Here is a recursive solution, using the Euclidean algorithm.

 var gcd = function(a, b) {

  if (!b) {

    return a;

  }



  return gcd(b, a % b);

}

Our base case is when b is equal to 0. In this case, we return a.

When we're recursing, we swap the input arguments but we pass the remainder of a / b as the second argument.