<Route component={App}>
  <Route path="groups" components={Groups} />
  <Route path="users" components={Users}>
   <Route path="users/:userId" component={Profile} />
  </Route>
</Route>

<Route component={App}>
  <Route path="groups" components={Groups} />
  <Route path="users" components={Users}>
   //<Route path="users/:userId" component={Profile} />
  </Route>
</Route>

const Users = ({ match }) => (
 <div>
  <h2>Topics</h2>
  <Route path={`${match.url}/:userId`} component={Profile}/>
 </div>
) 

import React, { Component } from 'react';
import ReactDOM from 'react-dom';
// import { Router, Route, Link, Switch } from 'react-router';
import {
 HashRouter,
 Route,
 Link,
 Switch
} from 'react-router-dom';

class App extends Component {
 render() {
  return (
   <div>
    <h1>App</h1>
    <ul>
     <li><Link to="/">Home</Link></li>
     <li><Link to="/about">About</Link></li>
     <li><Link to="/inbox">Inbox</Link></li>
    </ul>
    {this.props.children}

   </div>
  );
 }
}

const About = () => (
 <div>
  <h3>About</h3>
 </div>
)

const Home = () => (
 <div>
  <h3>Home</h3>
 </div>
)

const Message = ({ match }) => (
 <div>
  <h3>new messages</h3>
  <h3>{match.params.id}</h3>
 </div>
)

const Inbox = ({ match }) => (
 <div>
  <h2>Topics</h2>
  <Route path={`${match.url}/messages/:id`} component={Message}/>

 </div>
) 

ReactDOM.render(
 (<HashRouter>
  <App>
    <Route exact path="/" component={Home} />
    <Route path="/about" component={About} />
    <Route path="/inbox" component={Inbox} />
  </App>
 </HashRouter>),
 document.getElementById('root')
);