Why do the vowels have to be in [ ]?

y use filter_map() when map() works just fine?

Would you mind to elaborate? It's almost the same solution in C.

I find it very funny that I'm always overthinking things having a C background, does anyone have any tips?

Codewars could have a button for bad solutions

string.toCharArray() underneath makes a copy of the String character array (defensive programming)
If we prepare array by making new char[string.lenth()] and then iterate trought string using chatAt(i) we could avoid one array copy