_require__.c = installedModules;
    // 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 = "";
    // 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 = 0);
})
([
(function(module, exports, __webpack_require__) {
    "use strict";
    __webpack_require__.e/* import() */(0).then(__webpack_require__.bind(null, 1)).then(foo => {
        console.log(foo());
    })
    __webpack_require__.e/* import() */(1).then(__webpack_require__.bind(null, 2)).then(bar => {
        console.log(bar());
    })
})
]);

編譯后的代碼，整體跟前兩篇文章中使用commonjs和es6 module編寫的代碼編譯后的結(jié)構(gòu)差別不大，都是通過IFFE的方式啟動代碼，然后使用webpack實(shí)現(xiàn)的require和exports實(shí)現(xiàn)的模塊化。

而對于code splitting的支持，區(qū)別在于這里使用__webpack_require__.e實(shí)現(xiàn)動態(tài)加載模塊和實(shí)現(xiàn)基于promise的模塊導(dǎo)入。

所以首先分析__webpack_require__.e函數(shù)的定義，這個函數(shù)實(shí)現(xiàn)了動態(tài)加載：

__webpack_require__.e = function requireEnsure(chunkId) {
    // 1、緩存查找
    var installedChunkData = installedChunks[chunkId];
    if(installedChunkData === 0) {
        return new Promise(function(resolve) { resolve(); });
    }
    if(installedChunkData) {
        return installedChunkData[2];
    }
    // 2、緩存模塊
    var promise = new Promise(function(resolve, reject) {
        installedChunkData = installedChunks[chunkId] = [resolve, reject];
    });
    installedChunkData[2] = promise;
    // 3、加載模塊
    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 + "" + ({"0":"foo"}[chunkId]||chunkId) + ".bundle.js";
    // 4、異常處理
    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;
        }
    };
    head.appendChild(script);
    // 5、返回promise
    return promise;
};

代碼大致邏輯如下：

緩存查找：從緩存installedChunks中查找是否有緩存模塊，如果緩存標(biāo)識為0，則表示模塊已加載過，直接返回promise；如果緩存為數(shù)組，表示緩存正在加載中，則返回緩存的promise對象

如果沒有緩存，則創(chuàng)建一個promise，并將promise和resolve、reject緩存在installedChunks中

構(gòu)建一個script標(biāo)簽，append到head標(biāo)簽中，src指向加載的模塊腳本資源，實(shí)現(xiàn)動態(tài)加載js腳本

添加script標(biāo)簽onload、onerror 事件，如果超時或者模塊加載失敗，則會調(diào)用reject返回模塊加載失敗異常

如果模塊加載成功，則返回當(dāng)前模塊promise，對應(yīng)于import()

以上便是模塊加載的過程，當(dāng)資源加載完成，模塊代碼開始執(zhí)行，那么我們來看一下模塊代碼的結(jié)構(gòu)：

webpackJsonp([0],[
/* 0 */,
/* 1 */
/***/ (function(module, exports, __webpack_require__) {
"use strict";
exports.foo = function () {
    return 2;
}
/***/ })
]);

可以看到，模塊代碼不僅被包在一個函數(shù)中（用來模擬模塊作用域），外層還被當(dāng)做參數(shù)傳入webpackJsonp中。那么這個webpackJsonp函數(shù)的作用是什么呢？

其實(shí)這里的webpackJsonp類似于jsonp中的callback，作用是作為模塊加載和執(zhí)行完成的回調(diào)，從而觸發(fā)import的resolve。

具體細(xì)看webpackJsonp代碼來分析：

window["webpackJsonp"] = function webpackJsonpCallback(chunkIds, moreModules, executeModules) {
    var moduleId, chunkId, i = 0, resolves = [], result;
    // 1、收集模塊resolve
    for(;i < chunkIds.length; i++) {
        chunkId = chunkIds[i];
        if(installedChunks[chunkId]) {
            resolves.push(installedChunks[chunkId][0]);
        }
        installedChunks[chunkId] = 0;
    }
    // 2、copy模塊到modules
    for(moduleId in moreModules) {
        if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {
            modules[moduleId] = moreModules[moduleId];
        }
    }
    if(parentJsonpFunction) parentJsonpFunction(chunkIds, moreModules, executeModules);
    // 3、resolve import
    while(resolves.length) {
        resolves.shift()();
    }
};

代碼大致邏輯如下：

根據(jù)chunkIds收集對應(yīng)模塊的resolve，這里的chunkIds為數(shù)組是因?yàn)?b>require.ensure是可以實(shí)現(xiàn)異步加載多個模塊的，所以需要兼容

把動態(tài)模塊添加到IFFE的modules中，提供其他CMD方案使用模塊

直接調(diào)用resolve，完成整個異步加載

webpack通過__webpack_require__.e函數(shù)實(shí)現(xiàn)了動態(tài)加載，再通過webpackJsonp函數(shù)實(shí)現(xiàn)異步加載回調(diào)，把模塊內(nèi)容以promise的方式暴露給調(diào)用方，從而實(shí)現(xiàn)了對code splitting的支持。