server.js

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

const port = parseInt(process.env.PORT, 10) || 3000
const dev = process.env.NODE_ENV !== 'production'
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:${port}`)
      })
  })