数组的遍历，排序方法

join的方法

将数组拼接成字符串
var arr = ["html","css","js"];
// join可以将数组拼接成一个字符
console.log(arr.join("$"))
console.log(arr.join("|"))
console.log(arr.join("  "))
console.log(arr.join("*"))
console.log(typeof arr.join(""))

2sort

arr.sort((a,b)=>a-b);
arr.sort((a,b)=>b-a);

3求和reduce

特点：只能对简单的值求和[1,2,3]

4,reverse

特点：能够改变数组的结构

5遍历

 var arr = ["html","css","js"];
        arr.reverse();
        console.log(arr);

for in方法

 //for in
        var arr = ["html","css","js"];
        for(var key in arr){
            console.log(key)//key获取的是数组的下标
            //console.log(arr[key])
        }
        var obj = {
            name:"list",
            age:18
        }
        for(var k in obj){
            console.log(obj[k])
        }

获取下标的方法

 var arr =[3,4,5];
        for(var value of arr){
            console.log(value)
            //获取下标怎么获取
            //indexOF
            console.log(arr.indexOf(value))
        }

for each方法

 var arr = ["html","css","js"];
        arr.forEach(function(item){
            console.log(item)
        })

方法2

  var arr = [2,3,4];
        var newArr = [];
        arr.forEach(item=>{
            console.log(item)
            newArr.push(item*2)
        })
        console.log(newArr)

map遍历

   var arr = [
            {name:"html",value:"12px"},
            {name:"css",value:"13px"},
            {name:"js",value:"14px"}
    ]
       arr.map(item=>{
           var value = parseInt(item.value)*2;
           item.value = value;
       })
       console.log(arr)

some 遍历 数组中有一项满足某个条件时 就返回true

var arr = [3,4,5,6];
        var res = arr.some(item=>{
           return item>1;
        })
        console.log(res);

every遍历 数组中每一项都要满足条件 才返回true

var  arr = [2,3,4];
        var res = arr.every(item=>{
            return item>1;
        })
        console.log(res)

filter遍历 只要达到某个条件 就会返回一个新的数组

var  arr = [3,4,5,6];
        // filter  只要达到某个条件  就会返回一个新的数组
        var res = arr.filter(item=>{
            return item>4;
        })
        console.log(res)

find 遍历 返回数组中满足条件的某个值 只返回一个值

var arr=[2,3,4,3];
       var res= arr.find(item=>{
           return item==3;
  })
   console.log(res)

findIndex遍历 返回数组中满足条件的数组的下标

var arr =[2,3,4];
        // findIndex 返回数组中满足条件的值的下标
        var index = arr.findIndex(item=>{
            return item == 4;
        })
        console.log(index)

 <input type="text" id="app">
    <script>
        var  arr = [
            {name:"李四",id:"1001"},
            {name:"王四",id:"1002"},
            {name:"张四",id:"1003"},
            {name:"赵四",id:"1004"}
        ]
        /* 
        1.要获取数组中的值
        2.根据id值   获取数组中对应项的下标
        3.删除对应的项
        */
        var app = document.getElementById("app");
       app.onkeydown = function(event){
           if(event.keyCode==13){
               var id =Number(this.value);
               var index =arr.findIndex(item=>{
                   return item.id == id;
               })
               arr.splice(index,1);
               console.log(arr)
           }
       }
    </script>