express router lazy loading

lazyLoad.ts

import express from 'express';

/**
 * lazy load express router
 * @param resolver
 * @example
 * ```js
 * import { lazyLoad } from "./lazyLoad"
 * router.use(
 *   '/path_to_router',
 *   lazyLoad(() => import('./path_to_router')),
 * );
 * ```
 */
export function lazyLoad(resolver: () => Promise<{ default: express.Router } | express.Router>) {
  const lazyRouter = express.Router();
  let alreadyLoaded = false;
  lazyRouter.use((req, res, next) => {
    if (alreadyLoaded) {
      return next();
    } else {
      // initial startup
      resolver()
        .then(router => {
          alreadyLoaded = true;
          if ('default' in router) {
            lazyRouter.use(router.default);
          } else {
            lazyRouter.use(router);
          }
          next();
        })
        .catch(error => {
          next(error);
        });
    }
  });
  return lazyRouter;
}