LazyRouter.ts

import { Router } from "express";

export function LazyRouter(
  loader: () => Promise<{ default: Router }>,
  { preload }: { preload?: boolean } = {}
) {
  const loadRouter = () => (router ||= loader().then((m) => m.default));
  let router: Promise<Router> | null = preload ? loadRouter() : null;

  return Router().use((req, res, next) =>
    loadRouter()
      .then((r) => r(req, res, next))
      .catch(next)
  );
}