Subclassing ES6 objects with ES5 syntax.

C. Scott Ananian ecmascript at cscott.net
Wed Apr 29 18:12:24 UTC 2015


On Wed, Apr 29, 2015 at 2:07 PM, Mark S. Miller <erights at google.com> wrote:

> Hi Scott, I think your approach is on the right track. How about the
> following?
>
> Anyone see a way to attack it?
>
>
>
> const goodPromises = new WeakSet();
> class DefensivePromise {
>   constructor(x) {
>     super(x);
>     if (new.target === DefensivePromise) {
>       Object.freeze(this);
>       goodPromises.add(this);
>     }
>   }
>   static resolve(x) {
>     if (goodPromises.has(x)) {
>       return x;  // should be equiv to super.resolve(x);
>     }
>     return new DefensivePromise(r => {r(x)});
>   }
> }
>

Assuming that you don't export DefensivePromise to the attacker, this is
fine.  Otherwise, I think this is still vulnerable to Reflect.construct
lying about new.target:
```
class BadPromise extends DefensivePromise {
  then(r) { r(); r(); }
}
var bp = Reflect.construct(BadPromise, DefensivePromise);
```

Since it's `Promise.then` you care about, I think the approach in my
previous message (where `then` is tested directly) is preferable.
 --scott
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