title: why router is undefined in react js?
body: <p>could you please tell me why router is undefined in react js .I already import react-router.js file in my codepen</p>
  
  <p>here is my code
  <a href="http://codepen.io/naveennsit/pen/pymqPa?editors=1010" rel="nofollow">http://codepen.io/naveennsit/pen/pymqPa?editors=1010</a></p>
  
  <pre><code>class App extends React.Component {
  
      render() {
          return &lt;Router&gt;
            &lt;Route path='/' component={first}&gt;&lt;/Route&gt;
           &lt;Route path='/about' component={second}&gt;&lt;/Route&gt;
          &lt;/Router&gt;
      }
  
  }
  
  class second extends React.Component {
  
      render() {
          return &lt;label&gt;second component&lt;/label&gt;
      }
  
  }
  class first extends React.Component {
  
      render() {
          return &lt;label&gt;first component&lt;/label&gt;
      }
  
  }
  
  React.render( &lt; App / &gt; , document.getElementById('app'))
  </code></pre>