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node_modules Scope Hoisting May 21, 2017
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README.md

This example demonstrates Scope Hoisting in combination with Code Splitting.

This is the dependency graph for the example: (solid lines express sync imports, dashed lines async imports)

All modules except cjs are EcmaScript modules. cjs is a CommonJs module.

The interesting thing here is that putting all modules in single scope won't work, because of multiple reasons:

  • Modules lazy, c, d and cjs need to be in a separate chunk
  • Module shared is accessed by two chunks (different scopes)
  • Module cjs is a CommonJs module

webpack therefore uses a approach called "Partial Scope Hoisting" or "Module concatenation", which chooses the largest possible subsets of ES modules which can be scope hoisted and combines them with the default webpack primitives.

While module concatenation identifiers in modules are renamed to avoid conflicts and internal imports are simplified. External imports and exports from the root module use the existing ESM constructs.

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

module.exports = {
	// mode: "development" || "production",
	optimization: {
		usedExports: true,
		concatenateModules: true,
		occurrenceOrder: true // To keep filename consistent between different modes (for example building only)
	}
};

dist/output.js

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/******/ 	// install a JSONP callback for chunk loading
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/******/ 		var chunkIds = data[0];
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/******/ 			chunkId = chunkIds[i];
/******/ 			if(installedChunks[chunkId]) {
/******/ 				resolves.push(installedChunks[chunkId][0]);
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/******/ 			if(Object.prototype.hasOwnProperty.call(moreModules, moduleId)) {
/******/ 				modules[moduleId] = moreModules[moduleId];
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/******/ 		if(parentJsonpFunction) parentJsonpFunction(data);
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/******/ 		module.l = true;
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/************************************************************************/
/******/ ([
/* 0 */
/*!********************************************!*\
  !*** ./node_modules/shared.js + 1 modules ***!
  \********************************************/
/*! exports provided: x, y */
/*! exports used: x, y */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";

// 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 y */__webpack_require__.d(__webpack_exports__, "b", function() { return y; });
// shared module
var x = "x";



/***/ }),
/* 1 */
/*!********************************!*\
  !*** ./example.js + 2 modules ***!
  \********************************/
/*! no exports provided */
/*! all exports used */
/*! ModuleConcatenation bailout: Cannot concat with ./node_modules/shared.js because of ./node_modules/c.js */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
__webpack_require__.r(__webpack_exports__);

// 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() */ 1).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);
});


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

dist/1.output.js

(window["webpackJsonp"] = window["webpackJsonp"] || []).push([[1],[
/* 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: Cannot concat with ./node_modules/cjs.js (<- Module is not an ECMAScript module) */
/*! ModuleConcatenation bailout: Cannot concat with ./node_modules/shared.js */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
__webpack_require__.r(__webpack_exports__);
var d_namespaceObject = {};
__webpack_require__.r(d_namespaceObject);
__webpack_require__.d(d_namespaceObject, "a", function() { return a; });

// EXTERNAL MODULE: ./node_modules/cjs.js
var cjs = __webpack_require__(2);

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





/***/ })
]]);

Minimized

(window.webpackJsonp=window.webpackJsonp||[]).push([[1],[,,function(n,r){r.c="e"},function(n,r,t){"use strict";t.r(r);var c={};t.r(c),t.d(c,"a",function(){return e});var o=t(2),u=t(0),d=String.fromCharCode(o.c.charCodeAt(0)-2),e="d";t.d(r,"c",function(){return d}),t.d(r,"x",function(){return u.a}),t.d(r,"y",function(){return u.b}),t.d(r,"d",function(){return c})}]]);

Info

Unoptimized

Hash: 0a1b2c3d4e5f6a7b8c9d
Version: webpack 4.28.0
      Asset      Size  Chunks             Chunk Names
1.output.js   1.9 KiB       1  [emitted]  
  output.js  9.41 KiB       0  [emitted]  main
Entrypoint main = output.js
chunk    {0} output.js (main) 372 bytes >{1}< [entry] [rendered]
    > .\example.js main
 [0] ./node_modules/shared.js + 1 modules 100 bytes {0} [built]
     [exports: x, y]
     [all exports used]
     harmony side effect evaluation shared [1] ./example.js + 2 modules 3:0-23
     harmony export imported specifier shared [1] ./example.js + 2 modules 3:0-23
     harmony side effect evaluation shared [3] ./lazy.js + 2 modules 6:0-30
     harmony export imported specifier shared [3] ./lazy.js + 2 modules 6:0-30
     harmony export imported specifier shared [3] ./lazy.js + 2 modules 6:0-30
     |    2 modules
 [1] ./example.js + 2 modules 272 bytes {0} [built]
     [no exports]
     single entry .\example.js  main
     | ./example.js 161 bytes [built]
     |     [no exports]
     |     single entry .\example.js  main
     |     + 2 hidden modules
chunk    {1} 1.output.js 273 bytes <{0}> [rendered]
    > ./lazy [] 4:0-16
 [3] ./lazy.js + 2 modules 231 bytes {1} [built]
     [exports: d, c, x, y]
     import() ./lazy  ./example.js 4:0-16
     | ./lazy.js 57 bytes [built]
     |     [exports: d, c, x, y]
     |     import() ./lazy  ./example.js 4:0-16
     |     + 2 hidden modules
     + 1 hidden module

Production mode

Hash: 0a1b2c3d4e5f6a7b8c9d
Version: webpack 4.28.0
      Asset       Size  Chunks             Chunk Names
1.output.js  369 bytes       1  [emitted]  
  output.js   2.17 KiB       0  [emitted]  main
Entrypoint main = output.js
chunk    {0} output.js (main) 372 bytes >{1}< [entry] [rendered]
    > .\example.js main
 [0] ./node_modules/shared.js + 1 modules 100 bytes {0} [built]
     [exports: x, y]
     [all exports used]
     harmony side effect evaluation shared [1] ./example.js + 2 modules 3:0-23
     harmony export imported specifier shared [1] ./example.js + 2 modules 3:0-23
     harmony side effect evaluation shared [3] ./lazy.js + 2 modules 6:0-30
     harmony export imported specifier shared [3] ./lazy.js + 2 modules 6:0-30
     harmony export imported specifier shared [3] ./lazy.js + 2 modules 6:0-30
     |    2 modules
 [1] ./example.js + 2 modules 272 bytes {0} [built]
     [no exports]
     single entry .\example.js  main
     | ./example.js 161 bytes [built]
     |     [no exports]
     |     single entry .\example.js  main
     |     + 2 hidden modules
chunk    {1} 1.output.js 273 bytes <{0}> [rendered]
    > ./lazy [] 4:0-16
 [3] ./lazy.js + 2 modules 231 bytes {1} [built]
     [exports: d, c, x, y]
     import() ./lazy  ./example.js 4:0-16
     | ./lazy.js 57 bytes [built]
     |     [exports: d, c, x, y]
     |     import() ./lazy  ./example.js 4:0-16
     |     + 2 hidden modules
     + 1 hidden module