reduce

 reduce(callback,initiaValue)会传入两个变量，回调函数(callback)和初始值(initiaValue)。1.只传回调函数 prev只取数组的第一个元素 next从数组的第二个元素开始一直往下取var arr = ["a", "b", "c", "d", "e", "f"];function noPassValue() {return arr.reduce(function(prev, next) {console.log("prev:", prev);console.log("next:", next);// =>//   prev: a;//   next: b;//   prev: a;//   next: c;//   prev: a;//   next: d;//   prev: a;//   next: e;//   prev: a;//   next: f;return prev;});}console.log(noPassValue());=> a2.传入回调函数和初始值时 prev只取初始值 next从数组的第一个元素开始一直往下取var arr = ["a", "b", "c", "d", "e", "f"];function passValue() {return arr.reduce(function(prev, next) {console.log("prev:", prev);console.log("next:", next);// =>// prev: {}// next: a// prev: {a: 1}// next: b// prev: {a: 1, b: 1}// next: c// prev: {a: 1, b: 1, c: 1}// next: d// prev: {a: 1, b: 1, c: 1, d: 1}// next: e// prev: {a: 1, b: 1, c: 1, d: 1, e: 1}// next: fprev[next] = 1;return prev;}, {});}console.log(passValue());=> {a: 1, b: 1, c: 1, d: 1, e: 1, f: 1}reduce的应用1.数组求和var arr = [1, 2, 3, 4];var sum = arr.reduce(function(total, currentValue) {console.log(total, currentValue);// => 1, 2 3, 3 6, 4return total + currentValue; // => 每次把相加后返回的数据作为数组的第一个元素});console.log(sum); // => 10var eachSum = 0;arr.forEach(function(currentValue) {eachSum += currentValue;});console.log(eachSum); // => 102.合并二维数组 与 Object.entries(obj)互逆var twoArr = [["mu", "zi"],["dig", "big"],["lucky", "jiji"]];var oneArr = twoArr.reduce(function(total, currentValue) {console.log(total);// =>  ["mu", "zi"] ["mu", "zi", "dig", "big"]return total.concat(currentValue);});console.log(oneArr);// =>  ["mu", "zi", "dig", "big", "lucky", "jiji"]3.统计一个数组中有多少个重复的单词不用reduce时：var arr = ["apple", "orange", "apple", "orange", "pear", "orange"];function getWordCnt() {var obj = {};for (var i = 0, l = arr.length; i < l; i++) {var item = arr[i];obj[item] = obj[item] + 1 || 1;}return obj;}console.log(getWordCnt()); // => {apple: 2, orange: 3, pear: 1}// 用reduce时：var arr = ["apple", "orange", "apple", "orange", "pear", "orange"];function getWordCnt() {return arr.reduce(function(prev, next) {prev[next] = prev[next] + 1 || 1;return prev;}, {});}console.log(getWordCnt()); // => {apple: 2, orange: 3, pear: 1}