This is really weird from a mod at 1kyu.. Have YOU tried the code without them? It works perfectly fine, and of course it does, because before the reduce he's calling a map, and what's returned in the callback from map is always a number, the cast back to number happens when you do the power, there's no need to cast again in the reduce.

If you don't fill, the array is considered empty for .map, so .map returns an empty array. You can fill it anything, even undefined, but you have to do it or otherwise map doesn't do anything.