function rot13(str) {
  var newString = '';
  var newCharCode;
  var newChar;

  for (let i = 0; i < str.length; i++) {
    newCharCode = charTransform(str[i]);
    newChar = String.fromCharCode(newCharCode);
    newString += newChar;
  }

  return newString;
}

function charTransform(char) {
  var transformKey = 13;

  var capitalMax = 90;
  var capitalMin = 65
  var lowerCaseMax = 122;
  var lowerCaseMin = 97;

  var charCode = char.charCodeAt(0);

  if (char.match(/[A-Z]/)) {
    charCode += transformKey;

    if (charCode > capitalMax) {
      charCode = capitalMin + (charCode - capitalMax) - 1;
    }
  }

  if (char.match(/[a-z]/)) {
    charCode += transformKey;

    if (charCode > lowerCaseMax) {
      charCode = lowerCaseMin + (charCode - lowerCaseMax) - 1;
    }
  }

  return charCode;
}