Clarify behaviour of constructor in class Foo extends Event {}

[jakearchibald]

class Foo extends Event {
  hello() { return 'hello'; }
}
const foo = new Foo('bar');
console.log(foo.constructor);

Chrome & Firefox say Foo, Edge & Safari say Event.

console.log(foo.hello());

Chrome & Firefox say hello, Edge & Safari throw (undefined is not a function).

The Chrome/Firefox behaviour is preferable, as it allows you to create custom event objects.

[jakearchibald]

WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=183629
Edge bug: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/16379447/

[annevk]

@domenic why is it again that we can just say "Interface() returns a new Interface" in prose and the subclassing falls out naturally? It seems a little weird.

[jakearchibald]

In Safari/Edge, it seems like new Event() is behaving like:

class Event {
  constructor() {
    return Object.create(Event.prototype);
  }
}

Which you could argue is correct according to:

event = new Event(type [, eventInitDict])
Returns a new event…

Although this is in a non-normative section. There aren't any normative steps for the event constructor.

[annevk]

There are: https://dom.spec.whatwg.org/#constructing-events. They seem to cover the inheritance aspect actually. But we don't cover it for EventTarget so it's still a little weird all around how to go about this.

[jakearchibald]

Ah, sorry. The constructor in the IDL links to the non-normative section.

[domenic]

This is all defined in the IDL for [Constructor]. I can find it when back at a computer.

[annevk]

https://heycam.github.io/webidl/#interface-object I guess, though I can't quite figure out where that deals with subclassing.

[annevk]

The constructor in the IDL links to the non-normative section.

It links nowhere in the IDL. The non-normative version links to the IDL though. That seems worth fixing.

[domenic]

Hmm yeah I guess that's broken right now. @tobie and I talked about how to fix it a month or so ago. (Roughly, instead of converting R to O, O should be created by ObjectCreate(new.target.prototype), and save R as an internal [[IDLValue]] slot.)

[annevk]

I filed #604 on [Constructor] not linking and whatwg/webidl#533 on [Constructor] not taking into account subclasses properly.

Bugs were already filed. Interested in adding a web-platform-tests test?

[jakearchibald]

I can write some tests for Event (and I'll check there's some for EventTarget). Or do we need to do something more general?

[annevk]

Just for Event for now I think (pretty sure EventTarget is covered). Thanks!

[jakearchibald]

Turns out it's already covered http://w3c-test.org/dom/events/Event-subclasses-constructors.html

[annevk]

\o/