Summary of usage of reduce in javascript (with code)

This article brings you a summary of the usage of reduce in JavaScript (with code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Recently, I often see others using reduce to process data in projects. It is very awesome and dreamy. It is better to think about it yourself.

Look at w3c syntax first

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。
*/

Common usage

Array summation

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

Maximum value of array

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

Advanced usage

Usage in array objects

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

Find the number of occurrences of letters in a string

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

Array to array

<!-- 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;}, []);

Array to object

<!-- 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;}, {});

Advanced Usage

Multi-dimensional overlay execution operation

<!-- 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);

Flat a multi-dimensional array

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

Object array deduplication

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;
  }, []);

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