router-fetch-example.js

 /* ===== Backend GET ===== */
// ... 
router.get('/endPoint', (request, response) => {
   // Send some info
  return response.send({
    message: "RiceApps rocks!"
  })
})
app.use('/endpoint', someRouter);
// ...
app.listen(9000);
console.log("App listening on port 9000");

/* ====== Frontend GET ====== */
const URL = 'http://localhost:9000/endpoint' // Some endpoint to hit (e.g "https://somewebsite.com/api/endpoint")
fetch(URL, { method: 'GET' })
.then(response => response.json()) // Get json of response
.then((responseJson) => {
  console.log(responseJson.message)
  //You will probably want to update state e.g:
  setState({
    loading: false,
    message: responeJson.message
  })
})

Also, there's probably some syntactical issue in what I wrote above, so take it with a grain of salt. Reading up on the javascript fetch documentation would be good to do :)