Extension of javascript Set adding union, intersection & difference member functions

sset.js

class SSet extends Set {
  union(a) {
    return new SSet([...this, ...a]);
  }
  intersection(a) {
    return new SSet([...this].filter(x => a.has(x)));
  }
  difference(a) {
    return new SSet([...this].filter(x => !a.has(x)));
  }
  log() {
    console.log(Array.from(this));
  }
  toArray() {
    return Array.from(this);
  }
}