Edit in JSFiddle

var setA = new Set([1, 9, 11]);
console.log('Set A =>', [...setA]);
var setB = new Set([1, 4, 8, 9, 11]);
console.log('Set B =>', [...setB]);

function union(setA, setB) {
  return new Set([...setA, ...setB]);
}

const unionSet = union(setA, setB);
console.log(' A union B =>', ...unionSet);