const book1 = { title: 'Pride and Prejudice', author: 'Jane Austen' }; const book2 = { title: 'The Catcher in the Rye', author: 'J.D. Salinger' }; const book3 = { title: 'Gulliver’s Travels', author: 'Jonathan Swift' }; const library = new WeakMap(); library.set(book1, true); library.set(book2, false); library.set(book3, true); console.log(library); // library.set('The Grapes of Wrath', false); book1 = null; console.log(library);