js中reduce的神奇用法最近经常在项目中经常看到别人用reduce处理数据，很是牛掰，很梦幻， 不如自己琢磨琢磨。先看w3c语法array.reduce(function(total, currentValue, currentIndex, arr), initialValue);/* total: 必需。初始值, 或者计算结束后的返回值。 currentValue： 必需。当前元素。 currentIndex： 可选。当前元素的索引； arr： 可选。当前元素所属的数组对象。 initialValue: 可选。传递给函数的初始值，相当于total的初始值。*/常见用法数组求和const arr = [12, 34, 23];const sum = arr.reduce((total, num) => total + num);<!– 设定初始值求和 –>const arr = [12, 34, 23];const sum = arr.reduce((total, num) => total + num, 10); // 以10为初始值求和<!– 对象数组求和 –>var result = [ { subject: ‘math’, score: 88 }, { subject: ‘chinese’, score: 95 }, { subject: ’english’, score: 80 }];const sum = result.reduce((prev, cur) => prev + cur.score, 0); const sum = result.reduce((prev, cur) => prev + 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<!– 比如生成“老大、老二和老三” –>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((prev, cur) => {prev[cur] ? prev[cur]++ : prev[cur] = 1; return prev;}, {});数组转数组<!– 按照一定的规则转成数组 –>var arr1 = [2, 3, 4, 5, 6]; // 每个值的平方var newarr = arr1.reduce((prev, cur) => {prev.push(cur * cur); return prev;}, []);数组转对象<!– 按照id 取出stream –>var streams = [{name: ‘技术’, id: 1}, {name: ‘设计’, id: 2}];var obj = streams.reduce((prev, cur) => {prev[cur.id] = cur; return prev;}, {});高级用法多维的叠加执行操作<!– 各科成绩占比重不一样， 求结果 –>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((prev, cur) => dis[cur.subject] * cur.score + prev, 0);<!– 加大难度， 商品对应不同国家汇率不同，求总价格 –>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((prev1, cur1) => Object.keys(rates).reduce((prev2, cur2) => { console.log(prev1, cur1, prev2, cur2); prev1[${cur2}Total] += cur1.price * rates[cur2]; return prev1;}, {}), initialState);var manageReducers = function() { return function(state, item) { return Object.keys(rates).reduce((nextState, key) => { state[${key}Total] += item.price * rates[key]; return state; }, {}); }};var res1= prices.reduce(manageReducers(), initialState);扁平一个多维数组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; }, []);