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) {  
  var string = array. join("");
  for (var i = 0; i < string. length; i++) {                    
    if (string. charCodeAt(i + 1) - string. charCodeAt(i) != 1) {
      return String. fromCharCode(string. charCodeAt(i) + 1);
    } 
  }
  return undefined 
}