EventEmitter: Inconsistent handling of once() handlers

[Flarna]

• Version: 6.2.0 (with references to others)
• Platform: Windows 64 bit
• Subsystem: EventEmitter

It seems EventEmitter has an inconsistent handling of listeners installed via once().

• a listener listening on 'newListener' gets the function registered via once()
• a listener listening on 'removeListener' gets the function registered via once() since 6.2.0 because of events: pass the original listener added by EventEmitter#once to the removeListener handler #6394, in all other node versions the wrapper g() was returned.
• listeners() returns the wrapper g()

I think #5564 was targeting this topic also but was closed because #6394 was merged.

Not sure if this is a but or intended - or if changing this is seen as breaking change or not.
If it is kept like it is I would vote for updating documentation accordingly.

[cjihrig]

It was decided in #6394 that the previous behavior was a bug, and therefor not a breaking change.

What did you have in mind as a documentation update?

[Flarna]

#6394 has changed/corrected the behavior of signaling which listener has been removed via 'removeListener' event.
The listeners() function has not been changed in #6394, it still returns the wrapped function.

I think we should either document that listeners() is not returning the registered listener or correct it to actually return the listener.

[cjihrig]

Ah, yea, @addaleax pinged @omsmith about that in #5564 (comment).

[jasnell]

to be honest, if we could do so in a way that didn't kill perf I'd be ok with listeners being modified to not return the wrapper given that the wrapper really is an internal implementation detail.

[Flarna]

Is it really proven that adding a simple unwrap in listeners() kills performance?

I would wonder that listeners() is an API which is used in performance relevant code as it creates a copy of an array. Quite a lot work - for actually getting list of functions back which has maybe "wrong" entries inside in all node versions till now.

Most use cases I have seen till now are in the end just interested in the size of the array which is available much cheaper using listenerCount() where whether copy nor unwrap is needed.

Not sure if this is the right place to ask: But as this and #6394 are seen as bugs are there also ported back to Node 4?