Strange

I agree, somewhere several exit points are even necessary (for example, recursion). But this specific example can be simplified a little, at least for readability.

What are the advantages of "var" over "let"?

A bit hard to read the code. I would also recommend using a single exit point from the function, this helps a lot

I should broke this to 2-3 rows, thanks

My bad

Oh, I didn’t think about performance. Thanks for the advice

I think const is better

You can remove comment btw

I think we can't reuse sumNumber, so the idea of dividing into 2 functions seems strange to me
btw unnecessary comment