数组的方法

1 增

push();
unshift();
concat();
splice(index,0,...value) //从某个下标的前面增加

1-1 push 从后添加

     // 1.可以添加一个值，可以添加多个值
        // 2.不能拼接数组
      var arr=[1,2,3,4];
        // var b=[5,6,7];
        arr.push(5);
        console.log(arr);//1 2 3 4 5

1-2 unshift 从前添加

                 var arr=[1,2,3,4];
        arr.unshift(0);
        console.log(arr);//0,1,2,3,4
        var a=[1,2,3];
        var b=[4,5,6];
        b.unshift(...a);//向前添加数组需要展开
        console.log(b);//1,2,3,4,5,

        // ...arr  可以将一个数组展开
        var arr=[1,2,3];
        console.log(arr);//1,2,3
        console.log(...arr);//
        var a=[1,2,3];
        var b=[4,5,6];
        a.push(...b);
        console.log(a);//1,2,3,4,5,6

2 删

pop();
shift();
splice(index,howmany);

2-1 pop从后删除

                var arr=[1,2,3,4];
        arr.pop();//1,2,3
        console.log(arr);

2-2 shift 从前删除

var arr=[1,2,3,4];
 arr.shift();//2,3,4
 console.log(arr);

2-3 splice(startIndex，长度） 定点删除

var arr = [1,2,3,4,5]
arr.splice(3,1)
console.log(arr);  // [1,2,3,5]

3 改

3-1 splice(startIndex，长度，元素…) 修改 先删除后插入

splice(startindex,howmany,...value)

                var arr=[1,2,3,4,5];
        // arr[2]=7;  1,2,7,4,5
        //splice (startindex,howmany,...value)
        arr.splice(2,1,7,8);
        console.log(arr);//1,2,7,8,4,5

4 查

4-1 arr[index] 查询对应下标的值

//arr[index] 查询对应下标的值
var arr=[1,2,3];
console.log(arr[0]);  //1;

4-2 indexOf(value) 查询对应值的下标

var arr=[1,2,3,4];
console.log(arr.indexOf(1)); // 0
//如果没有对应的值,则返回-1

4-3 slice(start,end) 截取 (包含开始，不含结束)

// slice(startIndex,endIndex) 截取 包含开始不含结束
// slice(startIndex) 截取从startIndex开始到后面的所有值
var arr = [1,2,3,4]
console.log(arr.slice(1,3));  // [2,3]
console.log(arr.slice(1)); // [2,3,4]

4-4 includes(value) 判断数组是否包含某个值

// 返回boolean
var arr = [1,2,3,4]
console.log(arr.includes(5)); // false

jie

indexOf();
slice(startIndex,endIndex)
includes() //判断数组是否包含某个值

                var arr=[1,2,3,4];
        //slice(startIndex,endIndex)不包括最后一个
        console.log(arr.indexOf(4));//3
        console.log(arr.slice(1,3));//2,3
        console.log(arr.slice(1));//2,3,4

5 join

                var p=document.getElementById("p");
        var arr=[1994,"html","css","js"];
        var b=arr.join();
        console.log(b);//   1994,html,css,js
        var str=arr.join("/");
        p.innerHTML=str;
        console.log(str);    //  1994/html/css/js
 例子
            var data={
            country:"美国",
            year:"1994",
            story:"犯罪"
        }
        var arr=[]
        for(var i in data){
            arr.push(data[i])
        }
        var str=arr.join("/");
        console.log(str);  //美国/1994/犯罪

6 sort 升序降序

                //升序
        // arr.sort((a,b)=>{
        //     return a-b;
        // })
        var arr=[4,1,6,12,7,9];
        var res=arr.sort((a,b)=>{
            return b-a;
        })
        console.log(res); //[12, 9, 7, 6, 4, 1]
  例子(年龄排序)
           var students=[
            {name:"zhang",age:19},
                {name:"meng",age:21},
            {name:"li",age:8},
            {name:"liu",age:14},
        ]
        var result=students.sort((a,b)=>{
            return a["age"]-b["age"]
        })
        console.log(result)
  例子(外卖)
                    <button id="like">like</button>
    <button id="price">price </button>
    <button id="distance">distance</button>
    <script>
        var like=document.getElementById("like");
        var sales=[
            {name:"zs",like:56,distance:90,price:75},
            {name:"ls",like:89,distance:56,price:45},
            {name:"ww",like:76,distance:50,price:65},
        ]
        function addOrder(arr,value){
            if(Array.isArray(arr)){
                var res =arr.sort((a,b)=>{
                return a[value]-b[value];
            })
            return res;
            }else{
                return null;
            }
        }
        like.onclick=function(){
            console.log(addOrder(sales,"like"))
        }
        price.onclick=function(){
            console.log(addOrder(sales,"price"))
        }
        distance.onclick=function(){
            console.log(addOrder(sales,"distance"))
        }

7 reduce 求和

            1.    var arr=[1,2,3,4];
        var sum =arr.reduce((a,b)=>{
            return a+b;
        })
        console.log(sum);//10
        console.log(arr); //1,2,3,4
      2. var arr=[1,2,3,4];
        var sum=0;
        for(var i=0;i<arr.length;i++){
            sum=sum+arr[i];
        }
        console.log(sum);

8 取最大值

var arr=[1,2,3];
console.log(Math.max(45,2,10));//45
console.log(Math.max(...arr));//3
 for(var i=1;i<arr.length;i++){
            if(maxNum<arr[i]){
                maxNum=arr[i]
            }
        }

9 reverse 数组的反转

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

10.数组去重

                var arr=[1,2,3,2,4,8,6,4];
        // arr=[1,2,3,4,6,8]
        var res=[];
        arr.forEach(item=>{
            if(!res.includes(item)){
                res.push(item);
            }
        })
        console.log(res)

set方法

                var arr=[1,2,3,2,4,8,6,4];
        var s=new Set(arr);    
        console.log([...s])

将一个对象数组中的某个键添加到另一个数组中（按下标来) 根据某个定值来判断从那里截取（添加）

                let flag = data.activities.length;
        data.activities.forEach((item, index) => {
          if (item.content == e.orderStateName) {  //content定值
            flag = index;
            item.timestamp = res.data[index].flowLogTIme;  //定值项添加键值
          }
          if (index < flag) {
            item.timestamp = res.data[index].flowLogTIme);  //小于定值的下标的添加
          }
        });