reduce()用法总结

简单应用

//求和
let sum = arr.reduce(function(prev, cur, index, array) {
    return prev + cur
}, 0)
console.log(sum)

//求最大值
let max = arr.reduce(function(prev, cur) {
    return Math.max(prev, cur)
})
console.log(max)

//数组去重
let res = arr.reduce(function(prev, cur) {
    prev.indexOf(cur) == -1 && prev.push(cur)
    return prev
}, [])
console.log(res)

 高级应用

reduce函数的返回结果类型和传入的初始值相同，上个实例中初始值为number类型，同理，初始值也可为object类型。

var items = [10, 120, 1000];

// our reducer function
var reducer = function add(sumSoFar, item) {
  sumSoFar.sum = sumSoFar.sum + item;
  return sumSoFar;
};

// do the job
var total = items.reduce(reducer, {sum: 0});

console.log(total); // {sum:1130}

计算每个元素出现的次数

let names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

let nameNum = names.reduce((pre,cur)=>{
  if(cur in pre){
    pre[cur]++
  }else{
    pre[cur] = 1 
  }
  return pre
},{})
console.log(nameNum); //{Alice: 2, Bob: 1, Tiff: 1, Bruce: 1}

多数组转化为一维数组

let arr = [[0, 1], [2, 3], [4,[5,6,7]]]
const newArr = function(arr){
   return arr.reduce((pre,cur)=>pre.concat(Array.isArray(cur)?newArr(cur):cur),[])
}
console.log(newArr(arr)); //[0, 1, 2, 3, 4, 5, 6, 7]