server.js

const { createServer } = require('http'); /* eslint-disable-line*/
const { parse } = require('url'); /* eslint-disable-line*/
const next = require('next'); /* eslint-disable-line*/

const dev = process.env.NODE_ENV !== 'production';
const hostname = 'localhost';
const port = 3000;
// when using middleware `hostname` and `port` must be provided below
const app = next({ dev, hostname, port });
const handle = app.getRequestHandler();

app.prepare().then(() => {
  createServer(async (req, res) => {
    try {
      // Be sure to pass `true` as the second argument to `url.parse`.
      // This tells it to parse the query portion of the URL.
      const url = req.url ?? '';
      if (url !== '') {
        const parsedUrl = parse(url, true);
        const { pathname, query } = parsedUrl;

        if (pathname === '/a') {
          await app.render(req, res, '/a', query);
        } else if (pathname === '/b') {
          await app.render(req, res, '/b', query);
        } else {
          await handle(req, res, parsedUrl);
        }
      }
    } catch (err) {
      // console.error('Error occurred handling', req.url, err)
      res.statusCode = 500;
      res.end('internal server error');
    }
  }).listen(port, () => {
    // console.log(`> Ready on http://${hostname}:${port}`)
  });
});