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
  })
})

This is just a high level example. Essentially, you will boot up your node server, running on some port (e.g 9000). That contains some router set up via node & express.

Then, on the front end, you can use the fetch function to hit that end point and read any data sent by the back end.

Fetch returns a promise, which means you have to handle the synchronicity somehow. In essence, fetch returns a promise. You can either chain .then() calls or use await:

const response = await fetch(URL, {method: 'GET'})
const json = await response.json()
...

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 :)