How to determine which close button is pressed by multiple tabs

I found a lot of code to detect the close button of browsers, but I got an event when a closed tab was called. I just want the main event of the close button to alert the user to stay on this page or not. I found this code.

<script type="text/javascript"> 
window.onbeforeunload = function (evt){
    var message = 'Are you sure you want to leave?'; 
    if (typeof evt == 'undefined') { 
        evt = window.event; 
    } 
    if (evt) { 
        evt.returnValue = message; 
    } return message;  
   } 
</script>

It warns that users close or stay by closing the tab.

Now I want to detect the closing event of the main browser. Since the found closed event is also used to close tabs.

Can someone help me on this issue?