Find missing letter in list of alphabets

Question

I am trying to solve the following issue:

Find the missing letter in the passed letter range and return it. If all letters are present in the range, return undefine

Answer 1

Another function that may help:

var alphabet = "abcdefgheijklmnopqrstuvwxyz";
function fearNotLetter(a) {
  function letterIndex(text, index) {
    var letter = text.charAt(0);
    if (alphabet.indexOf(letter) !== index) { return alphabet.charAt(index); } else { return letterIndex(text.substring(1), index + 1) }
  }
  if (alphabet.indexOf(a) === -1) {
    return letterIndex(a, alphabet.indexOf(a.charAt(0)));
  }
  return undefined;
}
fearNotLetter("abc"); //Undefined
fearNotLetter("abce"); //d
fearNotLetter("fgi"); //h