exploring what a `currentThis` might look like in JS

gistfile1.js

// This is a theoretical exploration of something that could
// be added to JS. It borrows from JS events which have both a
// `target` and `currentTarget` element, which distinguish between
// the direct target of an event and the delegated parent handling
// the event, respectively.

// The idea is to distinguish between `this` and `currentThis`,
// respectively, as...
//
// `this`: the initial containing object for a call that initiates a
// traversal of the [[Prototype]] chain (just as it currently does)
//
// `currentThis`: the actual link in the [[Prototype]] chain where the
// traversal resolution is completed.

// Notice below that with `bar1` for example, `this` will point to
// `bar1` even when using the delegated `identify()` function found
// on `Foo`, but `currentThis` will point to `Foo`, since that's
// where `identify()` was found in the chain.


// So... what if:

var Foo = Object.create(null);
Foo.me = "Foo";
Foo.identify = function() {
	// whenever Foo#identify() is called, `currentThis` will 
	// always be `Foo`, but `this` will continue to follow the 
	// established rules for determining a `this` binding.
	console.log("Me: " + this.me + "; Current: " + currentThis.me);
};

var Bar = Object.create(Foo);
Bar.me = "Bar";
Bar.another = function() {
	console.log("Current: " + currentThis.me);
}

var bar1 = Object.create(Bar);
bar1.me = "bar1";
var bar2 = Object.create(Bar);
bar2.me = "bar2";

Foo.identify();  // "Me: Foo; Current: Foo"
Bar.identify();  // "Me: Bar; Current: Foo"
Bar.another();   // "Current: Bar"
bar1.identify(); // "Me: bar1; Current: Foo"
bar2.identify(); // "Me: bar2; Current: Foo"
bar1.another();  // "Current: Bar"
bar2.another();  // "Current: Bar"

@getify

Whether a particular function uses super (or currentThis) can be statically determined when the function is parsed and that fact can be record somewhere. Most likely in some sort of descriptor that is accessible relative to the address of the function.

However, at a call-site we generally don't know what particular function is being called because the function is obtained via a property lookup or a parameter value, etc. So if the call site wants to only pass the currentThis``value to functions that actually use it, it needs to inspect the target function's descriptor to see if it usescurrentThisand then conditionally pass thecurrentThis` value based upon what was recorded in the descriptor.

There are a lot of degrees of variability at this "close to the metal" level of design, but a reasonable simplification of the implementation alternatives are:

1. Always unconditionally pass the currentThis value. This has an idealized cost of 1 memory store on every call.
2. Only pass currentThis if the target function is flagged as needing it. This has an idealized cost of 1 memory read + 1 test + 1 conditional branch on every call. If the branch is taken because the target function actually needs currentThis then there will be a store and a branch back to the fast path.

Stores are expensive, but so are reads and branches, so at this level of abstraction we might as well say that the two alternatives add approximately the same base overhead to every function call and that overhead is approximately that of one additional argument.

Yes, I was referring to __proto__ tampering that might take place after the call but before the reference to super. An arbitrary large amount of code might execute between those two points include code that does loading, code-generation, dynamic mix-ins, etc.

It's not always obvious what the best semantics are in situations like this. However, in general JS property accesses are defined to reflect the state of objects at the point when the access is made. This allows for the possibility that object structure is dynamically highly mutable. It is probably best to minimize exceptions to that principle.