javascript中reduce的用法总结（附代码）

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。
*/

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

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

<!-- 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;
 }
}, '');

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

<!-- 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;}, []);

<!-- 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;}, {});

<!-- 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);

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

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;
 }, []);