the biggest problem in your code is that you split(' ') on the space character, so you end up with a single chunk, since there are no spaces in the input strings.

Oh, true. The question was has anybody else seen the same problem: the function returns an empty string here, but it works in vscode

What is your question tho? If the question is "why my code does not work" then we do not know, and we cannot tell without seeing your code.

onlyDuplicates('abccdefee'): expected '' to equal 'cceee'
running the same code in VSCode seems to filter ok
can't figure out what went wrong O_o

loved this kata, so much fun <3