一文快速弄懂webpack动态import原理一文快速弄懂webpack动态import原

前言
例子
1. 模块加载
2. jsonp动态加载script
3. 执行异步脚本
4. webpackJsonpCallback
5. 执行异步模块代码
流程图
总结

前言在vue中我们经常用到动态导入页面组件，那么它是如何实现的呢，本文将通过简单的案例，快速了解实现原理

例子

// index.jsimport('./test').then(fn => {console.log(fn.default()); })

// test.jsexport default function func() {return 1}

打包后的代码包含两个文件 bundle.js 和 0.js
点击展开bundle.js

/******/ (function(modules) { // webpackBootstrap/******/// install a JSONP callback for chunk loading/******/function webpackJsonpCallback(data) {/******/var chunkIds = data[0]; /******/var moreModules = data[1]; /******//******//******/// add "moreModules" to the modules object,/******/// then flag all "chunkIds" as loaded and fire callback/******/var moduleId, chunkId, i = 0, resolves = []; /******/for(; i < chunkIds.length; i++) {/******/chunkId = chunkIds[i]; /******/if(Object.prototype.hasOwnProperty.call(installedChunks, chunkId) && 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(data); /******//******/while(resolves.length) {/******/resolves.shift()(); /******/}/******//******/}; /******//******//******/// The module cache/******/var installedModules = {}; /******//******/// object to store loaded and loading chunks/******/// undefined = chunk not loaded, null = chunk preloaded/prefetched/******/// Promise = chunk loading, 0 = chunk loaded/******/var installedChunks = {/******/"main": 0/******/}; /******//******//******//******/// script path function/******/function jsonpScriptSrc(chunkId) {/******/return __webpack_require__.p + "" + chunkId + ".bundle.js"/******/}/******//******/// 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 promises = []; /******//******//******/// JSONP chunk loading for javascript/******//******/var installedChunkData = https://www.it610.com/article/installedChunks[chunkId]; /******/if(installedChunkData !== 0) { // 0 means"already installed"./******//******/// a Promise means "currently loading"./******/if(installedChunkData) {/******/promises.push(installedChunkData[2]); /******/} else {/******/// setup Promise in chunk cache/******/var promise = new Promise(function(resolve, reject) {/******/installedChunkData = https://www.it610.com/article/installedChunks[chunkId] = [resolve, reject]; /******/}); /******/promises.push(installedChunkData[2] = promise); /******//******/// start chunk loading/******/var script = document.createElement('script'); /******/var onScriptComplete; /******//******/script.charset = 'utf-8'; /******/script.timeout = 120; /******/if (__webpack_require__.nc) {/******/script.setAttribute("nonce", __webpack_require__.nc); /******/}/******/script.src = https://www.it610.com/article/jsonpScriptSrc(chunkId); /******//******/// create error before stack unwound to get useful stacktrace later/******/var error = new Error(); /******/onScriptComplete = function (event) {/******/// avoid mem leaks in IE./******/script.onerror = script.onload = null; /******/clearTimeout(timeout); /******/var chunk = installedChunks[chunkId]; /******/if(chunk !== 0) {/******/if(chunk) {/******/var errorType = event && (event.type ==='load' ? 'missing' : event.type); /******/var realSrc = https://www.it610.com/article/event && event.target && event.target.src; /******/error.message ='Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realSrc + ')'; /******/error.name = 'ChunkLoadError'; /******/error.type = errorType; /******/error.request = realSrc; /******/chunk[1](error); /******/}/******/installedChunks[chunkId] = undefined; /******/}/******/}; /******/var timeout = setTimeout(function(){/******/onScriptComplete({ type: 'timeout', target: script }); /******/}, 120000); /******/script.onerror = script.onload = onScriptComplete; /******/document.head.appendChild(script); /******/}/******/}/******/return Promise.all(promises); /******/}; /******//******/// 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, { enumerable: true, get: getter }); /******/}/******/}; /******//******/// define __esModule on exports/******/__webpack_require__.r = function(exports) {/******/if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {/******/Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' }); /******/}/******/Object.defineProperty(exports, '__esModule', { value: true }); /******/}; /******//******/// create a fake namespace object/******/// mode & 1: value is a module id, require it/******/// mode & 2: merge all properties of value into the ns/******/// mode & 4: return value when already ns object/******/// mode & 8|1: behave like require/******/__webpack_require__.t = function(value, mode) {/******/if(mode & 1) value = https://www.it610.com/article/__webpack_require__(value); /******/if(mode & 8) return value; /******/if((mode & 4) && typeof value ==='object' && value && value.__esModule) return value; /******/var ns = Object.create(null); /******/__webpack_require__.r(ns); /******/Object.defineProperty(ns, 'default', { enumerable: true, value: value }); /******/if(mode & 2 && typeof value != 'string') for(var key in value) __webpack_require__.d(ns, key, function(key) { return value[key]; }.bind(null, key)); /******/return ns; /******/}; /******//******/// 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; }; /******//******/var jsonpArray = window["webpackJsonp"] = window["webpackJsonp"] || []; /******/var oldJsonpFunction = jsonpArray.push.bind(jsonpArray); /******/jsonpArray.push = webpackJsonpCallback; /******/jsonpArray = jsonpArray.slice(); /******/for(var i = 0; i < jsonpArray.length; i++) webpackJsonpCallback(jsonpArray[i]); /******/var parentJsonpFunction = oldJsonpFunction; /******//******//******/// Load entry module and return exports/******/return __webpack_require__(__webpack_require__.s = "./src/index.js"); /******/ })/************************************************************************//******/ ({/***/ "./src/index.js":/*!**********************!*\!*** ./src/index.js ***!\**********************//*! no static exports found *//***/ (function(module, exports, __webpack_require__) {eval("__webpack_require__.e(/*! import() */ 0).then(__webpack_require__.bind(null, /*! ./test */ \"./src/test.js\")).then(function (fn) {\nconsole.log(fn[\"default\"]()); \n}); \n\n//# sourceURL=webpack:///./src/index.js?"); /***/ })/******/ });

点击展开0.js

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{/***/ "./src/test.js":/*!*********************!*\!*** ./src/test.js ***!\*********************//*! exports provided: default *//***/ (function(module, __webpack_exports__, __webpack_require__) {"use strict"; eval("__webpack_require__.r(__webpack_exports__); \n/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, \"default\", function() { return func; }); \nfunction func() {\nreturn 1; \n}\n\n//# sourceURL=webpack:///./src/test.js?"); /***/ })}]);

1. 模块加载 webpack通过__webpack_require__加载模块代码

// bundle.jsfunction __webpack_require__(moduleId)// 如果模块已经加载，直接返回模块导出 if(installedModules[moduleId]) {return installedModules[moduleId].exports; }// 模块导出和模块信息 var module = installedModules[moduleId] = {i: moduleId,l: false,exports: {} }// 执行模块代码 modules[moduleId].call(module.exports, module, module.exports, __webpack_require__) module.l = true // 标记模块已经加载完成 return module.exports; }

__webpack_require__('index.js')

然后执行index.js编译后的代码，如下。

Promise.all([__webpack_require__.e(0)]).then(__webpack_require__.bind(null, "./src/test.js")).then(function (fn) {console.log(fn.default()); }));

2. jsonp动态加载script 先一步步来，看下__webpack_require__.e这个方法，它是最先调用的。

// bundle.js__webpack_require__.e = function requireEnsure(chunkId) { var promises = [] var installedChunkData = https://www.it610.com/article/installedChunks[chunkId]; // 如果这个chunk已经加载过了就不需要加载了 if(installedChunkData !== 0) { // 0 means"already installed"if(installedChunkData) {promises.push(installedChunkData[2]); } else {// 为这个chunk创建一个promisevar promise = new Promise(function(resolve, reject) {// 记录这个chunk对应promise的resolve和reject方法installedChunkData = https://www.it610.com/article/installedChunks[chunkId] = [resolve, reject]; }); // promises数组里添加这个chunk对应的promisepromises.push(installedChunkData[2] = promise)// ============== 动态创建script =================var script = document.createElement('script'); var onScriptCompletescript.charset = 'utf-8'; script.timeout = 120; if (__webpack_require__.nc) {script.setAttribute("nonce", __webpack_require__.nc); }script.src = https://www.it610.com/article/jsonpScriptSrc(chunkId)// create error before stack unwound to get useful stacktrace latervar error = new Error(); // =================================================onScriptComplete = function (event) {// avoid mem leaks in IE.script.onerror = script.onload = null; clearTimeout(timeout); var chunk = installedChunks[chunkId]; if(chunk !== 0) {if(chunk) {var errorType = event && (event.type ==='load' ? 'missing' : event.type); var realSrc = https://www.it610.com/article/event && event.target && event.target.src; error.message ='Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realSrc + ')'; error.name = 'ChunkLoadError'; error.type = errorType; error.request = realSrc; chunk[1](error); }installedChunks[chunkId] = undefined; }}; var timeout = setTimeout(function(){onScriptComplete({ type: 'timeout', target: script }); }, 120000); script.onerror = script.onload = onScriptComplete; document.head.appendChild(script); } } return Promise.all(promises); };

总结一下，上述代码做的事情

如果chunk没有被加载过，会为这个chunk创建一个promise对象
将promise对象存在promises数组中
将promise的resolve 和 reject存在installedChunks[chunkId]中

3. 执行异步脚本经过上面的过程，会动态加载0.js的脚本代码

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{"./src/test.js":(function(module, __webpack_exports__, __webpack_require__) {"use strict"; eval("__webpack_require__.r(__webpack_exports__); \n/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, \"default\", function() { return func; }); \nfunction func() {\nreturn 1; \n}\n\n//# sourceURL=webpack:///./src/test.js?"); /***/ })}]);

可以看到window上有一个webpackJsonp数组，那么这个东西是从哪里来的呢？，我们来看下面的代码。

// bundle.jsvar jsonpArray = window["webpackJsonp"] = window["webpackJsonp"] || []; var oldJsonpFunction = jsonpArray.push.bind(jsonpArray); jsonpArray.push = webpackJsonpCallback; jsonpArray = jsonpArray.slice(); for(var i = 0; i < jsonpArray.length; i++) webpackJsonpCallback(jsonpArray[i]); var parentJsonpFunction = oldJsonpFunction;

其实一开始初始化时已经覆盖实现了webpackJsonp.push方法

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[0],{// test.js引入的模块代码})// 等价于webpackJsonpCallback([[0],{// test.js引入的模块代码})

下面再看看webpackJsonpCallback代码的实现

4. webpackJsonpCallback

// bundle.jsfunction webpackJsonpCallback(data) {// chunkid var chunkIds = data[0]; // chunkid对应的模块 var moreModules = data[1] var moduleId, chunkId, i = 0, resolves = []; for(; i < chunkIds.length; i++) {chunkId = chunkIds[i]; if(Object.prototype.hasOwnProperty.call(installedChunks, chunkId) && installedChunks[chunkId]) {// 收集chunk对应的resolve方法resolves.push(installedChunks[chunkId][0]); }// 标记该chunk已经加载installedChunks[chunkId] = 0; } for(moduleId in moreModules) {if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {// 添加chunk模块，到全局modules对象中modules[moduleId] = moreModules[moduleId]; } } if(parentJsonpFunction) parentJsonpFunction(data)// 依次执行chunk对应promise的resolve方法 while(resolves.length) {resolves.shift()(); }};

还是总结一下上面代码的过程

收集chunk对应的resolve方法, 前面执行__webpack_require__.e时放在了installedChunks[chunkId]里
将异步chunk下的所有模块添加到全局modules
依次执行chunk对应promise的resolve方法

5. 执行异步模块代码回到一开始index.js的代码

Promise.all([__webpack_require__.e(0)]).then(__webpack_require__.bind(null, "./src/test.js")).then(function (fn) {console.log(fn.default()); }));

经过上面的步骤，此时promise已经resolve了，__webpack_require__.bind(null, "./src/test.js") 会被执行，此时异步模块的代码已经在modules上了，所以可以直接加载。
最后，执行fn方法

console.log(fn.default());

流程图

文章图片

总结 webpack动态import的实现还是比较简单的，具体细节大家可以自己翻阅下打包后的代码~
【一文快速弄懂webpack动态import原理】到此这篇关于webpack动态import原理的文章就介绍到这了,更多相关webpack动态import原理内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！