server.js

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

const dev = process.env.NODE_ENV !== 'production'
const app = next({
  dev
})
const handle = app.getRequestHandler()
// default PORT is 3000
const PORT = process.env.PORT || 3000

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

    if (pathname === '/a') {
      app.render(req, res, '/a', query)
    } else if (pathname === '/b') {
      app.render(req, res, '/b', query)
    } else {
      handle(req, res, parsedUrl)
    }
  }).listen(PORT, (err) => {
    if (err) throw err
    console.log('> Ready on http://localhost:3000')
  })
})