ES.isCallable not spec compliant

[mhofman]

It looks like ES.isCallable simply leverages the is-callable package. However as noted in inspect-js/is-callable#16, that implementation is not equivalent to the IsCallable from the spec.

Class constructors do have a [[Call]] internal slot which makes them callable.
The [[Call]] internal method throws TypeError if [[FunctionKind]] == "classConstructor", but that's simply the implementation of the internal method.

[ljharb]

It's unfortunate that typeof x === 'function' is specced to only detect [[Call]], and not also [[Construct]]; this means that a constructor that can't be invoked does, as you indicate, have a throwing [[Call]]. It would have been much better to be able to have a function that has [[Construct]] and lacks [[Call]].

I'd be happy to change IsCallable to match the spec - ie, to be return typeof x === 'function' - if I found a way to make an IsConstructor check without relying on Proxy. Do you have any ideas?

[mhofman]

Why not use your toString / class regex logic from is-callable to implement IsConstructor ?

[ljharb]

It's not reliable due to many browsers not toStringing class constructors with class in them.

[mhofman]

This looks like an interesting approach: https://stackoverflow.com/a/46759625
Relies on Reflect.construct but not Proxy

[ljharb]

Is Reflect in any engine that lacks Proxy? If it’s not polyfillable it’s not much of an approach :-/

[ExE-Boss]

The current IsCallable spec only tests the presence of [[Call]] and not the value of [[IsClassConstructor]]: https://tc39.es/ecma262/#sec-iscallable.

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Also, there’s a simpler IsConstructor implementation:

const isConstructorMarker = {};
const badArrayLike = {
	get length() {
		throw isConstructorMarker;
	}
};

const IsConstructor = value => {
	try {
		Reflect.construct(value, badArrayLike);
	} catch (err) {
		return err === isConstructorMarker;
	}
};

This is because Reflect.construct ( target, argumentsList [ , newTarget ] ) checks IsConstructor(target) before doing Get(argumentsList, "length"): tc39/ecma262#1798 (comment).

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Although, it won’t work in environments that don’t support getters.