TIL-Lab49/Enforce request and response types when using express with TypeScript.

express.md

express exports Request and Response types for your request and response objects when handling express requests. It's done this for a while, but I recently learned that these types accept generics, which allow you get more specific about what you're accepting and responding with.

As the most simple possible example, let's imagine a RESTful API that responds with JSON:

router.get('/users', async (req, res, next) => {
  try {
    const users = await Users.getAll();
  
    res.status(200).json(users);
  } catch (err) {
    next(err);
  }
});

Pretty standard. But, you can take this a step further by doing:

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

router.get('/users', async (req: Request, res: Response<User[]>, next: NextFunction) => {

So now, if you do something like:

const resp = {
  success: true,
  users,
};

res.status(200).send(resp);

You'll get an error, because the object you're responding with is not User[], as specified in the type argument in Response.