underscore.js

//     Underscore.js 1.9.1
// 阅读源码，其实不少方法原生 js 已经提供，比如数组的 map，forEach 等
// 提高效率的做法，随着浏览器的更新，优势已经不明显
// 可能有一天，underscore 也会成为历史

(function() {

  // 基础配置
  // --------------

  // Establish the root object, `window` (`self`) in the browser, `global`
  // on the server, or `this` in some virtual machines. We use `self`
  // instead of `window` for `WebWorker` support.
  // 找出顶层对象。浏览器端是 window，node 端是 global
  // 用 self 不是 window 来判断，是因为 webworker 中没有 window
  var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this ||
            {};

  // 保存之前的 _ 变量
  var previousUnderscore = root._;

  // 减小 minified 体积
  var ArrayProto = Array.prototype, ObjProto = Object.prototype;
  var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;

  // 保持引用，提高访问 prototype 速度
  var push = ArrayProto.push,
      slice = ArrayProto.slice,
      toString = ObjProto.toString,
      hasOwnProperty = ObjProto.hasOwnProperty;

  // 需要用到的 es5 方法
  var nativeIsArray = Array.isArray,
      nativeKeys = Object.keys,
      nativeCreate = Object.create;

  // 一个空方法，在实现原型继承的时候会用到
  var Ctor = function(){};

  // Create a safe reference to the Underscore object for use below.
  var _ = function(obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
  };

  // 为 node.js 中使用。检查 nodeType 是为了保证 exports 和 module 不是 html 元素
  if (typeof exports != 'undefined' && !exports.nodeType) {
    if (typeof module != 'undefined' && !module.nodeType && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

  // 当前版本
  _.VERSION = '1.9.1';

  // 返回绑定 context 的函数
  // 区别对待了 call 和 apply。其实随着浏览器发展，这么做的优势已经不明显
  // 没记错的话，vue 源码中有类似的实现，不过已经加了注释，表示保留该方法只是历史原因
  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);
      };
      // The 2-argument case is omitted because we’re not using it.
      case 3: return function(value, index, collection) {
        return func.call(context, value, index, collection);
      };
      case 4: return function(accumulator, value, index, collection) {
        return func.call(context, accumulator, value, index, collection);
      };
    }
    return function() {
      return func.apply(context, arguments);
    };
  };

  var builtinIteratee;

  // An internal function to generate callbacks that can be applied to each
  // element in a collection, returning the desired result — either `identity`,
  // an arbitrary callback, a property matcher, or a property accessor.
  var cb = function(value, context, argCount) {
    if (_.iteratee !== builtinIteratee) return _.iteratee(value, context);
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    if (_.isObject(value) && !_.isArray(value)) return _.matcher(value);
    return _.property(value);
  };

  // 回调生成器的包装函数，简化调用
  _.iteratee = builtinIteratee = function(value, context) {
    return cb(value, context, Infinity);
  };

  // Some functions take a variable number of arguments, or a few expected
  // arguments at the beginning and then a variable number of values to operate
  // on. This helper accumulates all remaining arguments past the function’s
  // argument length (or an explicit `startIndex`), into an array that becomes
  // the last argument. Similar to ES6’s "rest parameter".
  // 和 es6 中的 rest 参数类似
  var restArguments = function(func, startIndex) {
    startIndex = startIndex == null ? func.length - 1 : +startIndex;
    return function() {
      var length = Math.max(arguments.length - startIndex, 0),
          rest = Array(length),
          index = 0;
      for (; index < length; index++) {
        rest[index] = arguments[index + startIndex];
      }
      switch (startIndex) {
        case 0: return func.call(this, rest);
        case 1: return func.call(this, arguments[0], rest);
        case 2: return func.call(this, arguments[0], arguments[1], rest);
      }
      var args = Array(startIndex + 1);
      for (index = 0; index < startIndex; index++) {
        args[index] = arguments[index];
      }
      args[startIndex] = rest;
      return func.apply(this, args);
    };
  };

  // 创建一个对象的继承对象
  var baseCreate = function(prototype) {
    // 不是对象，返回空对象
    if (!_.isObject(prototype)) return {};
    // 支持 create 用 create
    if (nativeCreate) return nativeCreate(prototype);
    // 不支持的话，用原型
    Ctor.prototype = prototype;
    var result = new Ctor;
    Ctor.prototype = null;
    return result;
  };

  // 返回一个闭包函数
  var shallowProperty = function(key) {
    // 返回 key 的属性值
    return function(obj) {
      return obj == null ? void 0 : obj[key];
    };
  };
  // 返回对象是否有某个属性
  var has = function(obj, path) {
    return obj != null && hasOwnProperty.call(obj, path);
  }
  // 深度获取对象的属性
  // 比如：
  // obj = {
  //   far: {
  //     bar: {
  //       name: 'bar'
  //     }
  //   }
  // }
  // deepGet(obj, ['far', 'bar', 'name'])
  var deepGet = function(obj, path) {
    var length = path.length;
    for (var i = 0; i < length; i++) {
      if (obj == null) return void 0;
      obj = obj[path[i]];
    }
    return length ? obj : void 0;
  };

  // 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 = shallowProperty('length');
  // 判断是否是类数组对象，有 length 属性，length 类型为 number 且大于0 小于 MAX_ARRAY_INDEX
  var isArrayLike = function(collection) {
    var length = getLength(collection);
    return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
  };

  // 数组方法
  // --------------------

  // forEach 的实现。
  // for 循环遍历类数组对象，keys 遍历对象
  _.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;
  };

  // map 实现
  _.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;
  };

  // reduce 生成器，dir 大于0，从左到右
  var createReduce = function(dir) {
    // 之前的版本使用 arguments.length，有性能问题
    // https://github.com/jashkenas/underscore/pull/1991/commits/2d7a68b34042efd6a3f4be2500e59c158ceccba1
    var reducer = function(obj, iteratee, memo, initial) {
      var keys = !isArrayLike(obj) && _.keys(obj),
          length = (keys || obj).length,
          index = dir > 0 ? 0 : length - 1;
      if (!initial) {
        memo = obj[keys ? keys[index] : index];
        index += dir;
      }
      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) {
      var initial = arguments.length >= 3;
      return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial);
    };
  };

  // 从左到右
  _.reduce = _.foldl = _.inject = createReduce(1);

  // 从右到左
  _.reduceRight = _.foldr = createReduce(-1);

  // 返回第一个通过检测的值，类似 indexOf 的作用
  _.find = _.detect = function(obj, predicate, context) {
    var keyFinder = isArrayLike(obj) ? _.findIndex : _.findKey;
    var key = keyFinder(obj, predicate, context);
    if (key !== void 0 && key !== -1) return obj[key];
  };

  // 返回所有通过检测的值，就是实现了filter
  _.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;
  };

  // filter 之外的值
  _.reject = function(obj, predicate, context) {
    return _.filter(obj, _.negate(cb(predicate)), context);
  };

  // every 的实现
  _.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;
  };

  // some 的实现
  _.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;
  };

  // 数组的 includes 方法
  _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) {
    if (!isArrayLike(obj)) obj = _.values(obj);
    if (typeof fromIndex != 'number' || guard) fromIndex = 0;
    return _.indexOf(obj, item, fromIndex) >= 0;
  };

  // Invoke a method (with arguments) on every item in a collection.
  _.invoke = restArguments(function(obj, path, args) {
    var contextPath, func;
    if (_.isFunction(path)) {
      func = path;
    } else if (_.isArray(path)) {
      contextPath = path.slice(0, -1);
      path = path[path.length - 1];
    }
    return _.map(obj, function(context) {
      var method = func;
      if (!method) {
        if (contextPath && contextPath.length) {
          context = deepGet(context, contextPath);
        }
        if (context == null) return void 0;
        method = context[path];
      }
      return method == null ? method : method.apply(context, args);
    });
  });

  // 返回对象的属性，key 是数组的话，调用 deepGet
  _.pluck = function(obj, key) {
    return _.map(obj, _.property(key));
  };

  // 返回对象列表中有指定键值对的对象
  // obj 应为一个对象类数组
  _.where = function(obj, attrs) {
    return _.filter(obj, _.matcher(attrs));
  };

  // 返回第一个符合键值对的值
  _.findWhere = function(obj, attrs) {
    return _.find(obj, _.matcher(attrs));
  };

  // 返回 obj 中最大值
  _.max = function(obj, iteratee, context) {
    var result = -Infinity, lastComputed = -Infinity,
        value, computed;
    if (iteratee == null || typeof iteratee == 'number' && typeof obj[0] != 'object' && obj != null) {
      obj = isArrayLike(obj) ? obj : _.values(obj);
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value != null && value > result) {
          result = value;
        }
      }
    } else {
      iteratee = cb(iteratee, context);
      _.each(obj, function(v, index, list) {
        computed = iteratee(v, index, list);
        if (computed > lastComputed || computed === -Infinity && result === -Infinity) {
          result = v;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // 返回最小值
  _.min = function(obj, iteratee, context) {
    var result = Infinity, lastComputed = Infinity,
        value, computed;
    if (iteratee == null || typeof iteratee == 'number' && typeof obj[0] != 'object' && obj != null) {
      obj = isArrayLike(obj) ? obj : _.values(obj);
      for (var i = 0, length = obj.length; i < length; i++) {
        value = obj[i];
        if (value != null && value < result) {
          result = value;
        }
      }
    } else {
      iteratee = cb(iteratee, context);
      _.each(obj, function(v, index, list) {
        computed = iteratee(v, index, list);
        if (computed < lastComputed || computed === Infinity && result === Infinity) {
          result = v;
          lastComputed = computed;
        }
      });
    }
    return result;
  };

  // 打乱数组
  _.shuffle = function(obj) {
    return _.sample(obj, Infinity);
  };

  // Sample **n** random values from a collection using the modern version of the
  // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle).
  // If **n** is not specified, returns a single random element.
  // The internal `guard` argument allows it to work with `map`.
  // 打乱数组
  _.sample = function(obj, n, guard) {
    if (n == null || guard) {
      if (!isArrayLike(obj)) obj = _.values(obj);
      return obj[_.random(obj.length - 1)];
    }
    var sample = isArrayLike(obj) ? _.clone(obj) : _.values(obj);
    var length = getLength(sample);
    n = Math.max(Math.min(n, length), 0);
    var last = length - 1;
    for (var index = 0; index < n; index++) {
      var rand = _.random(index, last);
      var temp = sample[index];
      sample[index] = sample[rand];
      sample[rand] = temp;
    }
    return sample.slice(0, n);
  };

  // 返回排序后的数组副本
  _.sortBy = function(obj, iteratee, context) {
    var index = 0;
    iteratee = cb(iteratee, context);
    return _.pluck(_.map(obj, function(value, key, list) {
      return {
        value: value,
        index: index++,
        criteria: iteratee(value, key, 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');
  };

  // 工具方法
  var group = function(behavior, partition) {
    return function(obj, iteratee, context) {
      var result = partition ? [[], []] : {};
      iteratee = cb(iteratee, context);
      _.each(obj, function(value, index) {
        var key = iteratee(value, index, obj);
        behavior(result, value, key);
      });
      return result;
    };
  };

  // 归类集合
  // groupBy 函数的必要参数有两个
  // obj 需要进行遍历的类数组，iterate 对数组每一项进行的操作
  // iterate 不一定需要是 function，因为group 方法中进行了如下操作：
  // iteratee = cb(iteratee, context);
  // 如果 iterate 是 function，
  // _.groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); });
  // => {1: [1.3], 2: [2.1, 2.4]}
  // iterate 可以是一个表示 key 的字符串
  // _.groupBy(['one', 'two', 'three'], 'length');
  // => {3: ["one", "two"], 5: ["three"]}
  _.groupBy = group(function(result, value, key) {
    if (has(result, key)) result[key].push(value); else result[key] = [value];
  });

  // 与 groupBy 类似，键唯一的时候使用
  _.indexBy = group(function(result, value, key) {
    result[key] = value;
  });

  // 计数
  _.countBy = group(function(result, value, key) {
    if (has(result, key)) result[key]++; else result[key] = 1;
  });

  var reStrSymbol = /[^\ud800-\udfff]|[\ud800-\udbff][\udc00-\udfff]|[\ud800-\udfff]/g;
  // 类数组转数组
  _.toArray = function(obj) {
    if (!obj) return [];
    if (_.isArray(obj)) return slice.call(obj);
    if (_.isString(obj)) {
      // Keep surrogate pair characters together
      return obj.match(reStrSymbol);
    }
    if (isArrayLike(obj)) return _.map(obj, _.identity);
    return _.values(obj);
  };

  // 返回数组长度或对象key数量
  _.size = function(obj) {
    if (obj == null) return 0;
    return isArrayLike(obj) ? obj.length : _.keys(obj).length;
  };

  // 根据 partition 条件，把数组一分为二
  _.partition = group(function(result, value, pass) {
    result[pass ? 0 : 1].push(value);
  }, true);

  // 数组方法
  // ---------------

  // 返回数组第一个元素
  _.first = _.head = _.take = function(array, n, guard) {
    if (array == null || array.length < 1) return n == null ? void 0 : [];
    if (n == null || guard) return array[0];
    return _.initial(array, array.length - n);
  };

  // 返回除最后一个元素外，所有元素
  _.initial = function(array, n, guard) {
    return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
  };

  // 返回最后的 n 个元素
  _.last = function(array, n, guard) {
    if (array == null || array.length < 1) return n == null ? 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.
  _.rest = _.tail = _.drop = function(array, n, guard) {
    return slice.call(array, n == null || guard ? 1 : n);
  };

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

  // 扁平化数组
  var flatten = function(input, shallow, strict, output) {
    output = output || [];
    var idx = output.length;
    for (var i = 0, length = getLength(input); i < length; i++) {
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
        // Flatten current level of array or arguments object.
        if (shallow) {
          var j = 0, len = value.length;
          while (j < len) output[idx++] = value[j++];
        } else {
          flatten(value, shallow, strict, output);
          idx = output.length;
        }
      } else if (!strict) {
        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);
  };

  // 返回不包含指定值的数组
  _.without = restArguments(function(array, otherArrays) {
    return _.difference(array, otherArrays);
  });

  // 数组去重，如果已经排序，效率更高
  _.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 = [];
    for (var i = 0, length = getLength(array); i < length; i++) {
      var value = array[i],
          computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted && !iteratee) {
        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;
  };

  // 返回数组并集。flatten 方法扁平化数组，uniq 去重
  _.union = restArguments(function(arrays) {
    return _.uniq(flatten(arrays, true, true));
  });

  //去交集
  _.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;
      var j;
      for (j = 1; j < argsLength; j++) {
        if (!_.contains(arguments[j], item)) break;
      }
      if (j === argsLength) result.push(item);
    }
    return result;
  };

  // 返回 array 存在，rest 中不存在的数据
  _.difference = restArguments(function(array, rest) {
    rest = flatten(rest, true, true);
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    });
  });

  // 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;
  };

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

  // 数组转换为对象
  _.object = function(list, values) {
    var result = {};
    for (var i = 0, length = getLength(list); i < length; i++) {
      if (values) {
        result[list[i]] = values[i];
      } else {
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

  // 返回的函数用来生成 findIndex 和 findLastIndex 函数.
  var createPredicateIndexFinder = function(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;
    };
  };

  // indexOf
  _.findIndex = createPredicateIndexFinder(1);
  _.findLastIndex = createPredicateIndexFinder(-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.
  _.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;
  };

  // Generator function to create the indexOf and lastIndexOf functions.
  var createIndexFinder = function(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;
    };
  };

  // 返回第一个、最后一个符合条件的元素
  _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
  _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);

  // 类似Python 的 range 函数，返回一组整数
  _.range = function(start, stop, step) {
    if (stop == null) {
      stop = start || 0;
      start = 0;
    }
    if (!step) {
      step = stop < start ? -1 : 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;
  };

  // 一维数组转换为二维，每个数组元素包含 count 个 原数组元素
  // chunk([1,2,3,4,5], 2) =》 [[1, 2], [3, 4], [5]]
  _.chunk = function(array, count) {
    if (count == null || count < 1) return [];
    var result = [];
    var i = 0, length = array.length;
    while (i < length) {
      result.push(slice.call(array, i, i += count));
    }
    return result;
  };

  // 函数相关的方法
  // ------------------

  // 根据参数判断是否调用构造函数
  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;
  };

  // bind 方法
  _.bind = restArguments(function(func, context, args) {
    if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
    var bound = restArguments(function(callArgs) {
      return executeBound(func, bound, context, this, args.concat(callArgs));
    });
    return bound;
  });

  // 为函数填充部分参数
  _.partial = restArguments(function(func, boundArgs) {
    var placeholder = _.partial.placeholder;
    var bound = function() {
      var position = 0, length = boundArgs.length;
      var args = Array(length);
      for (var i = 0; i < length; i++) {
        args[i] = boundArgs[i] === placeholder ? arguments[position++] : boundArgs[i];
      }
      while (position < arguments.length) args.push(arguments[position++]);
      return executeBound(func, bound, this, this, args);
    };
    return bound;
  });

  _.partial.placeholder = _;

  // 绑定 this
  _.bindAll = restArguments(function(obj, keys) {
    keys = flatten(keys, false, false);
    var index = keys.length;
    if (index < 1) throw new Error('bindAll must be passed function names');
    while (index--) {
      var key = keys[index];
      obj[key] = _.bind(obj[key], obj);
    }
  });

  // 缓存函数运行结果
  _.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;
  };

  // 延迟 wait 毫秒执行函数
  _.delay = restArguments(function(func, wait, args) {
    return setTimeout(function() {
      return func.apply(null, args);
    }, wait);
  });

  // 当前调用栈清空后再调用函数，类似 setTimeout 延迟 0 秒
  _.defer = _.partial(_.delay, _, 1);

  // throttle debounce 函数的节流与防抖
  _.throttle = function(func, wait, options) {
    var timeout, context, args, result;
    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;
    };

    var throttled = 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;
    };

    throttled.cancel = function() {
      clearTimeout(timeout);
      previous = 0;
      timeout = context = args = null;
    };

    return throttled;
  };


  _.debounce = function(func, wait, immediate) {
    var timeout, result;

    var later = function(context, args) {
      timeout = null;
      if (args) result = func.apply(context, args);
    };

    var debounced = restArguments(function(args) {
      if (timeout) clearTimeout(timeout);
      if (immediate) {
        var callNow = !timeout;
        timeout = setTimeout(later, wait);
        if (callNow) result = func.apply(this, args);
      } else {
        timeout = _.delay(later, wait, this, args);
      }

      return result;
    });

    debounced.cancel = function() {
      clearTimeout(timeout);
      timeout = null;
    };

    return debounced;
  };

  // 将第一个函数 function 封装到函数 wrapper 里面, 并把函数 function 作为第一个参数传给 wrapper.
  _.wrap = function(func, wrapper) {
    return _.partial(wrapper, func);
  };

  // 否定版本 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, 也就是一个函数执行完之后把返回的结果再作为参数赋给下一个函数来执行. 以此类推. 在数学里, 把函数 f(), g(), 和 h() 组合起来可以得到复合函数 f(g(h()))。
  _.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;
    };
  };

  // 执行 times 次后，才会真正执行
  _.after = function(times, func) {
    return function() {
      if (--times < 1) {
        return func.apply(this, arguments);
      }
    };
  };

  // 至多调用 times - 1 次
  _.before = function(times, func) {
    var memo;
    return function() {
      if (--times > 0) {
        memo = func.apply(this, arguments);
      }
      if (times <= 1) func = null;
      return memo;
    };
  };

  // 至多调用一次
  _.once = _.partial(_.before, 2);

  _.restArguments = restArguments;

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

  // Keys in IE < 9 that won't be iterated by `for key in ...` and thus missed.
  var hasEnumBug = !{toString: null}.propertyIsEnumerable('toString');
  var nonEnumerableProps = ['valueOf', 'isPrototypeOf', 'toString',
    'propertyIsEnumerable', 'hasOwnProperty', 'toLocaleString'];

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

    // Constructor is a special case.
    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);
      }
    }
  };

  // `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.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // for in 遍历，所有属性，包括原型
  _.allKeys = function(obj) {
    if (!_.isObject(obj)) return [];
    var keys = [];
    for (var key in obj) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
  };

  // 返回所有属性值
  _.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;
  };

  // 与 map 方法相比，遍历的是一个对象
  _.mapObject = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys = _.keys(obj),
        length = keys.length,
        results = {};
    for (var index = 0; index < length; index++) {
      var currentKey = keys[index];
      results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
    }
    return results;
  };

  // 返回键值对
  _.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;
  };

  // 互换键值对，值应是可序列化的
  _.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;
  };

  // 返回 obj 的 所有方法名
  _.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
  };

  // An internal function for creating assigner functions.
  var createAssigner = function(keysFunc, defaults) {
    return function(obj) {
      var length = arguments.length;
      if (defaults) obj = Object(obj);
      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 (!defaults || obj[key] === void 0) obj[key] = source[key];
        }
      }
      return obj;
    };
  };

  // 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);

  // 返回第一个 符合 predicate 的 key
  _.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;
    }
  };

  // obj 是否 有 key 属性.
  var keyInObj = function(value, key, obj) {
    return key in obj;
  };

  // 返回只有 keys 属性的 obj 副本.
  _.pick = restArguments(function(obj, keys) {
    var result = {}, iteratee = keys[0];
    if (obj == null) return result;
    if (_.isFunction(iteratee)) {
      if (keys.length > 1) iteratee = optimizeCb(iteratee, keys[1]);
      keys = _.allKeys(obj);
    } else {
      iteratee = keyInObj;
      keys = flatten(keys, false, false);
      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;
  });

  // 返回过滤出除去 keys 后的 obj
  _.omit = restArguments(function(obj, keys) {
    var iteratee = keys[0], context;
    if (_.isFunction(iteratee)) {
      iteratee = _.negate(iteratee);
      if (keys.length > 1) context = keys[1];
    } else {
      keys = _.map(flatten(keys, false, false), String);
      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);

  // Object.create
  _.create = function(prototype, props) {
    var result = baseCreate(prototype);
    if (props) _.extendOwn(result, props);
    return result;
  };

  // 浅拷贝
  _.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 obj。这种方法的主要意图是作为函数链式调用 的一环, 为了对此对象执行操作并返回对象本身。
  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

  // 返回对象是否有给定键值对
  _.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`.
  var eq, deepEq;
  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;
    // `null` or `undefined` only equal to itself (strict comparison).
    if (a == null || b == null) return false;
    // `NaN`s are equivalent, but non-reflexive.
    if (a !== a) return b !== b;
    // Exhaust primitive checks
    var type = typeof a;
    if (type !== 'function' && type !== 'object' && typeof b != 'object') return false;
    return deepEq(a, b, aStack, bStack);
  };

  // Internal recursive comparison function for `isEqual`.
  deepEq = function(a, b, aStack, bStack) {
    // 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;
      case '[object Symbol]':
        return SymbolProto.valueOf.call(a) === SymbolProto.valueOf.call(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;
  };

  // 深度遍历判断是否相等
  _.isEqual = function(a, b) {
    return eq(a, b);
  };

  // 是否为空数组或空对象
  _.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;
  };

  // 是否是 DOM element?
  _.isElement = function(obj) {
    return !!(obj && obj.nodeType === 1);
  };

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

  // 是否是 object?
  _.isObject = function(obj) {
    var type = typeof obj;
    return type === 'function' || type === 'object' && !!obj;
  };

  // is 方法: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError, isMap, isWeakMap, isSet, isWeakSet.
  _.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error', 'Symbol', 'Map', 'WeakMap', 'Set', 'WeakSet'], 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.
  if (!_.isArguments(arguments)) {
    _.isArguments = function(obj) {
      return has(obj, 'callee');
    };
  }

  // 是否为函数
  var nodelist = root.document && root.document.childNodes;
  if (typeof /./ != 'function' && typeof Int8Array != 'object' && typeof nodelist != 'function') {
    _.isFunction = function(obj) {
      return typeof obj == 'function' || false;
    };
  }

  // 无穷
  _.isFinite = function(obj) {
    return !_.isSymbol(obj) && isFinite(obj) && !isNaN(parseFloat(obj));
  };

  // 是否是 `NaN`?
  _.isNaN = function(obj) {
    return _.isNumber(obj) && isNaN(obj);
  };

  // 是否是一个 Boolean 值
  _.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
  };

  // null
  _.isNull = function(obj) {
    return obj === null;
  };

  // 是否为 undefined
  _.isUndefined = function(obj) {
    return obj === void 0;
  };

  // 检测对象是否有对应属性，可检测嵌套结构
  _.has = function(obj, path) {
    if (!_.isArray(path)) {
      return has(obj, path);
    }
    var length = path.length;
    for (var i = 0; i < length; i++) {
      var key = path[i];
      if (obj == null || !hasOwnProperty.call(obj, key)) {
        return false;
      }
      obj = obj[key];
    }
    return !!length;
  };

  // 工具函数
  // -----------------

  // 防止命名冲突，类似 jQuery 的 noConflict 方法
  _.noConflict = function() {
    root._ = previousUnderscore;
    return this;
  };

  // 默认迭代器，什么都没做
  _.identity = function(value) {
    return value;
  };

  // 始终返回相同的值
  _.constant = function(value) {
    return function() {
      return value;
    };
  };

  // 空函数
  _.noop = function(){};

  // Creates a function that, when passed an object, will traverse that object’s
  // properties down the given `path`, specified as an array of keys or indexes.
  _.property = function(path) {
    if (!_.isArray(path)) {
      return shallowProperty(path);
    }
    return function(obj) {
      return deepGet(obj, path);
    };
  };

  // Generates a function for a given object that returns a given property.
  _.propertyOf = function(obj) {
    if (obj == null) {
      return function(){};
    }
    return function(path) {
      return !_.isArray(path) ? obj[path] : deepGet(obj, path);
    };
  };

  // 闭包，返回的函数检测传入参数是否有指定键值对
  _.matcher = _.matches = function(attrs) {
    attrs = _.extendOwn({}, attrs);
    return function(obj) {
      return _.isMatch(obj, attrs);
    };
  };

  // Run a function **n** times.
  _.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;
  };

  // 返回 min 和 max 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.
  _.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);

  // Functions for escaping and unescaping strings to/from HTML interpolation.
  var createEscaper = function(map) {
    var escaper = function(match) {
      return map[match];
    };
    // Regexes for identifying a key that needs to be escaped.
    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);

  // Traverses the children of `obj` along `path`. If a child is a function, it
  // is invoked with its parent as context. Returns the value of the final
  // child, or `fallback` if any child is undefined.
  _.result = function(obj, path, fallback) {
    if (!_.isArray(path)) path = [path];
    var length = path.length;
    if (!length) {
      return _.isFunction(fallback) ? fallback.call(obj) : fallback;
    }
    for (var i = 0; i < length; i++) {
      var prop = obj == null ? void 0 : obj[path[i]];
      if (prop === void 0) {
        prop = fallback;
        i = length; // Ensure we don't continue iterating.
      }
      obj = _.isFunction(prop) ? prop.call(obj) : prop;
    }
    return obj;
  };

  // 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 escapeRegExp = /\\|'|\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(escapeRegExp, 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 offset.
      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';

    var render;
    try {
      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;
  };

  // Add a "chain" function. Start chaining a wrapped Underscore object.
  _.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.

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

  // 自有函数添加到 _
  _.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 chainResult(this, func.apply(_, args));
      };
    });
    return _;
  };

  // Add all of the Underscore functions to the wrapper object.
  _.mixin(_);

  // 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 chainResult(this, obj);
    };
  });

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

  // amd 注册方式，通常情况下是注册为一个匿名的模块，不过 underscore 足够出名，
  // 所以可以有名字
  if (typeof define == 'function' && define.amd) {
    define('underscore', [], function() {
      return _;
    });
  }
}());