Join And Split Methods On Multidimensional Arrays ?

I am working on a project which requires a method to join or split multidimensional arrays according to its arguments, which would be string delimiters for the various dimensions of the array.

An example of the join function might be:

The split function:

This would turn 'str' into an array like this:

I think I may have found the problem (but not the solution) - it looks as though javascript won't perform a method on an array which is part of another array. Firefox error console gives me "arr[0].join is not a function".