function isEven(num) {
  if (num == 0)
    return true;
  if (num == 1)
    return false;
  if (num < 0)
    return "??";
  else return isEven(num - 2);
}

console.log(isEven(50));
// → true
console.log(isEven(75));
// → false
console.log(isEven(-1));
// → ??