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最近經(jīng)常在項(xiàng)目中經(jīng)常看到別人用reduce處理數(shù)據(jù)，很是牛掰，很夢(mèng)幻， 不如自己琢磨琢磨。

先看w3c語(yǔ)法

array.reduce(function(total, currentValue, currentIndex, arr), initialValue);
/*
  total: 必需。初始值, 或者計(jì)算結(jié)束后的返回值。
  currentValue： 必需。當(dāng)前元素。
  currentIndex： 可選。當(dāng)前元素的索引；                     
  arr： 可選。當(dāng)前元素所屬的數(shù)組對(duì)象。
  initialValue: 可選。傳遞給函數(shù)的初始值，相當(dāng)于total的初始值。
*/

常見用法

數(shù)組求和

const arr = [12, 34, 23];
const sum = arr.reduce((total, num) => total + num);
<!-- 設(shè)定初始值求和 -->
const arr = [12, 34, 23];
const sum = arr.reduce((total, num) => total + num, 10);  // 以10為初始值求和
<!-- 對(duì)象數(shù)組求和 -->
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);  // 總分扣除10分

數(shù)組最大值

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

進(jìn)階用法

數(shù)組對(duì)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;
  }
}, '');

求字符串中字母出現(xiàn)的次數(shù)

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

數(shù)組轉(zhuǎn)數(shù)組

<!-- 按照一定的規(guī)則轉(zhuǎn)成數(shù)組 -->
var arr1 = [2, 3, 4, 5, 6]; // 每個(gè)值的平方
var newarr = arr1.reduce((accumulator, cur) => {accumulator.push(cur * cur); return accumulator;}, []);

數(shù)組轉(zhuǎn)對(duì)象

<!-- 按照id 取出stream -->
var streams = [{name: '技術(shù)', id: 1}, {name: '設(shè)計(jì)', id: 2}];
var obj = streams.reduce((accumulator, cur) => {accumulator[cur.id] = cur; return accumulator;}, {});

高級(jí)用法

多維的疊加執(zhí)行操作

<!-- 各科成績(jī)占比重不一樣， 求結(jié)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((accumulator, cur) => dis[cur.subject] * cur.score + accumulator, 0);

<!-- 加大難度， 商品對(duì)應(yīng)不同國(guó)家匯率不同，求總價(jià)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((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);

扁平一個(gè)多維數(shù)組

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

對(duì)象數(shù)組去重

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;
  }, []);

參考文章
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
https://www.liaoxuefeng.com/wiki/001434446689867b27157e896e74d51a89c25cc8b43bdb3000/001435119854495d29b9b3d7028477a96ed74db95032675000
https://aotu.io/notes/2016/04/14/js-reduce/index.html