Javascript String succ()

String.prototype.succ = function() {
  return this.slice(0, this.length - 1) +
    String.fromCharCode(this.charCodeAt(this.length - 1) + 1);
};

// I like extending core libraries.
// Copyright (c) Paul Mucur 2008

String.prototype.succ = function() {
  if (this.isNumber()) {
    return Number(this) + 1;
  } else {/*from  ww w.  ja v a  2 s  .  com*/
   var alphabet = 'abcdefghijklmnopqrstuvwxyz';
   var result = "";
   for (var i = 0; i < this.length; i++) {
     if (alphabet.indexOf(this.charAt(i).toLowerCase()) != -1) {
      if (this.charAt(i).isUpperCase()) {
       result += alphabet.charAt(alphabet.toUpperCase().indexOf(this.charAt(i)) + 1).toUpperCase();
      } else {
       result += alphabet.charAt(alphabet.indexOf(this.charAt(i)) + 1);
      }
    } else {
      result += this.charAt(i);
    }
   }
   return result;
  }
};