events: Alias EventEmitter.protoype.removeListener as EventEmitter.prototype.off

[yosuke-furukawa]

EventEmitter should have off alias for io.js beginners, preventing event leaks, API symmetry etc....

I understand the historical reason why events does not have off alias.

nodejs/node-v0.x-archive#3338
nodejs/node-v0.x-archive#5352

Of course, I understand the API of events is frozen in io.js. However io.js have already added the new API getMaxListeners.
And this change does not break backward compatibility. I really would like to add this alias in io.js.

I know io.js follows semantic versioning. If this changes is not patch version (v1.0.x), I can wait for io.js to merge this change until minor version up (v1.1.x).

[quantizor]

I like this -- it feels like a more natural antonym to the .on() method. Certainly less verbose! 👍

[tellnes]

+1 for a less verbose name
+1 for consistency with other enviroments
-1 for backwardscompatibility (it will probably break someones code, ref nodejs/node-v0.x-archive#3922)
-1 for making the api more cluttered

[toastynerd]

-1 I don't really thinks it makes sense to 'off' an event.

[ruimarinho]

+1 for API symmetry/consistency. Several libs implement the on/off pattern, so it's really weird to use on but not off.

[rvagg]

it's not really symmetry though is it? the "on" is meant to red like: "for this object, on this event, do this", and "off" doesn't quite fit.

[yosuke-furukawa]

Some famous libraries have on and off. So I feel on 's antonym is off.

jQuery has on and off for aliasing addEventListener, removeEventListener.

other front-end libraries also have on and off. e.g. React-event, Vue.js

Some EventEmitter forks have off alias.

• EventEmitter2
• Wolfy87/EventEmitter
• primus/EventEmitter3

So I feel EventEmitter need to have off method.
I am afraid that some developers think removeListener is not important.

[wavded]

off does feel odd IMO, but whatevs, I've seen un as another shortened alias in libraries too. if clarity is a thing, i've always thought bind and unbind were pretty clear.

[yosuke-furukawa]

I also have seen un. However I have ever seen off so many times. Actually, I also think bind and unbind
were clear. However io.js should not change the existing alias on.

[Fishrock123]

.forget()

[sam-github]

-1. on is a nice alias for addListener, and it gets called a lot. removeListener... not so often. increasing the number of aliases confuses the API. node picked a name, lets stick with it.

And off is weird. event listeners aren't light switches. "on" has a number of meanings, "off" is the opposite of only some of those meanings. "under" is also an antonym of "on"... but neither "under" nor "off" are antonyms of the meaning of "on" used by EE.

[jonathanong]

+0, but i would say this is semver-major. not a big fan of aliases.

[cjihrig]

-1. I think it should be semver-minor because it's not a breaking change. Can we set up some sort of vote so that we can either land or close this?

[rvagg]

@cjihrig I believe there's enough -1's to just close this

[yosuke-furukawa]

hm, no problem. Thank you for your comments.

[ljharb]

This came up again in a very large codebase where we're migrating to an EventEmitter - everything already uses "on" and "off", like most event libs, and we were shocked to find that "on" exists but "off" doesn't. The fact that it's an alias is irrelevant to discoverability or usability imo - if we don't like aliases, great - let's remove "on". If we can't or don't want to remove it (which is fine), then why would it be hard to add a (semver-minor, not major) alias that makes the API more consistent?

[ChALkeR]

+0 from me.

[Fishrock123]

I'm going to re-quote @sam-github from above:

And off is weird. event listeners aren't light switches. "on" has a number of meanings, "off" is the opposite of only some of those meanings. "under" is also an antonym of "on"... but neither "under" nor "off" are antonyms of the meaning of "on" used by EE.

If we are going to add something else I think it should be .forget().

(Even then I'm still -1 though.)

[ljharb]

If we have "forget" we'd need the corresponding english antonym "remember" - which wouldn't make any sense, so I don't think that would work.

[ChALkeR]

@Fishrock123 on/off is rather popular naming scheme for events in various libraries, while on/forget isn't. When people see that there is an on, some suppose that there should be an off.

Refs:

• https://www.npmjs.com/package/event-emitter#usage (3 214 916 downloads / month)
• https://www.npmjs.com/package/eventemitter2#emitter-off-event-listener (2 392 113 downloads / month)
• https://www.npmjs.com/package/eventemitter3 also supports .off, (2 693 198 downloads / month)
• http://api.jquery.com/off/

Edit: updated, added eventemitter3 to the list.

[ljharb]

I'd be surprised if anyone, lacking knowledge of this particular core module quirk, would expect there not to be an off if there's an on.

[Fishrock123]

Again, see the quote, whether or not there's an on, off doesn't make sense.

[ljharb]

@Fishrock123 it doesn't make sense to you, or to @sam-github. It makes perfect sense to many developers. For example, I might see an event emitter, while not as a light switch, as a switch on a walkie talkie - where when "on", i hear things that come in on the channel, but when i want to stop listening, i turn it "off". There's many kinds of on/off switches so I'm sure we can anecdotally come up with a bunch that do, and a bunch that don't, make sense as metaphors for event emitters, but I don't see how that's convincing or valuable in either direction.

[Fishrock123]

Ah, I get it, that makes sense I suppose.

[ChALkeR]

@Fishrock123 The fact how exactly the method opposite to on should be named to be semantically correct isn't actually relevant here.

cat is an animal, and mount looks like a mountain.

Almost everyone out there is using on/off names for the event systems in js libraries. Check out jQuery, event-emmiter, or EventEmitter2.

The only reason for adding an off would be to be more compatible and obvious here, given the naming scheme that other libraries are using. There are exactly no reasons for adding a forget, because the actual functionality is already present.

[thefourtheye]

on kind of gives me a feel that the expression is readable. For example, when I see something like this, inputStream.on('data', ...), in my mind, I read it like "data event on inputStream" or "on data event". But, I am not able to imagine having off like this. (PS: Not a native English speaker)

[ljharb]

Everyone clearly has lots of (valid) colliding subjective opinions on whether "on" or "off" makes sense or not, and on what metaphor makes the most sense for events, et cetera. I don't think we're going to resolve anything to anyone's satisfaction by throwing personal aesthetic preferences back and forth :-)

Personally I think all of the above is irrelevant. Unless we're talking about removing "on" - which would be a conceptually major, semver-major breaking change - we're stuck with it for the forseeable future.

Therefore, the real question is, what should the API look like? Currently there's a nice pairing of addListener/removeListener. on exists as an alias for addListener - but there isn't a companion to "on" in the API - so it's asymmetric. "off" is the word that, objectively based on library usage, pairs best with "on". So, I think it makes sense to add "off" as an alias to removeListener.

[sam-github]

fwiw, I retract my -1. I'm now +/- 0. Maybe soeone would expect a .off() if there is a .on(). But maybe they wouldn't expect to have pass a function to it, and expect it to be the equivalent of removeAllListeners()

[ljharb]

Any hope of revisiting this, or at least re-gathering -1/0/+1s from folks?

[ChALkeR]

@ljharb Actually, I'm leaning towards a +1 on this. It will improve interoperability with all the other event emitter libraries on npm, and on/off is a de-facto standard for that.

[ChALkeR]

Just a note: this

Of course, I understand the API of events is frozen in io.js.

is not correct anymore. events module is Stable now, not Frozen.

[ljharb]

I'd still love to see this added.

[ChALkeR]

Updated stats at #540 (comment), also added eventemitter3 which also supports .off.

[Delapouite]

Adding this alias would also be handy in Socket.io which inherits from EventEmitter.

[ljharb]

Any chance this could get reopened/reconsidered?

[ljharb]

Update: #17156 will theoretically land this in node v10