generate control char of italian fiscal code

function controlChar(code) {
  let control = "";
  let numbers = "0123456789";
  let letters = "BCDFGHJKLMNPQRSTVWXYZ";
  let vowels = "AEIOU";
  let sum = 0;
  let parity = 0;
  let i;
  for (i = 0; i < 15; i++) {
    if (i % 2 == 0) {
      sum += numbers.indexOf(code[i]);
      parity += letters.indexOf(code[i]);
    } else {
      sum += letters.indexOf(code[i]);
      parity += numbers.indexOf(code[i]);
    }
  }
  sum += parity;
  control = letters[sum % 26];
  return control;
}

controlChar("RSSMRA74A45H501")