multidimensional global arrays in Jitter Javascript

I'm trying to figure out how to create a multidimensional array of global variables in Javascript (with Jitter installed). I thought I had a variation of the following code working at one point, but something's very wrong now. The idea is the top-level array tracks generations of multilayer visual compositions, while the nested array tracks the individual layers and their attributes:

var layer = 1;
var gen = 0; //

g = new Global("params");
g.gen = new Array();
    for (gen = 0; gen < 10; gen++) { // number of generations
        g.xdim[gen] = new Array();
        g.ydim[gen] = new Array();
    }

When this is set up right, I should be able to read and write values such as:

gen = 7;
layer = 3;
g.xdim[gen][layer] = 42;

// using above vars, this would set g.xdim[7][3] = 42;

BTW, in a perfect world, I wouldn't have to dimension the number of generations in advance.

Any insight greatly appreciated!

Jeff