That's true. However this solution is really old, probably written before var became depricated.

It is better not to use var and replace it with let or const, but in this case let. It is better not to use var at all

True, but this was probably a really old answer. That's why I think Codewars shouldn't obfuscate dates. I don't know how exactly, but I'm sure there's a way to solve the issue of really outdated answers (that were great at the time) being presented as best practices to beginners especially in JS and Python katas.

What about number which is a string? ( ͡° ͜ʖ ͡°)

Your code is quite self-explanatory, no need for the comments :)

a little over-engineering

What I did; wasn't even aware of the ES6 functionality up to this point

"Do not mutate the original array/list".

When using splice you change at all the original array.

This 'splice method' add or remove an element from the original array. So it's wrong!

Yes it could. But try to keep in mind: is just for teaching good practices and how to inits variables