What about errors on the prototype chain?

[mgaudet]

Looking at the Firefox implementation, it appears that we explicitly handle the prototype chain in our getter implementation:

This means that

let o = {__proto__: new Error}
o.stack

works; as I read the spec text on this proposal it would currently throw however because o lacks an [[ErrorData]] internal slot.

[ljharb]

Your analysis is correct. Is there code on the web that relies on this working? It's very uncommon in the language to have accessors that don't brand check.

[mgaudet]

No idea. We don't have telemetry here to tell us.

[ljharb]

What is the appetite for shipping with the check with a pre-agreement to remove it if there's web compat issues?

Meaning, we could easily have two specs - main, plus a PR that removes the checks - and in theory a flag could guard the two variants that could be flipped post-deploy?

[mgaudet]

My appetite is fairly low; I wouldn't be opposed to trying to get some telemetry but not sure when I can slot that in.

[nicolo-ribaudo]

Note that the current spec text returns undefined, rather than throwing.

If E does not have an [[ErrorData]] internal slot, return undefined.

[ljharb]

Indeed, closing makes sense, I believe.

[nicolo-ribaudo]

Well, it still behaves differently from Firefox and Chrome where o.stack returns a string and not undefined :)

(I have no opinion on what we should do, I'd go with the current behavior if it's web-compatible)

[mgaudet]

I'd leave this open until we had any idea at all what is web compatible.

[ljharb]

Sounds good

[mgaudet]

(It does appear that Chrome also works this way)