Ecma262: Remove [[HomeObject]] from function Environment Records

[jorendorff]

env.[[HomeObject]] is always equal to env.[[FunctionObject]].[[HomeObject]], so it's redundant.

(I wonder if this is a leftover artifact from an early draft in which [[HomeObject]] was an extra implicit parameter passed to every function, independent of [[FunctionObject]], more like [[ThisValue]].)

[jorendorff]

I submitted this issue because the presence of [[HomeObject]] in Table 15 confused a person in the real world this morning. :)

[allenwb]

I agree there is no observable difference between these two specification alternative.

But, there is a subtle difference of intent that may be communicated by the original formulation.

Environments represent a set of binding and once established those bindings are generally resolved without reference to the function/block that instantiated the binding. [[HomeObject]] was conceptualize as a special binding and as such it was explicitly included in the environment. Just like each attempt to resolve a binding doesn't specify an indirection through [[FunctionObject]].[[Environment]] accessing the home object bind didn't indirect through [[FunctionObject]].[[HomeObject]]

[ljharb]

What's the value of avoiding the indirection through [[FunctionObject]]?

[jorendorff]

But, there is a subtle difference of intent that may be communicated by the original formulation.

Can we just add a note that says this? Or [[HomeObject]] could be specified as a special binding, if it really works that way. There's nothing wrong with the point being made. But the spec as it stands is confusing.

[syg]

The editor group is in favor of @jorendorff's original suggestion of removing the indirection. Wanna take a stab at a PR @jorendorff?

[jorendorff]

Do you mean removing env.[[HomeObject]]? If so, sure, I can take a second to do that.

[bathos]

Is this not different when the FunctionObject is an arrow function?

const object = Object.setPrototypeOf({
  method() {
    return (() => super.homeObjectPrototypeProperty)();
  }
}, { homeObjectPrototypeProperty: 7 });

object.method(); // 7

(I’m guessing not, given the prior discussion, but I’d have previously assumed that was the reason they were distinct, and I’m having trouble tracing how the right [[HomeObject]] ends up used here.)

[devsnek]

It's always an alias, see https://tc39.es/ecma262/#sec-newfunctionenvironment, the only place where [[HomeObject]] is ever set on an environment.

[bathos]

Thanks @devsnek. I’m still having trouble figuring out how/where the super reference above gets resolved, but can see that you’re correct.

[devsnek]

@bathos the super env is resolved by performing GetThisEnvronment()

[jorendorff]

OK, filed a PR.

The table of function Environment Record methods has gotten out of date, so I updated it.

But, I think that table has outlived its usefulness, and it would be more helpful to distribute the comments in the Purpose column to the sections defining those methods. Separate issue, though.