reduce比
filter它不仅仅建立用于计数的临时数组，在这里更合适。
var dataset = [2,2,4,2,6,4,7,8];var search = 2;var count = dataset.reduce(function(n, val) { return n + (val === search);}, 0);console.log(count);在ES6中：
let count = dataset.reduce((n, x) => n + (x === search), 0);
请注意，使用自定义匹配谓词进行扩展很容易，例如，对具有特定属性的对象进行计数：
people = [ {name: 'Mary', gender: 'girl'}, {name: 'Paul', gender: 'boy'}, {name: 'John', gender: 'boy'}, {name: 'Lisa', gender: 'girl'}, {name: 'Bill', gender: 'boy'}, {name: 'Maklatura', gender: 'girl'}]var numBoys = people.reduce(function (n, person) { return n + (person.gender == 'boy');}, 0);console.log(numBoys);{x:count ofxs}在javascript中，对所有项目进行计数（即使对象类似）很复杂，因为对象键只能是字符串，因此无法可靠地对具有混合类型的数组进行计数。不过，以下简单的解决方案在大多数情况下仍然可以正常使用：count = function (ary, classifier) { classifier = classifier || String; return ary.reduce(function (counter, item) { var p = classifier(item); counter[p] = counter.hasOwnProperty(p) ? counter[p] + 1 : 1; return counter; }, {})};people = [ {name: 'Mary', gender: 'girl'}, {name: 'Paul', gender: 'boy'}, {name: 'John', gender: 'boy'}, {name: 'Lisa', gender: 'girl'}, {name: 'Bill', gender: 'boy'}, {name: 'Maklatura', gender: 'girl'}];// If you don't provide a `classifier` this simply counts different elements:cc = count([1, 2, 2, 2, 3, 1]);console.log(cc);// With a `classifier` you can group elements by specific property:countByGender = count(people, function (item) { return item.gender});console.log(countByGender);2017更新
在ES6中，您可以使用
Map对象可靠地计算任意类型的对象。
class Counter extends Map { constructor(iter, key=null) { super(); this.key = key || (x => x); for (let x of iter) { this.add(x); } } add(x) { x = this.key(x); this.set(x, (this.get(x) || 0) + 1); }}// again, with no classifier just count distinct elementsresults = new Counter([1, 2, 3, 1, 2, 3, 1, 2, 2]);for (let [number, times] of results.entries()) console.log('%s occurs %s times', number, times);// counting objectspeople = [ {name: 'Mary', gender: 'girl'}, {name: 'John', gender: 'boy'}, {name: 'Lisa', gender: 'girl'}, {name: 'Bill', gender: 'boy'}, {name: 'Maklatura', gender: 'girl'}];chessChampions = { 2010: people[0], 2012: people[0], 2013: people[2], 2014: people[0], 2015: people[2],};results = new Counter(Object.values(chessChampions));for (let [person, times] of results.entries()) console.log('%s won %s times', person.name, times);// you can also provide a classifier as in the abovebyGender = new Counter(people, x => x.gender);for (let g of ['boy', 'girl']) console.log("there are %s %ss", byGender.get(g), g);的类型感知实现
Counter可以如下所示（Typescript）：
type CounterKey = string | boolean | number;interface CounterKeyFunc<T> { (item: T): CounterKey;}class Counter<T> extends Map<CounterKey, number> { key: CounterKeyFunc<T>; constructor(items: Iterable<T>, key: CounterKeyFunc<T>) { super(); this.key = key; for (let it of items) { this.add(it); } } add(it: T) { let k = this.key(it); this.set(k, (this.get(k) || 0) + 1); }}// example:interface Person { name: string; gender: string;}let people: Person[] = [ {name: 'Mary', gender: 'girl'}, {name: 'John', gender: 'boy'}, {name: 'Lisa', gender: 'girl'}, {name: 'Bill', gender: 'boy'}, {name: 'Maklatura', gender: 'girl'}];let byGender = new Counter(people, (p: Person) => p.gender);for (let g of ['boy', 'girl']) console.log("there are %s %ss", byGender.get(g), g);
