How to control the page before closing the window

Question

I am using the function

 window.onbeforeunload = function(){
  if(sessionStorage.hasOwnProperty("teststatred")){
            validNavigation = true;
            leave_message = "You are in the middle of the test, Are you sure do you want to close the window?"
        }
        if (validNavigation)
        {
            if (!e) e = window.event;              
            e.cancelBubble = true;
            e.returnValue = leave_message;               
            if (e.stopPropagation) {
                e.stopPropagation();
                e.preventDefault();
            }             
            return leave_message;
        }
 }

This function will display a window of two buttons: Leave this page and Stay on this page. How can I execute a function when the user clicks on Leave this page or 'Stay on this page'?

Answer 1

Unfortunately, you cannot, because the only way would be to use confirm(), alert(), or prompt() and they will all be ignored:

'Since 25 May 2011, the HTML5 specification states that calls to window.showModalDialog(), window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event.'

https://developer.mozilla.org/en-US/docs/Web/API/window.onbeforeunload?redirectlocale=en-US&redirectslug=DOM%2Fwindow.onbeforeunload

However, it does say may, so it wouldn't hurt to give it a try and prompt something.