Subclassing ES6 objects with ES5 syntax.

Erik Arvidsson erik.arvidsson at gmail.com
Fri Apr 24 01:24:18 UTC 2015


new.target is available in functions.

On Thu, Apr 23, 2015, 21:02 C. Scott Ananian <ecmascript at cscott.net> wrote:

> 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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>
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