Making a javascript function that returns itself where it is an instance of itself

Question

In Javascript, is it at all possible to make function Foo() such that both (Foo() == Foo), and (Foo() instanceof Foo)?

This almost works:

function Foo() { Object.setPrototypeOf(Foo,this); return Foo; }

so new Foo() instanceof Foo in this case, but if I just call Foo without new, it doesn't work.

`Object.setPrototypeOf(Foo,this);` sets *Foo*'s prototype to an instance of *Foo*, which still has the original *Foo* prototype as its `[[Prototype]]` (because it was constructed before you swapped the prototype). Subsequent calls will keep inserting new objects on the prototype chain. Messy. Call it without *new* and things really go to pot! — RobG, May 03 '17 at 01:38

score 3 · Accepted Answer · answered May 03 '17 at 01:48

3

You'll need to do

function Foo() { return Foo; }             // Foo() === Foo
Object.setPrototypeOf(Foo, Foo.prototype); // Foo instanceof Foo

Remember that a instanceof B is equivalent to B.prototype.isPrototypeOf(a)

answered May 03 '17 at 01:48

Bergi

3

And of course, please never use this anywhere for real… – Bergi May 03 '17 at 01:51
@RobG [`isProtototypeOf`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isPrototypeOf) also searches the chain – Bergi May 03 '17 at 12:05
Hmm, one day I'll work out how to read the spec, or it will be written in a way that is reasonably understandable… – RobG May 03 '17 at 23:30

1 Answers1