function GCD (a, b) {
  let _r = a % b;
  if (_r === 0 || _r === 1) return b;
  return GCD(b, _r);
}

console.log(GCD(21, 14)); //-> 7
console.log(GCD(60, 96)); //-> 12
console.log(GCD(14, 23)); //-> 1