CloseEvent.initCloseEvent() - Web APIs
syntax event.initmouseevent(type, canbubble, cancelable, wasclean, reasoncode, reason); parameters type the string to set the event's type to.
... wasclean whether or not the connection was cleanly closed.

CloseEvent() - Web APIs
closeeventinit optional is a closeeventinit dictionary, having the following fields: "wasclean", optional and defaulting to false, of type long, indicates if the connection has been closed cleanly or not.

CloseEvent - Web APIs
closeevent.wasclean read only returns a boolean that indicates whether or not the connection was cleanly closed.