gistfile1.txt

//Assuming your file structure looks like below, you
//should be able to serve the index.html from server/app.js
//**************************************************//
//
//  Project/
//    server/
//      app.js
//      /* the rest of your backend */
//    client/
//      /* all client side files */
//    dist/
//      index.html
//      bundle.js
//      bundle.css
//      /* whatever assets your build packaged */
//
//**************************************************//
//
// the only file in dist that you should be making yourself
// is dist/index.html, where you link the bundle.js and bundle.css
// files. usually through >_: create-react-app the command to build
// the bundle is `npm run build`, but you can check your package.json
// for extra help.

app.use('/', (req, res) => {
  res.sendFile(path.join(__dirname, '../dist', 'index.html');
}

sidenote. dist stands for distribution. makes it a little easier to understand that the client code is not served, but instead the bundled styles and javascript is served to clients.