server.js

const { createServer } = require("http");
const { parse } = require("url");
const next = require("next");
const Cors = require("cors");

function initMiddleware(middleware) {
  return (req, res) =>
    // eslint-disable-next-line no-undef
    new Promise((resolve, reject) => {
      middleware(req, res, (result) => {
        if (result instanceof Error) {
          return reject(result);
        }
        return resolve(result);
      });
    });
}

const cors = initMiddleware(
  Cors({
    methods: ["GET", "POST", "PUT", "DELETE"],
    origin: "*",
  })
);

const dev = process.env.NODE_ENV !== "production";
const hostname = "localhost";
const port = 3000;
const app = next({ dev, hostname, port });
const handle = app.getRequestHandler();

app.prepare().then(() => {
  createServer(async (req, res) => {
    await cors(req, res);

    try {
      const parsedUrl = parse(req.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, (err) => {
    if (err) throw err;
    console.log(`> Ready on http://${hostname}:${port}`);
  });
});