server.js

import express from 'express';
import React from 'react';
import { ServerStyleSheet } from 'styled-components';
import { renderToString } from 'react-dom/server';
import { StaticRouter } from 'react-router-dom';
import path from 'path';
import fs from 'fs';
import App from './src/App';

const app = express();

app.use(express.static('./build', { index: false }))

app.get('/*', (req, res) => {
	const sheet = new ServerStyleSheet();

	const reactApp = renderToString(
		sheet.collectStyles(
			<StaticRouter location={req.url}>
				<App />
			</StaticRouter>
		)
	);

	const templateFile = path.resolve('./build/index.html');
	fs.readFile(templateFile, 'utf8', (err, data) => {
		if (err) {
			return res.status(500).send(err);
		}

		return res.send(
			data.replace('<div id="root"></div>', `<div id="root">${reactApp}</div>`)
				.replace('{{ styles }}', sheet.getStyleTags())
		)
	});
});

app.listen(8080, () => {
	console.log('Server is listening on port 8080');
});