Using create-react-app with React Router + Express.js

routes.js

// src/routes.js
import React from 'react';
import { Router, Route } from 'react-router';

import App from './components/App';
import About from './components/About';
import NotFound from './components/NotFound';

const Routes = (props) => (
  <Router {...props}>
    <Route path="/" component={App} />
    <Route path="/about" component={About} />
    <Route path="*" component={NotFound} />
  </Router>
);

export default Routes;

With react-router-dom

import React from 'react';
import { Route, Switch } from 'react-router-dom'
import App from './components/App';
import About from './components/About';
import NotFound from './components/NotFound';
const Routes = (props) => (
  <Switch>
    <Route exact path="/" component={App} />
    <Route path="/about" component={About} />
    <Route component={NotFound} />
  </Switch>
);
export default Routes;

Guide for migration