underscore.js源码解析【'_'对象定义及内部函数】

(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.
  /*
    判断root是window/global/this
  */
  var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this;

  // Save the previous value of the `_` variable.
  /*
    保存全局下之前的'_'变量
    若变量冲突，则可以利用此变量进行恢复
  */
  var previousUnderscore = root._;

  // Save bytes in the minified (but not gzipped) version:
  /*
    原型赋值，以便压缩
  */
  var ArrayProto = Array.prototype, ObjProto = Object.prototype;
  var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;

  // Create quick reference variables for speed access to core prototypes.
  /*
    将内置对象原型中的常用方法赋值给引用变量，以便更方便的引用
  */
  var push = 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.
  /*
    定义了一些ECMAScript 5方法
  */
  var nativeIsArray = Array.isArray,
      nativeKeys = Object.keys,
      nativeCreate = Object.create;

  // Naked function reference for surrogate-prototype-swapping.
  /*
    定义一个裸函数
  */
  var Ctor = function(){};

  // Create a safe reference to the Underscore object for use below.
  /*
    创建'_'函数（对象）
  */
  var _ = function(obj) {
    /*
      如果'_'的prototype在obj的原型链上，直接返回obj
    */
    if (obj instanceof _) return obj;
    /*
      如果不在，new一个'_'对象
    */
    if (!(this instanceof _)) return new _(obj);
    /*
      如果都不是，则将obj的引用放在_.wrapped属性中????????????????????????????????????????????????????????????????
    */
    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.)
  /*
    判断宿主
  */
  if (typeof exports != 'undefined' && !exports.nodeType) {
    if (typeof module != 'undefined' && !module.nodeType && module.exports) {//node
      exports = module.exports = _;
    }
    exports._ = _;
  } else {//浏览器，放在window的属性下
    root._ = _;
  }

  // Current version.
  _.VERSION = '1.8.3';

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.
  /*
    用于内部使用的高阶函数，传入回调函数
    主要用来执行函数并改变所执行函数的作用域，最后加了一个argCount参数来指定参数个数
  */
  var optimizeCb = function(func, context, argCount) {
    if (context === void 0) return func;
    /*
      对参数个数小于等于4的情况进行分类处理
    */
    switch (argCount == null ? 3 : argCount) {
      case 1: return function(value) {
        return func.call(context, value);
      };
      // The 2-parameter case has been omitted only because no current consumers
      // made use of it.
      case 3: return function(value, index, collection) {
        return func.call(context, value, index, collection);
      };
      case 4: return function(accumulator, value, index, collection) {
        return func.call(context, accumulator, value, index, collection);
      };
    }
    /*
      当argCount不为null或1/3/4时，直接调用func.apply(context, arguments)
    */
    return function() {
      return func.apply(context, arguments);
    };
  };

  var builtinIteratee;

  // An internal function to generate callbacks that can be applied to each
  // element in a collection, returning the desired result — either `identity`,
  // an arbitrary callback, a property matcher, or a property accessor.
  /*
    判断参数类型
  */
  var cb = function(value, context, argCount) {
    if (_.iteratee !== builtinIteratee) return _.iteratee(value, context);
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);// 参数是函数，返回optimizeCb函数的执行结果
    if (_.isObject(value)) return _.matcher(value);// 参数是对象，则返回一个能判断对象是否相等的函数
    return _.property(value);// 默认返回一个获取对象属性的函数
  };

  // External wrapper for our callback generator. Users may customize
  // `_.iteratee` if they want additional predicate/iteratee shorthand styles.
  // This abstraction hides the internal-only argCount argument.
  _.iteratee = builtinIteratee = function(value, context) {
    return cb(value, context, Infinity);
  };

  // Similar to ES6's rest param (http://ariya.ofilabs.com/2013/03/es6-and-rest-parameter.html)
  // This accumulates the arguments passed into an array, after a given index.
  /*
    类似于es6中的rest param(function f(a, b, ...args) {})
    把startIndex后面的参数放到一个数组里，并作为参数传给fn
  */
  var restArgs = function(func, startIndex) {
    startIndex = startIndex == null ? func.length - 1 : +startIndex;//'+'相当于Number()
    return function() {
      var length = Math.max(arguments.length - startIndex, 0),
          rest = Array(length),
          index = 0;
      for (; index < length; index++) {
        rest[index] = arguments[index + startIndex];
      }
      switch (startIndex) {
        case 0: return func.call(this, rest);
        case 1: return func.call(this, arguments[0], rest);
        case 2: return func.call(this, arguments[0], arguments[1], rest);
      }
      /*
        将startIndex之前的参数放进数组中
      */
      var args = Array(startIndex + 1);
      for (index = 0; index < startIndex; index++) {
        args[index] = arguments[index];
      }
      args[startIndex] = rest;
      return func.apply(this, args);
    };
  };

  // An internal function for creating a new object that inherits from another.
  /*
    创建一个继承自另一个对象的新对象（原型继承）
  */
  var baseCreate = function(prototype) {
    if (!_.isObject(prototype)) return {};
    if (nativeCreate) return nativeCreate(prototype);//如果存在Object.create，则直接使用
    Ctor.prototype = prototype;
    var result = new Ctor;
    Ctor.prototype = null;
    return result;
  };

  /*
    获取对象及其原型链上的方法或属性
  */
  var property = function(key) {
    return function(obj) {
      return obj == null ? void 0 : obj[key];
    };
  };

    // Helper for collection methods to determine whether a collection
  // should be iterated as an array or as an object.
  // Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
  // Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
  /*
    判断类数组类型(有length属性，并且0 < length < MAX_ARRAY_INDEX)
  */
  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
  var getLength = property('length');
  var isArrayLike = function(collection) {
    var length = getLength(collection);
    return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
  };

......

}());

小结：

1.闭包

整个函数在一个闭包中，避免污染全局变量。通过传入this（其实就是window对象）来改变函数的作用域。和jquery的自执行函数其实是异曲同工之妙。这种传入全局变量的方式一方面有利于代码阅读，另一方面方便压缩。
underscore写法：

(function(){
    ...
}.call(this));

jquery写法：

(function(window, undefined) {
    ...
})(window);

2.格式

var
nativeIsArray      = Array.isArray,
nativeKeys         = Object.keys,
nativeBind         = FuncProto.bind,
nativeCreate       = Object.create;

3.高阶函数的使用

var property = function(key) {
    return function(obj) {
      return obj == null ? void 0 : obj[key];
    };
  };

参考资料：http://yalishizhude.github.io/2015/09/22/underscore-source/