关于reduce的介绍及用法说明

目录

• 参数介绍
• 举例说明
• 1.把数组合并为一个对象
• 2.去重
• 3.求和
• 4.求最大值和最小值
• 5.扁平数组
• 6.求字符串中各个字符出现的次数 

array.reduce((pre, cur, index, arr)=>{
    ...
}, init);

 const b = [1, 2, 3, 4]
 b.reduce((x, y) => console.log(x, y), 0)

//正常使用forEach循环
let arr = [{ a: 1, b: 10 }, { a: 2, b: 20 }, { a: 3, b: 30 }]
let obj = {}
arr.forEach((item, index) => {
    Object.keys(item).forEach(each => {
        obj[`${each}${index + 1}`] = item[each]
    })
})
console.log(obj);    //{a1:1,a2:2,a3:3,b1:10,b2:20,b3:30}

//使用reduce遍历，可以看出区别在于初始值，以及reduce中必须要return你想要的值
let a = arr.reduce((pre, cur, index) => {
    Object.keys(cur).forEach((item) => {
        pre[`${item}${index + 1}`] = cur[item]
    })
    return pre
}, {})
console.log(a);     //{a1:1,a2:2,a3:3,b1:10,b2:20,b3:30}

let arr2 = [9, 4, 3, 6, 9];
//基本类型去重
let set = arr2.reduce((pre, cur) => {
    !pre.includes(cur) && pre.push(cur)
    return pre
}, [])
console.log(set);  //[9,4,3,6]

let list = [
    {
        subject: "数学",
        marks: 80
    },
    {
        subject: "语文",
        marks: 90
    },
    {
        subject: "英语",
        marks: 80
    },
]
function arrSet(value,arr) {
    let obj = {}
    let res = arr.reduce((pre, cur) => {
        if (!obj[cur[value]]) {
            obj[cur[value]] = true
            pre.push(cur)
        }
        return pre
    }, [])
    return res
}
let result = arrSet("marks",list);
console.log(result);  //[{subject:"数学",marks:80}，{subject:"语文"：90}]

let arr2 = [9, 4, 3, 6, 9];
let sum = arr2.reduce((pre, cur) => {
    return pre + cur
},0)
console.log(sum)   //31

let arr = [
  {
    value: 45,
  },
  {
    value: 88,
  },
  {
    value: 101,
  },
];
let newArr = arr.reduce((pre, cur) => {
  return pre + cur.value;
}, 0);
console.log(newArr);   //234

let arr2 = [9, 4, 3, 6, 9];
let max = arr2.reduce((prev, cur)=> {
    return Math.max(prev, cur);
})
let min= arr2.reduce((prev, cur)=> {
    return Math.min(prev, cur);
})
console.log(max,min);   //9,3

let arr = [[1, 2, 8], [3, 4, 9], [5, 6, 10]];
let res = arr.reduce((pre, cur) => {
    return pre.concat(cur)
},[]);
console.log(res)  //[1, 2, 8, 3, 4, 9, 5, 6, 10]

let arr = [[1, [2, 8]], [3, 4, 9], [5, [6, 10]]]
function fn(arr) {
  return arr.reduce((pre, cur) => {
    return pre.concat(Array.isArray(cur) ? fn(cur) : cur);
  }, []);
}
const newArr = fn(arr);
console.log(newArr); //[1, 2, 8, 3, 4, 9, 5, 6, 10]

const str = 'jordanbryantjamescurrydurant';
const res1 = str.split('').reduce((pre, cur) => {
    pre[cur] ? pre[cur]++ : pre[cur] = 1;
    return pre;
}, {});
console.log(res1);  //{j:2,o:1,r:5,d:2,...}
let arr = []
let num = Math.max(...new Set(Object.values(res1)))
let value;
for (let keys in res1) {
    if (res1[keys] == num) {
        value = keys
        break
    }
}
console.log(`出现最多的字母为${value},出现的次数为${num}次`); //出现最多的字母为r,出现的次数为5次