Static function inheritance

I've created a few classes as Controllers for my routes in Node.js and I'd like them to be able to work on a singleton basis (not required, it's an extra)

Main Controller class

class RouteController {

    static getInstance() {
        if (!RouteController.singleton) {
            RouteController.singleton = new RouteController();
        }

        return RouteController.singleton;
    }
}

Now I want to create a route for example viewing posts and I want that class to have the singleton pattern as well, but not cop

PostController class

class PostController extends RouteController {

}

If I do this and check it console.log(RouteController.getInstance() === PostController.getInstance()) it returns true when I want each (sub)class to have their own singleton instance.

How can I do this correctly?

via CreasolDev