rot13.js

function rot13(encodedStr) {
  var alphabet = "abcdefghijklmnopqrstuvwxyz".toUpperCase().split("");
  var codeArr = encodedStr.split("");
  var decodedArr = [];
  decodedArr =  codeArr.map(function(a) {
    if(alphabet.indexOf(a)!==-1) {
      if(alphabet.indexOf(a)<13) {
        return alphabet[26+alphabet.indexOf(a)-13];
      } else {
        return alphabet[alphabet.indexOf(a)-13];
      }
    } else {
      return a;
    }
  });
  return decodedArr.join("");
}

console.log(rot13("GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK."));

Python translation:

def decrypt(encodedStr):
  alphabet = list("abcdefghijklmnopqrstuvwxyz".upper())
  coded = list(encodedStr)
  def decrypter(character):
    if character in alphabet:
      if alphabet.index(character) < 13:
        return alphabet[26+alphabet.index(character)-13]
      else:
        return alphabet[alphabet.index(character)-13]
    else:
      return character
  return ''.join(map(lambda a : decrypter(a), coded))