Is there a way to extend Router using only ES6 sublass syntax?

[GabenGar]

I've looked at this and this discussions and the code examples use util.inherits() which is discouraged by NodeJS itself. So is there a modern way without relying on goofy legacy syntax of constructor functions and prototype? My usecase is I want to extend a request for a certain route so I could assign the result of parsing the search query down the middleware chain in that router only. It's typescript so no willy-nilly assignments to req without typing them first either.

[markcellus]

Hmm the title says you'd like to extend Router but description says

I want to extend a request for a certain route

Do you want to extend Router or Request? Here is a way to extend using Router (not sure if the ExpressJS team recommends it though):

import { Router } from 'express';

export class CustomRouter {
    constructor() {
        Object.setPrototypeOf(this, Router.prototype);
        return this;
    }
}

[GabenGar]

Actually both, since I wanted a router-specific request, the shape of which should be expressed in its methods. However I found a way to sorta accomplish what I want through Typescript types without juggling prototype chains at runtime.

[GabenGar]

I guess the original problem stems from the fact that the docs use the examples of middleware, which assign values directly to req object. This is somewhat of an antipattern, if only because it makes harder to tell apart the details of an http request and related business logic. And the typings don't allow to change the Request shape on per-route basis, so it's pretty annoying to express in types without extending Application/Router object/class directly.
There is however Response.locals, the docs for which make it sound like it's something made for templates but the important part is here:

The variables set on res.locals are available within a single request-response cycle, and will not be shared between requests.

Which is more-less equivalent to assigning to req directly.
What is also important is that use() method and its derivatives share the same generic signature <PathParams, ResBody, ReqBody, SearchParams, Locals = Record<string, any>>, the last being the type for res.locals. That means the shape of res.locals can be declared on per-route basis, so no router/application extensions needed (but it does require passing the entire tuple of generics to every route).