Are There Any Js Objects For Which Iscallable Is False But Isconstructor Is True?

The ECMAScript specification function IsCallable returns true iff its argument has a [[Call]] internal method. It is used in several places in the specification, such as in the de

Solution 1:

Restatement: Is it possible for an object X to return true for IsConstructor(X) but false for IsCallable(X)? I.e., is it possible for an object X to have a [[Construct]] internal method but not have a [[Call]] internal method?

The ECMAScript spec isn't as clear-cut on this point as it could be.

(1) 6.1.7.2 "Object Internal Methods and Internal Slots" says:

A function object is an object that supports the [[Call]] internal method. A constructor (also referred to as a constructor function) is a function object that supports the [[Construct]] internal method.

From this we can conclude that if such an object X does exist, it is clearly not a 'function object' and thus also not a 'constructor'. I.e. IsConstuctor(X) would return true for an object that isn't deemed a 'constructor', which would be odd.

(2) Note that in the clause that defines IsConstructor, the preamble says that it determines whether its argument "is a function object with a [[Construct]] internal method", but the algorithm doesn't have an explicit check that the argument is a function object. This suggests that (the spec-writer thought that) having a [[Construct]] internal method is sufficient to guarantee that the argument is a function object, i.e. that it has a [[Call]] internal method.

(3) The only point to having a [[Construct]] internal method is to invoke it in the Construct abstract operation. Similar to point (2), the preamble says that the operation "is used to call the [[Construct]] internal method of a function object", but the algorithm doesn't explicitly check that F is a function object.

So I believe the answer is that, while the spec doesn't explicitly say that such objects can't exist, it fairly strongly implies/assumes that they don't.

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UPDATE (2018-05-22):

6.1.7.2 "Object Internal Methods and Internal Slots" has now been modified to say (emphasis mine):

A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.