Seeint this solution, I realize that I know nothing about Javascript :/

And since when being a mod makes me perfect? I stand corrected, it's not needed in this case.

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.

Unary operator to convert from strings to numbers, have you tried the code without them?

Can't understand why you're using +a + +b in your reduce function, instead using simple notation a + b.