带你一步步分析webpack是如何执行打包产物的

引入关系如图所示：
在这里插入图片描述
圈出来文件d是异步导入的文件。
wepback版本如图所示：

执行打包命令，产物如下图：

会生成两个js文件，一个是入口文件打包的testxx.js，还有一个是异步文件d生成的src_d_js.js。
打包后的内容如下所示，先贴代码，后面再分析：


(() => { // webpackBootstrap"use strict";var __webpack_modules__ = ({"./src/c.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {eval("__webpack_require__.r(__webpack_exports__);\n/* harmony export */ __webpack_require__.d(__webpack_exports__, {\n/* harmony export */   \"a\": () => (/* binding */ a),\n/* harmony export */   \"b\": () => (/* binding */ b)\n/* harmony export */ });\nconst a = function () {\n  console.log(1);\n}\n\nconst b = function () {\n  console.log(2);\n}\n\na()\nb()\n\n__webpack_require__.e(/*! import() */ \"src_d_js\").then(__webpack_require__.bind(__webpack_require__, /*! ./d */ \"./src/d.js\")).then(res => {\n  console.log(res);\n})\n\n\n//# sourceURL=webpack://test-webpack/./src/c.js?");}),"./src/index.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {eval("__webpack_require__.r(__webpack_exports__);\n/* harmony import */ var _c__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! ./c */ \"./src/c.js\");\n\r\n(0,_c__WEBPACK_IMPORTED_MODULE_0__.a)()\n\n//# sourceURL=webpack://test-webpack/./src/index.js?");})});/************************************************************************/// The module cachevar __webpack_module_cache__ = {};// The require functionfunction __webpack_require__(moduleId) {// Check if module is in cachevar cachedModule = __webpack_module_cache__[moduleId];if (cachedModule !== undefined) {return cachedModule.exports;}// Create a new module (and put it into the cache)var module = __webpack_module_cache__[moduleId] = {// no module.id needed// no module.loaded neededexports: {}};// Execute the module function__webpack_modules__[moduleId](module, module.exports, __webpack_require__);// Return the exports of the modulereturn module.exports;}// expose the modules object (__webpack_modules__)__webpack_require__.m = __webpack_modules__;/************************************************************************//* webpack/runtime/define property getters */(() => {// define getter functions for harmony exports__webpack_require__.d = (exports, definition) => {for (var key in definition) {if (__webpack_require__.o(definition, key) && !__webpack_require__.o(exports, key)) {Object.defineProperty(exports, key, { enumerable: true, get: definition[key] });}}};})();/* webpack/runtime/ensure chunk */(() => {__webpack_require__.f = {};// This file contains only the entry chunk.// The chunk loading function for additional chunks__webpack_require__.e = (chunkId) => {return Promise.all(Object.keys(__webpack_require__.f).reduce((promises, key) => {__webpack_require__.f[key](chunkId, promises);return promises;}, []));};})();/* webpack/runtime/get javascript chunk filename */(() => {// This function allow to reference async chunks__webpack_require__.u = (chunkId) => {// return url for filenames based on templatereturn "" + chunkId + ".js";};})();/* webpack/runtime/global */(() => {__webpack_require__.g = (function () {if (typeof globalThis === 'object') return globalThis;try {return this || new Function('return this')();} catch (e) {if (typeof window === 'object') return window;}})();})();/* webpack/runtime/hasOwnProperty shorthand */(() => {__webpack_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))})();/* webpack/runtime/load script */(() => {var inProgress = {};var dataWebpackPrefix = "test-webpack:";// loadScript function to load a script via script tag__webpack_require__.l = (url, done, key, chunkId) => {if (inProgress[url]) { inProgress[url].push(done); return; }var script, needAttach;if (key !== undefined) {var scripts = document.getElementsByTagName("script");for (var i = 0; i < scripts.length; i++) {var s = scripts[i];if (s.getAttribute("src") == url || s.getAttribute("data-webpack") == dataWebpackPrefix + key) { script = s; break; }}}if (!script) {needAttach = true;script = document.createElement('script');script.charset = 'utf-8';script.timeout = 120;if (__webpack_require__.nc) {script.setAttribute("nonce", __webpack_require__.nc);}script.setAttribute("data-webpack", dataWebpackPrefix + key);script.src = url;}inProgress[url] = [done];var onScriptComplete = (prev, event) => {// avoid mem leaks in IE.script.onerror = script.onload = null;clearTimeout(timeout);var doneFns = inProgress[url];delete inProgress[url];script.parentNode && script.parentNode.removeChild(script);doneFns && doneFns.forEach((fn) => (fn(event)));if (prev) return prev(event);};var timeout = setTimeout(onScriptComplete.bind(null, undefined, { type: 'timeout', target: script }), 120000);script.onerror = onScriptComplete.bind(null, script.onerror);script.onload = onScriptComplete.bind(null, script.onload);needAttach && document.head.appendChild(script);};})();/* webpack/runtime/make namespace object */(() => {// define __esModule on exports__webpack_require__.r = (exports) => {if (typeof Symbol !== 'undefined' && Symbol.toStringTag) {Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });}Object.defineProperty(exports, '__esModule', { value: true });};})();/* webpack/runtime/publicPath */(() => {var scriptUrl;if (__webpack_require__.g.importScripts) scriptUrl = __webpack_require__.g.location + "";var document = __webpack_require__.g.document;if (!scriptUrl && document) {if (document.currentScript)scriptUrl = document.currentScript.srcif (!scriptUrl) {var scripts = document.getElementsByTagName("script");if (scripts.length) scriptUrl = scripts[scripts.length - 1].src}}// When supporting browsers where an automatic publicPath is not supported you must specify an output.publicPath manually via configuration// or pass an empty string ("") and set the __webpack_public_path__ variable from your code to use your own logic.if (!scriptUrl) throw new Error("Automatic publicPath is not supported in this browser");scriptUrl = scriptUrl.replace(/#.*$/, "").replace(/\?.*$/, "").replace(/\/[^\/]+$/, "/");__webpack_require__.p = scriptUrl;})();/* webpack/runtime/jsonp chunk loading */(() => {// no baseURI// object to store loaded and loading chunks// undefined = chunk not loaded, null = chunk preloaded/prefetched// [resolve, reject, Promise] = chunk loading, 0 = chunk loadedvar installedChunks = {"testxx": 0};__webpack_require__.f.j = (chunkId, promises) => {// JSONP chunk loading for javascriptvar installedChunkData = __webpack_require__.o(installedChunks, chunkId) ? installedChunks[chunkId] : undefined;if (installedChunkData !== 0) { // 0 means "already installed".// a Promise means "currently loading".if (installedChunkData) {promises.push(installedChunkData[2]);} else {if (true) { // all chunks have JS// setup Promise in chunk cachevar promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));promises.push(installedChunkData[2] = promise);// start chunk loadingvar url = __webpack_require__.p + __webpack_require__.u(chunkId);// create error before stack unwound to get useful stacktrace latervar error = new Error();var loadingEnded = (event) => {if (__webpack_require__.o(installedChunks, chunkId)) {installedChunkData = installedChunks[chunkId];if (installedChunkData !== 0) installedChunks[chunkId] = undefined;if (installedChunkData) {var errorType = event && (event.type === 'load' ? 'missing' : event.type);var realSrc = event && event.target && event.target.src;error.message = 'Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realSrc + ')';error.name = 'ChunkLoadError';error.type = errorType;error.request = realSrc;installedChunkData[1](error);}}};__webpack_require__.l(url, loadingEnded, "chunk-" + chunkId, chunkId);} else installedChunks[chunkId] = 0;}}};// install a JSONP callback for chunk loadingvar webpackJsonpCallback = (parentChunkLoadingFunction, data) => {var [chunkIds, moreModules, runtime] = data;// add "moreModules" to the modules object,// then flag all "chunkIds" as loaded and fire callbackvar moduleId, chunkId, i = 0;if (chunkIds.some((id) => (installedChunks[id] !== 0))) {for (moduleId in moreModules) {if (__webpack_require__.o(moreModules, moduleId)) {__webpack_require__.m[moduleId] = moreModules[moduleId];}}if (runtime) var result = runtime(__webpack_require__);}if (parentChunkLoadingFunction) parentChunkLoadingFunction(data);for (; i < chunkIds.length; i++) {chunkId = chunkIds[i];if (__webpack_require__.o(installedChunks, chunkId) && installedChunks[chunkId]) {installedChunks[chunkId][0]();}installedChunks[chunkId] = 0;}}var chunkLoadingGlobal = self["webpackChunktest_webpack"] = self["webpackChunktest_webpack"] || [];chunkLoadingGlobal.forEach(webpackJsonpCallback.bind(null, 0));chunkLoadingGlobal.push = webpackJsonpCallback.bind(null, chunkLoadingGlobal.push.bind(chunkLoadingGlobal));})();// startup// Load entry module and return exports// This entry module can't be inlined because the eval devtool is used.var __webpack_exports__ = __webpack_require__("./src/index.js");})();

1.加载入口文件
在这里插入图片描述

index.js文件会引入c.js文件并执行a方法

会先执行

先执行__webpack_require__.r方法，定义了一些属性：

然后执行__webpack_require__导入c.js：

然后执行下面的方法：

__webpack_modules__[moduleId](module, module.exports, __webpack_require__);

继续执行下面c.js里面的方法：

 ((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {__webpack_require__.r(__webpack_exports__);__webpack_require__.d(__webpack_exports__, { "a": () => (a), "b": () => (b)});const a = function () {console.log(1);}const b = function () {console.log(2);}a()b()__webpack_require__.e("src_d_js").then(()=>{return __webpack_require__.bind(__webpack_require__,"./src/d.js")}).then(res => { console.log(res);})})

该函数会先执行.d方法，该方法已定义，但是是没添加到exports上，则会添加到exports：
在这里插入图片描述
然后执行：

__webpack_require__.e(/*! import() */ "src_d_js")

在这里插入图片描述
在这个方法中主要是执行__webpack_require.f.j方法：

__webpack_require__.f.j = (chunkId, promises) => {// 判断是否加载过var installedChunkData = __webpack_require__.o(installedChunks, chunkId) ? installedChunks[chunkId] : undefined;if (installedChunkData !== 0) { // 0 means "already installed".// a Promise means "currently loading".if (installedChunkData) {promises.push(installedChunkData[2]);} else {if (true) { // all chunks have JS// setup Promise in chunk cachevar promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));promises.push(installedChunkData[2] = promise);// start chunk loadingvar url = __webpack_require__.p + __webpack_require__.u(chunkId);// create error before stack unwound to get useful stacktrace latervar error = new Error();var loadingEnded = (event) => {if (__webpack_require__.o(installedChunks, chunkId)) {installedChunkData = installedChunks[chunkId];if (installedChunkData !== 0) installedChunks[chunkId] = undefined;if (installedChunkData) {var errorType = event && (event.type === 'load' ? 'missing' : event.type);var realSrc = event && event.target && event.target.src;error.message = 'Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realSrc + ')';error.name = 'ChunkLoadError';error.type = errorType;error.request = realSrc;installedChunkData[1](error);}}};__webpack_require__.l(url, loadingEnded, "chunk-" + chunkId, chunkId);} else installedChunks[chunkId] = 0;}}};

webpack_require.f.j方法主要是将设置一个promise，并将resolve，reject和本身放入installedChunkData，promises一开始是空列表，然后放入promise。随后调用__webpack_require__.l方法，并将要加载的文件url、加载完成事件等作为参数传入：

__webpack_require__.l(url, loadingEnded, "chunk-" + chunkId, chunkId);

接下来调用__webpack_require__.l方法：
在这里插入图片描述
该方法会根据url判断是否已经加载该script，如果没有就会去加载：

超时时间原本是120ms，本人调试就延迟了时间，这里会创建script并设置url为要加载的文件。

然后会监听该script的onload事件，当加载完毕会调用onScriptComplete函数。在调用该函数加载完毕之前会继续执行下图代码返回promises，该promises就是一开始installedChunks列表里放的[resolve,reject]。
在这里插入图片描述那么这个resolve什么时候执行呢？我们接着往下看，此时该文件已加载：

在这里插入图片描述
加载完成后就会执行onload函数的回调：onScriptComplete函数。
如果已经超时的话就好执行setTimeout函数，相当于加载失败直接回调了：

在这里插入图片描述
此时event就是：

如果没有超时则会加载script的内容，此时内容如下：

就是d.js模块的内容：

"use strict";
(self["webpackChunktest_webpack"] = self["webpackChunktest_webpack"] || []).push([["src_d_js"], {"./src/d.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {eval("__webpack_require__.r(__webpack_exports__);\n/* harmony export */ __webpack_require__.d(__webpack_exports__, {\n/* harmony export */   \"a\": () => (/* binding */ a),\n/* harmony export */   \"b\": () => (/* binding */ b)\n/* harmony export */ });\nconst a = function () {\n  console.log(1);\n}\n\nconst b = function () {\n  console.log(2);\n}\n\na()\nb()\n\n//# sourceURL=webpack://test-webpack/./src/d.js?");/***/})}]);

该模块可以看出会先执行webpackChunktest_webpack方法，那么该方法又从哪里来的呢？其实在一开始就定义了该函数：
在这里插入图片描述
并且设置覆盖了push方法为webpackJsonpCallback。该函数的作用是在modules记录该模块，并且执行resolve回调。如果该模块id在installedChunks里面（执行过__webpack_require__.f.j方法就会在该数组里面，相当于import），此时就会执行第一个函数，该函数就是promise的resolve。
在这里插入图片描述
此时会执行zhen方法，于是执行__webpack_require方法：

于是又到了我们熟悉的内容，上面已经分析过了，此时不再赘述。

执行完上面的模块，就会执行第二个then方法：

然后还有最后一个onload回调，由于script是宏任务，所以会执行完所有的微任务，再执行onload回调。
在这里插入图片描述
onload回调会移除该script和超时定时器。然后执行fn(event)，fn是loadingEnded，event是script.onload或script.onerror。正常执行完后会在webpackJsonpCallback里将installedChunks[chunkId] = 0，而installedChunkData会获取installedChunks[chunkId]的值。如果installedChunkData不是0说明没有执行webpackJsonpCallback，而webpackJsonpCallback的执行条件是已经script下载了内容才会执行。
在这里插入图片描述
全篇至此结束，还有些细节可能还没分析，有时间会再出文章解释说明，请各位读者持续关注。