const weakSet = new WeakSet();
console.log(weakSet); // WeakSet {}
const obj = {a:1};
// add to a weakSet
weakSet.add(obj);
console.log(weakSet); // WeakSet {{a: 1}}
// check if an element is in Set
console.log(weakSet.has(obj)); // true
// delete elements
weakSet.delete(obj);
console.log(weakSet); // WeakSet {}