next.js/examples/with-sitemap-and-robots-express-server/server/app.js

const express = require('express')
const next = require('next')
const sitemapAndRobots = require('./sitemapAndRobots')

const dev = process.env.NODE_ENV !== 'production'

const port = process.env.PORT || 8000
const ROOT_URL = dev
  ? `http://localhost:${port}`
  : 'https://sitemap-robots.now.sh'

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

// Nextjs's server prepared
app.prepare().then(() => {
  const server = express()

  sitemapAndRobots({ server })

  server.get('*', (req, res) => handle(req, res))

  // starting express server
  server.listen(port, err => {
    if (err) throw err
    console.log(`> Ready on ${ROOT_URL}`) // eslint-disable-line no-console
  })
})