delete刪除數(shù)組元素

var a = [1,2,3,4,5];
delete a[0];
console.log(a); // [undefined, 2,3,4,5]

用 delete 的區(qū)別（in 操作符）

var a = ['x','y','z'];
delete a[1];
1 in a; // false， 1 是 key

var a = ['x','y','z'];
a[1] = undefined;
1 in a; // true

arr.length -= 1; // 出去末尾， arr.length = arr.length - 1;