rot13.js

let encryptee = "日本語English"
//Rot13 Encode
function encode(encryptee) {
  return encryptee.replace(/[A-Z]/g, function(c) {
    var s = c.charCodeAt(0);
    return String.fromCharCode(s < 78 ? s + 13 : s - 13);
  }).replace(/[a-z]/g, function(c) {
    var s = c.charCodeAt(0);
    return String.fromCharCode(s < 110 ? s + 13 : s - 13);
  }).replace(/[ぁ-ゖ]/g, function(c) {
    var s = c.charCodeAt(0);
    return String.fromCharCode(s < 12366 ? s + 73 : s - 13);
  }).replace(/[ァ-ヶ]/g, function(c) {
    var s = c.charCodeAt(0);
    return String.fromCharCode(s < 12463 ? s + 73 : s - 13);
  }).replace(/[一-龠]/g, function(c) {
    var s = c.charCodeAt(0);
    return String.fromCharCode(s < 19982 ? s + 20217 : s - 13);
  });
}

// decode
function decode(decryptee) {
  return decryptee.replace(/[a-zA-Z]/g, function (c) {
      var s = c.charCodeAt(0);
      return String.fromCharCode((s <= 90 ? 90 : 122) >= (s + 13) ? s + 13 : s - 13);
  }).replace(/[ぁ-ゖ]/g, function (c) {
      var s = c.charCodeAt(0);
      return String.fromCharCode(s <= 12425 ? s + 13 : s - 73);
  }).replace(/[ァ-ヶ]/g, function (c) {
      var s = c.charCodeAt(0);
      return String.fromCharCode(s <= 12521 ? s + 13 : s - 73);
  }).replace(/[一-龠]/g, function (c) {
      var s = c.charCodeAt(0);
      return String.fromCharCode(s <= 40184 ? s + 13 : s - 20217);
  })
}