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：
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 cache
var __webpack_module_cache__ = {};
// The require function
function __webpack_require__(moduleId) {
// Check if module is in cache
var 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 needed
exports: {}
};
// Execute the module function
__webpack_modules__[moduleId](module, module.exports, __webpack_require__);
// Return the exports of the module
return 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 template
return "" + 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.src
if (!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 loaded
var installedChunks = {
"testxx": 0
};
__webpack_require__.f.j = (chunkId, promises) => {
// JSONP chunk loading for javascript
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 cache
var promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));
promises.push(installedChunkData[2] = promise);
// start chunk loading
var url = __webpack_require__.p + __webpack_require__.u(chunkId);
// create error before stack unwound to get useful stacktrace later
var 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 loading
var webpackJsonpCallback = (parentChunkLoadingFunction, data) => {
var [chunkIds, moreModules, runtime] = data;
// add "moreModules" to the modules object,
// then flag all "chunkIds" as loaded and fire callback
var 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
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：
然后执行：
__webpack_require__.e(/*! import() */ "src_d_js")
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 cache
var promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));
promises.push(installedChunkData[2] = promise);
// start chunk loading
var url = __webpack_require__.p + __webpack_require__.u(chunkId);
// create error before stack unwound to get useful stacktrace later
var 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：
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]。
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：
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：
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：
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。
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。
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。
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。