> var a = [1,2]
undefined
> a[4] = 3;
3
> a[3] = 3;
3
> var a = [1,2]
undefined
> a[4] = 4;
4
> a[3] = 3;
3
> a
[1, 2, undefined, 3, 4]
> a.map(function(k){ console.info("k: " + k) })
k: 1
k: 2
k: 3
k: 4