null created by oliverjam - https://repl.it/GfI5/0

basic-rot13.js

const str = "SERR PBQR PNZC";

function rot13(string) {
  return string.split('')
               .map(word => {
                  return word.split('')
                      .map(letter => {
                        const letterCode = letter.charCodeAt(0);
                        if (! (letterCode >= 65 && letterCode <= 90)) return letter;
                        if (letterCode <= 77) return String.fromCharCode(letterCode + 13);
                        if (letterCode >= 78) return String.fromCharCode(letterCode - 13);
                       }).join('');
  }).join('');
}

rot13(str);