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

Here's my another answer in missing letters:

function fearNotLetter(str) {
  var alphabet = 'abcdefghijklmnopqrstuvwxyz';
  var first = alphabet.indexOf(str[0]);
  var last = alphabet.indexOf(str[str.length-1]);
  var alpha = alphabet.slice(first,last);

  for(var i=0;i