Subclassing ES6 objects with ES5 syntax.

[Kevin Smith]

>
> 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)});
>   }
> }
>
>
Basically you can't rely on new.target to mean what you think it means, in
the face of Reflect.construct.

Maybe this?

constructor(x) {
    super(x);
    // At this point you know that "this" is a Promise, but you don't
    // know if the prototype is set correctly, so:
    if (Object.getPrototypeOf(this) === DefensivePromise.prototype)
        gooPromises.add(Object.freeze(this));
}
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