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

How about this one? it finds all missing letters anywhere between the first and the last given letters:

function fearNotLetter(str) {
  var strArr = str.split('');
  var missingChars = [], i = 0;
  var nextChar = String.fromCharCode(strArr[i].charCodeAt(0)+1);
  while (i