Add support for the EventTarget once option

[Sceat]

As per the documentation this is something not browser related and should be supported to allow transparent polyfill usage

https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener

import WebSocket from 'ws'

new WebSocket().addEventListener('message', () => {}, { once: true })

[lpinca]

Well, the whole EventTarget is browser related. It is emulated for convenience and it's still there only for historical reasons. Also it doesn't make much sense to use once for the 'open', 'error', and 'close' events as they are emitted only once.

Do you really need this feature? In Node.js you can use ws.once().

[Sceat]

well to build an hybrid node/web client you wouldn't know if you can use .once as it doesn't exist in the browser :/ this option allow to be transparent and not care about which WebSocket object you use

[lpinca]

Also this need tests (also for removeEventListener to make it sure that is works for once events added with addEventListener).

[lpinca]

The documentation should also be updated https://github.com/websockets/ws/blob/master/doc/ws.md#websocketaddeventlistenertype-listener.

I think it does not worth the effort as it is only useful for the 'message' event and the same can be achieved by removing the listener from the inside of the listener itself but I have no objections.

Anyway if you want to add it we have to do it properly.

[Sceat]

Also this need tests (also for removeEventListener to make it sure that is works for once events added with addEventListener).

Resolved the previous issues, but i'm kinda not very good at testing stuff 😨

[Sceat]

rip coverage

[lpinca]

Thank you.