underscore-notes.js

//     Underscore.js 1.8.3
//     http://underscorejs.org
//     (c) 2009-2015 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
//     Underscore may be freely distributed under the MIT license.

(function() {
  
  // Baseline setup
  // --------------

  // 保存全局对象
  // 在浏览器中是 window
  // 在服务端是 exports

  // Establish the root object, `window` in the browser, or `exports` on the server.
  var root = this;

  // 保存全局对象上的_属性
  // 在后面的noConflict会有用

  // Save the previous value of the `_` variable.
  var previousUnderscore = root._;

  // 保存Array、Object、Function的prototype引用
  // 方便压缩而不是gzip

  // Save bytes in the minified (but not gzipped) version:
  var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;

  // 保存一些常见的数组或者对象方法引用

  // Create quick reference variables for speed access to core prototypes.
  var
    push             = ArrayProto.push,
    slice            = ArrayProto.slice,
    toString         = ObjProto.toString,
    hasOwnProperty   = ObjProto.hasOwnProperty;

  // 保存es5中常见的部分方法

  // All **ECMAScript 5** native function implementations that we hope to use
  // are declared here.
  var
    nativeIsArray      = Array.isArray,
    nativeKeys         = Object.keys,
    nativeBind         = FuncProto.bind,
    nativeCreate       = Object.create;

  // Naked function reference for surrogate-prototype-swapping.
  var Ctor = function(){};

  // _构造函数,实现了非new调用也可以返回实例对象，可以想想jQuery的使用方式

  // Create a safe reference to the Underscore object for use below.
  var _ = function(obj) {
    if (obj instanceof _) return obj; // 如果obj已经是_的实例，则直接返回
    if (!(this instanceof _)) return new _(obj); // 如果调用方式是_()的形式则手动new _()调用返回
    this._wrapped = obj; // 将obj数据存放在_wrapped属性上方便后面使用
  };

  // 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.
  if (typeof exports !== 'undefined') {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

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

  // 一个经过了优化的回调函数返回函数
  // 我觉得其实目的就是两个
  // 1 绑定this作用域
  // 2 尽量使用指定参数，而不是arguments

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.
  var optimizeCb = function(func, context, argCount) {
    if (context === void 0) return func;
    switch (argCount == null ? 3 : argCount) {
      case 1: return function(value) {
        return func.call(context, value);
      };
      case 2: return function(value, other) {
        return func.call(context, value, other);
      };
      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);
      };
    }
    return function() {
      return func.apply(context, arguments);
    };
  };

  // 内部函数cb
  // 如果value为空，就返回一个返回参数自身的回调函数
  // 如果value为一个函数，就返回一个绑定了this作用域的回调函数
  // 如果value为一个对象，就返回一个是否匹配属性的函数
  // 否则返回一个读取对象value属性的回调函数

  // A mostly-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.
  var cb = function(value, context, argCount) {
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    if (_.isObject(value)) return _.matcher(value);
    return _.property(value);
  };

  // 一个重要的内部函数用来生成可应用到集合中每个元素的回调
  // 调用了内部函数cb，解析请看上个函数

  _.iteratee = function(value, context) {
    return cb(value, context, Infinity);
  };

  // 这个函数非常重要，为下文中的extend、extendOwn、assign、defaults提供生成的来源
  // extend 拷贝obj后的参数对象到obj上，并且相同的属性后者会覆盖前者(这里的拷贝包括原型上的)
  // extendOwn、assign拷贝obj后的参数对象到obj上，并且相同的属性后者会覆盖前者(这里的拷贝不包括原型上的)
  // defaults 拷贝obj后的参数对象到obj上，并且相同的属性只会进行一次覆盖即前者key对应的value为undefined的时候(这里的拷贝包括原型上的)
  // 特别注意下划线中的extend、extendOwn、assign、defaults的拷贝不是深复制

  // An internal function for creating assigner functions.
  var createAssigner = function(keysFunc, undefinedOnly) {
    return function(obj) {
      var length = arguments.length;
      if (length < 2 || obj == null) return obj; // 当参数只有obj一个或者obj为空时 直接返回obj
      for (var index = 1; index < length; index++) { // 从第二个参数开始拷贝
        var source = arguments[index], // 取出其中一个source
            keys = keysFunc(source), // keysFunc对应的是_.keys || _.allKeys
            l = keys.length;
        for (var i = 0; i < l; i++) { // 循环往obj中拷贝
          var key = keys[i];
          // 因为extend和extendOwn、assign传的undefinedOnly为undefined，而defaults传的是true，所以正好实现了几个函数对应的功能
          if (!undefinedOnly || obj[key] === void 0) obj[key] = source[key];
        }
      }
      return obj;
    };
  };

  // 常见的实现继承的方式之一

  // An internal function for creating a new object that inherits from another.
  var baseCreate = function(prototype) {
    if (!_.isObject(prototype)) return {}; // 如果prototype不是object类型直接返回空对象
    if (nativeCreate) return nativeCreate(prototype); // 如果原生支持create则用原生的
    Ctor.prototype = prototype; // 将prototype赋值为Ctor构造函数的原型
    var result = new Ctor; // 创建一个Ctor实例对象
    Ctor.prototype = null; // 为了下一次使用，将原型清空
    return result; // 最后将实例返回
  };

  // 返回一个能够读取obj对象的key属性的函数

  var property = function(key) {
    return function(obj) {
      return obj == null ? void 0 : obj[key]; // 如果传入的obj为null或者undefined就返回undefined,否则返回obj的key属性
    };
  };

  // 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
  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
  var getLength = property('length');

  // 判断对象是否为类数组
  // 判断的依据是对象含有的length属性值是否是有意义的正数
  // 其实函数和window对象都有length属性，所以这里判断是不够严谨的

  var isArrayLike = function(collection) {
    var length = getLength(collection);
    return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
  };

  // Collection Functions
  // --------------------

  // 模拟数组的forEach方式

  // The cornerstone, an `each` implementation, aka `forEach`.
  // Handles raw objects in addition to array-likes. Treats all
  // sparse array-likes as if they were dense.
  _.each = _.forEach = function(obj, iteratee, context) {
    iteratee = optimizeCb(iteratee, context); // 绑定作用后的函数
    var i, length;
    if (isArrayLike(obj)) { // 如果是类数组类型的obj
      for (i = 0, length = obj.length; i < length; i++) {
        iteratee(obj[i], i, obj);
      }
    } else { // 支持对象类型的数据迭代
      var keys = _.keys(obj);
      for (i = 0, length = keys.length; i < length; i++) {
        iteratee(obj[keys[i]], keys[i], obj);
      }
    }
    return obj;
  };

  // 模拟原生数组的map方法，不同之处在于其还可以对对象起作用

  // Return the results of applying the iteratee to each element.
  _.map = _.collect = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context); // 老规矩，先进行一下作用域绑定
    var keys = !isArrayLike(obj) && _.keys(obj),  // 非类数组对象就用keys读取obj的key
        length = (keys || obj).length,
        results = Array(length);
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index; // 数组是0，1，2 对象是对应的key
      results[index] = iteratee(obj[currentKey], currentKey, obj); // 拿到回调的返回值给result赋值
    }
    return results;
  };

  // 用来创建reduce、reduceRight的函数
  // 最后reduce、reduceRight不仅仅支持数组，对象也支持
  // 看着比较复杂，但其实在理解原生的reduce和reduceRight的基础上去看这个函数还是很清晰的

  // Create a reducing function iterating left or right.
  function createReduce(dir) {
    // Optimized iterator function as using arguments.length
    // in the main function will deoptimize the, see #1991.
    function iterator(obj, iteratee, memo, keys, index, length) { // 真正执行迭代的地方
      for (; index >= 0 && index < length; index += dir) {
        var currentKey = keys ? keys[index] : index; // 如果keys存在则认为是obj形式的参数，所以读取keys中的属性值，否则类数组只需要读取索引index即可
        memo = iteratee(memo, obj[currentKey], currentKey, obj); // 接着就是执行外部传入的回调了，并将结果赋值为memo，也就是我们最后要到的值
      }
      return memo;
    }

    return function(obj, iteratee, memo, context) {
      iteratee = optimizeCb(iteratee, context, 4); // 首先绑定一下this作用域
      var keys = !isArrayLike(obj) && _.keys(obj), // 如果不是类数组就读取其keys
          length = (keys || obj).length,
          index = dir > 0 ? 0 : length - 1; // 默认开始迭代的位置，从左边第一个开始还是右边第一个
      // Determine the initial value if none is provided.
      if (arguments.length < 3) { // 如果没有传入初始化值，则将第一个值(左边第一个或者右边第一个)作为初始值
        memo = obj[keys ? keys[index] : index];
        index += dir; // 从索引为1开始或者索引为length - 2开始迭代
      }
      return iterator(obj, iteratee, memo, keys, index, length); // 接着开始进入自定义的迭代函数，请往上看
    };
  }

  // 模拟数组的reduce函数，将数组或者对象中的元素进行处理，最后得到一个值

  // **Reduce** builds up a single result from a list of values, aka `inject`,
  // or `foldl`.
  _.reduce = _.foldl = _.inject = createReduce(1);

  // // 模拟数组的reduceRight函数，将数组或者对象中的元素进行处理，最后得到一个值

  // The right-associative version of reduce, also known as `foldr`.
  _.reduceRight = _.foldr = createReduce(-1);

  // 在obj中查找，返回第一个通过predicate迭代函数检测的元素值

  // Return the first value which passes a truth test. Aliased as `detect`.
  _.find = _.detect = function(obj, predicate, context) {
    var key;
    if (isArrayLike(obj)) {
      key = _.findIndex(obj, predicate, context);
    } else {
      key = _.findKey(obj, predicate, context);
    }
    if (key !== void 0 && key !== -1) return obj[key]; // 返回符合条件的value
  };

  // 模拟数组原生的filter方法
  // 原理很简单就是返回一个新的数组，数组中的值是回调中经过帅选条件后的值

  // Return all the elements that pass a truth test.
  // Aliased as `select`.
  _.filter = _.select = function(obj, predicate, context) {
    var results = [];
    predicate = cb(predicate, context);
    _.each(obj, function(value, index, list) {
      if (predicate(value, index, list)) results.push(value);
    });
    return results;
  };

  // 与filter功能相反
  // 过滤出不满足的条件的元素

  // Return all the elements for which a truth test fails.
  _.reject = function(obj, predicate, context) {
    return _.filter(obj, _.negate(cb(predicate)), context);
  };

  // 判断obj中的所有元素是不是都满足一个条件
  // 如果都满足就返回true
  // 只要有一个不满足就返回false

  // Determine whether all of the elements match a truth test.
  // Aliased as `all`.
  _.every = _.all = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = !isArrayLike(obj) && _.keys(obj), // 短路写法，非类数组则获取其keys
        length = (keys || obj).length; // 获取obj的length
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index; // keys若能转化为"真" 则说明obj是对象类型
      if (!predicate(obj[currentKey], currentKey, obj)) return false; // 只要有一个不满足就返回false，中断迭代
    }
    return true;
  };

  // 判断obj中是不是至少有一个元素满足一个条件
  // 如果有至少一个满足就返回true
  // 所有都不满足则返回false

  // Determine if at least one element in the object matches a truth test.
  // Aliased as `any`.
  _.some = _.any = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = !isArrayLike(obj) && _.keys(obj),
        length = (keys || obj).length;
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index;
      if (predicate(obj[currentKey], currentKey, obj)) return true; // 只要有一个满足条件就返回true
    }
    return false;
  };

  // 判断obj中是否含有item这个值

  // Determine if the array or object contains a given item (using `===`).
  // Aliased as `includes` and `include`.
  _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) {
    if (!isArrayLike(obj)) obj = _.values(obj); // 如果obj不是类数组，就取对象的值拼成数组
    if (typeof fromIndex != 'number' || guard) fromIndex = 0; // 默认fromIndex传入的不是数字就从0的位置开始查找，(guard后续分析到再看)
    return _.indexOf(obj, item, fromIndex) >= 0; // 判断调用indexOf方法返回查找后的索引值是否大于0(indexOf调用后不存在返回-1)
  };

  // 在obj的每个元素上执行method方法，第三个参数及之后的每个参数都会传递给method方法
  // method方法执行时，若没有特殊的处理，内部的this，是obj的每一项

  // Invoke a method (with arguments) on every item in a collection.
  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    var isFunc = _.isFunction(method); // 先判断一下method是不是一个函数
    return _.map(obj, function(value) {
      var func = isFunc ? method : value[method]; // 如果是函数就直接用，否则去value上读取
      return func == null ? func : func.apply(value, args); // 最后执行对应的方法，并将结果返回
    });
  };

  // 可以看做是map的简化版，用来获取数组中对象的某一属性值，最后返回该属性值的集合

  // Convenience version of a common use case of `map`: fetching a property.
  _.pluck = function(obj, key) {
    return _.map(obj, _.property(key)); // _.property(key)函数，返回一个能够获取obj对象key属性的函数
  };

  // 遍历obj中的每一个值, 返回一个数组
  // 数组中得到的值有一个特性，这些值都包含attrs的值

  // Convenience version of a common use case of `filter`: selecting only objects
  // containing specific `key:value` pairs.
  _.where = function(obj, attrs) {
    return _.filter(obj, _.matcher(attrs)); // matcher执行后返回的是一个函数
  };

  //遍历obj返回第一个包含attrs值的值

  // Convenience version of a common use case of `find`: getting the first object
  // containing specific `key:value` pairs.
  _.findWhere = function(obj, attrs) {
    return _.find(obj, _.matcher(attrs));
  };

  // 返回obj中的最大值。
  // 如果传递iteratee参数，iteratee将作为obj中每个值的排序依据。
  // 如果obj为空，将返回-Infinity，所以你可能需要事先用isEmpty检查 obj

  // Return the maximum element (or element-based computation).
  _.max = function(obj, iteratee, context) {
    var result = -Infinity, lastComputed = -Infinity,
        value, computed;
    if (iteratee == null && obj != null) { // 针对没有传入iteratee的情况
      obj = isArrayLike(obj) ? obj : _.values(obj); // 如果是类数组则直接进行下面的比较，否则拿到obj的values值得集合
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value > result) {
          result = value;
        }
      }
    } else { // 如果传了回调函数
      iteratee = cb(iteratee, context); // 绑定this作用域
      _.each(obj, function(value, index, list) {
        computed = iteratee(value, index, list); // 获取计算后的值
        if (computed > lastComputed || computed === -Infinity && result === -Infinity) { // 最后一个或运算是为了保证例如[{age: -Infinity}]类型，最后需要返回{age: -Infinity}，而不是-Infinity
          result = value;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // 返回obj中的小值。
  // 如果传递iteratee参数，iteratee将作为obj中每个值的排序依据。
  // 如果obj为空，将返回-Infinity，所以你可能需要事先用isEmpty检查 obj
  // 分析方法和max几乎一样

  // Return the minimum element (or element-based computation).
  _.min = function(obj, iteratee, context) {
    var result = Infinity, lastComputed = Infinity,
        value, computed;
    if (iteratee == null && obj != null) {
      obj = isArrayLike(obj) ? obj : _.values(obj);
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value < result) {
          result = value;
        }
      }
    } else {
      iteratee = cb(iteratee, context);
      _.each(obj, function(value, index, list) {
        computed = iteratee(value, index, list);
        if (computed < lastComputed || computed === Infinity && result === Infinity) {
          result = value;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // 返回一个随机打乱的obj副本(最后的返回值是一个数组)

  // Shuffle a collection, using the modern version of the
  // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
  _.shuffle = function(obj) {
    var set = isArrayLike(obj) ? obj : _.values(obj); // 如果是一个类数组，就直接用obj，否则读取其values
    var length = set.length;
    var shuffled = Array(length);
    for (var index = 0, rand; index < length; index++) {
      rand = _.random(0, index); // 这里很关键每次都是随机生成[0, index)中的某个值，index为0的时候就只能为0了
      if (rand !== index) shuffled[index] = shuffled[rand]; // 当随机出来的rand值和当前的index值不相等的时候，就把shuffled中index位置的值设置为rand位置的值
      shuffled[rand] = set[index]; // 把shuffled中随机出来的rand位置的值设置为obj中index位置的值
    }
    return shuffled;
  };

  // 从obj中返回一个随机打乱的obj副本
  // 如果没有传入n，则随机返回obj中的一个值
  // 如果传入了先随机打乱，再返回前n个值

  // Sample **n** random values from a collection.
  // If **n** is not specified, returns a single random element.
  // The internal `guard` argument allows it to work with `map`.
  _.sample = function(obj, n, guard) {
    if (n == null || guard) {
      if (!isArrayLike(obj)) obj = _.values(obj);
      return obj[_.random(obj.length - 1)]; // 对应返回其中一个值的情况
    }
    return _.shuffle(obj).slice(0, Math.max(0, n)); // 对应返回n个随机值的情况
  };

  // 返回一个排序之后的obj的副本

  // Sort the object's values by a criterion produced by an iteratee.
  _.sortBy = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    return _.pluck(_.map(obj, function(value, index, list) {
      return {
        value: value,
        index: index,
        criteria: iteratee(value, index, list)
      };
    }).sort(function(left, right) { // 这里后续还需要自己研读
      var a = left.criteria;
      var b = right.criteria;
      if (a !== b) {
        if (a > b || a === void 0) return 1;
        if (a < b || b === void 0) return -1;
      }
      return left.index - right.index;
    }), 'value');
  };

  // An internal function used for aggregate "group by" operations.
  var group = function(behavior) {
    return function(obj, iteratee, context) {
      var result = {};
      iteratee = cb(iteratee, context);
      _.each(obj, function(value, index) {
        var key = iteratee(value, index, obj);
        behavior(result, value, key);
      });
      return result;
    };
  };

  // 将一个集合分组为多个集合
  // 通过 iterator 返回的结果进行分组
  // 如果 iterator 是一个字符串而不是函数, 那么将使用 iterator 作为各元素的属性名来对比进行分组.

  // Groups the object's values by a criterion. Pass either a string attribute
  // to group by, or a function that returns the criterion.
  _.groupBy = group(function(result, value, key) {
    if (_.has(result, key)) result[key].push(value); else result[key] = [value];
  });

  // 和groupBy非常像
  // 不同的是如果知道obj的键是唯一的时候，可以用indexBy

  // Indexes the object's values by a criterion, similar to `groupBy`, but for
  // when you know that your index values will be unique.
  _.indexBy = group(function(result, value, key) {
    result[key] = value;
  });

  // 返回各组中的对象的数量的计数

  // Counts instances of an object that group by a certain criterion. Pass
  // either a string attribute to count by, or a function that returns the
  // criterion.
  _.countBy = group(function(result, value, key) {
    if (_.has(result, key)) result[key]++; else result[key] = 1;
  });

  // 将obj转化为数组

  // Safely create a real, live array from anything iterable.
  _.toArray = function(obj) {
    if (!obj) return [];
    if (_.isArray(obj)) return slice.call(obj); // 如果是数组 拷贝一份返回
    if (isArrayLike(obj)) return _.map(obj, _.identity); // 如果是类数组，使用map操作之后的数组返回
    return _.values(obj); // 其他情况获取values返回
  };

  // 返回obj的长度
  // 如果是类数组，直接返回length长度
  // 否则先获取keys，在返回长度

  // Return the number of elements in an object.
  _.size = function(obj) {
    if (obj == null) return 0;
    return isArrayLike(obj) ? obj.length : _.keys(obj).length;
  };

  // 分区：拆分一个数组为一个含有两个数组元素的二维数组
  // 其中第一个数组是通过了predicate判断的
  // 第二个则是未通过的

  // Split a collection into two arrays: one whose elements all satisfy the given
  // predicate, and one whose elements all do not satisfy the predicate.
  _.partition = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var pass = [], fail = []; // 准备好了两个容器来装通过和未通过的元素
    _.each(obj, function(value, key, obj) {
      (predicate(value, key, obj) ? pass : fail).push(value); // 通过 ？ 往pass里面添加元素 : 往fail里面添加
    });
    return [pass, fail];
  };

  // Array Functions
  // ---------------

  // 一个能够返回数组前n个元素的函数，默认是1

  // Get the first element of an array. Passing **n** will return the first N
  // values in the array. Aliased as `head` and `take`. The **guard** check
  // allows it to work with `_.map`.
  _.first = _.head = _.take = function(array, n, guard) {
    if (array == null) return void 0; // 如果array为空直接返回undefined
    if (n == null || guard) return array[0]; //如果没传n，直接返回数组第一个值
    return _.initial(array, array.length - n); // 结合initial，相当于(slice(0, array.length - (array.length - n))) => slice(0, n)
  };

  // 一个能够返回排除数组后面n个元素的数组的函数，默认是1
  // (n == null || guard ? 1 : n) 的结果是1或者n，至于n有什么作用到后面我们会说！！！先埋一个坑
  // Math.max其实是防止(array.length - x)减出来是个负数，如果是负数则取0

  // Returns everything but the last entry of the array. Especially useful on
  // the arguments object. Passing **n** will return all the values in
  // the array, excluding the last N.
  _.initial = function(array, n, guard) {
    return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
  };

  // 一个获取数组最后n个元素构成的数组，默认是最后一位

  // Get the last element of an array. Passing **n** will return the last N
  // values in the array.
  _.last = function(array, n, guard) {
    if (array == null) return void 0;
    if (n == null || guard) return array[array.length - 1];
    return _.rest(array, Math.max(0, array.length - n));
  };

  // 一个能够返回数组第n个元素之后的元素构成的数组，注意不包含第n个元素

  // Returns everything but the first entry of the array. Aliased as `tail` and `drop`.
  // Especially useful on the arguments object. Passing an **n** will return
  // the rest N values in the array.
  _.rest = _.tail = _.drop = function(array, n, guard) {
    return slice.call(array, n == null || guard ? 1 : n);
  };

  // 返回筛选掉数组的值为false副本
  // 这个时候_.identity的作用体现了

  // Trim out all falsy values from an array.
  _.compact = function(array) {
    return _.filter(array, _.identity);
  };

  // 将一个嵌套多层的数组转化为只有一层的数组，如果你传递 shallow参数，数组将进行一维的嵌套

  // Internal implementation of a recursive `flatten` function.
  var flatten = function(input, shallow, strict, startIndex) {
    var output = [], idx = 0;
    for (var i = startIndex || 0, length = getLength(input); i < length; i++) { // 对数组进行遍历
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) { // 如果是数组或者arguments对象则进行进一步判断
        //flatten current level of array or arguments object
        if (!shallow) value = flatten(value, shallow, strict); // shallow决定是否进行深度铺平，如果为false，则进行递归，一层层铺平
        var j = 0, len = value.length; // 否则直接将数组的值，添加在output上，而不会再理会value中是否有值为数组
        output.length += len;
        while (j < len) {
          output[idx++] = value[j++];
        }
      } else if (!strict) { // 否则直接给output赋值，并且给idx索引增1
        output[idx++] = value;
      }
    }
    return output;
  };

  // Flatten out an array, either recursively (by default), or just one level.
  _.flatten = function(array, shallow) {
    return flatten(array, shallow, false);
  };

  // 返回一个删除所有values值后的 array副本
  // 作用何difference类似，只不过difference要求只穿函数

  // Return a version of the array that does not contain the specified value(s).
  _.without = function(array) {
    return _.difference(array, slice.call(arguments, 1));
  };

  // 返回 array去重后的副本, 使用 === 做相等测试.
  // 如果您确定 array 已经排序, 那么给 isSorted 参数传递 true值, 此函数将运行的更快的算法.
  // 如果要处理对象元素, 传递 iteratee函数来获取要对比的属性.

  // Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  _.uniq = _.unique = function(array, isSorted, iteratee, context) {
    if (!_.isBoolean(isSorted)) {
      context = iteratee;
      iteratee = isSorted;
      isSorted = false;
    }
    if (iteratee != null) iteratee = cb(iteratee, context);
    var result = [];
    var seen = []; // 这里之前做一些参数处理
    for (var i = 0, length = getLength(array); i < length; i++) {
      var value = array[i],
          computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted) { // 因为已经确定了是排序过了 所以只需要和上一个数比较就好
        if (!i || seen !== computed) result.push(value);
        seen = computed;
      } else if (iteratee) { // 如果是回调函数的形式
        if (!_.contains(seen, computed)) { // seen用来缓存已经比较过的值
          seen.push(computed);
          result.push(value); // 缓存seen中没有则将数组中的值添加进去，从而达到去重
        }
      } else if (!_.contains(result, value)) { // 这种场景最简单 _.uniq([1, 2, 1, 3, 1, 4]) => [1, 2, 3, 4]
        result.push(value);
      }
    }
    return result;
  };

  // 返回传入的数组的并集
  // 原理很简单，先将所有传入的数组铺平得到一维的数组
  // 再进行去重，便得到并集

  // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  _.union = function() {
    return _.uniq(flatten(arguments, true, true));
  };

  // 返回传入 arrays（数组）交集
  // 既然是交集，肯定返回值在每个传入的数组中都存在
  // 其实我想是不是先找出传入的数组中length最短的那个，在进行后续的比较整个计算时间会短一点？

  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  _.intersection = function(array) {
    var result = [];
    var argsLength = arguments.length;
    for (var i = 0, length = getLength(array); i < length; i++) {
      var item = array[i];
      if (_.contains(result, item)) continue;
      for (var j = 1; j < argsLength; j++) {
        if (!_.contains(arguments[j], item)) break;
      }
      if (j === argsLength) result.push(item);
    }
    return result;
  };

  // 返回array中，其他的数组中不存在的值

  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  _.difference = function(array) {
    var rest = flatten(arguments, true, true, 1); // 先将第一个参数之后的输入值铺平为一维数组
    return _.filter(array, function(value){
      return !_.contains(rest, value); // 帅选array的值，不在铺平后的rest中
    });
  };

  // 和unzip的功能是一致的，区别在于使用方式
  // _.unzip([[], [], []])
  // _.zip([], [], [])

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  _.zip = function() {
    return _.unzip(arguments);
  };

  // 将每个arrays中相应位置的值合并在一起,类似下面的用法
  // [['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]]
  //  [["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]

  // Complement of _.zip. Unzip accepts an array of arrays and groups
  // each array's elements on shared indices
  _.unzip = function(array) {
    var length = array && _.max(array, getLength).length || 0;
    var result = Array(length);

    for (var index = 0; index < length; index++) {
      result[index] = _.pluck(array, index);
    }
    return result;
  };

  // 将数组转化为对象
  // 1. ([[], []])
  // 2. ([], [])

  // Converts lists into objects. Pass either a single array of `[key, value]`
  // pairs, or two parallel arrays of the same length -- one of keys, and one of
  // the corresponding values.
  _.object = function(list, values) {
    var result = {};
    for (var i = 0, length = getLength(list); i < length; i++) {
      if (values) { // 对应情况2的调用方式
        result[list[i]] = values[i];
      } else { // 对应情况1的调用方式
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

  // 创建findIndex和findLastIndex的函数封装
  // findIndex从0开始查找
  // findLastIndex从数组的length - 1开始查找

  // Generator function to create the findIndex and findLastIndex functions
  function createPredicateIndexFinder(dir) {
    return function(array, predicate, context) {
      predicate = cb(predicate, context);
      var length = getLength(array);
      var index = dir > 0 ? 0 : length - 1;
      for (; index >= 0 && index < length; index += dir) {
        if (predicate(array[index], index, array)) return index;
      }
      return -1;
    };
  }

  // Returns the first index on an array-like that passes a predicate test
  _.findIndex = createPredicateIndexFinder(1);
  _.findLastIndex = createPredicateIndexFinder(-1);

  // 使用二分查找法找寻指定值在array中的位置
  // 要搜索的obj不一定在array中，返回的index是值比较近的较小的index

  // Use a comparator function to figure out the smallest index at which
  // an object should be inserted so as to maintain order. Uses binary search.
  _.sortedIndex = function(array, obj, iteratee, context) {
    iteratee = cb(iteratee, context, 1);
    var value = iteratee(obj);
    var low = 0, high = getLength(array);
    while (low < high) {
      var mid = Math.floor((low + high) / 2);
      if (iteratee(array[mid]) < value) low = mid + 1; else high = mid;
    }
    return low;
  };

  // 创建indexOf和lastIndexOf的函数封装

  // Generator function to create the indexOf and lastIndexOf functions
  function createIndexFinder(dir, predicateFind, sortedIndex) {
    return function(array, item, idx) {
      var i = 0, length = getLength(array);
      if (typeof idx == 'number') {
        if (dir > 0) {
            i = idx >= 0 ? idx : Math.max(idx + length, i);
        } else {
            length = idx >= 0 ? Math.min(idx + 1, length) : idx + length + 1;
        }
      } else if (sortedIndex && idx && length) {
        idx = sortedIndex(array, item);
        return array[idx] === item ? idx : -1;
      }
      if (item !== item) {
        idx = predicateFind(slice.call(array, i, length), _.isNaN);
        return idx >= 0 ? idx + i : -1;
      }
      for (idx = dir > 0 ? i : length - 1; idx >= 0 && idx < length; idx += dir) {
        if (array[idx] === item) return idx;
      }
      return -1;
    };
  }

  // Return the position of the first occurrence of an item in an array,
  // or -1 if the item is not included in the array.
  // If the array is large and already in sort order, pass `true`
  // for **isSorted** to use binary search.
  _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
  _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);

  // 一个用来创建整数灵活编号的函数
  // _.range(10) [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
  // _.range(0, 10, 3) [ 0, 3, 6, 9 ]

  // Generate an integer Array containing an arithmetic progression. A port of
  // the native Python `range()` function. See
  // [the Python documentation](http://docs.python.org/library/functions.html#range).
  _.range = function(start, stop, step) {
    if (stop == null) {
      stop = start || 0;
      start = 0;
    }
    step = step || 1; // 做一些参数的处理

    var length = Math.max(Math.ceil((stop - start) / step), 0); // Math.ceil处理针对(10/3)除不尽的情况，并且需要向上取整
    var range = Array(length);

    for (var idx = 0; idx < length; idx++, start += step) {
      range[idx] = start;
    }

    return range;
  };

  // Function (ahem) Functions
  // ------------------

  // Determines whether to execute a function as a constructor
  // or a normal function with the provided arguments
  var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
    if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args); // 如果调用方式不是new func的形式就直接调用sourceFunc，并且给到对应的参数即可
    var self = baseCreate(sourceFunc.prototype); // 处理new调用的形式
    var result = sourceFunc.apply(self, args);
    if (_.isObject(result)) return result;
    return self;
  };

  // 绑定函数 func 到对象 context 上,
  // 也就是无论何时调用函数, 函数里的 this 都指向这个 context.

  // Create a function bound to a given object (assigning `this`, and arguments,
  // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
  // available.
  _.bind = function(func, context) {
    if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1)); // 如果原生支持bind函数，就用原生的，并将对应的参数传进去
    if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function'); // 如果传入的func不是一个函数类型 就抛出异常
    var args = slice.call(arguments, 2); // 把第三个参数以后的值存起来，接下来请看executeBound
    var bound = function() {
      return executeBound(func, bound, context, this, args.concat(slice.call(arguments)));
    };
    return bound;
  };

  // 局部应用一个函数填充在任意个数的arguments
  // 不改变其动态this值,和bind方法很相近
  // 可以在partial调用的时传进_来占位，进而用在传进的回调函数实际参数进行替换


  // Partially apply a function by creating a version that has had some of its
  // arguments pre-filled, without changing its dynamic `this` context. _ acts
  // as a placeholder, allowing any combination of arguments to be pre-filled.
  _.partial = function(func) {
    var boundArgs = slice.call(arguments, 1); // 获取除了传进回调函数之外的其他参数
    var bound = function() {
      var position = 0, length = boundArgs.length;
      var args = Array(length); // 先创建一个和boundArgs长度等长的空数组
      for (var i = 0; i < length; i++) { // 处理占位元素_
        args[i] = boundArgs[i] === _ ? arguments[position++] : boundArgs[i]; // 如果发现boundArgs中有_的占位元素，就依次用arguments中的元素进行替换boundArgs
      }
      while (position < arguments.length) args.push(arguments[position++]); // 把auguments中的其他元素添加到boundArgs中
      return executeBound(func, bound, this, this, args);
    };
    return bound;
  };

  // 把methodNames参数指定的一些方法绑定到object上，这些方法就会在对象的上下文环境中执行

  // Bind a number of an object's methods to that object. Remaining arguments
  // are the method names to be bound. Useful for ensuring that all callbacks
  // defined on an object belong to it.
  _.bindAll = function(obj) {
    var i, length = arguments.length, key;
    if (length <= 1) throw new Error('bindAll must be passed function names');
    for (i = 1; i < length; i++) { // 从第一个实参开始处理，这些便是需要绑定this作用域到obj的函数
      key = arguments[i];
      obj[key] = _.bind(obj[key], obj); // 调用内部的bind方法进行this绑定
    }
    return obj;
  };

  // memoize函数可以缓存某函数处理之后的结果，对于耗时较长的操作很有帮助
  // 如果传递了 hashFunction 参数，就用 hashFunction 的返回值作为key存储函数的计算结果
  // hashFunction 默认使用function的第一个参数作为key

  // Memoize an expensive function by storing its results.
  _.memoize = function(func, hasher) {
    var memoize = function(key) {
      var cache = memoize.cache;
      var address = '' + (hasher ? hasher.apply(this, arguments) : key); // 注意hasher，如果传了hasher，就用hasher()执行的结果作为缓存func()执行的结果的key
      if (!_.has(cache, address)) cache[address] = func.apply(this, arguments); // 如果没有在cache中查找到对应的key就去计算一次，并缓存下来
      return cache[address];
    };
    memoize.cache = {};
    return memoize; // 返回一个具有cache静态属性的函数
  };

  // 类似setTimeout，等待wait秒之后调用func。
  // 如果传递可选的参数arguments，当函数function执行时， arguments 会作为参数传入

  // Delays a function for the given number of milliseconds, and then calls
  // it with the arguments supplied.
  _.delay = function(func, wait) {
    var args = slice.call(arguments, 2); // 读取第三个参数开始的其他参数
    return setTimeout(function(){
      return func.apply(null, args); // 执行func并将参数传入
    }, wait);
  };

  // 延迟调用function
  // 这里使用了_.partial， _.delay，并且有一个_作为函数占位，等待_.defer实际传入
  // 类似使用延时为1的setTimeout方法。对于执行开销大的计算和无阻塞UI线程的HTML渲染时候非常有用

  // Defers a function, scheduling it to run after the current call stack has
  // cleared.
  _.defer = _.partial(_.delay, _, 1);

  // Returns a function, that, when invoked, will only be triggered at most once
  // during a given window of time. Normally, the throttled function will run
  // as much as it can, without ever going more than once per `wait` duration;
  // but if you'd like to disable the execution on the leading edge, pass
  // `{leading: false}`. To disable execution on the trailing edge, ditto.

  // 函数节流，非常重要。

  _.throttle = function(func, wait, options) {
    var context, args, result;
    // 存储定时器
    var timeout = null;
    // 记录上一次返回的节流函数执行时间
    var previous = 0;
    if (!options) options = {};
    var later = function() {
      previous = options.leading === false ? 0 : _.now();
      timeout = null;
      result = func.apply(context, args);
      if (!timeout) context = args = null;
    };
    return function() {
      var now = _.now();
      // 默认是会尽快执行传入的函数，但是如果传入leading为false则反之
      if (!previous && options.leading === false) previous = now;
      // 计算当前执行时间距离上次执行是否已经到了wait时间
      var remaining = wait - (now - previous);
      context = this;
      args = arguments;
      // remaining <= 0表示已经超过或者正好等于wait时间
      if (remaining <= 0 || remaining > wait) {
        // 定时器如果存在会先将其清除
        if (timeout) {
          clearTimeout(timeout);
          timeout = null;
        }
        // 将previous保存为当前时间，并执行callback
        previous = now;
        result = func.apply(context, args);
        if (!timeout) context = args = null;
      } else if (!timeout && options.trailing !== false) {
        timeout = setTimeout(later, remaining);
      }
      return result;
    };
  };

  // Returns a function, that, as long as it continues to be invoked, will not
  // be triggered. The function will be called after it stops being called for
  // N milliseconds. If `immediate` is passed, trigger the function on the
  // leading edge, instead of the trailing.
  _.debounce = function(func, wait, immediate) {
    var timeout, args, context, timestamp, result;

    var later = function() {
      var last = _.now() - timestamp;

      if (last < wait && last >= 0) {
        timeout = setTimeout(later, wait - last);
      } else {
        timeout = null;
        if (!immediate) {
          result = func.apply(context, args);
          if (!timeout) context = args = null;
        }
      }
    };

    return function() {
      context = this;
      args = arguments;
      timestamp = _.now();
      var callNow = immediate && !timeout;
      if (!timeout) timeout = setTimeout(later, wait);
      if (callNow) {
        result = func.apply(context, args);
        context = args = null;
      }

      return result;
    };
  };

  // 将第一个函数func封装到第二个函数wrapper里面
  // 可以在wrapper中的前面或者后面去执行func

  // Returns the first function passed as an argument to the second,
  // allowing you to adjust arguments, run code before and after, and
  // conditionally execute the original function.
  _.wrap = function(func, wrapper) {
    return _.partial(wrapper, func);
  };

  // Returns a negated version of the passed-in predicate.
  _.negate = function(predicate) {
    return function() {
      return !predicate.apply(this, arguments);
    };
  };

  // 返回函数集 functions 组合后的复合函数
  // 感觉这里有点类似管道的思想，前一个pipe的输出作为后一个pipe的输入，最后处理完成再将结果返回

  // Returns a function that is the composition of a list of functions, each
  // consuming the return value of the function that follows.
  _.compose = function() {
    var args = arguments;
    var start = args.length - 1; // 从最后一个参数开始处理
    return function() {
      var i = start;
      var result = args[start].apply(this, arguments); // 执行最后一个函数
      while (i--) result = args[i].call(this, result); // 从后往前一个个调用传进来的函数，并将上一次执行的结果作为参数传进下一个函数
      return result;
    };
  };

  // 创建一个函数只有在运行了times次之后，func才会执行
  // 如果在处理多个异步请求的时候，我们需要确保所有的请求都完成之后才执行func这个函数,_.after将非常有用

  // Returns a function that will only be executed on and after the Nth call.
  _.after = function(times, func) {
    return function() {
      if (--times < 1) {
        return func.apply(this, arguments);
      }
    };
  };

  // 创建一个函数，这个函数调用次数不超过times次
  // 如果次数 >= times 则最后一次调用函数的返回值将被记住并一直返回该值

  // Returns a function that will only be executed up to (but not including) the Nth call.
  _.before = function(times, func) {
    var memo;
    return function() {
      if (--times > 0) {
        memo = func.apply(this, arguments);
      }
      if (times <= 1) func = null;
      return memo;
    };
  };

  // 创建一个只能调用一次的函数
  // 重复调用改进的方法也没有效果，只会返回第一次执行时的结果。
  // 作为初始化函数使用时非常有用, 不用再设一个boolean值来检查是否已经初始化完成.
  // 突然想起自己现在维护的公司的一个项目初始化操作就可以用这个方法,太赞

  // Returns a function that will be executed at most one time, no matter how
  // often you call it. Useful for lazy initialization.
  _.once = _.partial(_.before, 2);

  // Object Functions
  // ----------------

  // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString'); // 判断浏览器是否存在枚举bug，如果有，在取反操作前会返回false
  var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
                      'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString']; // 所有需要处理的可能存在枚举问题的属性

  // 处理ie9以下的一个枚举bug

  function collectNonEnumProps(obj, keys) {
    var nonEnumIdx = nonEnumerableProps.length;
    var constructor = obj.constructor;
    var proto = (_.isFunction(constructor) && constructor.prototype) || ObjProto;  // 读取obj的原型

    // Constructor is a special case.
    var prop = 'constructor'; // 这里我也有个疑问，对于constructor属性为什么要单独处理？
    if (_.has(obj, prop) && !_.contains(keys, prop)) keys.push(prop);

    while (nonEnumIdx--) {
      prop = nonEnumerableProps[nonEnumIdx];
      if (prop in obj && obj[prop] !== proto[prop] && !_.contains(keys, prop)) { // nonEnumerableProps中的属性出现在obj中，并且不是原型中，再者keys中不存在，就添加进去
        keys.push(prop);
      }
    }
  }

  // 获取对象obj的所有键
  // 这里的键不包括原型上的

  // Retrieve the names of an object's own properties.
  // Delegates to **ECMAScript 5**'s native `Object.keys`
  _.keys = function(obj) {
    if (!_.isObject(obj)) return []; // 如果obj不是object类型直接返回空数组
    if (nativeKeys) return nativeKeys(obj); // 如果浏览器支持原生的keys方法，则使用原生的keys
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys.push(key); // 注意这里1、for in会遍历原型上的键，所以用_.has来确保读取的只是对象本身的属性
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys); // 这里主要处理ie9以下的浏览器的bug，会将对象上一些本该枚举的属性认为不可枚举，详细可以看collectNonEnumProps分析
    return keys;
  };

  // 获取对象obj的所有的键
  // 与keys不同，这里包括继承来的key

  // Retrieve all the property names of an object.
  _.allKeys = function(obj) {
    if (!_.isObject(obj)) return [];
    var keys = [];
    for (var key in obj) keys.push(key); // 直接读遍历取到的key，包括原型上的
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys); // 同样处理一下有枚举问题的浏览器
    return keys;
  };

  // 获取对象obj所有值(上面是获取键)
  // 注意这里不包括原型上的值，仅仅是obj自身的属性

  // Retrieve the values of an object's properties.
  _.values = function(obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var values = Array(length);
    for (var i = 0; i < length; i++) {
      values[i] = obj[keys[i]];
    }
    return values;
  };

  // 作用类似于map，不同的是map返回的是数组
  // mapObject返回的是对象

  // Returns the results of applying the iteratee to each element of the object
  // In contrast to _.map it returns an object
  _.mapObject = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys =  _.keys(obj),
          length = keys.length,
          results = {},
          currentKey;
      for (var index = 0; index < length; index++) {
        currentKey = keys[index];
        results[currentKey] = iteratee(obj[currentKey], currentKey, obj);  // 将原obj的每个key的值转化为经过iteratee处理后的值
      }
      return results;
  };

  // 把一个对象转变为一个[key, value]形式的数组。
  // 实现的思路很简单，获取obj的所有key，生成一个等长的数组，然后进行[key, value]循环操作

  // Convert an object into a list of `[key, value]` pairs.
  _.pairs = function(obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var pairs = Array(length);
    for (var i = 0; i < length; i++) {
      pairs[i] = [keys[i], obj[keys[i]]];
    }
    return pairs;
  };

  // 返回一个object副本，使其键（keys）和值（values）对换
  // 注意这里必须确保object里所有的值都是唯一的且可以序列号成字符串.

  // Invert the keys and values of an object. The values must be serializable.
  _.invert = function(obj) {
    var result = {};
    var keys = _.keys(obj);
    for (var i = 0, length = keys.length; i < length; i++) {
      result[obj[keys[i]]] = keys[i];
    }
    return result;
  };

  // 返回对象obj中经过排序的函数集

  // Return a sorted list of the function names available on the object.
  // Aliased as `methods`
  _.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key); // 是函数，就装载进去
    }
    return names.sort(); // 最后返回经过排序的数组
  };

  // Extend a given object with all the properties in passed-in object(s).
  _.extend = createAssigner(_.allKeys);

  // Assigns a given object with all the own properties in the passed-in object(s)
  // (https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/assign)
  _.extendOwn = _.assign = createAssigner(_.keys);

  // 对象版本的findIndex
  // 如果对象obj中有符合predicate函数执行后的值则将key返回

  // Returns the first key on an object that passes a predicate test
  _.findKey = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = _.keys(obj), key;
    for (var i = 0, length = keys.length; i < length; i++) { // 对对象进行遍历，有符合predicate函数执行的value，则将key返回
      key = keys[i];
      if (predicate(obj[key], key, obj)) return key;
    }
  };

  // 返回object的一个副本
  // 特征是过滤出pick(obj, 'key1', 'key2'), key1, key2指定的属性值
  // 或者接收一个函数来指定挑选哪一个

  // Return a copy of the object only containing the whitelisted properties.
  _.pick = function(object, oiteratee, context) {
    var result = {}, obj = object, iteratee, keys;
    if (obj == null) return result;
    if (_.isFunction(oiteratee)) { // 如果oiteratee是一个函数
      keys = _.allKeys(obj); // 取出obj所有的key，包括原上的
      iteratee = optimizeCb(oiteratee, context); // 绑定一下this作用域，返回的还是一个函数
    } else {
      keys = flatten(arguments, false, false, 1); // 否则将arguments铺平，得到要挑选的key
      iteratee = function(value, key, obj) { return key in obj; };
      obj = Object(obj);
    }
    for (var i = 0, length = keys.length; i < length; i++) { // 进行循环处理
      var key = keys[i];
      var value = obj[key];
      if (iteratee(value, key, obj)) result[key] = value;
    }
    return result; // 得到结果
  };

  // 和pick刚好相反
  // 返回一个object副本
  // 只过滤出除去keys(有效的键组成的数组)参数指定的属性值
  // 或者接受一个判断函数，指定忽略哪个key

   // Return a copy of the object without the blacklisted properties.
  _.omit = function(obj, iteratee, context) {
    if (_.isFunction(iteratee)) {
      iteratee = _.negate(iteratee); // negate
    } else {
      var keys = _.map(flatten(arguments, false, false, 1), String); // //把arguments其余参数转化成字符串
      iteratee = function(value, key) {
        return !_.contains(keys, key);
      };
    }
    return _.pick(obj, iteratee, context);
  };

  // Fill in a given object with default properties.
  _.defaults = createAssigner(_.allKeys, true);

  // 模拟Object.create,第一个参数是要继承的原型，第二个参数是对象本身的属性
  // 继承的原型的实现主要利用了baseCreate，下文会解析
  // 然后将props合并到baseCreate返回的对象

  // Creates an object that inherits from the given prototype object.
  // If additional properties are provided then they will be added to the
  // created object.
  _.create = function(prototype, props) {
    var result = baseCreate(prototype);
    if (props) _.extendOwn(result, props);
    return result;
  };

  // 浅拷贝函数
  // 实现的原理很简单，如果obj是个数组类型，就是直接复制一份
  // 否则调动_.extend拷贝一份

  // Create a (shallow-cloned) duplicate of an object.
  _.clone = function(obj) {
    if (!_.isObject(obj)) return obj;
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

  // 用obj作为参数来调用函数interceptor，然后返回object
  // 经常作为链式调用的一环，对obj进行了某些操作后，又返回obj，方便下一次操作

  // Invokes interceptor with the obj, and then returns obj.
  // The primary purpose of this method is to "tap into" a method chain, in
  // order to perform operations on intermediate results within the chain.
  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

  // 判断attrs对象上的属性是不是全在object中
  // 注意不仅仅是在object中查找，还包括其原型

  // Returns whether an object has a given set of `key:value` pairs.
  _.isMatch = function(object, attrs) {
    var keys = _.keys(attrs), length = keys.length;
    if (object == null) return !length;
    var obj = Object(object); // 防止object不是对象
    for (var i = 0; i < length; i++) {
      var key = keys[i];
      if (attrs[key] !== obj[key] || !(key in obj)) return false; // 如果key对应的值不相等 或者key不在object中就返回false（其实是不是先判断key是否在object中会更快?）
    }
    return true;
  };


  // Internal recursive comparison function for `isEqual`.
  var eq = function(a, b, aStack, bStack) {
    // Identical objects are equal. `0 === -0`, but they aren't identical.
    // See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
    if (a === b) return a !== 0 || 1 / a === 1 / b;
    // A strict comparison is necessary because `null == undefined`.
    if (a == null || b == null) return a === b;
    // Unwrap any wrapped objects.
    if (a instanceof _) a = a._wrapped;
    if (b instanceof _) b = b._wrapped;
    // Compare `[[Class]]` names.
    var className = toString.call(a);
    if (className !== toString.call(b)) return false;
    switch (className) {
      // Strings, numbers, regular expressions, dates, and booleans are compared by value.
      case '[object RegExp]':
      // RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
      case '[object String]':
        // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
        // equivalent to `new String("5")`.
        return '' + a === '' + b;
      case '[object Number]':
        // `NaN`s are equivalent, but non-reflexive.
        // Object(NaN) is equivalent to NaN
        if (+a !== +a) return +b !== +b;
        // An `egal` comparison is performed for other numeric values.
        return +a === 0 ? 1 / +a === 1 / b : +a === +b;
      case '[object Date]':
      case '[object Boolean]':
        // Coerce dates and booleans to numeric primitive values. Dates are compared by their
        // millisecond representations. Note that invalid dates with millisecond representations
        // of `NaN` are not equivalent.
        return +a === +b;
    }

    var areArrays = className === '[object Array]';
    if (!areArrays) {
      if (typeof a != 'object' || typeof b != 'object') return false;

      // Objects with different constructors are not equivalent, but `Object`s or `Array`s
      // from different frames are.
      var aCtor = a.constructor, bCtor = b.constructor;
      if (aCtor !== bCtor && !(_.isFunction(aCtor) && aCtor instanceof aCtor &&
                               _.isFunction(bCtor) && bCtor instanceof bCtor)
                          && ('constructor' in a && 'constructor' in b)) {
        return false;
      }
    }
    // Assume equality for cyclic structures. The algorithm for detecting cyclic
    // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.

    // Initializing stack of traversed objects.
    // It's done here since we only need them for objects and arrays comparison.
    aStack = aStack || [];
    bStack = bStack || [];
    var length = aStack.length;
    while (length--) {
      // Linear search. Performance is inversely proportional to the number of
      // unique nested structures.
      if (aStack[length] === a) return bStack[length] === b;
    }

    // Add the first object to the stack of traversed objects.
    aStack.push(a);
    bStack.push(b);

    // Recursively compare objects and arrays.
    if (areArrays) {
      // Compare array lengths to determine if a deep comparison is necessary.
      length = a.length;
      if (length !== b.length) return false;
      // Deep compare the contents, ignoring non-numeric properties.
      while (length--) {
        if (!eq(a[length], b[length], aStack, bStack)) return false;
      }
    } else {
      // Deep compare objects.
      var keys = _.keys(a), key;
      length = keys.length;
      // Ensure that both objects contain the same number of properties before comparing deep equality.
      if (_.keys(b).length !== length) return false;
      while (length--) {
        // Deep compare each member
        key = keys[length];
        if (!(_.has(b, key) && eq(a[key], b[key], aStack, bStack))) return false;
      }
    }
    // Remove the first object from the stack of traversed objects.
    aStack.pop();
    bStack.pop();
    return true;
  };

  // Perform a deep comparison to check if two objects are equal.
  _.isEqual = function(a, b) {
    return eq(a, b);
  };

  // 判断obj是否不包含可枚举的属性

  // Is a given array, string, or object empty?
  // An "empty" object has no enumerable own-properties.
  _.isEmpty = function(obj) {
    if (obj == null) return true;
    if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0; // 如果是类数组直接判断length长度是否为0
    return _.keys(obj).length === 0; // 如果是对象，则先获取自身的keys，在判断length长度是否为0来确定
  };

  // 判断obj是不是dom元素，主要通过nodeType来做检测

  // Is a given value a DOM element?
  _.isElement = function(obj) {
    return !!(obj && obj.nodeType === 1);
  };

  // 判断obj是否是数组

  // Is a given value an array?
  // Delegates to ECMA5's native Array.isArray
  _.isArray = nativeIsArray || function(obj) {
    return toString.call(obj) === '[object Array]';
  };

  // 判断obj是不是对象
  // 注意不是判断Object类型，js中函数也是个对象
  // 注意!!obj 这里是为了排除null (typeof null => object)

  // Is a given variable an object?
  _.isObject = function(obj) {
    var type = typeof obj;
    return type === 'function' || type === 'object' && !!obj;
  };

  // 判断obj是否是'Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error'函数

  // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.
  _.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error'], function(name) {
    _['is' + name] = function(obj) {
      return toString.call(obj) === '[object ' + name + ']';
    };
  });

  // 有些浏览器(例如<ie9)没有Arguments这种类型，通过检测是否有callee属性来判断是否是Arguments

  // Define a fallback version of the method in browsers (ahem, IE < 9), where
  // there isn't any inspectable "Arguments" type.
  if (!_.isArguments(arguments)) {
    _.isArguments = function(obj) {
      return _.has(obj, 'callee');
    };
  }

  // Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8,
  // IE 11 (#1621), and in Safari 8 (#1929).
  if (typeof /./ != 'function' && typeof Int8Array != 'object') {
    _.isFunction = function(obj) {
      return typeof obj == 'function' || false;
    };
  }

  // 判断obj是否是无穷大，这里使用了原生的isFinite函数
  // 后面还对NaN做了排除，作用何在？兼容？

  // Is a given object a finite number?
  _.isFinite = function(obj) {
    return isFinite(obj) && !isNaN(parseFloat(obj));
  };

  // 判断obj是不是NaN(注意这里和原生的isNaN的作用和含义是不一样的)
  // 符合NaN的条件是：为数字，并且不等于自身(那只有NaN了)

  // Is the given value `NaN`? (NaN is the only number which does not equal itself).
  _.isNaN = function(obj) {
    return _.isNumber(obj) && obj !== +obj; // +'1' => 1 将obj转化为数字的方法之一
  };

  // 判断是否是布尔类型
  // 布尔类型不是true or false嘛，为啥还要加最后的判断呢？亦或是只用最后的判断是不是也可以

  // Is a given value a boolean?
  _.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
  };

  // 判断obj是否是null
  // 注意这里的三等号，意味着只判断null，不包括undefined

  // Is a given value equal to null?
  _.isNull = function(obj) {
    return obj === null;
  };

  // 判断obj是否为undefined
  // 注意这里的三等号，意味着只判断undefined，不包括null
  // void 0  执行完之后返回undefined，正好用来做判断

  // Is a given variable undefined?
  _.isUndefined = function(obj) {
    return obj === void 0;
  };

  // 判断obj是否拥有key属性
  // 仅仅在对象自身查找，不涉及原型

  // Shortcut function for checking if an object has a given property directly
  // on itself (in other words, not on a prototype).
  _.has = function(obj, key) {
    return obj != null && hasOwnProperty.call(obj, key);
  };

  // Utility Functions
  // -----------------

  // 防止全局变量冲突的非常经典的解决方案

  // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  // previous owner. Returns a reference to the Underscore object.
  _.noConflict = function() {
    root._ = previousUnderscore; // 将previousUnderscore(_)重新赋值给全局对象
    return this; // 并将_返回以供使用
  };

  // 一个返回自身参数的函数，作为underscore.js默认的迭代器存在

  // Keep the identity function around for default iteratees.
  _.identity = function(value) {
    return value;
  };

  // 返回一个函数，该函数返回调用_.constant(value)时传的参数value

  // Predicate-generating functions. Often useful outside of Underscore.
  _.constant = function(value) {
    return function() {
      return value;
    };
  };

  _.noop = function(){};

  _.property = property;

  // 与property函数相反
  // 返回一个函数，专门用来读取obj对象的属性

  // Generates a function for a given object that returns a given property.
  _.propertyOf = function(obj) {
    return obj == null ? function(){} : function(key) {
      return obj[key];
    };
  };

  // 返回一个断言函数(返回值是布尔类型的函数)

  // Returns a predicate for checking whether an object has a given set of
  // `key:value` pairs.
  _.matcher = _.matches = function(attrs) {
    attrs = _.extendOwn({}, attrs); // 本地先保存一份attrs的副本
    return function(obj) {
      return _.isMatch(obj, attrs); // 判断attrs是否全在obj中
    };
  };

  // 调用给定的迭代函数n次
  // 每一次调用iteratee都传入一个索引值i，这个i就是0, 1, 2, 3 ... n
  // 最后将一个存着多次调用迭代函数返回值的数组返回

  // Run a function **n** times.
  _.times = function(n, iteratee, context) {
    var accum = Array(Math.max(0, n));
    iteratee = optimizeCb(iteratee, context, 1); // 绑定一下this作用域
    for (var i = 0; i < n; i++) accum[i] = iteratee(i); // 将每一次调用iteratee的返回值存入数组accum
    return accum; // 最后将结果返回
  };

  // random随机函数，返回[min, max)之间的一个整数
  // 如果没有传max，则默认从0开始计算

  // Return a random integer between min and max (inclusive).
  _.random = function(min, max) {
    if (max == null) {
      max = min;
      min = 0;
    }
    return min + Math.floor(Math.random() * (max - min + 1));
  };

  // 得到系统时间戳
  // 如果浏览器支持Date.now则用其，不支持就自己实现

  // A (possibly faster) way to get the current timestamp as an integer.
  _.now = Date.now || function() {
    return new Date().getTime();
  };

   // List of HTML entities for escaping.
  var escapeMap = {
    '&': '&amp;',
    '<': '&lt;',
    '>': '&gt;',
    '"': '&quot;',
    "'": '&#x27;',
    '`': '&#x60;'
  };
  var unescapeMap = _.invert(escapeMap);

  // 用来创建_.escape和_.unescape的函数，接收一个对象，返回一个函数

  // Functions for escaping and unescaping strings to/from HTML interpolation.
  var createEscaper = function(map) { // 由这个函数可以联想到dom操作中封装addClass和removeClass使用正则可以怎么做 嘎嘎嘎
    var escaper = function(match) {
      return map[match];
    };
    // Regexes for identifying a key that needs to be escaped
    var source = '(?:' + _.keys(map).join('|') + ')'; // escape => (?:&|<|>|"|'|`) unescape反过来取对应的值
    var testRegexp = RegExp(source); // 得到接下来匹配的正则表达式
    var replaceRegexp = RegExp(source, 'g');
    return function(string) {
      string = string == null ? '' : '' + string;
      return testRegexp.test(string) ? string.replace(replaceRegexp, escaper) : string; // 将需要替换的key变成map中的值
    };
  };
  _.escape = createEscaper(escapeMap);
  _.unescape = createEscaper(unescapeMap);

  // 如果指定的对象object的属性property的值是一个函数，那么将在object上下文内调用它;
  // 否则，返回它。
  // 如果提供默认值，并且属性不存在，那么默认值将被返回。如果设置defaultValue是一个函数，它的结果将被返回。

  // If the value of the named `property` is a function then invoke it with the
  // `object` as context; otherwise, return it.
  _.result = function(object, property, fallback) {
    var value = object == null ? void 0 : object[property];
    if (value === void 0) {
      value = fallback;
    }
    return _.isFunction(value) ? value.call(object) : value;
  };

  // 生成唯一id

  // Generate a unique integer id (unique within the entire client session).
  // Useful for temporary DOM ids.
  var idCounter = 0;
  _.uniqueId = function(prefix) {
    var id = ++idCounter + '';
    return prefix ? prefix + id : id;
  };

  // By default, Underscore uses ERB-style template delimiters, change the
  // following template settings to use alternative delimiters.
  _.templateSettings = {
    evaluate    : /<%([\s\S]+?)%>/g,
    interpolate : /<%=([\s\S]+?)%>/g,
    escape      : /<%-([\s\S]+?)%>/g
  };

  // When customizing `templateSettings`, if you don't want to define an
  // interpolation, evaluation or escaping regex, we need one that is
  // guaranteed not to match.
  var noMatch = /(.)^/;

  // Certain characters need to be escaped so that they can be put into a
  // string literal.
  var escapes = {
    "'":      "'",
    '\\':     '\\',
    '\r':     'r',
    '\n':     'n',
    '\u2028': 'u2028',
    '\u2029': 'u2029'
  };

  var escaper = /\\|'|\r|\n|\u2028|\u2029/g;

  var escapeChar = function(match) {
    return '\\' + escapes[match];
  };

  // JavaScript micro-templating, similar to John Resig's implementation.
  // Underscore templating handles arbitrary delimiters, preserves whitespace,
  // and correctly escapes quotes within interpolated code.
  // NB: `oldSettings` only exists for backwards compatibility.
  _.template = function(text, settings, oldSettings) {
    if (!settings && oldSettings) settings = oldSettings;
    settings = _.defaults({}, settings, _.templateSettings);

    // Combine delimiters into one regular expression via alternation.
    var matcher = RegExp([
      (settings.escape || noMatch).source,
      (settings.interpolate || noMatch).source,
      (settings.evaluate || noMatch).source
    ].join('|') + '|$', 'g');

    // Compile the template source, escaping string literals appropriately.
    var index = 0;
    var source = "__p+='";
    text.replace(matcher, function(match, escape, interpolate, evaluate, offset) {
      source += text.slice(index, offset).replace(escaper, escapeChar);
      index = offset + match.length;

      if (escape) {
        source += "'+\n((__t=(" + escape + "))==null?'':_.escape(__t))+\n'";
      } else if (interpolate) {
        source += "'+\n((__t=(" + interpolate + "))==null?'':__t)+\n'";
      } else if (evaluate) {
        source += "';\n" + evaluate + "\n__p+='";
      }

      // Adobe VMs need the match returned to produce the correct offest.
      return match;
    });
    source += "';\n";

    // If a variable is not specified, place data values in local scope.
    if (!settings.variable) source = 'with(obj||{}){\n' + source + '}\n';

    source = "var __t,__p='',__j=Array.prototype.join," +
      "print=function(){__p+=__j.call(arguments,'');};\n" +
      source + 'return __p;\n';

    try {
      var render = new Function(settings.variable || 'obj', '_', source);
    } catch (e) {
      e.source = source;
      throw e;
    }

    var template = function(data) {
      return render.call(this, data, _);
    };

    // Provide the compiled source as a convenience for precompilation.
    var argument = settings.variable || 'obj';
    template.source = 'function(' + argument + '){\n' + source + '}';

    return template;
  };

  // 返回一个经过下划线包装过的实例
  // 这个实例调用下划线的方法会返回该实例本身
  // 直到调用value方法为止

  // Add a "chain" function. Start chaining a wrapped Underscore object.
  _.chain = function(obj) {
    var instance = _(obj); // 下划线包装一下
    instance._chain = true; // 设置是否链式调用的标志
    return instance; // 将实例返回
  };

  // OOP
  // ---------------
  // If Underscore is called as a function, it returns a wrapped object that
  // can be used OO-style. This wrapper holds altered versions of all the
  // underscore functions. Wrapped objects may be chained.

  // Helper function to continue chaining intermediate results.
  var result = function(instance, obj) {
    return instance._chain ? _(obj).chain() : obj;
  };

  // 扩展下划线库的一个方法，类似jQ中的extend

  // Add your own custom functions to the Underscore object.
  _.mixin = function(obj) {
    _.each(_.functions(obj), function(name) { // 拿到obj中的所有是函数类型的值组成数组并进行遍历
      var func = _[name] = obj[name]; // 获取functon name并设置为下划线的静态属性
      _.prototype[name] = function() { // 添加到下划线原型身上
        var args = [this._wrapped];
        push.apply(args, arguments);
        return result(this, func.apply(_, args)); // 这一句非常的重要
      };
    });
  };

  // Add all of the Underscore functions to the wrapper object.
  _.mixin(_); // 将下划线的静态方法都扩展到了其原型身上

  // Add all mutator Array functions to the wrapper.
  _.each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
    var method = ArrayProto[name];
    _.prototype[name] = function() {
      var obj = this._wrapped;
      method.apply(obj, arguments);
      if ((name === 'shift' || name === 'splice') && obj.length === 0) delete obj[0];
      return result(this, obj);
    };
  });

  // Add all accessor Array functions to the wrapper.
  _.each(['concat', 'join', 'slice'], function(name) {
    var method = ArrayProto[name];
    _.prototype[name] = function() {
      return result(this, method.apply(this._wrapped, arguments));
    };
  });

  // Extracts the result from a wrapped and chained object.
  _.prototype.value = function() {
    return this._wrapped;
  };

  // Provide unwrapping proxy for some methods used in engine operations
  // such as arithmetic and JSON stringification.
  _.prototype.valueOf = _.prototype.toJSON = _.prototype.value;

  _.prototype.toString = function() {
    return '' + this._wrapped;
  };

  // AMD registration happens at the end for compatibility with AMD loaders
  // that may not enforce next-turn semantics on modules. Even though general
  // practice for AMD registration is to be anonymous, underscore registers
  // as a named module because, like jQuery, it is a base library that is
  // popular enough to be bundled in a third party lib, but not be part of
  // an AMD load request. Those cases could generate an error when an
  // anonymous define() is called outside of a loader request.
  
  if (typeof define === 'function' && define.amd) {
    define('underscore', [], function() {
      return _;
    });
  }
}.call(this));