rot13.js

function reduceRot13Cipher(map, c, i, abc) {
  const off = 13;
  const len = abc.length;
  const j = (i + off) % len;
  map[c] = abc[j];
  return map;
}

const rot13 = (() => {
  const abc = 'abcdefghijklmnopqrstuvwxyz';
  const map = {};
  abc.split('').reduce(reduceRot13Cipher, map);
  abc.toUpperCase().split('').reduce(reduceRot13Cipher, map);

  return function rot13(msg) {
    const dec = msg.split('');
    const enc = dec.map(c => (c in map) ? map[c] : c);
    return enc.join('');
  }
})();