phpjs/functions/strings/str_rot13.js

php.js offers community built php functions in javascript

function str_rot13(str) {
  //  discuss at: http://phpjs.org/functions/str_rot13/
  // original by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
  // improved by: Ates Goral (http://magnetiq.com)
  // improved by: Rafał Kukawski (http://blog.kukawski.pl)
  // bugfixed by: Onno Marsman
  //   example 1: str_rot13('Kevin van Zonneveld');
  //   returns 1: 'Xriva ina Mbaariryq'
  //   example 2: str_rot13('Xriva ina Mbaariryq');
  //   returns 2: 'Kevin van Zonneveld'
  //   example 3: str_rot13(33);
  //   returns 3: '33'

  return (str + '')
    .replace(/[a-z]/gi, function(s) {
      return String.fromCharCode(s.charCodeAt(0) + (s.toLowerCase() < 'n' ? 13 : -13));
    });
}