Handling routes with a function instead of middleware

server.ts

import * as express from "express";

const server = express();

server.get("/posts", withAuthenticatedUser(function(req, res, user) {
  const posts = getPostsForUser(user);

  res.json(posts);
}));

server.listen(8080);

interface RequestHandlerWithUser {
  (req: express.Request, res: express.Response, user: User): void;
}

function withAuthenticatedUser(handler: RequestHandlerWithUser): express.RequestHandler {
  const routeHandler: express.RequestHandler = (req, res, next) => {
    findUser(req)
      .then(user => {
        if (!user) {
          res.redirect("/login");
          return;
        }

        handler(req, res, user);
      })
      .catch(next);
  };

  return routeHandler;
}

interface User {
  id: number;
  name: string;
}

async function findUser(req: express.Request): Promise<User | null> {
  if (req.query.online === "extremely") {
    return { id: 1, name: "Gary" };
  } else {
    return null;
  }
}

function getPostsForUser(user: User) {
  return [{ slug: "jsjsjs", user }];
}