[рони]

junior29,

function fun(a, b)
{ a > b || ([a, b] = [b, a]);
  let k = 1;
  for (let i = 2; i <= b; ) {
      if(!(b % i) && !(a % i) ) {
      b /= i;
      a /= i;
      k *= i;
      }
      else i++
  }
  return k
}
alert(fun(20, 30));