??????? 工作中需要用到underscore.js，发现这是一个包括了很多基本功能函数的js库，里面有很多实用的函数。而且它没有扩展 javascript的原生对象。主要涉及对Collection、Object、Array、Function的操作。
?????? 学习官方网址：http://underscorejs.org。
?????? Collection Functions (Arrays or Objects) 【集合函数】
??????? 1.each ? _.each(list, iterator, [context]) Alias: forEach
??????? 迭代list中的所有元素，按顺序用迭代器输出每个元素。如果传递了context参数，则把iterator绑定到context对象上。每次调用iterator都会传递三个参数：(element, index, list)。如果list是个JavaScript对象，iterator的参数是 (value, key, list)。存在原生的forEach方法，Underscore就委托给forEach。
//_.each
_.each([1, 2, 3], function(num){ console.log(num); });
// 1,2,3
_.each({one : 1, two : 2, three : 3}, function(num, key){ console.log(num); });
// 1,2,3
_.each({one : 1, two : 2, three : 3}, function(num, key){ console.log(key); });
// one,two,three
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??????? 2.map? _.map(list, iterator, [context]) Alias: collect
用转换函数把list中的每个值映射到一个新的数组。如果list是个JavaScript对象，iterator的参数是(value, key, list),这里的用法和each一样。 map 和 each 的区别就是map可以接受返回值。
//_.map
var r = _.map([1, 2, 3], function(num){ return num * 3; });
console.log(r); // [3, 6, 9]
var r = _.map({one : 1, two : 2, three : 3}, function(num, key){ return num * 3; });
console.log(r); // [3, 6, 9]
var r = _.map({one : 1, two : 2, three : 3}, function(num, key) {
return key;
});
console.log(r); //["one", "two", "three"]
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??????? 3.reduce? _.reduce(list, iterator, memo, [context]) Aliases: inject, foldl
reduce方法把列表中元素归结为一个简单的数值，。Memo是reduce函数的初始值，reduce的每一步都需要由iterator返回。
//_.reduce
var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);
console.log(sum); //6
var sum = _.reduce([2, 4, 8], function(memo, num){ return memo + num; }, 11);
console.log(sum); //25
???????? 这个函数有些浏览器提供了原生的，如下的underscore源代码可以很好的帮忙理解此函数。
// 代码的前面就声明了一个变量，检测是否支持原生reduce： var nativeReduce = ArrayProto.reduce;
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// **Reduce** builds up a single result from a list of values, aka `inject`,
// or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
_.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
var initial = arguments.length > 2;
if (obj == null) obj = [];
if (nativeReduce && obj.reduce === nativeReduce) {
if (context) iterator = _.bind(iterator, context);
return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
}
each(obj, function(value, index, list) {
if (!initial) {
memo = value;
initial = true;
} else {
memo = iterator.call(context, memo, value, index, list);
}
});
if (!initial) throw new TypeError(reduceError);
return memo;
};
???????? 解释上面的例子1就是：试用迭代器把obj（1,2,3）里面的元素相加，由于设置了初始值（0），那就先加初始值，每次的相加的值都存储在memo里面。所以结果是0+1+2+3=6。
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??????? 4.reduceRight? _.reduceRight(list, iterator, memo, [context]) Alias: foldr
??????? reducRight是从右侧开始组合的元素的reduce函数，如果存在JavaScript 1.8版本的reduceRight，则用其代替。Foldr在javascript中不像其它有懒计算的语言那么有用（lazy evaluation：一种求值策略，只有当表达式的值真正需要时才对表达式进行计算）。
//_.reduceRight
var list = [[0, 1], [2, 3], [4, 5]];
var flat = _.reduceRight(list, function(a, b) { return a.concat(b); }, []);
console.log(flat); // [4, 5, 2, 3, 0, 1]
var list = [[0],[11,22],[33,44,55]];
var flat = _.reduceRight(list, function(a, b) { return a.concat(b); }, []);
console.log(flat); // [33, 44, 55, 11, 22, 0]
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??????? 5.find? _.find(list, iterator, [context]) Alias: detect
??????? 遍历list，返回第一个通过iterator真值检测的元素值。如果找到匹配的元素立即返回，不会遍历整个list。
//_.find
var even = _.find([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
console.log(even); //2
var even = _.find([1, 2, 3, 4, 5, 6], function(num){ return num > 3; });
console.log(even); //4
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??????? 6.filter? _.filter(list, iterator, [context]) Alias: select
??????? 遍历list，返回包含所有通过iterator真值检测的元素值。如果存在原生filter方法，则委托给filter,
和find不同的是，它返回所有符合条件的值，返回一个数组。
//_.filter
var r = _.filter([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
console.log(r); //[2, 4, 6]
var r = _.filter([1, 2, 3, 4, 5, 6], function(num){ return num > 3; });
console.log(r); //[4, 5, 6]
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??????? 7.where?? _.where(list, properties)
??????? 遍历list，返回properties中所有键值对都相等的对象数组。
//_.where
var listOfPlays = [{title: "Cymbeline", author: "Shakespeare", year: 1611},
{title: "The Tempest", author: "Shakespeare", year: 1631},
{title: "The Tempest2", author: "Shakespeare", year: 1661},
{title: "The Tempest3", author: "Shakespeare3", year: 1611},
{title: "The Tempest4", author: "Shakespeare", year: 1611}];
var r = _.where(listOfPlays, {author: "Shakespeare", year: 1611});
console.log(r);
//[{title: "Cymbeline", author: "Shakespeare", year: 1611},{title: "The Tempest4", author: "Shakespeare", year: 1611}]
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??????? 8.findWhere?? _.findWhere(list, properties)
??????? 遍历list，返回properties中所有键值对都相等的第一个对象。
//_.findWhere
var listOfPlays = [{title: "Cymbeline", author: "Shakespeare0", year: 1611},
{title: "The Tempest", author: "Shakespeare", year: 1611},
{title: "The Tempest2", author: "Shakespeare", year: 1661},
{title: "The Tempest3", author: "Shakespeare3", year: 1611},
{title: "The Tempest4", author: "Shakespeare", year: 1611}];
var r = _.findWhere(listOfPlays, {author: "Shakespeare", year: 1611});
console.log(r); //{title: "The Tempest", author: "Shakespeare", year: 1611}
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??????? 9.reject ? _.reject(list, iterator, [context])
??????? 返回那么没有通过iterator真值检测的元素数组，filter的相反函数。
//_.reject
var r = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
console.log(r); //[1, 3, 5]
var r = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num > 3; });
console.log(r); //[1, 2, 3]
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??????? 10.every??? _.every(list, [iterator], [context]) Alias: all
??????? 如果list中的所有元素都通过iterator的真值检测就返回true。如果存在原生的every方法，则委托给every。
//_.every
var r = _.every([2, 22, 12, 4, 34, 68], function(num){ return num % 2 == 0; });
console.log(r); //true
var r = _.every([2, 22, 12, 3, 34, 68], function(num){ return num % 2 == 0; });
console.log(r); //false
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??????? 11.some? _.some(list, [iterator], [context])? Alias: any
??????? 如果有任何一个元素通过通过 iterator 的真值检测就返回true。如果存在原生的some方法，则委托给some。
//_.some
var r = _.some([null, 0, 'yes', false]);
console.log(r); //true
var r = _.some([2, 1, 13, 6, 37, 68], function(num){ return num % 2 == 0; });
console.log(r); //true
var r = _.some([2, 1, 13, 7, 37, 68], function(num){ return num % 3 == 0; });
console.log(r); //false
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??????? 12.contains??? _.contains(list, value) ?? Alias: include
??????? 如果list包含指定的value则返回true，使用===检测是否相等。如果list 是数组，内部使用indexOf判断。
//_contains
var r = _.contains([1, 2, 3], 3);
console.log(r); //true
var r = _.contains({one:1,two:2},1);
console.log(r); //true
var r = _.contains({one:2,two:3},1);
console.log(r); //false
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??????? 13.