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 isSubset(subset, set) {
  for (let elem of subset) {
    if (!set.has(elem)) {
      return false;
    }
  }
  return true;
}

console.log('is A subset of B=>', isSubset(setA, setB));