Function.isCallable() / Function.isConstructor() example matrix

[aedart]

Hi

Would it be possible to provide an example matrix with various argument types and the expected result of isCallable() and isConstructor()?

Currently, I'm looking at Core-JS' implementation of this proposal and I'm wondering how built-in classes, e.g. Array, Proxy...etc, are expected to behave when tested via these methods.

[caitp]

It will take some work to put together a complete matrix, I don't know when I'll have the time to complete this. But my vision overall is:

Refers to isCallable() specifically:

• Most values with a [[Call]] internal method would return true
• Class constructors would return false
• A bound function would produce a value consistent with the target of the bound function
• Returns true for Proxy objects which have an "apply" trap, or whose target has a [[Call]] method that does not statically throw (e.g, returns false for class constructors or builtins which emulate that behaviour)
• Anything without a [[Call]] internal method, would return false (this is the case for most values, such as numbers, strings, most objects)

Similar logic would apply to isConstructor. If I get some time, and someone is interested in championing this in tc39, I'm happy to draft a matrix to help out

[aedart]

The reason why I'm asking about this, is to determine whether the implementation by Core-JS actually satisfies your vision, or if it is lacking. I'm particularly concerned with the about the "isClassConstructor" part in Core-JS (not intended as criticism of the author).

I guess what I'm trying to ask for, is if this proposal can be polyfilled. If so, perhaps that could be a place to start and continue to ask tc39 champions for help (sorry, I'm not much help here because I'm not skilled enough).

In any case, I do hope that this proposal gets through. JavaScript developers could really benefit from this type of "simple" functionality.

[caitp]

I believe this would have false negatives, in the case where a class constructor is bound. OTOH, a bound class constructor should fail the toString test, and the property descriptor test

I may be wrong about that, ill take a look not on mobile in a bit to be sure. Probably does not have false positives, though

[caitp]

Yeah,

class C {
  constructor() {
    console.log("C construct");
  }
}

let ctor = C.prototype.constructor.bind(null);

ctor(); // throws TypeError as it should
new ctor(); // returns instance of the class, invokes ctor
`${ctor}` // "function() {\n    [native code]\n}" 

(https://jsfiddle.net/utm79h2e/)

[zloirock]

Yep, AFAIK we have no way to detect bound classes. So it's marked sham: true.

[caitp]

yeah, it's not fully polyfillable, but it gets pretty close.

[zloirock]

BTW I'm planning to remove proposals that have not progressed for a long time from core-js@4. Do you have any plans to advance it?

[caitp]

It needs a champion to really get it anywhere, I can talk to my colleagues about raising it in committee, but it's not a super high priority atm

[aedart]

Thanks for your input. I really hope that this proposal will get through.