app.js

// @flow 
import express from 'express'
import myRoute from './myroute'

const app = express()

app.use('/my-route', myRoute())

myroute.js

// @flow 
import express from 'express'

const router = express.Router()

export default () => {
  router.get('/', (req, res, next) => {
    // respond to GET on /my-route
  })

  return router
}