How to call a function on beforeclose event in bpopup?

I want to call a function before closing the bpopup. I have used following code:

$('#casepromptPopup').bPopup({
    escClose: false,
    closeClass: 'b-close',
    modalClose: false,
    onClose: function() {                         
        confirm("Are you sure you wish to quit and grade your performance?");

I want to show the confirm message before popup close. But onClose method fires after the popup close.

Solution

While I'm not a fan of plugin modifications, it seems that there's no other way to achieve this.

Modify the bpopup.js:

1. Find function close() and add if(o.confirmClose()){ :

function close() {
    if(o.confirmClose()){
        // rest of the original function
    }
}

2. Find $.fn.bPopup.defaults = { (at the bottom) and add:
```
confirmClose: function(){return true;}
```

Now you can add confirmation into your bpopup options:

$('#casepromptPopup').bPopup({
    escClose     : false,
    closeClass   : 'b-close',
    confirmClose : function(){                         
        return confirm("Are you sure you wish to quit and grade your performance?");
    }
});

JSFiddle