First non-repeating character.js

/*
Description:
Write a function named first_non_repeating_letter that takes a string input, and returns the first character that is not repeated anywhere in the string.

For example, if given the input 'stress', the function should return 't', since the letter t only occurs once in the string, and occurs first in the string.

As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input 'sTreSS' should return 'T'.

If a string contains all repeating characters, it should return None.
*/

function firstNonRepeatingLetter(s) {
  return s[s.toLowerCase().split('').findIndex(letter => s.toLowerCase().split('').filter(l => l === letter).length === 1 )] || '';
}