阅读一些著名框架类库的源码，就好像和一个个大师对话，你会学到很多。为什么是 underscore？最主要的原因是 underscore 简短精悍（约 1.5k 行），封装了 100 多个有用的方法，耦合度低，非常适合逐个方法阅读，适合楼主这样的 JavaScript 初学者。从中，你不仅可以学到用 void 0 代替 undefined 避免 undefined 被重写等一些小技巧 ，也可以学到变量类型判断、函数节流&函数去抖等常用的方法，还可以学到很多浏览器兼容的 hack，更可以学到作者的整体设计思路以及 API 设计的原理（向后兼容）。

<a href="http://www.css88.com/doc/underscore/">Underscore文档</a>
<a href="http://www.qdfuns.com/notes/17398/441f8b402179ea95dd833bee9842bad2.html">解读教程</a>

//     Underscore.js 1.8.3
//提供了一整套函数式编程的实用功能，但是没有扩展任何JavaScript内置对象
//它弥补了部分jQuery没有实现的功能,同时又是Backbone.js必不可少的部分。
//闭包
//整个函数在一个闭包中，避免污染全局变量。通过传入this（其实就是window对象）来改变函数的作用域。和jquery的自执行函数其实是异曲同工之妙。
//这种传入全局变量的方式一方面有利于代码阅读，另一方面方便压缩。

//集合类方法、数组类方法、对象类方法、函数类方法、实用工具方法、链式调用
//undefined在es3中不是关键字原因导致的。

//es5中那些方法的模拟、常见的谓词函数is开头的那些、函数节流和函数去抖、函数模板。
//1.短路语句的理解
//2.call和apply的应用（this指向问题）
//3.闭包的应用
//4.定时器的应用
//5.正则表达式的掌握
//6.函数式编程抽象思维
(function() {

    // Baseline setup
    // --------------

    // 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.
    //缓存this，浏览器的window或者服务端的exports
    var root = typeof self == 'object' && self.self === self && self ||
        typeof global == 'object' && global.global === global && global ||
        this ||
        {};

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

    //原型复制(便于压缩)
    //Array,Object,Function这些本质都是函数，获取函数原型属性prototype也是为了便于压缩。
    //Object.prototype.xxx这种代码是不可压缩的，Object,prototype这些名字改了浏览器就不认得了。
    //上面的代码中创建了ObjProto之后，源生代码经过压缩之后，ObjProto就可能命名成a变量，那么原来的代码就压缩成a.xxx
    var ArrayProto = Array.prototype, ObjProto = Object.prototype;
    var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;

    // Create quick reference variables for speed access to core prototypes.
    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.
    var nativeIsArray = Array.isArray,
        nativeKeys = Object.keys,
        nativeCreate = Object.create;

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

    // Create a safe reference to the Underscore object for use below.
    //下划线库的局部变量_,注意是个函数(构造函数)
    var _ = function(obj) {
        if (obj instanceof _) return obj;//如果参数是自己的实例，那就返回参数。
        if (!(this instanceof _)) return new _(obj);//如果你调用_，不是用new的，函数内部自动会给你new
        this._wrapped = obj;
    };
    /*
     //看下Person类
     <script>
     var Person = function(name){
         if(!(this instanceof Person)){
            return new Person(name);
         }
         this.name = name;
     }
     var p = new Person('laoyao');
     alert(p.name);
     var p = Person('xxx');
     alert(p.name);
     </script>
     //当然了，你也可以反过来用，例如一个函数不让它new，比如alert，
     <script>
     var MyAlert = function(str){
         if(this instanceof MyAlert){
            throw new Error("MyAlert is not a constructor");
         }
         alert(str);
     }
     MyAlert(111);//弹出
     new MyAlert(222);//报错
     </script>
     */

    // Export the Underscore object for **Node.js**, with
    // backwards-compatibility for their old module API. If we're in
    // the browser, add `_` as a global object.
    // (`nodeType` is checked to ensure that `module`
    // and `exports` are not HTML elements.)
    if (typeof exports != 'undefined' && !exports.nodeType) {
        if (typeof module != 'undefined' && !module.nodeType && module.exports) {
            exports = module.exports = _;
        }
        exports._ = _;
    } else {
        root._ = _;
    }

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

    // Internal function that returns an efficient (for current engines) version
    // of the passed-in callback, to be repeatedly applied in other Underscore
    // functions.
    var optimizeCb = function(func, context, argCount) {
        if (context === void 0) return func;
        switch (argCount) {
            case 1: return function(value) {
                return func.call(context, value);
            };
            // The 2-parameter case has been omitted only because no current consumers
            // made use of it.
            case null:
            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);
    };

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

    // Similar to ES6's rest param (http://ariya.ofilabs.com/2013/03/es6-and-rest-parameter.html)
    // This accumulates the arguments passed into an array, after a given index.
    var restArgs = 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);
        };
    };

    // 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;
    };

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

    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');
    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 = _.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.
    _.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.
    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)
        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);
        };
    };

    // **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`.
    _.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`.
    _.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.
    _.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`.
    _.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`.
    _.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`.
    _.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 = restArgs(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);
        });
    });

    // Convenience version of a common use case of `map`: fetching a property.
    _.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.
    _.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.
    _.findWhere = function(obj, attrs) {
        return _.find(obj, _.matcher(attrs));
    };

    // Return the maximum element (or element-based computation).
    _.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).
    _.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](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`.
    _.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);
    };

    // Sort the object's values by a criterion produced by an iteratee.
    _.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.
    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.
    _.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.
    _.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.
    _.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;
    // 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.
    _.partition = group(function(result, value, pass) {
        result[pass ? 0 : 1].push(value);
    }, true);

    // 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) {
        if (array == null || array.length < 1) 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.
    _.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.
    _.last = function(array, n, guard) {
        if (array == null || array.length < 1) 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.
    _.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);
    };

    // Internal implementation of a recursive `flatten` function.
    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);
    };

    // Return a version of the array that does not contain the specified value(s).
    _.without = restArgs(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.
    // Aliased as `unique`.
    _.uniq = _.unique = function(array, isSorted, iteratee, context) {
        if (!_.isBoolean(isSorted)) {
            context = iteratee;
            iteratee = isSorted;
            isSorted = false;
        }
        if (iteratee != null) iteratee = cb(iteratee, context);
        var result = [];
        var seen = [];
        for (var i = 0, length = getLength(array); 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.
    _.union = restArgs(function(arrays) {
        return _.uniq(flatten(arrays, true, true));
    });

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

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

    // 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;
    };

    // Split an **array** into several arrays containing **count** or less elements
    // of initial array.
    _.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;
    };

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

    // Determines whether to execute a function as a constructor
    // or a normal function with the provided arguments.
    var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
        if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args);
        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.
    _.bind = restArgs(function(func, context, args) {
        if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
        var bound = restArgs(function(callArgs) {
            return executeBound(func, bound, context, this, args.concat(callArgs));
        });
        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 by default, allowing any combination of arguments to be
    // pre-filled. Set `_.partial.placeholder` for a custom placeholder argument.
    _.partial = restArgs(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 = _;

    // Bind a number of an object's methods to that object. Remaining arguments
    // are the method names to be bound. Useful for ensuring that all callbacks
    // defined on an object belong to it.
    _.bindAll = restArgs(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);
        }
    });

    // Memoize an expensive function by storing its results.
    _.memoize = function(func, hasher) {
        var memoize = function(key) {
            var cache = memoize.cache;
            var address = '' + (hasher ? hasher.apply(this, arguments) : key);
            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.
    _.delay = restArgs(function(func, wait, args) {
        return setTimeout(function() {
            return func.apply(null, args);
        }, wait);
    });

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

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

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

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

        var debounced = restArgs(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;
    };

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

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

    // Returns a function that is the composition of a list of functions, each
    // consuming the return value of the function that follows.
    _.compose = function() {
        var args = arguments;
        var start = args.length - 1;
        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.
    _.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.
    _.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.
    _.once = _.partial(_.before, 2);

    _.restArgs = restArgs;

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

    // 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);
            }
        }
    };

    // Retrieve the names of an object's own properties.
    // Delegates to **ECMAScript 5**'s native `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;
    };

    // Retrieve all the property names of an object.
    _.allKeys = function(obj) {
        if (!_.isObject(obj)) return [];
        var keys = [];
        for (var key in obj) keys.push(key);
        // Ahem, IE < 9.
        if (hasEnumBug) collectNonEnumProps(obj, keys);
        return keys;
    };

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

    // Returns the results of applying the iteratee to each element of the object.
    // In contrast to _.map it returns an object.
    _.mapObject = function(obj, iteratee, context) {
        iteratee = cb(iteratee, context);
        var keys = _.keys(obj),
            length = keys.length,
            results = {};
        for (var index = 0; index < length; index++) {
            var currentKey = keys[index];
            results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
        }
        return results;
    };

    // Convert an object into a list of `[key, value]` pairs.
    // The opposite of _.object.
    _.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.
    _.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`.
    _.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);

    // Returns the first key on an object that passes a predicate test.
    _.findKey = function(obj, predicate, context) {
        predicate = cb(predicate, context);
        var keys = _.keys(obj), key;
        for (var i = 0, length = keys.length; i < length; i++) {
            key = keys[i];
            if (predicate(obj[key], key, obj)) return key;
        }
    };

    // Internal pick helper function to determine if `obj` has key `key`.
    var keyInObj = function(value, key, obj) {
        return key in obj;
    };

    // Return a copy of the object only containing the whitelisted properties.
    _.pick = restArgs(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;
    });

    // Return a copy of the object without the blacklisted properties.
    _.omit = restArgs(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);

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

    // Create a (shallow-cloned) duplicate of an object.
    _.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.
    _.tap = function(obj, interceptor) {
        interceptor(obj);
        return obj;
    };

    // Returns whether an object has a given set of `key:value` pairs.
    _.isMatch = function(object, attrs) {
        var keys = _.keys(attrs), length = keys.length;
        if (object == null) return !length;
        var obj = Object(object);
        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;
    };

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

    // Is a given array, string, or object empty?
    // An "empty" object has no enumerable own-properties.
    _.isEmpty = function(obj) {
        if (obj == null) return true;
        if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
        return _.keys(obj).length === 0;
    };

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

    /*    判断obj是否是数组。
    如果浏览器中有isArray(es5)就用原生的。
    经典判断是否是数组方法。
    不考虑复用性，也可以这么写
    return {}.toString.call(obj) === '[object Array]';
    判断其他对象，下划线库也是通用的实现。*/
    _.isArray = nativeIsArray || function(obj) {
        return toString.call(obj) === '[object Array]';
    };

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

    // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError, isMap, isWeakMap, isSet, isWeakSet.
    /*    下划线判断常见对象的实现。
    使用了自己身的each。*/
    _.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 + ']';
        };
    });

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

    // Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8,
    // IE 11 (#1621), Safari 8 (#1929), and PhantomJS (#2236).
    var nodelist = root.document && root.document.childNodes;
    if (typeof /./ != 'function' && typeof Int8Array != 'object' && typeof nodelist != 'function') {
        _.isFunction = function(obj) {
            return typeof obj == 'function' || false;
        };
    }
    //Arguments兼容结束

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

    // Is the given value `NaN`?
    _.isNaN = function(obj) {
        return _.isNumber(obj) && isNaN(obj);
    };

    //判断是否是布尔类型
    //布尔类型不是就是true和false吗，为啥源码中有两个或呢。
    _.isBoolean = function(obj) {
        return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
    };

    //判断obj是否是null。
    //注意源码中是三等号。所以如果obj是undefined，返回的是false；
    _.isNull = function(obj) {
        return obj === null;
    };

    // 判断obj是否是undefined
    _.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).
    _.has = function(obj, path) {
        if (!_.isArray(path)) {
            return obj != null && hasOwnProperty.call(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;
    };

    // Utility Functions
    // -----------------
    //实用功能部分

    //防止全局变量冲突的典型解决方案。跟jquery的$一样
    //放弃Underscore 的控制变量"_"。返回Underscore 对象的引用。
    _.noConflict = function() {
        root._ = previousUnderscore;
        return this;
    };
    //var underscore = _.noConflict();设置underscore为_

    //一个返回自身的函数，正如网站里说的一样，看似没鸟用，迭代器里有用（后面会讲）
    //这个函数看似无用, 但是在Underscore里被用作默认的迭代器iterator.
    _.identity = function(value) {
        return value;
    };
    /*var stooge = {name: 'moe'};
    stooge === _.identity(stooge);
    => true*/

    //返回一个能返回参数本身的函数。
    _.constant = function(value) {
        return function() {
            return value;
        };
    };

    /*
    noop仅仅是一个空函数而已。除了少了几个字母以外，我们平时也很少用的。
    但是这么做是有好处的，
    比如我们一般写的接口（等着被覆盖的话）
    */
    _.noop = function(){};
    /*var Parent = {
        doSomething : function(){},
        doSomethingElse : function(){}
    };
    var Parent = {
        doSomething : _.noop,
        doSomethingElse : _.noop
    };*/

    _.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);
        };
    };

    // Returns a predicate for checking whether an object has a given set of
    // `key:value` pairs.
    _.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;
    };

    /*    random的作用是取min和max之间的一个数。如果只传一个参数，是返回0和其之间的随机数的。
    官方说是整数，那得要求你传进来的min是整数才行。Math.random()返回的一个[0.0,1.0)之间的随机数。注意取不到1的。
    比如max是5，min是3，所以最后返回的数字是3,4,5.
        要求3,4,5是等概率的，因此[0.0,1.0)变成[0.0,3.0)才行。加3，再变成[3.0,6.0)。
    最后向下取整，是为了[3.0,4.0)之间的值为3，是为了[4.0,5.0)之间的值为4，[5.0,6.0)为5.这么做才是等概率的。
    当然了，你传min等于5，max等于3,会弹一直返回4的。所以这个api是有点问题的。
    是不是要做一个大小比较呢。如果要真正返回随机整数怎么写呢？比如2.1和5.3之间的随机整数。自己研究吧。*/
    _.random = function(min, max) {
        if (max == null) {
            max = min;
            min = 0;
        }
        return min + Math.floor(Math.random() * (max - min + 1));
    };

    /*    获取系统时间戳。
    如果有Date.now存在，就用那个，不存在只能自己写了。
    一般动态创建dom的需要id时，我一般都用时间戳加上个前缀来做的。
    不然还累加计数器变量，由于闭包会产生问题的。还是时间戳来的酷点。*/
    _.now = Date.now || function() {
        return new Date().getTime();
    };

    // List of HTML entities for escaping.
    var escapeMap = {
        '&': '&',
        '<': '<',
        '>': '>',
        '"': '"',
        "'": ''',
        '`': '`'
    };
    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;
    };

    // Add your own custom functions to the Underscore object.
    _.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);
    };

    //对amd支持
    if (typeof define == 'function' && define.amd) {
        define('underscore', [], function() {
            return _;
        });
    }
}());