next_letter.js

function LetterChanges(str) { 

   var letters = str.split('');
   for (var i = 0; i < letters.length; i++) {
     var letter = letters[i];
     if(/[a-yA-Y]/.test(letter)){
       letters[i] = String.fromCharCode(letter.charCodeAt(letter.length-1)+1);
     }
   }
    
   return letters.join(''); 
         
}