Is a class's heritage referenced from the wrong scope?

[bradzacher]

Am I misunderstanding the spec here?
I'm new to attempting to read the ECMAScript spec, so I wouldn't be surprised if I'm wrong...

To me it looks like the superclass is referenced from within the class's scope:

https://www.ecma-international.org/ecma-262/8.0/#sec-runtime-semantics-classdefinitionevaluation

1. Let lex be the LexicalEnvironment of the running execution context.
2. Let classScope be NewDeclarativeEnvironment(lex).
3. ...
4. If className is not undefined, then
   a) ...
5. If ClassHeritage_opt is not present, then
   a) ...
6. Else,
   a) Set the running execution context's LexicalEnvironment to classScope.
   b) Let superclass be the result of evaluating ClassHeritage.
   c) ...
7. ...

When superclass is evaluated, it is specifically within classScope. So this means the reference should be placed within the class scope, right?

However, the scope manager references the superclass before it nests the class scope (i.e. it is referenced from the parent's scope):

eslint-scope/lib/referencer.js

Lines 264 to 280 in dbddf14

	visitClass(node) {
	if (node.type === Syntax.ClassDeclaration) {
	this.currentScope().__define(node.id,
	new Definition(
	Variable.ClassName,
	node.id,
	node,
	null,
	null,
	null
	));
	}
	this.visit(node.superClass);
	this.scopeManager.__nestClassScope(node);

---

I tried to dig through the old escope history, but there's no information attached to the commit (estools/escope@c3e23e2)

[nzakas]

I think the only answer we can give here is we don't know. We forked escope in order to have a better way to get it updated to fit in with our release cycle, but we still don't know all the dark secrets contained within.

[bradzacher]

It's not a huge deal ultimately - there's still a reference to it, and I don't think anyone consuming this package is explicitly asserting a reference comes from a specific scope.

I was just working on a ground-up reimplementation which supports typescript (typescript-eslint/typescript-eslint#1939) and noticed this. I have some snapshot tests to validate the structure of the scope. When I added a snapshot test for the heritage, I noticed the placement of the reference and was confused.

I was mainly hoping someone more familiar with the spec could tell me if I'm interpreting the JS spec correctly or not 😅

[mysticatea]

I'm sorry for my overlooked.

I think @bradzacher is right. The references of the extends clause should be in the class scope. Therefore, in the following rare case:

const A = class A extends A {}

the third A must be resolved to the second A rather than the first A. But, the current eslint-scope resolves it to the first A. It looks like a bug.

[nzakas]

@mysticatea is this something you feel like addressing? If not, I can try to do it

[mdjermanovic]

@nzakas are you maybe working on this? If not, I could take it.

[nzakas]

I'm not. Please feel free.

[mdjermanovic]

I'm working on this.