WeakSet.prototype.add()

const weakset1 = new WeakSet();
const object1 = {};

weakset1.add(object1);
console.log(weakset1.has(object1));
// Expected output: true

try {
  weakset1.add(1);
} catch (error) {
  console.log(error);
  // Expected output (Chrome): TypeError: Invalid value used in weak set
  // Expected output (Firefox): TypeError: WeakSet value must be an object, got 1
  // Expected output (Safari): TypeError: Attempted to add a non-object key to a WeakSet
}

var ws = new WeakSet();

ws.add(window); //window オブジェクトを WeakSet へ追加

ws.has(window); // true

// WeakSet は引数としてオブジェクトのみを取ります。
ws.add(1);
// 結果は "TypeError: Invalid value used in weak set" (Chrome)
// "TypeError: 1 is not a non-null object" (Firefox)