has sort method stability been fixed in more recents node versions tho

It appears that sorting has become stable since node v12 according to MDN. So maybe you're using js version that supports stable sorting locally.

If there are two or more characters with the same l return the first in order of appearance.

Your method uses sort, and sort is not stable. Not a kata issue unfortunately, you need to slightly alter your approach.

Your recursion isn't going as well as you want to. Getting undefined means the function ended without a return (or with a return with no value, or return with value undefined, but these two cases obviously aren't happening here). Hint: Each function can only return for its own call.

Not directly telling you so that you figure it out by thinking and learn something :)

Without seeing your code we can only guess, are you using recursion? If so, maybe you're doing it wrong. If not, are you sure your function returns the result?

After seeing your code, yes, you're doing recursion wrong. Also, don't use global vars like that, they keep their value between tests and you'll get wrong results when your function is called several times in a row.

Please see if the last bullet point of this paragraph of FAQ helps: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution#when-i-print-my-answer-it-looks-exactly-the-same-as-the-expected-output-yet-tests-fail