创建-增-删-改-查

var arr3 =  [1,2,3,4,6];
var result = arr3.splice(2,0,"sdf") //第二个位置,删除0个元素,插入值
console.log(arr3)  // [1, 2, "sdf", 3, 4, 6]

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

var arr2 =  [1,2,3,4,6];
var result = arr2.splice(1,3)
console.log(result)  // [2, 3, 4] 删除的值
console.log(arr2)  // [1, 6] 删除后得到的值
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var arr3 =  [1,2,3,4,6];
var result = arr3.splice(1,3,"sdf")
console.log(result)  // [2, 3, 4] 删除的值
console.log(arr3)  // [1, "sdf", 6] 删除后插入的值

var arr = [1, 2, 3, 4];
delete arr[0];
console.log(arr);   //[undefined, 2, 3, 4]
delete删除之后数组长度不变，只是被删除元素被置为undefined了

var t = [1,23,5,6,7];
t.length =3;
console.log(t)   // [1,23,5]

区别：
var arr = [1,23,5,6,7];
var arr1 = arr;
arr.length = 0;
console.log(arr)  // []
console.log(arr1) // []
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var arr2 = [1,23,5,6,7]; 
var arr3 = arr2;
arr2 = [];
console.log(arr2)  // []
console.log(arr3) // [1, 23, 5, 6, 7]