array length

Hello

As I read in Internets, array.length can cut array to size. I need this to save only 100 last readings. But using lenght can, I think, cause some memory issues.
code:

a = new Array();
a = [1, 2, 3, 4, 5, 6];
console.log(a);

console requests:

>[ 1, 2, 3, 4, 5, 6 ]
=undefined
>a.length
=6
>a
=[ 1, 2, 3, 4, 5, 6 ]
>a.length = 2
=2
>a
=[ 1, 2, 3, 4, 5, 6, "length": 2 ]
> 

I can get a real array size only before trying to cut it.