JavaScript function to replicate Ruby's "succ()" method.

gistfile1.js

String.prototype.succ = function() {
  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;
};

String.prototype.isUpperCase = function() {
  return this.toUpperCase() == this;
};

/* 
 * "a".succ()        => "b"
 * "b".succ()        => "c"
 * "C".succ()        => "D"
 * "def".succ()      => "efg"
 * "gHi".succ()      => "hIj"
 * "£$^".succ()      => "£$^"
 * "abc£$^".succ()   => "bcd£$^"
 * "c".isUpperCase() => false
 * "C".isUpperCase() => true
 */