Add [[PlatformBrand]] internal slot to all platform objects

[whatwg]

Add [[PlatformBrand]] internal slot to all platform objects

[dominiccooney]

Is the [[PlatfromBrand]] on the instance (for example a text node in the DOM, not the Text interface or Text interface prototype object?)

For what it is worth Blink implements the check for HTMLElement in custom elements as if the brand is on the instance; see CustomElementDefinition::checkConstructorResult. Sorry for the C++, but basically it checks the element instance is a HTMLElement; it doesn't consult JavaScript prototypes or anything.

[annevk]

Yes, on the instance. All internal slots are I think.

[annevk]

(Once we add this StructuredSerialize can be simplified a little bit as all platform objects will have this slot and therefore can be treated like promises or WeakMap objects.)

[annevk]

The other thing that would be useful here is some shorthands around it. When you check against the list of platform brands, there's a difference if the matched value is first in the list or somewhere else (or last and somewhere else, depending on order).

E.g., in DOM we sometimes want to match DocumentFragment, but not ShadowRoot (a subclass).

[littledan]

It sounds like we'd have two "entrypoints" to [[PlatformBrand]]:

• In WebIDL, when evaluating the phrase, "is a platform object that implements"
• In all the other specs, when they use the phrase, "a new XYZ object"

Is that what you were picturing, @annevk? The first one will be pretty easy to update, but the second would be a lot of work to go around fixing up. Would it be reasonable to add a "create a new platform object of interface |interface|" algorithm to WebIDL and update all the other specs to call into that?

[annevk]

Yeah, I think that's reasonable. We also want "x is a XYZ" formalized as a shorthand for "is a platform object that implements" for use in other standards.

I'm not a 100% sure

E.g., in DOM we sometimes want to match DocumentFragment, but not ShadowRoot (a subclass).

is true anymore btw. I kinda hope it's not.