Underscore是一个 JavaScript 工具库，它提供了一整套函数式编程的实用功能，但是没有扩展任何 JavaScript 内置对象。 他解决了这个问题：“如果我面对一个空白的 HTML 页面，并希望立即开始工作，我需要什么？” 他弥补了jQuery没有实现的功能，同时又是Backbone必不可少的部分。

首先分析结构部分

(function() {

//Baseline setup

//--------------

//Establish the root object, `window` (`self`) in the browser, `global`

//on the server, or `this` in some virtual machines. We use `self`

//instead of `window` for `WebWorker` support.

varroot=typeofself=='object'&&self.self===self&&self||

typeofglobal=='object'&&global.global===global&&global||

this||

{};

//Save the previous value of the `_` variable.

varpreviousUnderscore=root._;

//Save bytes in the minified (but not gzipped) version:

varArrayProto=Array.prototype, ObjProto=Object.prototype;

varSymbolProto=typeofSymbol!=='undefined'?Symbol.prototype:null;

//Create quick reference variables for speed access to core prototypes.

varpush=ArrayProto.push,

slice=ArrayProto.slice,

toString=ObjProto.toString,

hasOwnProperty=ObjProto.hasOwnProperty;

//All **ECMAScript 5** native function implementations that we hope to use

//are declared here.

varnativeIsArray=Array.isArray,

nativeKeys=Object.keys,

nativeCreate=Object.create;

//Naked function reference for surrogate-prototype-swapping.

varCtor=function(){};

//Create a safe reference to the Underscore object for use below.

var_=function(obj) {

if(objinstanceof_)returnobj;

if(!(thisinstanceof_))returnnew_(obj);

this._wrapped=obj;

};

//Export the Underscore object for **Node.js**, with

//backwards-compatibility for their old module API. If we're in

//the browser, add `_` as a global object.

//(`nodeType` is checked to ensure that `module`

//and `exports` are not HTML elements.)

if(typeofexports!='undefined'&&!exports.nodeType) {

if(typeofmodule!='undefined'&&!module.nodeType&&module.exports) {

exports=module.exports=_;

}

exports._=_;

}else{

root._=_;

}

//Current version.

_.VERSION='1.8.3';