摘要：為情況下，直接返回執行過程，對右側表達式求值。包裝執行函數上下文創建迭代函數，可選遍歷方向是的簡寫。遍歷整個，返回匹配參數所列出的所有鍵值對的第一個值。傳遞一個數字表示從中返回個隨機元素。對集合每個元素調用函數后，得到。

// Internal function that returns an efficient (for current engines) version
// of the passed-in callback, to be repeatedly applied in other Underscore
// functions.
// 內部復用函數，用于改變函數上下文。

var optimizeCb = function(func, context, argCount) {
    //context為undefined情況下，直接返回func
    // void 0
    // 執行過程：1，對右側表達式求值。2.返回undefined
    // 為何這樣多此一舉：js中，undefined不是保留字，可以創建windows.undefined=anything.void返回undefined較安全。
    // 用處：1返回undefined。2a標簽不跳轉href=javascript:(void 0)3img空圖片src=javascript:(void 0)
    if (context === void 0) return func;
    //有傳入context情況下，將func上下文改為context
    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);
            };
            //_.reduce中使用
        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.
// 內部函數，生成適用于集合每個元素的回調函數
// 傳入null等空值，得到等價函數
// 傳入function，得到綁定了上下文的函數
// 傳入對象，得到匹配函數
// 傳入其他，得到屬性訪問函數

var cb = function(value, context, argCount) {
    //?？是何用意？？
    if (_.iteratee !== builtinIteratee) return _.iteratee(value, context);
    //返回null
    if (value == null) return _.identity;
    //如果是函數，綁定函數上下文
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    //如果是對象，返回匹配函數
    if (_.isObject(value) && !_.isArray(value)) return _.matcher(value);
    //返回獲取屬性值函數
    return _.property(value);
};

// External wrapper for our callback generator. Users may customize
// _.iteratee if they want additional predicate/iteratee shorthand styles.
// This abstraction hides the internal-only argCount argument.
// 外部函數，返回cb函數結果

_.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".
// 用于實參個數不定的函數的固定參數及剩余參數整理。
// example:_.invoke
// func.length=3，so startIndex=2,即從第三個參數開始，都存入rest變量中，并作為第三個參數傳入。第一二個參數不變。
// example:_.without
// func.length=2，so startIndex=1,即從第二個參數開始，都存入rest變量中，并作為第二個參數傳入第一個參數不變。
// example:_.union
// func.length=1，so startIndex=0,即將所有參數都傳入rest變量中，并作為第一個參數傳入。
// 最后一種情況，func形參個數大于3，不再使用call手動傳入參數
// 而是利用apply的數組參數特性，將參數保存在數組中，再傳入函數。

var restArguments = function(func, startIndex) {
    console.log("func2")
    //func.length->形參個數        
    startIndex = startIndex == null ? func.length - 1 : +startIndex;
    return function() { //func3
        // arguments.length->實參個數
        // 個人理解：此處arguments指的是最貼近的這個閉包function在執行時傳入的參數，并不是func在執行時傳入的參數個數。
        console.log("func3")
        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);
    };
};

// Invoke a method (with arguments) on every item in a collection.
// 在集合的每個元素上調用方法path，參數為arguments
// _.invoke([[3,4,5],[65,33,3]],"sort")函數運行順序
// 在underscore.js加載編譯時，restArgument被執行，console func2。并返回匿名函數func3
// 在_.invoke()被執行，即匿名函數func3被執行，console func3。此時的arguments=[[[3,4,5],[65,33,3]],"sort"]。返回匿名函數func執行的值
// 執行func，console func。參數obj = arguments[0]=[[[3,4,5],[65,33,3]],path = arguemnts[1]="sort",args = rest = []
// console func4
// console func4

_.invoke = restArguments(function(obj, path, args) { //func
    console.log("func")
    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) { //func4
        console.log("func4")
        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);
    });
});

// An internal function for creating a new object that inherits from another.

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;
};

// 偏函數與柯里函數區別
// 偏函數：傳入某些固定參數后，返回包含固定參數，并可傳入其他參數的函數
// 柯里函數：將n元參數的函數改變為n次函數，每次都只能傳入一個參數。
// 訪問屬性函數

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

//判斷是否存在自有屬性path

var has = function(obj, path) {
    return obj != null && hasOwnProperty.call(obj, path);
}

//訪問對象的深度多級屬性 函數

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/~jo...
// Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
// js 最大的精確整數值

var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
var getLength = shallowProperty("length");
var isArrayLike = function(collection) {
    var length = getLength(collection);
    return typeof length == "number" && length >= 0 && length <= MAX_ARRAY_INDEX;
};

// Collection Functions
// --------------------
// The cornerstone, an each implementation, aka forEach.
// Handles raw objects in addition to array-likes. Treats all
// sparse array-likes as if they were dense.
// each與map區別
// each中iteratee函數執行結果不保存，返回obj本身
// map中iteratee函數執行結果保存到數組results中，并返回。

_.each = _.forEach = function(obj, iteratee, context) {
    iteratee = optimizeCb(iteratee, context);
    var i,
        keys = !isArrayLike(obj) && _.keys(obj),
        length = (keys || obj).length;
    for (i = 0; i < length; i++) {
        var currentKey = keys ? keys[i] : i;
        iteratee(obj[currentKey], currentKey, obj);
    }
    return obj;
};

// Return the results of applying the iteratee to each element.

_.map = _.collect = function(obj, iteratee, context) {
    //包裝執行函數上下文
    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.
// 創建迭代函數，可選遍歷方向

var createReduce = function(dir) {
    // Wrap code that reassigns argument variables in a separate function than
    // the one that accesses `arguments.length` to avoid a perf hit. (#1991)
    // perf hit是performance hit的簡寫。
    // arguments的使用，會導致一些性能問題。如chrome nodejs的v8引擎在使用arguments會代碼優化步驟被跳過。
    var reducer = function(obj, iteratee, memo, initial) { //func2
        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;
    };
    // 此處返回func1函數，作用域中包含initial變量，存放了arguments.length，
    // 用意就在于將arguments變量從reducer函數中分離出來，
    // 并在調用reducer函數時，釋放argumenrs變量，避免性能問題。
    return function(obj, iteratee, memo, context) { //func1
        var initial = arguments.length >= 3;
        return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial);
    };
};

// Reduce builds up a single result from a list of values, aka inject,
// or foldl.

_.reduce = _.foldl = _.inject = createReduce(1);

// The right-associative version of reduce, also known as foldr.

_.reduceRight = _.foldr = createReduce(-1);

// Return the first value which passes a truth test. Aliased as detect.
// 返回第一個斷言問真的值

_.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];
};

// Return all the elements that pass a truth test.
// Aliased as select.
// 返回所有斷言為真的值組成的數組，中途不中斷

_.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.
// 返回所有斷言為假的值，
// 內部直接調用filter

_.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.
// 判斷是否所有斷言函數都為真，出現假立即返回

_.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.
// 檢測是否至少有一個值斷言為真，出現真立即返回

_.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 item (using ===).
// Aliased as includes and include.
// 判斷從fromIndex索引開始，是否包含item

_.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;
};

// Convenience version of a common use case of map: fetching a property.
// 萃取數組對象中某屬性值，返回一個數組

_.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.
// 遍歷list中的每一個值，返回一個數組，這個數組里的元素包含 properties 所列出的鍵 - 值對。

_.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.
// 遍歷整個list，返回 matches（匹配） properties參數所列出的所有 鍵 - 值 對的第一個值。

_.findWhere = function(obj, attrs) {
    return _.find(obj, _.matcher(attrs));
};

// Return the maximum element (or element-based computation).
// 返回list中的最大值。如果傳遞iteratee參數，iteratee將作為list中每個值的排序依據。
// 如果list為空，將返回-Infinity，所以你可能需要事先用isEmpty檢查 list 。

_.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;
};

// Return the minimum element (or element-based computation).
// 返回list中的最小值。如果傳遞iteratee參數，iteratee將作為list中每個值的排序依據。
// 如果list為空，將返回Infinity，所以你可能需要事先用isEmpty檢查 list 。

_.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;
};

// Shuffle a collection.

_.shuffle = function(obj) {
    return _.sample(obj, Infinity);
};

// Sample n random values from a collection using the modern version of the
// Fisher-Yates shuffle.
// If n is not specified, returns a single random element.
// The internal guard argument allows it to work with map.
// 從 list中產生一個隨機樣本。傳遞一個數字表示從list中返回n個隨機元素。否則將返回一個單一的隨機項。

_.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);
    //取一個符合0-length的合理值
    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);
};

// Sort the object"s values by a criterion produced by an iteratee.
// 返回一個（穩定的）排序后的list拷貝副本。
// 如果傳遞iteratee參數，iteratee將作為list中每個值的排序依據。
// 用來進行排序迭代器也可以是屬性名稱的字符串(比如 length)。

_.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");
};

// An internal function used for aggregate "group by" operations.
// 對集合每個元素調用iteratee函數后，得到key。
// 然后再根據behavior進行分組

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;
    };
};

// Groups the object"s values by a criterion. Pass either a string attribute
// to group by, or a function that returns the criterion.
// 把一個集合分組為多個集合，通過 iterator 返回的結果進行分組.
// 如果 iterator 是一個字符串而不是函數, 那么將使用 iterator 作為各元素的屬性名來對比進行分組。

_.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.
// 給定一個list，和 一個用來返回一個在列表中的每個元素鍵 的iterator 函數（或屬性名）， 返回一個每一項索引的對象。
// 和groupBy非常像，但是當你知道你的鍵是唯一的時候可以使用indexBy 。

_.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.
// 排序一個列表組成多個組，并且返回各組中的對象的數量的計數。
// 類似groupBy，但是不是返回列表的值，而是返回在該組中值的數目。

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

var reStrSymbol = /¹|ud800-udbff|[ud800-udfff]/g;
// Safely create a real, live array from anything iterable.

_.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);
};

// Return the number of elements in an object.

_.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.
// 將 list 拆分為兩個數組：第一個數組其元素都滿足predicate迭代函數， 而第二個的所有元素均不能滿足predicate迭代函數。
// predicate 通過 iteratee 進行轉換，以簡化速記語法。

_.partition = group(function(result, value, pass) {
    result[pass ? 0 : 1].push(value);
}, true);