function gcd(a, b) { //greatest common divisor
  if(isNaN(a) || isNaN(b)){
    //throw new Error("a or b should be a number");
    //console.log('a or b should be a number');
    return false;
  }
  if(Math.floor(a) !== a || Math.floor(b) !== b){
    //throw new Error("a or b should be an integer");
    //console.log('a or b should be an integer');
    return false;
  }

  while(b !== 0){
    let r = a % b;
    a = b;
    b = r;
  }
  return (a<0) ? -a : a;
}

function lcm(a, b) { //least common multiple
  return Math.abs(a)*Math.abs(b)/ gcd(a, b);
}

function smallestMult(n) {

  let myLcm  = 1;
  let i;
  for(i=2; i<=n; i++){
    myLcm = lcm(myLcm, i);
  }

  console.log(myLcm);
  return myLcm;
}

smallestMult(20);