underscore.js

//     Underscore.js 1.8.2
//     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
  // --------------
  // 基线设置

  // Establish the root object, `window` in the browser, or `exports` on the server.
  // window在浏览器或exports服务器上建立根对象。
  var root = this;

  // Save the previous value of the `_` variable.
  // 保存_变量的上一个值。
  var previousUnderscore = root._;

  // Save bytes in the minified (but not gzipped) version:
  // 在最小化（但不是gzip））版本中保存字节：
  var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;

  // Create quick reference variables for speed access to core prototypes.
  // 创建速度访问核心原型的快速参考变量。
  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.
  // 我们希望使用的所有ECMAScript 5本机功能实现都在此声明。
  var
    nativeIsArray      = Array.isArray,
    nativeKeys         = Object.keys,
    nativeBind         = FuncProto.bind,
    nativeCreate       = Object.create;

  // Naked function reference for surrogate-prototype-swapping.
  // 用于替代原型交换的裸函数参考。
  var Ctor = function(){};

  // Create a safe reference to the Underscore object for use below.
  // 创建一个安全的引用到Underscore对象以供下面使用。
  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.
  // 导出Node.js的Underscore对象，具有旧require()API的向后兼容性。如果我们在浏览器中，添加_为全局对象。
  if (typeof exports !== 'undefined') {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

  // Current version.
  // 当前版本
  _.VERSION = '1.8.2';

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.
  // 内部函数返回一个有效的（针对当前引擎）的传入回调版本，可以在其他的Underscore函数中重复应用。
  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);
    };
  };

  // 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.
  // 一个主要的内部函数，用于生成可应用于集合中每个元素的回调，返回所需的结果 - 身份，任意回调，属性匹配器或属性访问器。
  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);
  };
  _.iteratee = function(value, context) {
    return cb(value, context, Infinity);
  };

  // An internal function for creating assigner functions.
  // 用于创建分配器功能的内部功能。
  var createAssigner = function(keysFunc, undefinedOnly) {
    return function(obj) {
      var length = arguments.length;
      if (length < 2 || obj == null) return obj;
      for (var index = 1; index < length; index++) {
        var source = arguments[index],
            keys = keysFunc(source),
            l = keys.length;
        for (var i = 0; i < l; i++) {
          var key = keys[i];
          if (!undefinedOnly || obj[key] === void 0) obj[key] = source[key];
        }
      }
      return obj;
    };
  };

  // An internal function for creating a new object that inherits from another.
  // 用于创建从另一个继承的新对象的内部函数。
  var baseCreate = function(prototype) {
    if (!_.isObject(prototype)) return {};
    if (nativeCreate) return nativeCreate(prototype);
    Ctor.prototype = prototype;
    var result = new Ctor;
    Ctor.prototype = null;
    return result;
  };

  // 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
  // 帮助收集方法来确定一个集合是否应该被迭代为一个数组或一个对象相关：http : //people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
  var isArrayLike = function(collection) {
    var length = collection != null && collection.length;
    return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
  };

  // Collection Functions
  // --------------------
  // 收藏此功能

  // 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.
  // 基石，一个each执行，又名forEach。处理原始对象除了数组喜欢。对待所有稀疏阵列，就像它们是密集的一样。
  _.each = _.forEach = function(obj, iteratee, context) {
    iteratee = optimizeCb(iteratee, context);
    var i, length;
    if (isArrayLike(obj)) {
      for (i = 0, length = obj.length; i < length; i++) {
        iteratee(obj[i], i, obj);
      }
    } else {
      var keys = _.keys(obj);
      for (i = 0, length = keys.length; i < length; i++) {
        iteratee(obj[keys[i]], keys[i], obj);
      }
    }
    return obj;
  };

  // Return the results of applying the iteratee to each element.
  // 返回将iteratee应用于每个元素的结果。
  _.map = _.collect = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys = !isArrayLike(obj) && _.keys(obj),
        length = (keys || obj).length,
        results = Array(length);
    for (var index = 0; index < length; index++) {
      var currentKey = keys ? keys[index] : index;
      results[index] = iteratee(obj[currentKey], currentKey, obj);
    }
    return results;
  };

  // Create a reducing function iterating left or right.
  // 创建一个向左或向右迭代的缩小函数。
  function createReduce(dir) {
    // Optimized iterator function as using arguments.length
    // in the main function will deoptimize the, see #1991.
    // 在主函数中使用arguments.length的优化迭代器函数将优化，参见＃1991。
    function iterator(obj, iteratee, memo, keys, index, length) {
      for (; index >= 0 && index < length; index += dir) {
        var currentKey = keys ? keys[index] : index;
        memo = iteratee(memo, obj[currentKey], currentKey, obj);
      }
      return memo;
    }

    return function(obj, iteratee, memo, context) {
      iteratee = optimizeCb(iteratee, context, 4);
      var keys = !isArrayLike(obj) && _.keys(obj),
          length = (keys || obj).length,
          index = dir > 0 ? 0 : length - 1;
      // Determine the initial value if none is provided.
      // 如果没有提供，确定初始值。
      if (arguments.length < 3) {
        memo = obj[keys ? keys[index] : index];
        index += dir;
      }
      return iterator(obj, iteratee, memo, keys, index, length);
    };
  }

  // **Reduce** builds up a single result from a list of values, aka `inject`,
  // or `foldl`.
  // 减少从值列表aka inject或foldl。建立一个单一的结果。
  _.reduce = _.foldl = _.inject = createReduce(1);

  // The right-associative version of reduce, also known as `foldr`.
  // 权利关联版本减少，也称为foldr。
  _.reduceRight = _.foldr = createReduce(-1);

  // Return the first value which passes a truth test. Aliased as `detect`.
  // 返回通过真实测试的第一个值。别名为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];
  };

  // Return all the elements that pass a truth test.
  // Aliased as `select`.
  // 返回通过真相测试的所有元素。别名为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;
  };

  // Return all the elements for which a truth test fails.
  // 返回真相测试失败的所有元素。
  _.reject = function(obj, predicate, context) {
    return _.filter(obj, _.negate(cb(predicate)), context);
  };

  // Determine whether all of the elements match a truth test.
  // Aliased as `all`.
  // 确定所有元素是否匹配真实测试。别名为all。
  _.every = _.all = 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 false;
    }
    return true;
  };

  // Determine if at least one element in the object matches a truth test.
  // Aliased as `any`.
  // 确定对象中至少有一个元素是否与真实测试相匹配。别名为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;
    }
    return false;
  };

  // Determine if the array or object contains a given value (using `===`).
  // Aliased as `includes` and `include`.
  // 确定数组或对象是否包含给定值（使用===）。别名为includes和include。
  _.contains = _.includes = _.include = function(obj, target, fromIndex) {
    if (!isArrayLike(obj)) obj = _.values(obj);
    return _.indexOf(obj, target, typeof fromIndex == 'number' && fromIndex) >= 0;
  };

  // Invoke a method (with arguments) on every item in a collection.
  // 在集合中的每个项目上调用一个方法（带参数）
  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    var isFunc = _.isFunction(method);
    return _.map(obj, function(value) {
      var func = isFunc ? method : value[method];
      return func == null ? func : func.apply(value, args);
    });
  };

  // Convenience version of a common use case of `map`: fetching a property.
  // 方便版本的常用用例map：获取属性。
  _.pluck = function(obj, key) {
    return _.map(obj, _.property(key));
  };

  // Convenience version of a common use case of `filter`: selecting only objects
  // containing specific `key:value` pairs.
  // 常见用例的便利版本filter：仅选择包含特定key:value对的对象。
  _.where = function(obj, attrs) {
    return _.filter(obj, _.matcher(attrs));
  };

  // Convenience version of a common use case of `find`: getting the first object
  // containing specific `key:value` pairs.
  // 常用的便利版本是find：获取包含特定key:value对的第一个对象。
  _.findWhere = function(obj, attrs) {
    return _.find(obj, _.matcher(attrs));
  };

  // Return the maximum element (or element-based computation).
  // 返回最大元素（或基于元素的计算）。
  _.max = 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;
  };

  // Return the minimum element (or element-based computation).
  // 返回最小元素（或基于元素的计算）。
  _.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;
  };

  // Shuffle a collection, using the modern version of the
  // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
  // 洗牌，使用现代版的 Fisher-Yates洗牌。
  _.shuffle = function(obj) {
    var set = isArrayLike(obj) ? obj : _.values(obj);
    var length = set.length;
    var shuffled = Array(length);
    for (var index = 0, rand; index < length; index++) {
      rand = _.random(0, index);
      if (rand !== index) shuffled[index] = shuffled[rand];
      shuffled[rand] = set[index];
    }
    return shuffled;
  };

  // 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`.
  // 样品ñ从集合随机值。如果未指定n，则返回单个随机元素。内部guard参数允许它使用map。
  _.sample = function(obj, n, guard) {
    if (n == null || guard) {
      if (!isArrayLike(obj)) obj = _.values(obj);
      return obj[_.random(obj.length - 1)];
    }
    return _.shuffle(obj).slice(0, Math.max(0, n));
  };

  // Sort the object's values by a criterion produced by an iteratee.
  // 通过迭代产生的标准对对象的值进行排序
  _.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) {
      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.
  // 用于聚合“分组”操作的内部函数。
  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;
    };
  };

  // Groups the object's values by a criterion. Pass either a string attribute
  // to group by, or a function that returns the criterion.
  // 按标准对对象的值进行分组。将一个字符串属性传递给group by或返回该标准的函数。
  _.groupBy = group(function(result, value, key) {
    if (_.has(result, key)) result[key].push(value); else result[key] = [value];
  });

  // Indexes the object's values by a criterion, similar to `groupBy`, but for
  // when you know that your index values will be unique.
  // 以类似于groupBy但是当您知道您的索引值将是唯一的标准来索引对象的值时
  _.indexBy = group(function(result, value, key) {
    result[key] = value;
  });

  // 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.
  // 按照某个标准对按组分组的实例进行计数。将一个字符串属性传递给count或返回该标准的函数。
  _.countBy = group(function(result, value, key) {
    if (_.has(result, key)) result[key]++; else result[key] = 1;
  });

  // Safely create a real, live array from anything iterable.
  // 从任何可迭代的安全地创建一个真实的，活的数组。
  _.toArray = function(obj) {
    if (!obj) return [];
    if (_.isArray(obj)) return slice.call(obj);
    if (isArrayLike(obj)) return _.map(obj, _.identity);
    return _.values(obj);
  };

  // Return the number of elements in an object.
  // 返回对象中的元素数。
  _.size = function(obj) {
    if (obj == null) return 0;
    return isArrayLike(obj) ? obj.length : _.keys(obj).length;
  };

  // Split a collection into two arrays: one whose elements all satisfy the given
  // predicate, and one whose elements all do not satisfy the predicate.
  // 将集合拆分成两个数组：其元素都满足给定谓词，其元素全部不满足谓词的元素。
  _.partition = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var pass = [], fail = [];
    _.each(obj, function(value, key, obj) {
      (predicate(value, key, obj) ? pass : fail).push(value);
    });
    return [pass, fail];
  };

  // Array Functions
  // ---------------
  // 数组函数

  // 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`.
  // 获取数组的第一个元素。传递n将返回数组中的前N个值。别名为head和take。该警卫检查允许它与合作_.map。
  _.first = _.head = _.take = function(array, n, guard) {
    if (array == null) return void 0;
    if (n == null || guard) return array[0];
    return _.initial(array, array.length - n);
  };

  // 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.
  // 返回除数组的最后一个条目以外的所有内容。对参数对象特别有用。传递n将返回数组中的所有值，不包括最后一个N.
  _.initial = function(array, n, guard) {
    return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
  };

  // Get the last element of an array. Passing **n** will return the last N
  // values in the array.
  // 获取数组的最后一个元素。传递n将返回数组中的最后N个值。
  _.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));
  };

  // 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.
  // 返回除数组的第一个条目以外的所有内容。别名为tail和drop。对参数对象特别有用。传递n将返回数组中的其余N个值。
  _.rest = _.tail = _.drop = function(array, n, guard) {
    return slice.call(array, n == null || guard ? 1 : n);
  };

  // Trim out all falsy values from an array.
  // 修剪数组中的所有的伪造值
  _.compact = function(array) {
    return _.filter(array, _.identity);
  };

  // Internal implementation of a recursive `flatten` function.
  // 内部执行递归函数flatten
  var flatten = function(input, shallow, strict, startIndex) {
    var output = [], idx = 0;
    for (var i = startIndex || 0, length = input && input.length; i < length; i++) {
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
        //flatten current level of array or arguments object
        // 展开数组或参数对象的当前级别
        if (!shallow) value = flatten(value, shallow, strict);
        var j = 0, len = value.length;
        output.length += len;
        while (j < len) {
          output[idx++] = value[j++];
        }
      } else if (!strict) {
        output[idx++] = value;
      }
    }
    return output;
  };

  // Flatten out an array, either recursively (by default), or just one level.
  // 展开一个数组，递归（默认）或只有一个级别。
  _.flatten = function(array, shallow) {
    return flatten(array, shallow, false);
  };

  // Return a version of the array that does not contain the specified value(s).
  // 返回不包含指定值的数组的版本。

  _.without = function(array) {
    return _.difference(array, slice.call(arguments, 1));
  };

  // 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`.
  // 生成一个无重复的版本的阵列。如果阵列已被排序，则可以选择使用更快的算法。别名为unique。
  _.uniq = _.unique = function(array, isSorted, iteratee, context) {
    if (array == null) return [];
    if (!_.isBoolean(isSorted)) {
      context = iteratee;
      iteratee = isSorted;
      isSorted = false;
    }
    if (iteratee != null) iteratee = cb(iteratee, context);
    var result = [];
    var seen = [];
    for (var i = 0, length = array.length; i < length; i++) {
      var value = array[i],
          computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted) {
        if (!i || seen !== computed) result.push(value);
        seen = computed;
      } else if (iteratee) {
        if (!_.contains(seen, computed)) {
          seen.push(computed);
          result.push(value);
        }
      } else if (!_.contains(result, value)) {
        result.push(value);
      }
    }
    return result;
  };

  // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  // 生成一个包含union的数组：来自所有传入数组的每个不同的元素。
  _.union = function() {
    return _.uniq(flatten(arguments, true, true));
  };

  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  // 生成包含所有传入数组之间共享的每个项目的数组。
  _.intersection = function(array) {
    if (array == null) return [];
    var result = [];
    var argsLength = arguments.length;
    for (var i = 0, length = array.length; 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;
  };

  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  // 取一个数组和其他数组之间的区别。只有第一个数组中存在的元素才会保留。
  _.difference = function(array) {
    var rest = flatten(arguments, true, true, 1);
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    });
  };

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  // 将多个列表压缩到单个数组中 - 共享索引的元素一起使用。
  _.zip = function() {
    return _.unzip(arguments);
  };

  // Complement of _.zip. Unzip accepts an array of arrays and groups
  // each array's elements on shared indices
  // _.zip的补充。Unzip接受一个数组数组，并对共享索引上的每个数组的元素进行分组
  _.unzip = function(array) {
    var length = array && _.max(array, 'length').length || 0;
    var result = Array(length);

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

  // 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.
  // 将列表转换为对象。传递一个[key, value] 对的单个数组，或两个相同长度的并行数组 - 一个键和一个对应的值。
  _.object = function(list, values) {
    var result = {};
    for (var i = 0, length = list && list.length; i < length; i++) {
      if (values) {
        result[list[i]] = values[i];
      } else {
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

  // 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.
  // 返回数组中第一个项目的位置，如果该项目不包括在数组中，则返回-1。如果阵列中的排序顺序大，已经，通过true 对isSorted使用二进制搜索。
  _.indexOf = function(array, item, isSorted) {
    var i = 0, length = array && array.length;
    if (typeof isSorted == 'number') {
      i = isSorted < 0 ? Math.max(0, length + isSorted) : isSorted;
    } else if (isSorted && length) {
      i = _.sortedIndex(array, item);
      return array[i] === item ? i : -1;
    }
    if (item !== item) {
      return _.findIndex(slice.call(array, i), _.isNaN);
    }
    for (; i < length; i++) if (array[i] === item) return i;
    return -1;
  };

  _.lastIndexOf = function(array, item, from) {
    var idx = array ? array.length : 0;
    if (typeof from == 'number') {
      idx = from < 0 ? idx + from + 1 : Math.min(idx, from + 1);
    }
    if (item !== item) {
      return _.findLastIndex(slice.call(array, 0, idx), _.isNaN);
    }
    while (--idx >= 0) if (array[idx] === item) return idx;
    return -1;
  };

  // Generator function to create the findIndex and findLastIndex functions
  // Generator函数创建findIndex和findLastIndex函数
  function createIndexFinder(dir) {
    return function(array, predicate, context) {
      predicate = cb(predicate, context);
      var length = array != null && array.length;
      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
  // 返回通过谓词测试的数组上的第一个索引
  _.findIndex = createIndexFinder(1);

  _.findLastIndex = createIndexFinder(-1);

  // 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.
  // 使用比较器函数找出要插入对象的最小索引，以保持顺序。使用二进制搜索。
  _.sortedIndex = function(array, obj, iteratee, context) {
    iteratee = cb(iteratee, context, 1);
    var value = iteratee(obj);
    var low = 0, high = array.length;
    while (low < high) {
      var mid = Math.floor((low + high) / 2);
      if (iteratee(array[mid]) < value) low = mid + 1; else high = mid;
    }
    return low;
  };

  // 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).
  // 生成包含算术进度的整数数组。原生Python range()函数的端口。请参阅 Python文档。
  _.range = function(start, stop, step) {
    if (arguments.length <= 1) {
      stop = start || 0;
      start = 0;
    }
    step = step || 1;

    var length = Math.max(Math.ceil((stop - start) / step), 0);
    var range = Array(length);

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

    return range;
  };

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

  // Determines whether to execute a function as a constructor
  // or a normal function with the provided arguments
  // 确定是否使用提供的参数作为构造函数或正常函数执行函数
  var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
    if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args);
    var self = baseCreate(sourceFunc.prototype);
    var result = sourceFunc.apply(self, args);
    if (_.isObject(result)) return result;
    return self;
  };

  // Create a function bound to a given object (assigning `this`, and arguments,
  // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
  // available.
  // 创建绑定到给定对象的函数（赋值this和参数，可选）。代表ECMAScript 5的本机（Function.bind如果可用）。
  _.bind = function(func, context) {
    if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
    if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
    var args = slice.call(arguments, 2);
    var bound = function() {
      return executeBound(func, bound, context, this, args.concat(slice.call(arguments)));
    };
    return bound;
  };

  // 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.
  // 通过创建一个已经预先填充了一些参数的版本，而不改变它的动态this上下文来部分地应用一个函数。_作为占位符，允许任何参数的组合预先填充。
  _.partial = function(func) {
    var boundArgs = slice.call(arguments, 1);
    var bound = function() {
      var position = 0, length = boundArgs.length;
      var args = Array(length);
      for (var i = 0; i < length; i++) {
        args[i] = boundArgs[i] === _ ? arguments[position++] : boundArgs[i];
      }
      while (position < arguments.length) args.push(arguments[position++]);
      return executeBound(func, bound, this, this, args);
    };
    return bound;
  };

  // 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.
  // 将一些对象的方法绑定到该对象。剩余的参数是要绑定的方法名称。有助于确保对象上定义的所有回调属于它。
  _.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++) {
      key = arguments[i];
      obj[key] = _.bind(obj[key], obj);
    }
    return obj;
  };

  // Memoize an expensive function by storing its results.
  // 通过存储其结果来记录一个昂贵的功能
  _.memoize = function(func, hasher) {
    var memoize = function(key) {
      var cache = memoize.cache;
      var address = '' + (hasher ? hasher.apply(this, arguments) : key);
      if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
      return cache[address];
    };
    memoize.cache = {};
    return memoize;
  };

  // Delays a function for the given number of milliseconds, and then calls
  // it with the arguments supplied.
  // 延迟给定的毫秒数的函数，然后使用提供的参数调用它
  _.delay = function(func, wait) {
    var args = slice.call(arguments, 2);
    return setTimeout(function(){
      return func.apply(null, args);
    }, wait);
  };

  // Defers a function, scheduling it to run after the current call stack has
  // cleared.
  // 定义一个函数，调度它在当前调用栈清除后运行
  _.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.
  // 返回一个函数，当被调用时，在给定的时间窗口中最多只能触发一次。通常，节流功能将尽可能多地运行，而不会每次wait持续不止一次; 但是如果你想禁用在前沿的执行，请通过 {leading: false}。要在后端禁用执行，请同时进行。
  _.throttle = function(func, wait, options) {
    var context, args, result;
    var timeout = null;
    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();
      if (!previous && options.leading === false) previous = now;
      var remaining = wait - (now - previous);
      context = this;
      args = arguments;
      if (remaining <= 0 || remaining > wait) {
        if (timeout) {
          clearTimeout(timeout);
          timeout = null;
        }
        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.
  // 返回一个函数，只要它继续被调用，就不会被触发。该函数将在停止被调用N毫秒后被调用。如果immediate通过，则触发前沿上的功能，而不是尾部。
  _.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;
    };
  };

  // 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.
  // 返回作为参数传递给第二个函数的第一个函数，允许您调整参数，在前后运行代码，并有条件地执行原始函数。
  _.wrap = function(func, wrapper) {
    return _.partial(wrapper, func);
  };

  // Returns a negated version of the passed-in predicate.
  // 返回传入谓词的否定版本
  _.negate = function(predicate) {
    return function() {
      return !predicate.apply(this, arguments);
    };
  };

  // Returns a function that is the composition of a list of functions, each
  // consuming the return value of the function that follows.
  // 返回一个函数，该函数是函数列表的组成，每个函数消耗后面的函数的返回值。
  _.compose = function() {
    var args = arguments;
    var start = args.length - 1;
    return function() {
      var i = start;
      var result = args[start].apply(this, arguments);
      while (i--) result = args[i].call(this, result);
      return result;
    };
  };

  // Returns a function that will only be executed on and after the Nth call.
  // 返回一个仅在第N个调用之后执行的函数。
  _.after = function(times, func) {
    return function() {
      if (--times < 1) {
        return func.apply(this, arguments);
      }
    };
  };

  // Returns a function that will only be executed up to (but not including) the Nth call.
  // 返回一个只能执行第（N个）调用（但不包括）的函数。
  _.before = function(times, func) {
    var memo;
    return function() {
      if (--times > 0) {
        memo = func.apply(this, arguments);
      }
      if (times <= 1) func = null;
      return memo;
    };
  };

  // Returns a function that will be executed at most one time, no matter how
  // often you call it. Useful for lazy initialization.
  // 返回一个最多一次执行的函数，无论你多久调用一次。用于延迟初始化。
  _.once = _.partial(_.before, 2);

  // Object Functions
  // ----------------
  // 对象函数

  // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  // IE <9中的键不会被迭代for key in ...而被忽略。
  var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString');
  var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
                      'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];

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

    // Constructor is a special case.
    // 构造函数是一个特例
    var prop = 'constructor';
    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)) {
        keys.push(prop);
      }
    }
  }

  // Retrieve the names of an object's own properties.
  // Delegates to **ECMAScript 5**'s native `Object.keys`
  // 检索对象自己的属性的名称。代表ECMAScript 5的 本地 Object.keys
  _.keys = function(obj) {
    if (!_.isObject(obj)) return [];
    if (nativeKeys) return nativeKeys(obj);
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys.push(key);
    // Ahem, IE < 9.
    // 咳咳, IE < 9
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // Retrieve all the property names of an object.
  //  检索对象的所有属性名称
  _.allKeys = function(obj) {
    if (!_.isObject(obj)) return [];
    var keys = [];
    for (var key in obj) keys.push(key);
    // Ahem, IE < 9.
    // 咳咳, IE < 9
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // Retrieve the values of an object's properties.
  // 检索对象属性的值
  _.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;
  };

  // Returns the results of applying the iteratee to each element of the object
  // In contrast to _.map it returns an object
  // 返回将iteratee应用于对象的每个元素的结果与_.map相反，它返回一个对象
  _.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);
      }
      return results;
  };

  // Convert an object into a list of `[key, value]` pairs.
  // 将对象转换[key, value]成对列表。
  _.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;
  };

  // Invert the keys and values of an object. The values must be serializable.
  // 反转对象的键和值。值必须是可序列化的。
  _.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;
  };

  // Return a sorted list of the function names available on the object.
  // Aliased as `methods`
  // 返回对象上可用的功能名称的排序列表。别人为methods
  _.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
  };

  // Extend a given object with all the properties in passed-in object(s).
  // 使用传入对象中的所有属性扩展给定对象。
  _.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)
  // 使用传入的对象中的所有属性分配给定对象
  _.extendOwn = _.assign = createAssigner(_.keys);

  // Returns the first key on an object that passes a predicate test
  // 返回通过谓词测试的对象上的第一个键
  _.findKey = function(obj, predicate, context) {
    predicate = cb(predicate, context);
    var keys = _.keys(obj), key;
    for (var i = 0, length = keys.length; i < length; i++) {
      key = keys[i];
      if (predicate(obj[key], key, obj)) return key;
    }
  };

  // Return a copy of the object only containing the whitelisted properties.
  // 返回仅包含许可列表属性的对象的副本。
  _.pick = function(object, oiteratee, context) {
    var result = {}, obj = object, iteratee, keys;
    if (obj == null) return result;
    if (_.isFunction(oiteratee)) {
      keys = _.allKeys(obj);
      iteratee = optimizeCb(oiteratee, context);
    } else {
      keys = flatten(arguments, false, false, 1);
      iteratee = function(value, key, obj) { return key in obj; };
      obj = Object(obj);
    }
    for (var i = 0, length = keys.length; i < length; i++) {
      var key = keys[i];
      var value = obj[key];
      if (iteratee(value, key, obj)) result[key] = value;
    }
    return result;
  };

   // Return a copy of the object without the blacklisted properties.
   // 返回没有黑名单属性的对象的副本。
  _.omit = function(obj, iteratee, context) {
    if (_.isFunction(iteratee)) {
      iteratee = _.negate(iteratee);
    } else {
      var keys = _.map(flatten(arguments, false, false, 1), String);
      iteratee = function(value, key) {
        return !_.contains(keys, key);
      };
    }
    return _.pick(obj, iteratee, context);
  };

  // Fill in a given object with default properties.
  // 使用默认属性填充给定对象
  _.defaults = createAssigner(_.allKeys, true);

  // Creates an object that inherits from the given prototype object.
  // If additional properties are provided then they will be added to the
  // created object.
  // 创建从给定的原型对象继承的对象。如果提供其他属性，那么它们将被添加到创建的对象。
  _.create = function(prototype, props) {
    var result = baseCreate(prototype);
    if (props) _.extendOwn(result, props);
    return result;
  };

  // Create a (shallow-cloned) duplicate of an object.
  // 创建对象的（浅克隆）副本。
  _.clone = function(obj) {
    if (!_.isObject(obj)) return obj;
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

  // 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.
  // 调用与obj的拦截器，然后返回obj。该方法的主要目的是“挖掘”一个方法链，以便对链中的中间结果执行操作。
  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

  // Returns whether an object has a given set of `key:value` pairs.
  // 返回对象是否具有给定的一组key:value对
  _.isMatch = function(object, attrs) {
    var keys = _.keys(attrs), length = keys.length;
    if (object == null) return !length;
    var obj = Object(object);
    for (var i = 0; i < length; i++) {
      var key = keys[i];
      if (attrs[key] !== obj[key] || !(key in obj)) return false;
    }
    return true;
  };


  // Internal recursive comparison function for `isEqual`.
  // 内部递归比较功能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).
    // 相同的对象是相等的。0 === -0，但它们不一样。见和谐egal建议。
    if (a === b) return a !== 0 || 1 / a === 1 / b;
    // A strict comparison is necessary because `null == undefined`.
    // 因为严格的比较是必要的null == undefined。
    if (a == null || b == null) return a === b;
    // Unwrap any wrapped objects.
    // 打开任何包裹的物体。
    if (a instanceof _) a = a._wrapped;
    if (b instanceof _) b = b._wrapped;
    // Compare `[[Class]]` names.
    // 比较[[Class]]名称
    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.
      // 字符串，数字，正则表达式，日期和布尔值按值进行比较。
      case '[object RegExp]':
      // RegExps are coerced to strings for comparison (Note: '' + /a/i === '/a/i')
      // RegExps被强制用于字符串进行比较（注意：“+ / a / i ==='/ a / i'）
      case '[object String]':
        // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
        // equivalent to `new String("5")`.
        // 原语及其相应的对象包装器是等效的; 因此，"5"相当于new String("5")。
        return '' + a === '' + b;
      case '[object Number]':
        // `NaN`s are equivalent, but non-reflexive.
        // Object(NaN) is equivalent to NaN
        // NaNs是等同的，但不反身。Object(NaN)相当于NaN
        if (+a !== +a) return +b !== +b;
        // An `egal` comparison is performed for other numeric values.
        // 一个egal比较是为其他数值来执行
        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.
        // 强制日期和布尔数字原始值。日期比较它们的毫秒表示。请注意，毫秒表示的无效日期NaN不等同。
        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.
      // 具有不同构造的对象是不等效的，但ObjectS或Array从不同的帧s为。
      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`.
    // 假设循环结构相等。用于检测循环结构的算法从ES 5.1节15.12.3中进行了抽象运算JO。

    // Initializing stack of traversed objects.
    // It's done here since we only need them for objects and arrays comparison.
    // 初始化遍历对象的堆栈。这是在这里完成的，因为我们只需要它们进行对象和数组比较
    aStack = aStack || [];
    bStack = bStack || [];
    var length = aStack.length;
    while (length--) {
      // Linear search. Performance is inversely proportional to the number of
      // unique nested structures.
      // 线性搜索。性能与独特嵌套结构的数量成反比。
      if (aStack[length] === a) return bStack[length] === b;
    }

    // Add the first object to the stack of traversed objects.
    // 将第一个对象添加到遍历对象的堆栈中。
    aStack.push(a);
    bStack.push(b);

    // Recursively compare objects and arrays.
    // 递归比较对象和数组。
    if (areArrays) {
      // Compare array lengths to determine if a deep comparison is necessary.
      // 比较阵列长度以确定是否需要进行深度比较
      length = a.length;
      if (length !== b.length) return false;
      // Deep compare the contents, ignoring non-numeric properties.
      // 深入比较内容，忽略非数字属性。
      while (length--) {
        if (!eq(a[length], b[length], aStack, bStack)) return false;
      }
    } else {
      // Deep compare objects.
      // 深度比较对象
      var keys = _.keys(a), key;
      length = keys.length;
      // Ensure that both objects contain the same number of properties before comparing deep equality.
      // 在比较深度相等之前，确保两个对象都包含相同数量的属性。
      if (_.keys(b).length !== length) return false;
      while (length--) {
        // Deep compare each member
        // 深入比较每个成员
        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.
    // 从堆叠的对象中移除第一个对象。
    aStack.pop();
    bStack.pop();
    return true;
  };

  // Perform a deep comparison to check if two objects are equal.
  // 执行深入的比较以检查两个对象是否相等。
  _.isEqual = function(a, b) {
    return eq(a, b);
  };

  // Is a given array, string, or object empty?
  // An "empty" object has no enumerable own-properties.
  // 给定的数组，字符串或对象为空？“空”对象没有可枚举的属性。
  _.isEmpty = function(obj) {
    if (obj == null) return true;
    if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
    return _.keys(obj).length === 0;
  };

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

  // Is a given value an array?
  // Delegates to ECMA5's native Array.isArray
  // 给定值是数组吗？代表ECMA5的本机Array.isArray
  _.isArray = nativeIsArray || function(obj) {
    return toString.call(obj) === '[object Array]';
  };

  // Is a given variable an object?
  // 给定值是一个对象吗?
  _.isObject = function(obj) {
    var type = typeof obj;
    return type === 'function' || type === 'object' && !!obj;
  };

  // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.
  // 添加一些isType方法：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 + ']';
    };
  });

  // Define a fallback version of the method in browsers (ahem, IE < 9), where
  // there isn't any inspectable "Arguments" type.
  // 在浏览器（ahem，IE <9）中定义方法的后备版本，其中没有可检查的“参数”类型。
  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).
  // isFunction适当时优化。解决旧版v8，IE 11（＃1621）和Safari 8（＃1929）中的一些类型的错误。
  if (typeof /./ != 'function' && typeof Int8Array != 'object') {
    _.isFunction = function(obj) {
      return typeof obj == 'function' || false;
    };
  }

  // Is a given object a finite number?
  // 给定的对象是有限数量吗？
  _.isFinite = function(obj) {
    return isFinite(obj) && !isNaN(parseFloat(obj));
  };

  // Is the given value `NaN`? (NaN is the only number which does not equal itself).
  // 是给定值NaN吗？（NaN是唯一不等于本身的数字）
  _.isNaN = function(obj) {
    return _.isNumber(obj) && obj !== +obj;
  };

  // Is a given value a boolean?
  // 给定值是一个布尔值吗？
  _.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
  };

  // Is a given value equal to null?
  // 给定值是否等于null？
  _.isNull = function(obj) {
    return obj === null;
  };

  // Is a given variable undefined?
  // 给定的变量是否定义？
  _.isUndefined = function(obj) {
    return obj === void 0;
  };

  // Shortcut function for checking if an object has a given property directly
  // on itself (in other words, not on a prototype).
  // 用于检查对象是否具有直接在自身上的给定属性（换句话说，不在原型上）的快捷方式功能。
  _.has = function(obj, key) {
    return obj != null && hasOwnProperty.call(obj, key);
  };

  // Utility Functions
  // -----------------
  // 实用功能

  // Run Underscore.js in *noConflict* mode, returning the `_` variable to its
  // previous owner. Returns a reference to the Underscore object.
  // 在noConflict模式下运行Underscore.js，将_变量返回给其先前的所有者。返回对Underscore对象的引用。
  _.noConflict = function() {
    root._ = previousUnderscore;
    return this;
  };

  // Keep the identity function around for default iteratees.
  // 保持默认迭代周围的身份功能。
  _.identity = function(value) {
    return value;
  };

  // Predicate-generating functions. Often useful outside of Underscore.
  // Predicate-generating 函数。经常在Underscore之外有用
  _.constant = function(value) {
    return function() {
      return value;
    };
  };

  _.noop = function(){};

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

  // Generates a function for a given object that returns a given property.
  // 为给定的对象生成返回给定属性的函数。
  _.propertyOf = function(obj) {
    return obj == null ? function(){} : function(key) {
      return obj[key];
    };
  };

  // Returns a predicate for checking whether an object has a given set of
  // `key:value` pairs.
  // 返回一个predicate，用于检查对象是否具有给定的一组 key:value对。
  _.matcher = _.matches = function(attrs) {
    attrs = _.extendOwn({}, attrs);
    return function(obj) {
      return _.isMatch(obj, attrs);
    };
  };

  // Run a function **n** times.
  // 运行一个函数n次
  _.times = function(n, iteratee, context) {
    var accum = Array(Math.max(0, n));
    iteratee = optimizeCb(iteratee, context, 1);
    for (var i = 0; i < n; i++) accum[i] = iteratee(i);
    return accum;
  };

  // Return a random integer between min and max (inclusive).
  // 返回最小和最大（含）之间的随机整数。
  _.random = function(min, max) {
    if (max == null) {
      max = min;
      min = 0;
    }
    return min + Math.floor(Math.random() * (max - min + 1));
  };

  // A (possibly faster) way to get the current timestamp as an integer.
  // 一个（可能更快）的方式来获取当前时间戳为整数。
  _.now = Date.now || function() {
    return new Date().getTime();
  };

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

  // Functions for escaping and unescaping strings to/from HTML interpolation.
  // 用于从/插入HTML插入转义字符串的功能。
  var createEscaper = function(map) {
    var escaper = function(match) {
      return map[match];
    };
    // Regexes for identifying a key that needs to be escaped
    // 用于识别需要转义的键的正则表达式
    var source = '(?:' + _.keys(map).join('|') + ')';
    var testRegexp = RegExp(source);
    var replaceRegexp = RegExp(source, 'g');
    return function(string) {
      string = string == null ? '' : '' + string;
      return testRegexp.test(string) ? string.replace(replaceRegexp, escaper) : string;
    };
  };
  _.escape = createEscaper(escapeMap);
  _.unescape = createEscaper(unescapeMap);

  // If the value of the named `property` is a function then invoke it with the
  // `object` as context; otherwise, return it.
  // 如果命名的值property是一个函数，然后使用objectas context 调用它 ; 否则返回
  _.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;
  };

  // Generate a unique integer id (unique within the entire client session).
  // Useful for temporary DOM ids.
  // 生成唯一的整数id（整个客户端会话中唯一）。适用于临时DOM ID。
  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.
  // 默认情况下，Underscore使用ERB样式模板分隔符，将以下模板设置更改为使用替代分隔符。
  _.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.
  // 自定义时templateSettings，如果不想定义内插，评估或转义正则表达式，则需要保证不匹配的内容。
  var noMatch = /(.)^/;

  // Certain characters need to be escaped so that they can be put into a
  // string literal.
  // 某些角色需要被转义，以便将它们放入一个字符串文字。
  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.
  // JavaScript微模板，类似于John Resig的实现。下划线模板处理任意分隔符，保留空格，并在内插代码中正确转义引号。注意：oldSettings仅存在向后兼容性。
  _.template = function(text, settings, oldSettings) {
    if (!settings && oldSettings) settings = oldSettings;
    settings = _.defaults({}, settings, _.templateSettings);

    // Combine delimiters into one regular expression via alternation.
    // 通过交替将分隔符合并成一个正则表达式。
    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.
    // 编译模板源，适当地转义字符串文字。
    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.
      // Adobe VM需要返回的匹配来产生正确的最差。
      return match;
    });
    source += "';\n";

    // If a variable is not specified, place data values in local scope.
    // 如果未指定变量，请将数据值放在本地作用域中
    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.
    // 提供编译源，以方便预编译。
    var argument = settings.variable || 'obj';
    template.source = 'function(' + argument + '){\n' + source + '}';

    return template;
  };

  // Add a "chain" function. Start chaining a wrapped Underscore object.
  // 添加“链”功能。开始链接一个包裹的Underscore对象。
  _.chain = function(obj) {
    var instance = _(obj);
    instance._chain = true;
    return instance;
  };

  // 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.
  // 如果将Underscore作为函数调用，则返回可以使用OO样式的包装对象。此包装器保留所有下划线功能的更改版本。包裹的对象可能被链接。

  // Helper function to continue chaining intermediate results.
  // 助手功能继续链接中间结果
  var result = function(instance, obj) {
    return instance._chain ? _(obj).chain() : obj;
  };

  // Add your own custom functions to the Underscore object.
  // 将自己的自定义函数添加到Underscore对象。
  _.mixin = function(obj) {
    _.each(_.functions(obj), function(name) {
      var func = _[name] = obj[name];
      _.prototype[name] = function() {
        var args = [this._wrapped];
        push.apply(args, arguments);
        return result(this, func.apply(_, args));
      };
    });
  };

  // Add all of the Underscore functions to the wrapper object.
  // 将所有的Underscore函数添加到包装器对象。
  _.mixin(_);

  // Add all mutator Array functions to the wrapper.
  // 将所有mutator Array函数添加到包装器
  _.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.
  // 将所有访问器Array函数添加到包装器。
  _.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.
  // 从包装和链接对象中提取结果。
  _.prototype.value = function() {
    return this._wrapped;
  };

  // Provide unwrapping proxy for some methods used in engine operations
  // such as arithmetic and JSON stringification.
  // 为引擎操作中使用的某些方法（如算术和JSON字符串）提供解包代理。
  _.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.
  // AMD注册发生在最后，与AMD加载程序兼容，这些加载程序可能不会对模块执行下一个转折语义。尽管AMD注册的一般做法是匿名的，但是将注册表作为一个命名模块，因为像jQuery一样，它是一个基础库，其流行度足以捆绑在第三方库中，但不是AMD加载请求的一部分。当在加载程序请求之外调用匿名define（）时，这些情况可能会产生错误。
  if (typeof define === 'function' && define.amd) {
    define('underscore', [], function() {
      return _;
    });
  }
}.call(this));