Woowww hooww hold on, why the best its O(n^2)

you can use '_' instead, it just a placeholder

its just because without it, the functions assumes W is the index and the last parameter is left off. The functions isnt aware of the parameter name, just the location of the parameter within the callback.

very clever, but by far the slowest I've seen in terms of performance

Generally one can find good and interesting solutions among the top voted ones, but it is not necessary the case of the best voted one.

I suggest you follow multiple highly skilled coders (like akar-0) and instead compare your solutions to them.

Well, you have thousands of katas, with all sort of problems and difficulty levels. I don't know what to recommend, try a bit of everything and see what's the best for you and what you prefer. All I can say is I learnt a lot from looking at other people solutions.

People vote anything "best practice". You should not care much about that, unfortunately it's the same on many katas.

Solutions with O(n^2) time complexity should never be the best practise in my opinion, if there is a better solution with the same readability(not mine, it looks bad without refactoring).
I see here very often, that not good solutions have been upvoted as hell.

It is kind of lame, because if you want to compare and fix your crappy solution you often only see other crappy solutions with slightly different syntax at the top.

Not efficient for larger words

do you mind sharing the solution with O(n) performance?

O(N²)

Can someone tell me what the time complexity of this code