`SubscriptionObserver` members should be optional explicitly

[tetsuharuohzeki]

By the current spec, SubscriptionObserver's members are treated optional actually in https://wicg.github.io/observable/#ref-for-dictdef-subscriptionobserver%E2%91%A1. But WebIDL is not so. It's ambiguous. https://wicg.github.io/observable/#dictdef-subscriptionobserver

They should be optional explicitly, I think.

[domfarolino]

But WebIDL is not so. It's ambiguous.

I don't think so? Web IDL is very clear that all dictionary members that are not marked as required are optional. I don't think the optional keyword can be used explicitly in dictionary syntax in Web IDL anyways. At least if it can, I never really see it (example). I don't think we need to do anything here.

[tetsuharuohzeki]

At least if it can, I never really see it (example).

Let's see https://dom.spec.whatwg.org/#dictdef-customeventinit

[domfarolino]

Let's see https://dom.spec.whatwg.org/#dictdef-customeventinit

I'm not sure I understand your point. No optional keyword is expressed there. In that spec, they just define a default value. I don't think we need one here is all.

[tetsuharuohzeki]

Let's see https://dom.spec.whatwg.org/#dictdef-customeventinit

I'm not sure I understand your point. No optional keyword is expressed there. In that spec, they just define a default value. I don't think we need one here is all.

@domfarolino

Did you know https://webidl.spec.whatwg.org/#ref-for-dictionary-member-optional%E2%91%A0 ?

---

The step 3 of https://wicg.github.io/observable/#observable-subscribe-to-an-observable says "If observer’s next is not null", so I'm talking about that the following code would be more nice to express what its would be.

dictionary SubscriptionObserver {
  SubscriptionObserverCallback next = null;
  SubscriptionObserverCallback error = null;
  VoidFunction complete = null;
};

How about do you think?

[domfarolino]

Hmm, I'm thinking instead of null checking in the spec, and just having all of these members default to null when they are not included, we should keep the dictionary IDL as-is, and then change the null-checks (aka this here, and this here for example) to contains checks instead. So it would be if |observer|[{{SubscriptionObserver/next}}] [=map/exists=], then foo. I think that's more idiomatic and a much more common pattern, especially if we don't need the values to default to null or anything in particular.