转自：https://juejin.cn/post/6844903669977055239 参考：https://www.jianshu.com/p/e375ba1cfc47

常见用法

数组求和

const arr = [12, 34, 23]; const sum = arr.reduce((total, num) => total + num);

const arr = [12, 34, 23]; const sum = arr.reduce((total, num) => total + num, 10); // 以10为

初始值求和

var result = [ { subject: 'math', score: 88 }, { subject: 'chinese', score: 95 }, { subject: 'english', score: 80 } ]; const sum = result.reduce((accumulator, cur) => accumulator + cur.score, 0); const sum = result.reduce((accumulator, cur) => accumulator + cur.score, -10); // 总分扣除10分

数组最大值

const a = [23,123,342,12]; const max = a.reduce(function(pre,cur,inde,arr){return pre>cur?pre:cur;}); // 342

进阶用法

数组对象中的用法

const objArr = [{name: '老大'}, {name: '老二'}, {name: '老三'}]; const res = objArr.reduce((pre, cur, index, arr) => { if (index === 0) { return cur.name; } else if (index === (arr.length - 1)) { return pre + '和' + cur.name; } else { return pre + '、' + cur.name; } }, '');

求字符串中字母出现的次数

const str = 'sfhjasfjgfasjuwqrqadqeiqsajsdaiwqdaklldflas-cmxzmnha'; const res = str.split('').reduce((accumulator, cur) => {accumulator[cur] ? accumulator[cur]++ : accumulator[cur] = 1; return accumulator;}, {});

数组转数组

var arr1 = [2, 3, 4, 5, 6]; // 每个值的平方 var newarr = arr1.reduce((accumulator, cur) => {accumulator.push(cur * cur); return accumulator;}, []);

数组转对象

var streams = [{name: '技术', id: 1}, {name: '设计', id: 2}]; var obj = streams.reduce((accumulator, cur) => {accumulator[cur.id] = cur; return accumulator;}, {});

高级用法

多维的叠加执行操作

var result = [ { subject: 'math', score: 88 }, { subject: 'chinese', score: 95 }, { subject: 'english', score: 80 } ]; var dis = { math: 0.5, chinese: 0.3, english: 0.2 }; var res = result.reduce((accumulator, cur) => dis[cur.subject] * cur.score + accumulator, 0);

var prices = [{price: 23}, {price: 45}, {price: 56}]; var rates = { us: '6.5', eu: '7.5', }; var initialState = {usTotal:0, euTotal: 0}; var res = prices.reduce((accumulator, cur1) => Object.keys(rates).reduce((prev2, cur2) => { console.log(accumulator, cur1, prev2, cur2); accumulator[${cur2}Total] += cur1.price * rates[cur2]; return accumulator; }, {}), initialState);

var manageReducers = function() { return function(state, item) { return Object.keys(rates).reduce((nextState, key) => { state[${key}Total] += item.price * rates[key]; return state; }, {}); } }; var res1= prices.reduce(manageReducers(), initialState);

扁平一个二维数组

var arr = [[1, 2, 8], [3, 4, 9], [5, 6, 10]]; var res = arr.reduce((x, y) => x.concat(y), []);

对象数组去重

const hash = {}; chatlists = chatlists.reduce((obj, next: Object) => { const hashId = ${next.topic}_${next.stream_id}; if (!hash[hashId]) { hash[${next.topic}_${next.stream_id}] = true; obj.push(next); } return obj; }, []);