Converting an object back from nsISupports to Javascript Object or Array ( { } or [ ] )

Hi,

    I am using the mozilla observer service for notification
between different windows, using javascript. One problem I face is,
when I called the notifyObservers interface like this:

    var subject = { test : 1 };
    observerService.notifyObservers( subject, "topic", "data" ); 
    
    I am only getting an nsISupports as the subject when the observers
receive the notification. My question is, how should I QueryInterface on
the subject to retrieve either a:

    1. Javascript object
    2. Javascript array
    
    Thanks in advance.
    
Regards,
    Kok Hoor