So, How About UMD模块-Universal Module Definition

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