How to control browser confirmation dialog on leaving page?

Question

I know there are a lot of questions regarding this but nothing is answering me right. I want to show a confirmation dialog when user leaves the page. If the user press Cance

Answer 1

 window.onbeforeunload = function() {
       if (data_needs_saving()) {
           return "Do you really want to leave our brilliant application?";
       } else {
          return;
       }
    };