[Old Pedant]

But a big problem with this approach is the *granularity*.

Just to pick the example you used, if we wanted "rare" to be 1%, and still assuming the same 3 weapons (4 occurrences), we would need 20 values in the rarity array.

And *still* combined.rare.sword would be 2% instead of 1%.

And so on.

I think I would opt for a much simpler yet more flexible system:

var weapons = {
    "sword" : [ 70, 28, 2 ],
    "bow" : [ 60, 30, 10 ],
    "staff" : [ 80, 19, 1 ]
};

where the 3 numbers are the percentage chances that a given weapon will be common, uncommon, or rare.

Completely flexible: You can decide what "rare" means on a per-weapon basis *and* you can easily see the distribution *and* you can easily change it.

Changing the distribution with RndMe's dual array system might turn into a real challenge to get the numbers you actually want.