JavaScript中reduce()详解及使用方法

 let arr = [1,2,3,4,5]
 arr.reduce((prev,cur)=>{
      return prev+cur
 })

let arr = [1,2,3,4,5]
arr.reduce((prev,cur)=>{
    return prev+cur
},10)

      var arr = [0,1,2,3];
 
      var value = arr.reduce((pre, cur) => {
        return pre + cur;
      }, 0);
      
      console.log(value);// 输出：6

      var arr = [{ x: 1 }, { x: 2 }, { x: 3 }];
 
      var value = arr.reduce((pre, cur) => {
        return pre + cur.x;
      }, 0);
      
      console.log(value);// 输出：6

      var arr = [
        [0, 1],
        [2, 3],
        [4, 5],
      ];
 
      var value = arr.reduce((pre, cur) => {
        return pre.concat(cur);
      }, []);
 
      console.log(value); // 输出：[0, 1, 2, 3, 4, 5]

      var arr = ["a", "b", "a", "c", "c", "c"];
 
      var value = arr.reduce((pre, cur) => {
        if (cur in pre) {
          pre[cur]++;
        } else {
          pre[cur] = 1;
        }
        return pre;
      }, {});
 
      console.log(value); // 输出：{a: 2, b: 1, c: 3}

      var people = [
        { name: "tom", age: 17, birthYear: 2021 },
        { name: "tony", age: 21, birthYear: 2021 },
        { name: "bob", age: 18, birthYear: 2020 },
        { name: "bob", age: 18, birthYear: 2021 },
        { name: "bob", age: 18, birthYear: 2019 },
      ];
 
      function groupBy(arr, property) {
        if (!Array.isArray(arr)) return [];
 
        return arr.reduce((pre, obj) => {
          var newObj = {
            [property]: obj[property],
            data: [obj],
          };
 
          if (!pre.length) {
            return [newObj];
          }
 
          for (let i = 0; i < pre.length; i++) {
            let item = pre[i];
            if (item[property] === obj[property]) {
              item.data = [...item.data, obj];
              return pre;
            }
          }
          return [...pre, newObj];
        }, []);
      }
 
      var value = groupBy(people, "birthYear");

      var arr = [1, 2, 3, 2, 2, 3, 5, null, null, 0, false, false, true];
 
      var value = arr.reduce((pre, cur) => {
        return pre.includes(cur) ? pre : pre.concat(cur);
      }, []);
 
      console.log(value);//[1, 2, 3, 5, null, 0, false, true]

（1）计算数组中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}


（2）数组去重
let arr = [1,2,3,4,4,1]
let newArr = arr.reduce((pre,cur)=>{
    if(!pre.includes(cur)){
      return pre.concat(cur)
    }else{
      return pre
    }
},[])
console.log(newArr);// [1, 2, 3, 4]

（3）将二维数组转化为一维

let arr = [[0, 1], [2, 3], [4, 5]]
let newArr = arr.reduce((pre,cur)=>{
    return pre.concat(cur)
},[])
console.log(newArr); // [0, 1, 2, 3, 4, 5]
（3）将多维数组转化为一维

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]

（4）、对象里的属性求和
var result = [
    {
        subject: 'math',
        score: 10
    },
    {
        subject: 'chinese',
        score: 20
    },
    {
        subject: 'english',
        score: 30
    }
];

var sum = result.reduce(function(prev, cur) {
    return cur.score + prev;
}, 0);
console.log(sum) //60