Javadoc
This method allows the registration of event listeners on the
event target. If an EventListener
is added to an
EventTarget
which is currently processing an event
the new listener will not be triggered by the current event.
If multiple identical EventListener
s are
registered on the same EventTarget
with the same
parameters the duplicate instances are discarded. They do not
cause the EventListener
to be called twice and
since they are discarded they do not need to be removed with
the removeEventListener
method.