在ES6模块解决方案出现之前,工具库或包常用三种解决方案提供对外使用的接口,最早是直接暴露给全局变量,比如我们熟知的Jquery暴露的全局变量是$,Lodash对外暴露的全局变量是_,后来出现了AMD和CommonJS(CMD的一种实现)两种常用的模块解决方案.

- 全局变量
// MyDependency is in your global scope
var MyModule = function() {
MyDependency.xxx()
};
- CommonJS
var MyDependency = require('my-dependency');
module.exports = function() {
MyDependency.xxx()
};
- AMD
define(['my-dependency'], function(MyDependency) {
return function() {
MyDependency.xxx()
};
});
为了同时兼容各种场景下的使用,业界提出了UMD的解决方案.遵循该标准的包可以通过上面三种方式引入.
以Lodash为例,其源码结构大致如下:
;(function(){
...
...
/** Detect free variable `global` from Node.js. */
var freeGlobal = typeof global == 'object' && global && global.Object === Object && global;
/** Detect free variable `self`. */
var freeSelf = typeof self == 'object' && self && self.Object === Object && self;
/** Used as a reference to the global object. */
var root = freeGlobal || freeSelf || Function('return this')();
/** Detect free variable `exports`. */
var freeExports = typeof exports == 'object' && exports && !exports.nodeType && exports;
/** Detect free variable `module`. */
var freeModule = freeExports && typeof module == 'object' && module && !module.nodeType && module;
function lodash(value) {
return xxxx
}
lodash.xxx= xxxxxx
lodash.prototype.xxxx= xxxxx
...
...
...
/*--------------------------------------------------------------------------*/
// 导出为AMD模块
if (typeof define == 'function' && typeof define.amd == 'object' && define.amd) {
root._ = lodash;
define(function() {
return lodash;
});
}
// 导出为CommonJS模块
else if (freeModule) {
(freeModule.exports = lodash)._ = lodash;
freeExports._ = lodash;
}
//导出到全局变量
else {
root._ = lodash;
}
}.call(this))
关于UMD的实现方式有很多种,都大同小异,下面是常见的实现方式.
(function (root, factory) {
if (typeof exports === 'object') {
// CommonJS
module.exports = factory(require('b'));
} else if (typeof define === 'function' && define.amd) {
// AMD
define(['b'], function (b) {
return (root.returnExportsGlobal = factory(b));
});
} else {
// Global Variables
root.returnExportsGlobal = factory(root.b);
}
}(this, function (b) {
// Your actual module
return {
b.xxx
};
}));
以Zepto为例,由于其仅用于浏览器端,因此不需要支持CommonJS.
/* Zepto v1.2.0 - zepto event ajax form ie - zeptojs.com/license */
;(function(global, factory) {
if (typeof define === "function" && define.amd)
define(function() {
return factory(global)
})
else factory(global)
})(this, function(window) {
var Zepto = (function() {
...
...
...
})()
window.Zepto = Zepto
window.$ === undefined && (window.$ = Zepto)()
return Zepto
})
Ramda的UMD封装通用性更强,还支持Babel转换ES6模块的方式.参见babel-plugin-transform-es2015-modules-commonjs
// Ramda v0.25.0
// https://github.com/ramda/ramda
// (c) 2013-2017 Scott Sauyet, Michael Hurley, and David Chambers
// Ramda may be freely distributed under the MIT license.
;(function(global, factory) {
typeof exports === "object" && typeof module !== "undefined"
? factory(exports) // CommonJS
: typeof define === "function" && define.amd
? define(["exports"], factory) // AMD
: factory((global.R = {})) // Global Variables
})(this, function(exports) {
"use strict"
exports.F = F
exports.T = T
exports.__ = __
exports.add = add
exports.addIndex = addIndex
exports.adjust = adjust
exports.all = all
// ...
// ...
exports.zipWith = zipWith
// 支持 Babel 转换的 ES6 module
Object.defineProperty(exports, "__esModule", { value: true })
})
当然实际开发过程中,我们不需要自己写样板代码,可以使用webpack+babel打包.例如我们设计一个简单的求和模块.
// sum.js
function Sum(a, b){
return a + b
}
export { Sum }
安装如下几个依赖
// package.json
"devDependencies": {
"@babel/core": "^7.1.6",
"babel-loader": "^8.0.4",
"babel-plugin-transform-es2015-modules-umd": "^6.24.1",
"webpack": "^4.25.1",
"webpack-cli": "^3.1.2"
}
webpack配置如如下:
var path = require('path')
module.exports = {
mode: 'development',
entry: './sum.js',
output: {
filename: 'sum.umd.js',
libraryTarget: 'umd',
library: 'UMDSUM',
path: path.resolve(__dirname, './dist'),
},
module: {
rules: [
{
test: /\.js$/,
use: ['babel-loader'],
},
]
},
devtool: 'source-map'
};
执行npx webpack打包结果如下:
(function webpackUniversalModuleDefinition(root, factory) {
if(typeof exports === 'object' && typeof module === 'object')
module.exports = factory();
else if(typeof define === 'function' && define.amd)
define([], factory);
else if(typeof exports === 'object')
exports["UMDSUM"] = factory();
else
root["UMDSUM"] = factory();
})(window, function() {
return /******/ (function(modules) { // webpackBootstrap
/******/ // The module cache
/******/ var installedModules = {};
/******/
/******/ // 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;
/******/ }
/******/
/******/
/******/ // 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 = __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 = "";
/******/
/******/
/******/ // Load entry module and return exports
/******/ return __webpack_require__(__webpack_require__.s = "./sum.js");
/******/ })
/************************************************************************/
/******/ ({
/***/ "./sum.js":
/*!****************!*\
!*** ./sum.js ***!
\****************/
/*! exports provided: Sum */
/***/ (function(module, __webpack_exports__, __webpack_require__) {
"use strict";
__webpack_require__.r(__webpack_exports__);
/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, "Sum", function() { return Sum; });
function Sum(a, b) {
return a + b;
}
/***/ })
/******/ });
});
//# sourceMappingURL=sum.umd.js.map
参考
Browserify and the Universal Module Definition