App.js

import React from 'react';
import { Link, Redirect, Route, Switch } from 'react-router-dom';

const Nav = () => (
  <ul>
    <Link to="/friends/"><li>Friends</li></Link>
    <Link to="/books/"><li>Books</li></Link>
  </ul>
)


const Friends = () => <h1>Friends</h1>
const Books = () => <h1>Books</h1>


const App = () => (
  <div>
    <Nav />
    <Switch>
      <Route exact path="/friends/" component={Friends} />
      <Route exact path="/books/" component={Books} />
      <Redirect exact from="/" to="/friends/" />
    </Switch>
  </div>
)


export default App