// best
function rot13(message) {
  var a = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
  var b = "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM";
  return message.replace(/[a-z]/gi, (c) => b[a.indexOf(c)]);
}