https://www.freecodecamp.org/challenges/caesars-cipher

caeserCipher.js

/*
Wirten by James Calleja 
32/11/2017
*/
function rot13(str) 
{ 
  var strArray = str.split("");
  var result = [];
  for (var i = 0; i < str.length; i++)
    {
      if (strArray[i].match(/[a-zA-Z]/)) //if isAlpha()
        {
          if (strArray[i].charCodeAt() -13 <= 64)
            {
              result.push(String.fromCharCode(strArray[i].charCodeAt() - 13 + 26));
            }
          else
            {
              result.push(String.fromCharCode(strArray[i].charCodeAt() - 13));
            }
        }
      else 
        {
          result.push(String.fromCharCode(strArray[i].charCodeAt()));
        }
    }
  return result.toString().replace(/,/g,"");
}

// Change the inputs below to test
rot13("SERR PBQR PNZC");