next.js/examples/custom-server-fastify/server.js

const fastify = require('fastify')
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(() => {
  const server = fastify()

  server.get('/a', (req, res) => {
    return app.render(req.req, res.res, '/a', req.query)
  })

  server.get('/b', (req, res) => {
    return app.render(req.req, res.res, '/b', req.query)
  })

  server.get('/*', (req, res) => {
    return handle(req.req, res.res)
  })

  server.listen(port, (err) => {
    if (err) throw err
    console.log(`> Ready on http://localhost:${port}`)
  })
})