My code for Caesars Cipher

My code for Caesars Cipher
0

#1
function rot13(str) { // LBH QVQ VG!
     
  return str.replace(/\w/g, function(letter){
    if (str.charCodeAt(str.indexOf(letter)) + 13 > 90) {
      return String.fromCharCode(str.charCodeAt(str.indexOf(letter)) - 13);
  } else {
    return String.fromCharCode(str.charCodeAt(str.indexOf(letter)) + 13);
  }
  });
  
}
// Change the inputs below to test
rot13("SERR PBQR PNZC");