JavaScript function that takes a ROT13 encoded string and decodes it

CaesarsCipher.js

function rot13(str) { 
  var result="";
  var rot=13;
  var s=(26-rot)%26; //s=rot; if you want to encrypt!
  for (var i=0;i<str.length;i++)
    {
      var c = str.charCodeAt(i);
      if (c>=65 && c<=90) 
        result=result+String.fromCharCode((c-65+s)%26+65);
      else 
        if (c>=97 && c<=122)
          result=result+String.fromCharCode((c-97+s)%26+97);
      else
        result=result+str.charAt(i);
    }
  
  return result;
}