手机旅游视频网站模板/seo是怎么优化的

CommonJs模块化实现原理

首先看一个案例
初始化项目

npm init
npm i webpack -D

目录结构如下：

webpack.config.js

const path = require("path");
module.exports = {mode: "development",entry: "./src/index.js",output: {path: path.resolve(__dirname, "dist"),filename: "[name].js",},devtool: "source-map"
};

src/index.js

const b = require("./b");
const a = require("./a");
console.log(a, b)

src/a.js

let a = "这是a"
module.exports = a;

src/b.js

let b = "这是b"
module.exports = b;

build.js

const { webpack } = require("webpack");
const webpackOptions = require("./webpack.config.js");const compiler = webpack(webpackOptions);compiler.run((err, stats) => {console.log(err)
});

进行node ./build.js后查看dist文件下

(() => { var __webpack_modules__ = ({"./src/a.js": ((module) => {let a = "这是a"module.exports = a;}),"./src/b.js":((module) => {let b = '这是b'module.exports = b;})});// 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;}var __webpack_exports__ = {};(() => {const b = __webpack_require__("./src/b.js");const a = __webpack_require__("./src/a.js");console.log(a, b);})();})()
;
//# sourceMappingURL=main.js.map

分析一下打包产物

首先看下 webpack_modules，我们在src/index.js中引入了a.js、b.js, webpack会把’src/a.js’、‘src/b.js’作为modules的key值，该模块内容作为modules的value值；

 var __webpack_modules__ = ({"./src/a.js": ((module) => {let a = "这是a"module.exports = a;}),"./src/b.js":((module) => {let b = '这是b'module.exports = b;})});

定义 __webpack_require__函数

	var __webpack_module_cache__ = {};function __webpack_require__(moduleId) {// 判断一下缓存中有没有当前modulevar cachedModule = __webpack_module_cache__[moduleId];if (cachedModule !== undefined) {// 缓存中有的话走缓存,返回cachedModule.exportsreturn cachedModule.exports;}// 缓存中没有就重新创建一个moudle，设置export对象，并放入缓存var module = __webpack_module_cache__[moduleId] = {exports: {}};// 执行模块代码, 传入当前module，根据需要传入module.exports, __webpack_require__，module.exports会在模块中赋值__webpack_modules__[moduleId](module, module.exports, __webpack_require__);// 返回module.exportsreturn module.exports;}

执行入口函数，为防止命名污染，封装成立即执行函数。

 (() => {const b = __webpack_require__("./src/b.js");const a = __webpack_require__("./src/a.js");console.log(a, b);})();

ES Module模块化原理

src/index.js

import a from './a'
import {b} from './b'
console.log(a, b);

src/a.js

const a = "这是a"
export default a

src/b.js

export const b = '这是b'

node ./build.js 之后main.js如下：

 (() => { "use strict";var __webpack_modules__ = ({"./src/a.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {__webpack_require__.r(__webpack_exports__);__webpack_require__.d(__webpack_exports__, {"default": () => (__WEBPACK_DEFAULT_EXPORT__)});const a = "这是a"const __WEBPACK_DEFAULT_EXPORT__ = (a);}),"./src/b.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {__webpack_require__.r(__webpack_exports__);__webpack_require__.d(__webpack_exports__, {b: () => (b)});const b = '这是b'})});var __webpack_module_cache__ = {};function __webpack_require__(moduleId) {var cachedModule = __webpack_module_cache__[moduleId];if (cachedModule !== undefined) {return cachedModule.exports;}var module = __webpack_module_cache__[moduleId] = {exports: {}};__webpack_modules__[moduleId](module, module.exports, __webpack_require__);return module.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_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))})();(() => {__webpack_require__.r = (exports) => {if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });}Object.defineProperty(exports, '__esModule', { value: true });};})();var __webpack_exports__ = {};(() => {__webpack_require__.r(__webpack_exports__);var _a__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__("./src/a.js");var _b__WEBPACK_IMPORTED_MODULE_1__ = __webpack_require__("./src/b.js");console.log(_a__WEBPACK_IMPORTED_MODULE_0__["default"], _b__WEBPACK_IMPORTED_MODULE_1__.b);})();})()
;
//# sourceMappingURL=main.js.map

如果是通过export default 方式导出的，那就在 exports 对象加一个 default 属性

var __webpack_modules__ = ({"./src/a.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {// 在 ESM 模式下声明 ESM 模块标识__webpack_require__.r(__webpack_exports__);// 将模块导出的内容附加的模块对象上,如果是通过export default导出，给exports的对象加default属性__webpack_require__.d(__webpack_exports__, {"default": () => (__WEBPACK_DEFAULT_EXPORT__)});const a = "这是a"const __WEBPACK_DEFAULT_EXPORT__ = (a);}),"./src/b.js":((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {__webpack_require__.r(__webpack_exports__);__webpack_require__.d(__webpack_exports__, {b: () => (b)});const b = '这是b'})});

通过__webpack_require__.r把模块标识为 ES Module
了解Symbol.toStringTag

(() => {__webpack_require__.r = (exports) => {if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });}Object.defineProperty(exports, '__esModule', { value: true });};})();

通过__webpack_require__.d对exports做代理

(() => {__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_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))})();

ES Module加载CommonJS实现原理
src/index.js

import b from './b'
console.log(b);

src/b.js

let b = '这是b'
module.exports = b;

node ./build.js之后

 (() => { var __webpack_modules__ = ({"./src/b.js": ((module) => {let b = '这是b'module.exports = b;})});var __webpack_module_cache__ = {};function __webpack_require__(moduleId) {var cachedModule = __webpack_module_cache__[moduleId];if (cachedModule !== undefined) {return cachedModule.exports;}var module = __webpack_module_cache__[moduleId] = {exports: {}};__webpack_modules__[moduleId](module, module.exports, __webpack_require__);return module.exports;}(() => {__webpack_require__.n = (module) => {var getter = module && module.__esModule ?() => (module['default']) :() => (module);__webpack_require__.d(getter, { a: getter });return getter;};})();(() => {__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_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))})();(() => {__webpack_require__.r = (exports) => {if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });}Object.defineProperty(exports, '__esModule', { value: true });};})();var __webpack_exports__ = {};(() => {"use strict";__webpack_require__.r(__webpack_exports__);var _b__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__("./src/b.js");var _b__WEBPACK_IMPORTED_MODULE_0___default = __webpack_require__.n(_b__WEBPACK_IMPORTED_MODULE_0__);console.log((_b__WEBPACK_IMPORTED_MODULE_0___default()));})();})()
;
//# sourceMappingURL=main.js.map

由此可以看出加了这一个步骤：
通过__webpack_require__.n判断模块是否是esModule返回module

	(() => {__webpack_require__.n = (module) => {var getter = module && module.__esModule ?() => (module['default']) :() => (module);__webpack_require__.d(getter, { a: getter });return getter;};})();

CommonJS加载ES Module的实现原理

src/index.js

const b = require('./b.js')
console.log(b);

src/b.js

let b = '这是b'
export default b;

node ./build.js之后

 (() => {var __webpack_modules__ = ({"./src/b.js": ((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {"use strict";__webpack_require__.r(__webpack_exports__);__webpack_require__.d(__webpack_exports__, {"default": () => (__WEBPACK_DEFAULT_EXPORT__)});let b = '这是b'const __WEBPACK_DEFAULT_EXPORT__ = (b);})});var __webpack_module_cache__ = {};function __webpack_require__(moduleId) {var cachedModule = __webpack_module_cache__[moduleId];if (cachedModule !== undefined) {return cachedModule.exports;}var module = __webpack_module_cache__[moduleId] = {exports: {}};__webpack_modules__[moduleId](module, module.exports, __webpack_require__);return module.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_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))})();(() => {__webpack_require__.r = (exports) => {if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });}Object.defineProperty(exports, '__esModule', { value: true });};})();var __webpack_exports__ = {};(() => {const b = __webpack_require__(/*! ./b.js */ "./src/b.js")console.log(b);})();})()
;
//# sourceMappingURL=main.js.map