Built-in module conventions

[Ms2ger]

There's been a number of discussions around conventions for built-in modules, in particular:

• Enumerability of methods/accessors (see also Introduce an UnenumerableOperations extended attribute #719)
• Same-realm brand checks (see also Introduce a SameRealmBrandCheck extended attribute #718)
• Freezing

Let's figure out what we want, and involve TC39 in the discussion as well, potentially at the 23-25 July 2019 F2F meeting.

CC @domenic @annevk @bzbarsky

[domenic]

For the record: in KV Storage, I've done the following:

• Enumerability of methods/accessors: chosen enumerable, like the web platform built-in globals, unlike JS-spec built-in globals. Whichever is chosen here, changing would be a very slight breaking change, that in practice is unlikely to break consumers, because enumerability of built-ins very rarely impacts JS code.
• Same-realm brand checks: adopted, via Introduce a SameRealmBrandCheck extended attribute #718. If we decided not to do this, it would be a backward-compatible loosening to change KV Storage.
• Freezing: not adopted, largely because there is no concrete proposal. If we decided to do this, it would be a backward-incompatible change with medium risk. Code that tries to monkeypatch KV Storage objects would start throwing errors.

Looking at the above, it may be more conservative to spec/implement KV Storage with freezing initially, hmm. But I am really unclear whether that's being seriously pursued. I will try to gather more information at TC39.

[ljharb]

Are web platform built-in globals made enumerable for intentional reasons, or to be consistent with legacy choices? If the former, what are the reasons?

[domenic]

My impression is legacy choices, but the decision predates me. @bzbarsky or @heycam are our best bets here, perhaps?

[bzbarsky]

I believe they were enumerable in all browsers when the Web IDL spec was written, and the spec just reflected that.

[ljharb]

In that case (re enumerability) it seems like a clean break opportunity for the web to start mirroring the language, and go non-enumerable?