Enable an event listener to be invoked just once

[annevk]

No description provided.

[annevk]

I wasn't able to find the original request for this (I thought there was even an issue), but if you search for "addeventlistener once" you'll find some Stack Overflow threads requesting this.

[ArkadiuszMichalski]

Probably this: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28455

[annevk]

You're amazing, marked the bug "MOVED".

[RByers]

This makes sense to me. I don't know if people would choose to use this (with feature detection / polyfill for the next several years) over just writing the remove code themselves. But it seems like a nice easy little feature to have - even just planning for the far future when developers can depend on it being there.

@dtapuska / @tkent-google, what do you think? Pretty trivial to add, right?

[tkent-google]

This looks a useful flag. LGTM.

[josh]

Spiked out a polyfill implementation for this https://github.com/josh/event-listener-options-polyfill/pull/1

(edit: d'oh, looks like @RByers is already working this)

[Daniel15]

The spec should clarify what happens if the callback throws an exception. Based on my interpretation of this:

When set to true, options’s once member indicates that the callback will only be invoked once after which the event listener will be removed

It sounds like the event listener will be removed even if it doesn't run to completion, but it would be good to clarify this 👍

[annevk]

Callbacks throwing never really does anything to event dispatch. (The exceptions are reported per https://dom.spec.whatwg.org/#concept-event-listener-inner-invoke.) Where would you expect to see this clarified?

[Daniel15]

Good catch, I missed that part of the spec.

[cvrebert]

Vendor feature request bugs:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1267505
• https://bugs.chromium.org/p/chromium/issues/detail?id=615384