数组越界问题

    var a = [];
    for (var i = 0; i < 10; i++) {
        a[i] = function () {
            console.log(i);
        }
    }
    a[6]();//10

等价于

   var a = [fn, fn, fn, fn, fn, fn, fn, fn, fn,fn];
    var i = 10;
    function fn() {
        console.log(i);
    }
    a[6]();//10

要想让a6输出的结果为6,将for循环的var改成let即可,修改代码如下:

    var a = [];
    for (let i = 0; i < 10; i++) {
        a[i] = function () {
            console.log(i);
        }
    }
    a[6]();//10

关于var 和 let 区别,感兴趣的可以自己百度一下.