Premature optimisation is a terrable thing, when they come with such a request I will simply refactor

I agree but this will never be production code anyway. This is codewars and the whole game is to produce short code, just to teach others clever ways of doing coding.

That's why there is eslint no-nested-ternary rule that prevents me to commit such rubbish to production code. However it is alright in codewars as this is a platform for streching the limits of what can be done coding wise. I have learned quite a lot from such short nonsense solutions.

It is an antipattern using switch with a true statement

You don't check if the number is Harshad at all.

This solution will replace uppercase letters as well.