[Option 3] Have a way to set the internal slot from JS

[lucacasonato]

This is needed to be able to:

• faithfully polyfill objects that are defined by specs using WebIDL
• implement objects that are defined by specs using WebIDL (Node.js and Deno do this extensively)

This likely boils down to something like Object.markInternal(v, bool) or by having a Symbol.notThenable property.

[bakkot]

I really don't like the idea of Object.markInternal since that means arbitrary JS can set this bit on arbitrary objects, possibly including those they don't own, possibly even in the middle of an await or something. The goal is to make the machinery easier to reason about and that makes it more complicated.

[ljharb]

Whether there's a way to set it or not, there definitely would need to be a way to detect it, since it would make all builtin prototypes exotic.

[bakkot]

For option 3 as currently proposed you could detect it by installing a getter for .then on the builtin and then seeing if Promise.resolve({ __proto__: builtin }) triggers the getter.

[ljharb]

True, but I don't think requiring that someone mutate a builtin, especially one someone could have frozen already, is a good API :-)

[bakkot]

For sufficiently niche use cases I'm OK with us not prioritizing the API being friendly.

[ljharb]

I agree with that! but requiring mutation i think is too much.