Caesar's Cipher

function rot13(str) { // LBH QVQ VG!
  debugger;
  var newArr = [];

  for(var i = 0; i < str.length; i++){
    var letter = str.charCodeAt(i);
    if (64 < letter && letter < 78){
      newArr.push(String.fromCharCode(letter+13));
    }  
      else if(77 < letter && letter < 91){
      newArr.push(String.fromCharCode(letter-13));
    } else {
      newArr.push(String.fromCharCode(letter));
    }
  }
  return newArr.join("");
}

// Change the inputs below to test
rot13("SERR PBQR PNZC");