增删改只有concat不能改变原数组，增删改查之外只有sort和reverse以及复杂数组时的map方法会改变

添加数组元素

push(在数组末尾添加数组元素)

var arr1 = [1,2,3,4];
var b = [5,6,7,8];
arr1.push(b);
b.push(9,10,11);//可同时push多个值
console.log(b);//[1,2,3,4,Array(4)]
console.log(b);//[5,6,7,8,9，10，11]

unshift(在数组开头添加数组元素)

var arr = [1,2,3];
var b = [4,5,6];
arr.unshift(...b);
console.log(arr);//[4,5,6,1,2,3]

concat

concat不会改变原数组

var arr = [1,2,3];
var c = [5,6,7]
var b = arr.concat(c);
console.log(arr);//[1,2,3]
console.log(b);//[1,2,3,5,6,7]

splice

在某个数组元素的前面添加

<script>
        var arr = [1,2,3,4]
        var b = [7,8,9]
        arr.splice(2,0,...b)
        console.log(arr)//[1,2,7,8,9,3,4]
</script>

数组展开

“…”加数组可将数组展开

var arr1 = [1,2,3,4];
var b = [5,6,7,8];
arr.push(...b);
console.log(arr);//[1,2,3,4,5,6,7,8]

删除数组元素

pop(在数组末尾删除数组元素)

shift(在数组开头删除数组元素)

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

定点删除splice(index,howmany)

index 从哪个下标开始 howmany 删除多少个

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

修改数组元素splice(index,howmany,value)

index 从哪个下标开始 howmany 删除多少个 value 插入的值(可以是多个值)

var arr = [1,2,3,4,5];
var b = [1,2,3,4,5];
arr.splice(2,1,7);
b.splice(2,3,7,8,9);
console.log(arr); //[1,2,7,4,5]
console.log(b); //[1,2,7,8,9]

查询数组元素

indexOf —查询数组某个值的下标

只会查询到元素出现第一次的下标

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

findIndex

只会查询到元素出现第一次的下标

var arr = [1,2,3,2];
var index = arr.findIndex(item=>{
  return item == 2;
})
console.log(index);//1

find

找到第一个符合条件的值并返回

        <script>
        var a = [1,2,3,4]
        var res = a.find(item=>{
            return item == 3
        })
        console.log(res) //3
        console.log(a)  
    </script>

截取数组片断

slice(startIndex,endIndex) 不包含endIndex
slice(startIndex) 截取从startIndex开始到后面所有的值

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

例子

<script>
        var arr = [1,2,3,4,5,6,7,8,9]
        //[[1,2,3],[4,5,6],[7,8,9]]
        var sum =[];
        for(var i=0;i<arr.length;i=i+3){
             var item = arr.slice(i,i+3);
             sum.push(item);
        }
        console.log(sum)
</script>