Ok, it's time to say Goodbye. This is the last recipe of our book. In every book that describes some new software technology, we can find the famous "Hello, world!" example. But there is never a "Goodbye, world!" program. That's the reason why we have added one such recipe . We will see how to handle the closing application. Sometimes users close the application before saving some important settings. In that case, we can alert users to stay on the page. We'll do it by using native JavaScript code, which will be added as a component to the Vaadin application.