fix: Add [ImplicitThis] behaviour

[ExE-Boss]

This implements the [ImplicitThis] behaviour present on all WebIDL attributes and operations.

Needed for jsdom/jsdom#2835
Will allow fixing jsdom/jsdom#2830

See whatwg/webidl#155 for details.

[domenic]

[ImplicitThis]

I think this phrasing is confusing since there is no extended attribute [ImplicitThis].

Fixes jsdom/jsdom#2830

This is not correct, right? I.e. merging this would not fix that bug.

---

As for the approach, I wish there were a way to generate this only for the cases where it matters, i.e. functions on the prototype chain of the global object. From what I understand that's more of how browser implementations do it. But this works I guess, and the generated code isn't too heavyweight. And there is some value in following the spec as close as possible. So I'm inclined to go with this approach, although it'd be good to get other maintainers' thoughts.

[ExE-Boss]

This is how whatwg/webidl#155 is specified to behave.

Case in point: https://heycam.github.io/webidl/#dfn-create-operation-function

[domenic]

Yes, as I said above,

I wish there were a way to generate this only for the cases where it matters, i.e. functions on the prototype chain of the global object. From what I understand that's more of how browser implementations do it. But this works I guess, and the generated code isn't too heavyweight. And there is some value in following the spec as close as possible

[ExE-Boss]

Right, I somehow missed the paragraph after the horizontal line.

As for jsdom/jsdom#2830, that will be fully fixed once this is released and jsdom/jsdom#2835 is merged.

[domenic]

I guess we should be sure not to merge this until Window.js has appropriate brand checks on all its methods; otherwise it would be a regression.

[domenic]

Wait, no, that's wrong, I was confused.

Is there any reason to hold off on merging this? I guess maybe because it adds to the output code size slightly for no observable gain at this point...

[ExE-Boss]

It removes the need to use the this.addEventListener = this.addEventListener.bind(this) hack in Window.js:

///// METHODS for [ImplicitThis] hack
// See https://lists.w3.org/Archives/Public/public-script-coord/2015JanMar/0109.html
this.addEventListener = this.addEventListener.bind(this);
this.removeEventListener = this.removeEventListener.bind(this);
this.dispatchEvent = this.dispatchEvent.bind(this);

[TimothyGu]

Looks good. One question.

[TimothyGu]

Can you tell me if my understanding is correct: this isn't needed because for unforgeable methods/properties this always points to obj anyway?

[ExE-Boss]

Well, yes, unless you were to use unforgeableMethod.call, in which case this will be whatever was passed to call as the thisArg parameter.

[ExE-Boss]

This is correct per spec: https://heycam.github.io/webidl/#es-stringifier, whatwg/webidl#855.