pre-Stage 3 review

[ljharb]

(you'll also need signoff from the designated reviewers, and from @zenparsing)

• I wonder if maybe "NullishExpression" should be "NullishORExpression", so it mirrors "LogicalORExpression"?

Otherwise it seems good.

(It may be useful to merge this and optional chaining into one stage 3 proposal, if both are ready to advance, especially since there's a lot of spec interaction between them)

[rkirsling]

I wonder if maybe "NullishExpression" should be "NullishORExpression", so it mirrors "LogicalORExpression"?

Now that you mention it, I think "coalescing" is really the more important word here—even "CoalesceExpression" would likely suffice for uniqueness.

[DanielRosenwasser]

Ugh, you're right but I was hoping to change it later. *gets to work*

[DanielRosenwasser]

Should I change the grammar flag to Coalesced?

[rkirsling]

Hmm, maybe just Coalesce? (Since I think the + is meant to be read like "has".)

[jridgewell]

I don't think Coalesce is a good flag name, it's not clear what it's doing. As for the Grammar name, maybe LogicalNullishExpression?

[DanielRosenwasser]

I think I'm going to run with CoalesceExpression with Coalesced as the grammar flag.

[DanielRosenwasser]

43 is merged. @rkirsling @codehag @zenparsing, would you be able to review the current spec text?

[rkirsling]

LGTM (and confirmed ease of implementation using JSC :grin:). Still rooting for #40!

[codehag]

LGTM!

[zenparsing]

Spec text 👍

[ljharb]

Fresh LGTM

(note that we should probably find a way to make sure that document.all ?? x evaluates and returns document.all, despite that document.all == null - or, maybe it should return x? i'm not sure)

[jridgewell]

(note that we should probably find a way to make sure that document.all ?? x evaluates and returns document.all, despite that document.all == null - or, maybe it should return x? i'm not sure)

We've already done that (it returns document.all). Does the current spec text not match that?

[ljharb]

Grammar isn't my strong suit, but I don't see where an object with an [[IsHTMLDDA]] internal slot gets special treatment - saying "is null or undefined" doesn't include document.all unless you use ToBoolean or the AbstractEqualityComparison algorithm.

Perhaps what's needed is an addition to https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot ?

[rkirsling]

Is there something that obliges us to give it special treatment here? If Annex B.3.7 states that document.all == null is true, it seems like that's all there is to say.

---

Edit: Er whoops, I was trying to say that document.all ?? x should thus be x, but I think @ljharb is right that "is undefined or null" in the spec text doesn't suffice to achieve this. If we want == null semantics then it seems we do need to explicitly reference Abstract Equality Comparison (both here and in Optional Chaining).

[jridgewell]

document.all ?? x is supposed to evaluate to document.all. That's why the examples and desugaring are using left === null || left === undefined.

Specifying "val is null or undefined" should be enough to distinguish that document.all is not null or undefined. Eg, Object.prototype.toString.call(document.all) (which is similarly spec'ed) and doesn't return the string "[object Undefined]" nor "[object Null]".

[ljharb]

Got it, thanks for clarifying :-)

I think you’re right that the current spec means that document.all ?? x will be document.all, and thus no changes are needed.

[rkirsling]

Specifying "val is null or undefined" should be enough to distinguish that document.all is not null or undefined.

That is true, but ignoring Annex B is effectively creating a special case here. In particular:

1. It does not correspond with existing ways of performing ?? or ?.: document.all[0] is valid on its own but none of the following return it.
   • (document.all || [])[0]
   • document.all && document.all[0]
   • document.all == null ? undefined : document.all[0]
2. It does not correspond with the stated semantics of the Optional Chaining README.
3. It kicks the burden to implementations instead: for instance, JSC bytecode literally has OpNeqNull.

If we're going to give document.all this kind of special treatment, we definitely need a strong reason.
(I don't think that Object.prototype.toString constitutes such a reason, because it is conceptually unrelated to nullishness and truthiness alike.)

[ljharb]

I think that this is a lack of special treatment, since document.all can be navigated into already without throwing.

[rkirsling]

So here's what WHATWG states as the goal of this bewildering "masquerade-as-undefined" behavior:

These special behaviors are motivated by a desire for compatibility with two classes of legacy content: one that uses the presence of document.all as a way to detect legacy user agents, and one that only supports those legacy user agents and uses the document.all object without testing for its presence first.

That is, the aim is to have feature checks for it always fail while having its actual functionality preserved.

Now, even if we throw out all presuppositions about desugaring and strictly focus on surface intent, I want to argue that document.all ?? [] and document.all?.[0] are nothing other than feature checks, but the situation admittedly gets rather muddy when it's reassigned to another variable.

Say we have a function third(x) which returns x[3] if possible and otherwise undefined. We surely would like to implement this as return x?.[3];. I suppose in that case having third(document.all) return undefined could be viewed as interfering with its Array-like functionality.

[ljharb]

There's not going to be any new code written with ?? tho, that's checking for legacy user agents with document.all.

[rkirsling]

Hmm, perhaps that is the best way to look at it. I guess I'm alright with this then, we just need to update the Optional Chaining README (at least, where it says "The desugaring is not exact in the sense that...").