Many frameworks out there now come with a service worker file already set-up for us. But knowing how to implement it with Vanilla JavaScript can help you understand PWAs. A Book instance is deleted from the Book.instances collection by first testing if the map has an element with the given key line 2, and then applying the JavaScript built-in delete operator:, which deletes a slot from an object, or, in our case, an element from a map: Pay close attention to the script tag with a type attribute