const path = require('path');

// Serve the static files from the React app
app.use(express.static(path.join(__dirname, 'react/build')));

// Routers etc. here
...

// Handles any requests that don't match the ones above
app.get('*', (req,res) => {
    res.sendFile(path.join(__dirname+'/react/build/index.html'));
});

// If Heroku
"scripts": {
    "start": "node index.js",
    "heroku-postbuild": "cd react && npm install --only=dev && npm install && npm run build",
    ...
  }