Symbol.hasInstance and [[Get]] vs [[GetOwnProperty]]

[/#!/JoePea]

I want to add also that using `super` makes it more readable, but still
requires boilerplate code, and is just as statically limited as using a
direct reference to a constructor ([I wish it wasn't](https://esdiscuss.org/
topic/the-super-keyword-doesnt-work-as-it-should)):

```js
class A {
    static [Symbol.hasInstance](obj) {
        if (this === A) return false
        else return super[Symbol.hasInstance](obj)
    }
}
```

*/#!/*JoePea

On Mon, Aug 15, 2016 at 6:45 PM, /#!/JoePea <joe at trusktr.io> wrote:

> It seems like using [[Get]] for looking up `@@hasInstance` can be
> confusing and requires devs to write extra code. For example, suppose we
> have the following code:
>
> ```js
> class A {
>     static [Symbol.hasInstance] (obj) {
>         return false
>     }
> }
>
> class B extends A {}
> class C extends B {}
>
> let c = new C
>
> console.log(c instanceof B) // false, but expected true!
> console.log(c instanceof A) // false, as expected
> ```
>
> The `c instanceof B` check (arguably unintuitively) fails. Defining a
> `Symbol.hasInstance` method on a base class causes `instanceof` checks on
> any subclasses to automatically fail unless the developer takes care to
> write an ugly and fragile workaround:
>
> ```js
> class A {
>     static [Symbol.hasInstance](obj) {
>         if (this === A) return false
>         else return Function.prototype[Symbol.hasInstance].call(this, obj)
>     }
> }
>
> class B extends A {}
> class C extends B {}
>
> let c = new C
>
> console.log(c instanceof B) // true, as expected
> console.log(c instanceof A) // false, as expected
> ```
>
> That seems likely to introduce errors or headaches.
>
> What if the lookup for `Symbol.hasInstance` on an object use
> [[GetOwnProperty]]? Then the first version without the conditional checks
> would not break subclasses.
>
> */#!/*JoePea
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20160815/07a87079/attachment-0001.html>