Next in Express example

registerNext.js

const next = require('next');
const pathToRegexp = require('path-to-regexp');
const { parse } = require('url');

module.exports = async function registerNextApp(nextOptions) {
  const nextApp = next(nextOptions);
  const requestHandler = nextApp.getRequestHandler();

  try {
    await nextApp.prepare();
  } catch (ex) {
    console.error('NEXT ERROR', ex.stack);
    process.exit(1);
  }

  // Return Express Router callback
  return (req, res) => {
    const parsedUrl = parse(req.url, true);
    const { pathname, query } = parsedUrl;

    if (pathname === '/a') {
      nextApp.render(req, res, '/b', query)
    } else if (pathname === '/b') {
      nextApp.render(req, res, '/a', query)
    } else {
      requestHandler(req, res, parsedUrl)
    }
  };
};

server.js

async function setupExpress() {
  const app = express();

  // Register other endpoints
  
  const clientHandler = await registerNext({ dev });
  app.get('*', clientHandler); // allow next to handle remaining requests
}