Set in javascript : Can you clarify why boolean is used while adding objects to the WeakMaps in the code below. I understand set takes two(key and value) arguments. The boolean values gets printed in the console as well…that is my doubt…

Thanks in Advance.

    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);
    
    WeakMap {Object {title: ‘Pride and Prejudice’, author: ‘Jane Austen’} => true, Object {title: ‘The Catcher in the Rye’, author: ‘J.D. Salinger’} => false, Object {title: ‘Gulliver’s Travels’, author: ‘Jonathan Swift’} => true}