It encapsulates the parameters associated with an event.
The [[sender]] property describes who raises the event.
And the [[handled]] property indicates if the event is handled.
If an event handler sets [[handled]] to be true, the rest of the
uninvoked handlers will no longer be called to handle the event.
Additionally, when attaching an event handler, extra data may be passed
and be available via the [[data]] property when the event handler is invoked.