That is, if I use the current time as an index into the array:
array[Date.getTime()] = value;
will the interpreter instantiate all the elements from 0 to now? Do different browsers do it differently?
I remember there used to be a bug in the AIX kernel, which would create pseudo-ttys on request, but if you did, say, "echo > /dev/pty10000000000" it would create /dev/pty0, /dev/pty1, .... and then fall over dead. It was fun at trade shows, but I don't want this to happen to my customers.
How exactly JavaScript arrays are implemented differs from browser to browser, but they generally fall back to a sparse implementation - most likely the same one used for property access of regular objects - if using an actual array would be inefficient.
You'll have to ask someone with more knowledge about specific implementations to answer what excatly triggers the shift from dense to sparse, but your example should be perfectly safe. If you want to get a dense array, you should call the constructor with an explicit length argument and hope you'll actually get one.
See this answer for a more detailed description by olliej.