webpack2 示例之：Scope Hoisting 和 Code Splitting

原文链接：https://github.com/webpack/we...
译者：@justjavac

这个示例演示了与代码拆分相结合的 Scope Hoisting。

这是示例的依赖图：（实线表示同步导入，虚线表示异步导入）

除 cjs 之外的所有模块都是 EcmaScript 模块。cjs 是 CommonJs 模块。

有趣的是，将所有模块放在单一范围（scope）内将无法正常工作，原因如下：

• 模块 lazy, c, d 和 cjs 需要在一个单独的块（chunk）
• 模块 shared 可以被两个 chunk 访问（不同的范围）
• 模块 cjs 是一个 CommonJs 模块

因此，Webpack 使用一种称为“局部范围提升（Partial Scope Hoisting）”或“模块级联”的方法，该方法选择可以可以覆盖 Scope Hoisting 范围的 ES 模块的最大子集，并将其与默认的 webpack 原语(primitives)相结合。

为了避免冲突，模块中的模块连接标识符被重命名，并简化了内部导入。根模块的外部导入和导出使用现有的 ESM 结构。

example.js

import { a, x, y } from "a";
import * as b from "b";

import("./lazy").then(function(lazy) {
    console.log(a, b.a(), x, y, lazy.c, lazy.d.a, lazy.x, lazy.y);
});

lazy.js

export * from "c";
import * as d from "d";
export { d };

a.js

// module a
export var a = "a";
export * from "shared";

b.js

// module b
export function a() {
    return "b";
};

c.js

// module c
import { c as e } from "cjs";

export var c = String.fromCharCode(e.charCodeAt(0) - 2);

export { x, y } from "shared";

d.js

// module d
export var a = "d";

cjs.js

// module cjs (commonjs)
exports.c = "e";

shared.js

// shared module
export var x = "x";
export * from "shared2";

shared2.js

// shared2 module
export var y = "y";

webpack.config.js

var webpack = require("../../");

module.exports = {
    plugins: [
        new webpack.optimize.ModuleConcatenationPlugin()
    ]
};

js/output.js

/******/ (function(modules) { /* webpackBootstrap */ })
......

/******/ ([
/* 0 */
/*!********************************************!*\
  !*** ./node_modules/shared.js + 1 modules ***!
  \********************************************/
/*! exports provided: x, y */
/*! exports used: x, y */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
Object.defineProperty(__webpack_exports__, "__esModule", { value: true });

// CONCATENATED MODULE: ./node_modules/shared2.js
// shared2 module
var y = "y";

// CONCATENATED MODULE: ./node_modules/shared.js
/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, "a", function() { return x; });
/* concated harmony reexport */__webpack_require__.d(__webpack_exports__, "b", function() { return y; });
// shared module
var x = "x";



/***/ }),
/* 1 */
/*!********************************!*\
  !*** ./example.js + 2 modules ***!
  \********************************/
/*! exports provided:  */
/*! all exports used */
/*! ModuleConcatenation bailout: Module is an entry point */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
Object.defineProperty(__webpack_exports__, "__esModule", { value: true });

// EXTERNAL MODULE: ./node_modules/shared.js + 1 modules
var shared = __webpack_require__(0);

// CONCATENATED MODULE: ./node_modules/a.js
// module a
var a = "a";


// CONCATENATED MODULE: ./node_modules/b.js
// module b
function b_a() {
    return "b";
};

// CONCATENATED MODULE: ./example.js



__webpack_require__.e/* import() */(0).then(__webpack_require__.bind(null, /*! ./lazy */ 3)).then(function(lazy) {
    console.log(a, b_a(), shared["a" /* x */], shared["b" /* y */], lazy.c, lazy.d.a, lazy.x, lazy.y);
});


/***/ })
/******/ ]);

js/0.output.js

webpackJsonp([0],[
/* 0 */,
/* 1 */,
/* 2 */
/*!*****************************!*\
  !*** ./node_modules/cjs.js ***!
  \*****************************/
/*! no static exports found */
/*! exports used: c */
/*! ModuleConcatenation bailout: Module is not an ECMAScript module */
/***/ (function(module, exports) {

// module cjs (commonjs)
exports.c = "e";


/***/ }),
/* 3 */
/*!*****************************!*\
  !*** ./lazy.js + 2 modules ***!
  \*****************************/
/*! exports provided: d, c, x, y */
/*! all exports used */
/*! ModuleConcatenation bailout: Module is referenced from these modules with unsupported syntax: ./example.js (referenced with import()) */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
Object.defineProperty(__webpack_exports__, "__esModule", { value: true });
var d_namespaceObject = {};
__webpack_require__.d(d_namespaceObject, "a", function() { return a; });

// EXTERNAL MODULE: ./node_modules/cjs.js
var cjs = __webpack_require__(2);
var cjs_default = /*#__PURE__*/__webpack_require__.n(cjs);

// EXTERNAL MODULE: ./node_modules/shared.js + 1 modules
var shared = __webpack_require__(0);

// CONCATENATED MODULE: ./node_modules/c.js
// module c


var c = String.fromCharCode(cjs["c"].charCodeAt(0) - 2);



// CONCATENATED MODULE: ./node_modules/d.js
// module d
var a = "d";

// CONCATENATED MODULE: ./lazy.js
/* concated harmony reexport */__webpack_require__.d(__webpack_exports__, "c", function() { return c; });
/* concated harmony reexport */__webpack_require__.d(__webpack_exports__, "x", function() { return shared["a" /* x */]; });
/* concated harmony reexport */__webpack_require__.d(__webpack_exports__, "y", function() { return shared["b" /* y */]; });
/* concated harmony reexport */__webpack_require__.d(__webpack_exports__, "d", function() { return d_namespaceObject; });





/***/ })
]);

Minimized

webpackJsonp([0],[,,function(n,r){r.c="e"},function(n,r,t){"use strict";Object.defineProperty(r,"__esModule",{value:!0});var e={};t.d(e,"a",function(){return d});var u=t(2),c=t(0),o=String.fromCharCode(u.c.charCodeAt(0)-2),d="d";t.d(r,"c",function(){return o}),t.d(r,"x",function(){return c.a}),t.d(r,"y",function(){return c.b}),t.d(r,"d",function(){return e})}]);

Info

Uncompressed

Hash: 6596ce0a50ccbbaa89c6
Version: webpack 3.5.1
      Asset     Size  Chunks             Chunk Names
0.output.js   1.9 kB       0  [emitted]  
  output.js  7.39 kB       1  [emitted]  main
Entrypoint main = output.js
chunk    {0} 0.output.js 286 bytes {1} [rendered]
    > [] 4:0-16
    [3] ./lazy.js + 2 modules 242 bytes {0} [built]
        [exports: d, c, x, y]
        import() ./lazy [] ./example.js 4:0-16
     + 1 hidden module
chunk    {1} output.js (main) 390 bytes [entry] [rendered]
    > main [] 
    [0] ./node_modules/shared.js + 1 modules 105 bytes {1} [built]
        [exports: x, y]
        [only some exports used: x, y]
        harmony import shared [1] ./example.js + 2 modules 3:0-23
        harmony import shared [3] ./lazy.js + 2 modules 6:0-30
    [1] ./example.js + 2 modules 285 bytes {1} [built]
        [no exports]

Minimized (uglify-js, no zip)

Hash: 6596ce0a50ccbbaa89c6
Version: webpack 3.5.1
      Asset       Size  Chunks             Chunk Names
0.output.js  364 bytes       0  [emitted]  
  output.js    1.66 kB       1  [emitted]  main
Entrypoint main = output.js
chunk    {0} 0.output.js 286 bytes {1} [rendered]
    > [] 4:0-16
    [3] ./lazy.js + 2 modules 242 bytes {0} [built]
        [exports: d, c, x, y]
        import() ./lazy [] ./example.js 4:0-16
     + 1 hidden module
chunk    {1} output.js (main) 390 bytes [entry] [rendered]
    > main [] 
    [0] ./node_modules/shared.js + 1 modules 105 bytes {1} [built]
        [exports: x, y]
        [only some exports used: x, y]
        harmony import shared [1] ./example.js + 2 modules 3:0-23
        harmony import shared [3] ./lazy.js + 2 modules 6:0-30
    [1] ./example.js + 2 modules 285 bytes {1} [built]
        [no exports]

---

欢迎关注我的公众号，关注前端文章：