server.js

const {createServer} = require('http');
const {parse} = require('url');
const next = require('next');

const dev = process.env.NODE_ENV !== 'production';
const port = process.env.PORT || 3000;

const app = next({dev});
const handle = app.getRequestHandler();

app.prepare().then(() => {
    createServer((req, res) => {
        const parsedUrl = parse(req.url, true);
        const {pathname, query} = parsedUrl;

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

}).listen(port, (err) => {
    if (err) throw err;
    console.log('> Ready on http://localhost:3000');
});
});