node.js – What would be the response if a middleware fails in a REST API?

I am reading a code that has two files like below:

first file that uses the currentuser middleware:

const router = express.Router();

router.get("/api/users/currentuser", currentUser, (req, res) => {
  res.send({ currentUser: req.currentUser || null });
});

export { router as currentUserRouter };

Second file that defines the middleware:

interface UserPayload {
  id: string;
  email: string;
}

declare global {
  namespace Express {
    interface Request {
      currentUser?: UserPayload;
    }
  }
}

export const currentUser = (
  req: Request,
  res: Response,
  next: NextFunction
) => {
  if (!req.session?.jwt) {
    return next();
  }

  try {
    const payload = jwt.verify(
      req.session.jwt,
      process.env.JWT_KEY!
    ) as UserPayload;
    req.currentUser = payload;
  } catch (err) {}

  next();
};

I understand that if there is a verified jwt token, the middleware will take the the payload out of it and add it to the req object. But what if it fails and it can’t add the payload/current user to the req? What would happen for the following request and what will the res object look like?

router.get("/api/users/currentuser", currentUser, (req, res) => {
  res.send({ currentUser: req.currentUser || null });
});

Could you edit this get request to show how can I catch the probable error if I am not the writer of the middleware?