Subclassing ES6 objects with ES5 syntax.

Andrea Giammarchi andrea.giammarchi at gmail.com
Fri Apr 24 11:13:02 UTC 2015


Not exactly ... if it's an Array, as example, the moment you
slice/map/splice/concat, etc will return an instanceof Array, not an
instance of whatever you have sublcassed.

Regards

On Fri, Apr 24, 2015 at 10:24 AM, Alex Kocharin <alex at kocharin.ru> wrote:

>
> I believe you can subclass anything using code like this:
>
> function MyPromise(executor) {
>   var self = new Promise(executor)
>   self.setPrototypeOf(self, MyPromise.prototype)
>   return self
> }
> Object.setPrototypeOf(MyPromise, Promise)
>
>
> ... and it can be easily subclassed itself in the same way.
>
>
> 24.04.2015, 04:02, "C. Scott Ananian" <ecmascript at cscott.net>:
>
> Is there any way to access `new.target` using ES5 syntax?
>
> It appears that the "correct" way to create a subclass using ES5 syntax is:
> ```
> function MyPromise(executor) {
>   var self = Reflect.construct(Promise, [executor], new.target);
>   return self;
> }
> Object.setPrototypeOf(MyPromise, Promise);
> ```
> But since `new.target` isn't accessible, we have to do something like:
> ```
> function MyPromise(executor) {
>   var self = Reflect.construct(Promise, [executor], MyPromise); // <-- THIS
>   return self;
> }
> Object.setPrototypeOf(MyPromise, Promise);
> ```
> which works for only a single level of subclassing.  That is, it allows us
> to create and instantiate MyPromise, but now nobody can subclass
> MyPromise.  That's too bad.
>
> Is there any way around this?
>   --scott
>
> ps. Use case: My `prfun` package on npm subclasses `Promise` in order to
> add all the useful utility helpers without stomping on the global `Promise`
> object.  I'd like to do so in a way which is compatible with both native
> ES6 promises (if they are available) and properly-written ES5 shims.
> ,
>
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