underscore.js 源码学习 array（未完待续）

    // Array Functions
    // ---------------
    // Get the first element of an array. Passing **n** will return the first N
    // values in the array. Aliased as `head` and `take`. The **guard** check
    // allows it to work with `_.map`.
    _.first = _.head = _.take = function(array, n, guard) {
        //如果array为空的情况下，n也为空返回undefined，n不为空返回[]
        if (array == null || array.length < 1) return n == null ? void 0 : [];
        //n为空，返回数组第一个元素
        if (n == null || guard) return array[0];
        //返回包含数组前n个元素的数组
        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.
    // 返回数组中除了最后一个元素外的其他全部元素。 在arguments对象上特别有用。
    // 传递 n参数将从结果中排除从最后一个开始的n个元素（注：排除数组后面的 n 个元素）。
    _.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.
        // 返回数组的后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.
        // 返回从n个索引开始的元素
    _.rest = _.tail = _.drop = function(array, n, guard) {
        return slice.call(array, n == null || guard ? 1 : n);
    };
        // Trim out all falsy values from an array.
        // 返回数组中的真值
       
    
         _.compact = function(array) {
                return _.filter(array, Boolean);
            };
        // Internal implementation of a recursive `flatten` function.
        // 将嵌套多层数组转换为一位数组
       
    
         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];
                    // 元素类型为数组或者参数
                    if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
                        // Flatten current level of array or arguments object.
                        // 传入shallow的情况下，只展开一层就不再深入
                        if (shallow) {
                            var j = 0,
                                len = value.length;
                            while (j < len) output[idx++] = value[j++];
                        } else {// 否则，迭代展开。
                            flatten(value, shallow, strict, output);
                            idx = output.length;
                        }
                    } else if (!strict) {//非严格模式下，将元素加入输出中。
                        output[idx++] = value;
                    }
                }
                return output;
            };
        // Flatten out an array, either recursively (by default), or just one level.
        // 展开嵌套的多层数组
    _.flatten = function(array, shallow) {
        return flatten(array, shallow, false);
    };
        // Return a version of the array that does not contain the specified value(s).
        // 返回一个删除所有values值后的 array副本。（注：使用===表达式做相等测试。）
    _.without = restArguments(function(array, otherArrays) {
        return _.difference(array, otherArrays);
    });
        // Produce a duplicate-free version of the array. If the array has already
        // been sorted, you have the option of using a faster algorithm.
        // The faster algorithm will not work with an iteratee if the iteratee
        // is not a one-to-one function, so providing an iteratee will disable
        // the faster algorithm.
        // Aliased as `unique`.
        // 返回 array去重后的副本, 使用 === 做相等测试. 
        // 如果您确定 array 已经排序, 那么给 isSorted 参数传递 true值, 此函数将运行的更快的算法. 
        // 如果要处理对象元素, 传递 iteratee函数来获取要对比的属性。
    _.uniq = _.unique = function(array, isSorted, iteratee, context) {
        // 类似jq，做参数兼容方案
        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;
            // 如果数组有序，且不存在迭代器
            if (isSorted && !iteratee) {
                // 如果计算结果不等于seen中，或者i为第一个元素，将元素存入result中
                if (!i || seen !== computed) result.push(value);
                // 将seen替换为最新结果
                seen = computed;
            } else if (iteratee) {//如果存在迭代器
                if (!_.contains(seen, computed)) {//如果seen中不包含计算结果
                    seen.push(computed);//计算结果存入seen中
                    result.push(value);//元素存入result中
                }
            } else if (!_.contains(result, value)) {//如果不存在迭代器
                result.push(value);
            }
        }
        return result;
    };
        // Produce an array that contains the union: each distinct element from all of
        // the passed-in arrays.
       
    
         _.union = restArguments(function(arrays) {
                // 将数组展开后去重
                return _.uniq(flatten(arrays, true, true));
            });
        // Produce an array that contains every item shared between all the
        // passed-in arrays.
        // 返回传入 arrays（数组）交集。结果中的每个值是存在于传入的每个arrays（数组）里。
    _.intersection = function(array) {
        var result = [];
        var argsLength = arguments.length;
        for (var i = 0, length = getLength(array); i < length; i++) {
            var item = array[i];
            // 元素已存在于result中，则继续下一个循环
            if (_.contains(result, item)) continue;
            var j;
            for (j = 1; j < argsLength; j++) {
                if (!_.contains(arguments[j], item)) break;
            }
            //如果所有参数都包含item，则item为交集元素
            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.
        // array中与rest不同的值
       
    
         _.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.
        // 将每个 arrays 中相应位置的值合并在一起。 
        // 当您有通过匹配数组索引进行协调的独立数据源时，这非常有用。 
        // 结合 apply 一起使用传入一个二维数组。
        // 如果你用来处理矩阵嵌套数组时，则可以使用它来转换矩阵。
    _.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.
       
        // 将多个数组，合成一个。
     _.zip = restArguments(_.unzip);
        // 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. Passing by pairs is the reverse of _.pairs.
        // 将数组转换为对象。传递任何一个单独[key, value]对的列表，或者一个键的列表和一个值得列表。成对（Pairs）传递 则是 pairs 的反函数。 
        // 如果存在重复键，最后一个值将被返回。
    _.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;
    };
        // Generator function to create the findIndex and findLastIndex functions.
    var createPredicateIndexFinder = function(dir) {
        return function(array, predicate, context) {
            predicate = cb(predicate, context);
            var length = getLength(array);
            //正向起始位置为0，反向起始位置为数组末尾length-1
            var index = dir > 0 ? 0 : length - 1;
            for (; index >= 0 && index < length; index += dir) {
                if (predicate(array[index], index, array)) return index;
            }
            return -1;
        };
    };
        // Returns the first index on an array-like that passes a predicate test.
    _.findIndex = createPredicateIndexFinder(1);
    _.findLastIndex = createPredicateIndexFinder(-1);
        // 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.
        // 假设array是有序，且是升序
        // 返回iteratee(obj)值在array中的iteratee(array(index))的位置index
    _.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);
            //存在初始位置
            if (typeof idx == 'number') {
                //正向，改变起始位置
                if (dir > 0) {
                    //起始位置 = idx为正?idx:取(length+idx)与0中的最大值
                    i = idx >= 0 ? idx : Math.max(idx + length, i);
                } else {//反向，改变数组长度
                    //lastIndexOf函数功能：
                    //lastIndexOf(searchValue,fromIndex)
                    //若在0-fromIndex之间存在searchValue,则返回最后一个出现的位置
                    //fromIndex为正值，范围则是0-abs(fromIndex)
                    //fromIndex为负值，范围则是0-(length-abs(fromIndex))??此处为什么有+1
                    //看了原生lastIndexOf中就是如此设置的。
                    //数组长度=idx为正?取(idx+1,length):idx + length + 1
                    length = idx >= 0 ? Math.min(idx + 1, length) : idx + length + 1;
                }
            //idx数值以外的值(isSorted=true)，默认array为有序数组，使用sortedIndex查找item位置
            } else if (sortedIndex && idx && length) {
                idx = sortedIndex(array, item);
                return array[idx] === item ? idx : -1;
            }
            //--------------自我理解可简写----------
            //查找值为NaN
            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;
            //--------------自我理解可简写为如下----------
            var iteratee = null;
            if(item!==item) iteratee = _.isNaN
            idx = predicateFind(slice.call(array, i, length), iteratee);
            return idx >= 0 ? idx + i : -1;
            
        };
    };
        // Return the position of the first occurrence of an item in an array,
        // or -1 if the item is not included in the array.
        // If the array is large and already in sort order, pass `true`
        // for **isSorted** to use binary search.
        // 如果array数值大且是升序，可以设置_.indexOf(array,item,isSorted=true)，使用二进制搜索。
    _.indexOf = createIndexFinder(1, _.findIndex, _.sortedIndex);
    _.lastIndexOf = createIndexFinder(-1, _.findLastIndex);
        // Generate an integer Array containing an arithmetic progression. A port of
        // the native Python `range()` function. See
        // [the Python documentation](http://docs.python.org/library/functions.html#range).
    _.range = function(start, stop, step) {
        if (stop == null) {
            stop = start || 0;
            start = 0;
        }
        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;
    };
        // Chunk a single array into multiple arrays, each containing `count` or fewer
        // items.
        // 将 array 分成多个数组，每个数组包含length 或更少的项。
    _.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;
    };