Converting String To DOM Object Reference?

I have searched the web and this forum for the answer to this but am unable to find a reasonable choice. So, this is the problem I am facing.

Code:
var str = "document.frames[0].document.frames[0]";
var str 1 = "document.frames[6]";
//Loop Starts
{
var obj1 = str + str1;

[Code]...

I know the easiest way to do this is to use eval() and it most definitely does work. The problem is that I have to use eval() inside a loop to check whether the reference is null and from what I hear, that is one of the most inefficient ways to code. I have searched for alternatives like using, var myFunc = new Function(string) and then calling myFunc(). But this does not work in my case. I cannot find any other alternative in this case. I guess one can say eval() is the only choice here but I am not too sure.