let str = '1234';
const arr1 =Array.from(str);
console.log(arr1);//(4) ['1', '2', '3', '4']
 
 
const Arr = {
      2:'a',
      3:'b',
      length:4,
}
console.log(Array.from(Arr));// (4) [undefined, undefined, 'a', 'b']

2.array.find()

Returns the value of the first element of the array that meets the criteria (array and object)

const arr = [1,2,3,4];//数组
let num = arr.find(item=>item==3);
console.log(num);//3
 
 
 
const arr1 = [    //对象
		{realname:"张三1",age:18},
		{realname:"张三2",age:17},
		{realname:"张三3",age:19},
		{realname:"张三4",age:17},
			];
console.log(arr1.find(item=>item.age==17));//age: 17 realname: "张三2"

3.array.findindex()

Find out where eligible members are located.

const arr = [1,2,3,4]; //数组
console.log(arr.findIndex(item=>item==4)); //3
		
const arrobj = [    //对象
		{realname:"张三1",age:18},
		{realname:"张三2",age:19},
		{realname:"张三3",age:15},
		{realname:"张三4",age:14},
			]
console.log(arrobj.findIndex(item=>item.age==19)); //1

4.array.includes()

Find out if an array contains a given value.

const arr = [1,2,3,4];
console.log(arr.includes(10));//有就返回true 没有就返回false

Two, Array extension

1.array.map()

return new array

const arr = [1,2,3,4];
const newarr = arr.map(item=>item+2);
console.log(newarr); //从新输出一个数组，不是改变原的数组

2. array.filter()

filter

const arr = [1,2,3,4,5,6,7];
const newarr = arr.filter(item=> item%2==0);
console.log(newarr);

3.array.reduce()

reduce

//total:即是初始值又是返回值
//currentValue:当前值
reduce第二个参数指定初始值
const arr = [1,2,3,4,5];
let sum = arr.reduce((total,currentValue)=>{
			return total + currentValue;
},10) //可以指定初始值
console.log(sum);

4.array.fill()

filling

let arr = [1,2,3,4,5,6,7];
	arr.fill('x',1,3);
	console.log(arr);//[1, 'x', 'x', 4, 5, 6, 7]

Three, string extension

1. Usage of template strings

function demo(){
		return "end";
			}
let es6 = "es6!";
let str = `hello,${es6},${demo()}`;
console.log(str);//hello,es6,end

2. StartsWith and endsWith usage

let str = "hello,es6!";
console.log(str.startsWith("hello"));//判断某个字符串前面是否包含hello 有就为true
console.log(str.endsWith("es6!"));//判断某个字符串后面是否包含es6 有就为true

3.repeat string repetition times

console.log("hello".repeat(4));//hellohellohellohello

Fourth, the traversal of the array

1.for of traverses the value of the array

const arr = ["a","b","c","d"];
   for(v of arr){
        console.log(v); //a,b,c,d
}

2. for in traversal index

const arr = ["a","b","c","d"];
    for(let k in arr){
     console.log(k); //0,1,2,3
        }

3.for of traversal object

   const Person={realname:'zs',age:20}
   const keys = Object.keys(Person);
   for(let k of keys){
   console.log(`k:${k}`,`v:${Person[k]}`)}; //k:realname v:zs k:age v:20

4. The usage of forEach

let arr = [1,2,3,4];
	arr.forEach((item,index)=>{
	console.log(`v:${item},k:${index}`);	
			})  //v:1,k:0  v:2,k:1 v:3,k:2 v:4,k:3

example

1. Find the first student with a passing test score in a group of students and output it to the page

<ul class="score"></ul> 
 
    <hr>
    <h1 class="username"></h1>
    <script>
        let person=[
            {realname:'张三',score:'40'},
            {realname:'李四',score:'40'},
            {realname:'王五',score:'60'},
            {realname:'赵六',score:'90'}
        ]
        let str = '';
        let userName='';
        for(i=0;i<person.length;i++){
            str = str + `<li>姓名：${person[i].realname},分数：${person[i].score}</li>`
        }
        document.querySelector('.score').innerHTML=str;
 
        userName = person.find(item=>item.score>=60)
        document.querySelector('.username').innerHTML=`姓名：${userName.realname},分数：${userName.score}`
    </script>

2. Find the first person who is older than the specified age (received by the input box on the page), and output the location of this person

    <ul class="age"></ul>
    <hr>
    <input type="text" placeholder="请输入年龄" value="" class="mark">
    <input type="button" value="查询" class="btn">
    <h1></h1>
    
    
    <script>
        let person=[
            {realname:'张三',age:'15'},
            {realname:'李四',age:'18'},
            {realname:'王五',age:'19'},
            {realname:'赵六',age:'20'}
        ]
        let str = '';
        for(i=0;i<person.length;i++){
            str = str + `<li>姓名：${person[i].realname},分数：${person[i].age}</li>`
        }
        document.querySelector('.age').innerHTML=str;
        btn = document.querySelector('.btn')
        
        btn.onclick=function(){
            let num;
            input=document.querySelector('.mark').value;//获得输入的值
            num = person.findIndex(item=>item.age==input)//得到下标
            if(num==-1){
            document.querySelector('h1').innerHTML='查无此人';
            }else{
                num++;
            document.querySelector('h1').innerHTML=`位置是${num}`
            }
        }
        
    </script>

3. Output a group of personnel information, the output to the page information is as follows (name, score, 60 points for passing or not)

    const person=[
            {realname:'zs1',age:20,score:50},
            {realname:'zs2',age:20,score:70},
            {realname:'zs3',age:20,score:80},
            {realname:'zs4',age:20,score:90}
        ]
    // 使用 for of 将内容显示在网页上
    // let arr='';
    // for(let v of person){
    //      arr =arr+ `<li>姓名：${v.realname},年龄：${v.age}</li>`
    // }
    // document.querySelector('ul').innerHTML=arr;
    let mark='';
    let newarr = person.filter(item=>{
    let rer= item.score>60?'及格':'不及格'
    mark+=`<li>姓名：${item.realname},年龄：${item.age},分数：${item.score},${rer}</li>`
    })
      document.querySelector('ul').innerHTML=mark;

Methods and extensions of Array objects in ES6, extension methods of strings, and traversal of arrays

1. Array object method

1.Array.from()