How can I handle type with middleware of express?

Question

I am using Typescript in Node.js. When you use Express middleware, you often transform the Request object. With Typescript, however, we could not track how the Request object was transformed. If you know the middleware that passed before, is there a way to find out the type of the request from it? If not possible in express, I would like to find another framework where it is possible. Is it possible in Nest (https://github.com/kamilmysliwiec/nest)?

Example Code

import { Request, Response, NextFunction } from 'express';

function userMiddleware(req: Request & User, res: Response, next: NextFunction) {
    req.user = {
        id: 'user_id',
    };
    next();
}

interface User {
    user: {
        id: string;
    }
}

interface Middleware {
    <T>(req: Request & T, res: Response, next: NextFunction): void;
}

class Controller {
    middleware = [userMiddleware];

    get = new GetMethod(this.middleware);

    post = (req: Request /* I don't know exact req type */, res: Response, next: NextFunction) => {
        console.log(req.user) // Error!
    }
}

class GetMethod {
    constructor(middleware: Middleware[]) {
        // How to deduce type of req from Middleware array?
    }
}

const controller = new Controller();

express.use('/', controller.middleware, controller.post);

I want to extract type information from Middleware list in Controller class.

Answer 1

First I think the right interface is

interface User {
  id: string;
}

---

Because they're callbacks they'll receive default Request that don't have user in its signature.

Therefore you have 2 options, do a type assertion, or to write a custom declaration. Both a fine if you do them properly.

---

Type assertion:

interface User {
  id: string;
}

const isObject = (value: unknown): value is {[key: string]: unknown} => {
  return value && typeof value === 'object';
};

const isReqWithUser = (req: Request): req is Request & {user: User} => {
  return isObject(req) && !!req.user;
}

class Controller {
  post = (req: Request, res: Response, next: NextFunction) => {
    if (isReqWithUser(req)) {
      console.log(req.user) // now it works
    }
    next();
  }
}

---

Custom declaration:

but we need to understand that user not always exist on the request and we should mark it optional.

interface User {
  id: string;
}

declare module 'express' {
  export interface Request {
    user?: User; // adding our custom declaration.
  }
}

class Controller {
  post = (req: Request, res: Response, next: NextFunction) => {
    console.log(req.user) // now it works
    next();
  }
}