ROT13 replaces each letter by its partner 13 characters further along the alphabet. For example, HELLO becomes URYYB (or, conversely, URYYB becomes HELLO again). Read more - https://en.wikipedia.org/wiki/ROT13

rot13.js

const rot13 = message => {
  const a = message.split('');
  return a.map(s => {
    const c = s.charCodeAt();
    if (c <= 65 || c >= 123 || s === ' ') return s;
    return String.fromCharCode(
      c <= 78 && c < 90 || c >= 97 && c < 110 ? c+13 : c-13
    );
  }).join('');
}