在Node.js中對模塊載入和執(zhí)行進行了包裝，使得模塊文件中的變量在一個閉包中，不會污染全局變量，和他人沖突。
前端模塊通常是我們開發(fā)人員為了避免和他人沖突才把模塊代碼放置在一個閉包中。
如何封裝Node.js和前端通用的模塊，我們可以參考Underscore.js 實現(xiàn)，他就是一個Node.js和前端通用的功能函數(shù)模塊，查看代碼：
// Create a safe reference to the Underscore object for use below.
var _ = function(obj) {
if (obj instanceof _) return obj;
if (!(this instanceof _)) return new _(obj);
this._wrapped = obj;
};
// Export the Underscore object for **Node.js**, with
// backwards-compatibility for the old `require()` API. If we're in
// the browser, add `_` as a global object via a string identifier,
// for Closure Compiler "advanced" mode.
if (typeof exports !== 'undefined') {
if (typeof module !== 'undefined' && module.exports) {
exports = module.exports = _;
}
exports._ = _;
} else {
root._ = _;
}
通過判斷exports是否存在來決定將局部變量 _ 賦值給exports，向后兼容舊的require() API，如果在瀏覽器中，通過一個字符串標識符“_”作為一個全局對象；完整的閉包如下：
(function() {
// Baseline setup
// --------------
// Establish the root object, `window` in the browser, or `exports` on the server.
var root = this;
// Create a safe reference to the Underscore object for use below.
var _ = function(obj) {
if (obj instanceof _) return obj;
if (!(this instanceof _)) return new _(obj);
this._wrapped = obj;
};
// Export the Underscore object for **Node.js**, with
// backwards-compatibility for the old `require()` API. If we're in
// the browser, add `_` as a global object via a string identifier,
// for Closure Compiler "advanced" mode.
if (typeof exports !== 'undefined') {
if (typeof module !== 'undefined' && module.exports) {
exports = module.exports = _;
}
exports._ = _;
} else {
root._ = _;
}
}).call(this);
通過function定義構(gòu)建了一個閉包，call(this)是將function在this對象下調(diào)用，以避免內(nèi)部變量污染到全局作用域。瀏覽器中，this指向的是全局對象（window對象），將“_”變量賦在全局對象上“root._”，以供外部調(diào)用。和Underscore.js 類似的Lo-Dash，也是使用了類似的方案，只是兼容了AMD模塊載入的兼容：
;(function() {
/** Used as a safe reference for `undefined` in pre ES5 environments */
var undefined;
/** Used to determine if values are of the language type Object */
var objectTypes = {
'boolean': false,
'function': true,
'object': true,
'number': false,
'string': false,
'undefined': false
};
/** Used as a reference to the global object */
var root = (objectTypes[typeof window] && window) || this;
/** Detect free variable `exports` */
var freeExports = objectTypes[typeof exports] && exports && !exports.nodeType && exports;
/** Detect free variable `module` */
var freeModule = objectTypes[typeof module] && module && !module.nodeType && module;
/** Detect the popular CommonJS extension `module.exports` */
var moduleExports = freeModule && freeModule.exports === freeExports && freeExports;
/*--------------------------------------------------------------------------*/
// expose Lo-Dash
var _ = runInContext();
// some AMD build optimizers, like r.js, check for condition patterns like the following:
if (typeof define == 'function' && typeof define.amd == 'object' && define.amd) {
// Expose Lo-Dash to the global object even when an AMD loader is present in
// case Lo-Dash was injected by a third-party script and not intended to be
// loaded as a module. The global assignment can be reverted in the Lo-Dash
// module by its `noConflict()` method.
root._ = _;
// define as an anonymous module so, through path mapping, it can be
// referenced as the "underscore" module
define(function() {
return _;
});
}
// check for `exports` after `define` in case a build optimizer adds an `exports` object
else if (freeExports && freeModule) {
// in Node.js or RingoJS
if (moduleExports) {
(freeModule.exports = _)._ = _;
}
// in Narwhal or Rhino -require
else {
freeExports._ = _;
}
}
else {
// in a browser or Rhino
root._ = _;
}
}.call(this));
再來看看Moment.js的封裝閉包主要代碼：
(function (undefined) {
var moment；
// check for nodeJS
var hasModule = (typeof module !== 'undefined' && module.exports)；
/************************************
Exposing Moment
************************************/
function makeGlobal(deprecate) {
var warned = false, local_moment = moment;
/*global ender:false */
if (typeof ender !== 'undefined') {
return;
}
// here, `this` means `window` in the browser, or `global` on the server
// add `moment` as a global object via a string identifier,
// for Closure Compiler "advanced" mode
if (deprecate) {
this.moment = function () {
if (!warned && console && console.warn) {
warned = true;
console.warn(
"Accessing Moment through the global scope is " +
"deprecated, and will be removed in an upcoming " +
"release.");
}
return local_moment.apply(null, arguments);
};
} else {
this['moment'] = moment;
}
}
// CommonJS module is defined
if (hasModule) {
module.exports = moment;
makeGlobal(true);
} else if (typeof define === "function" && define.amd) {
define("moment", function (require, exports, module) {
if (module.config().noGlobal !== true) {
// If user provided noGlobal, he is aware of global
makeGlobal(module.config().noGlobal === undefined);
}
return moment;
});
} else {
makeGlobal();
}
}).call(this);
從上面的幾個例子可以看出，在封裝Node.js和前端通用的模塊時，可以使用以下邏輯：
if (typeof exports !== "undefined") {
exports.** = **;
} else {
this.** = **;
}
即，如果exports對象存在，則將局部變量裝載在exports對象上，如果不存在，則裝載在全局對象上。如果加上ADM規(guī)范的兼容性，那么多加一句判斷：
if (typeof define === "function" && define.amd){}
