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

missingletters.js

function fearNotLetter(str) {
  var x = 0;
  var res = 0;
  for (var i=0; i<str.length; i++) {
    var y = str.charCodeAt(i);
    if(x !== 0){
      if(y !== x+1){
        res = x + 1;
        break;
      }
    }
    x = y;
  }
  
  if(res !== 0){
    str = String.fromCharCode(res);
  }
  else{
    str = undefined;
  }
  return str;
}

fearNotLetter("abce");