Multidimensional array variable size (JS)

The syntax I'm using with my array is this:

 var multiArray : String[,];

and usually I'd instantiate like so:

 multiArray = new String[4,4]; //or something similar

but I need to populate my array with an unknown amount of values, as they will be coming from a Split() string of variable length.

I know the length of the first dimension of the array but not the second.

 multiArray = new String(knownArray.length, variableLength);

I'm only able to get the length of the second dimensions after I've already assigned the first dimension of the multi-dimensional array. eg.:

 for(var i=0; i<knownArray.length; i++)
 {
     var sArray : String[] = knownArray[i].Split(","[0]);
     for(var x=0; x<sArray.length; x++)
     {
         multiArray[i,x] = sArray[x];
     }
 }

How would you suggest I acheive this functionality?