How to properly type express route handlers?

New to express, and I'm trying to understand the 'proper' way to have type-safe route handlers. How can I fix this? Clearly my code 'runs', but I'm not sure how to properly type handlers in express. Say I have a route 'events' and some methods:

app.use("/events", eventsRoutes);

within my eventsRoutes I define a simple get, but it has errors in my IDE:

//... 

router.get("/user/:userId", (req, res) => { // <- This is the error
  const { userId } = req.params;
  const collection = EVENTS_COLLECTION.get(userId);
  if (!collection) {
    return res.status(200).json([]);
  }

  // iterate the keys and return an array of events
  const eventsArray: Event[] = [];
  for (const [date, events] of collection.entries()) {
    events.forEach((event) => {
      event.date = new Date(date);
      eventsArray.push(event);
    });
  }

  res.status(200).json(eventsArray);
});

// ...

No overload matches this call. The last overload gave the following error.

Argument of type '(req: Request<{ userId: string; }, any, any, ParsedQs, Record<string, any>>, res: Response<any, Record<string, any>, number>) => Response<...> | undefined' is not assignable to parameter of type 'Application<Record<string, any>>'.
  Type '(req: Request<{ userId: string; }, any, any, ParsedQs, Record<string, any>>, res: Response<any, Record<string, any>, number>) => Response<...> | undefined' is missing the following properties from type 'Application<Record<string, any>>': init, defaultConfiguration, engine, set, and 63 more.

How can I fix this? Clearly my code 'runs', but I'm not sure how to extend or properly 'type' the Request object in express.

Solution

This type error means that something else was returned. In this case it should be:

  ...
  if (!collection) {
    res.status(200).json([]);
    return;
  }
  ...

A handler is a a function that returns undefined (void type), or a promise of undefined (Promise<void>) to make it compatible with async functions.

There is no need to explicitly type a handler, the type is inferred. But it's beneficial to match a type to spot a mistake easier:

router.get("/user/:userId", (req, res) => {
  ...
} satisfies RequestHandler);