console.log(difference([1, 2, 3], [100, 2, 1,9, 10]));

function difference(setA,setB)
{
    var union = new Set(setA);
    console.log('union 1st : '+setA);
    console.log('union 2nd : '+setB);
    
    console.log('-------------');
  for (var i=0; i<setB.length ; i++) {
    union.add(setB[i]);
  }
  return union;
}