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

  function fearNotLetter(str) {
  //transform the string to an array of characters    
  str = str.split('');

  //generate an array of letters from a to z
  var lettersArr = genCharArray('a', 'z');
  //transform the array of letters to string
  lettersArr = lettersArr.join('');
  //substr the a to z string starting from the first letter of the provided 
  string
  lettersArr = lettersArr.substr(lettersArr.indexOf(str[0]), str.length);
  //transform it again to an array of letters
  lettersArr = lettersArr.split('');
  //compare the provided str to the array of letters
  for(var i=0; i