title: TypeError: Class extends value undefined is not a constructor or null in react js
body: <p>I am a beginner in react and I am playing with an example in <code>https://jscomplete.com/repl.</code></p>
  
  <p>So far my code looks like :</p>
  
  <pre><code>let data = [
      {
      name:"Paul O’Shannessy",
      avatar_url:"https://avatars1.githubusercontent.com/u/8445?v=4",
          company_name:"Facebook"
    },
    {
      name:"Tom Preston-Werner",
      avatar_url:"https://avatars0.githubusercontent.com/u/1?v=4",
          company_name:"Facebook"  
    }
  ];
  
  const Card = (props) =&gt; {
      return (
      &lt;div style={{margin:'1em'}}&gt;
          &lt;img width="75" src={props.avatar_url} /&gt;
        &lt;div className="info" style={{display: 'inline-block',marginLeft: 10}}&gt;
          &lt;div style={{fontSize: '1.25em',fontWeight: 'bold'}}&gt;{props.name}&lt;/div&gt;
          &lt;div&gt;{props.company_name}&lt;/div&gt;
        &lt;/div&gt;
      &lt;/div&gt;
    );
  }
  
  const CardList = (props) =&gt; {
      return (
      &lt;div&gt;
          {props.cards.map((card) =&gt; &lt;Card {...card}/&gt;)}
      &lt;/div&gt;
    );
  }
  
  class Form extends React.component {
      render() {
      return (
          &lt;form&gt;
          &lt;input type="text" placeholder="Github Username" /&gt;
          &lt;button type="submit"&gt;Add Card&lt;/button&gt;
        &lt;/form&gt;
      );
    };
  }
  
  class App extends React.component {
      render() {
      return (
          &lt;div&gt;
          &lt;Form /&gt;
          &lt;CardList cards={data} /&gt;
        &lt;/div&gt;
      );
    };
  }
  
  ReactDOM.render(&lt;App /&gt;,mountNode);
  </code></pre>
  
  <p>But each time I run, I keep getting this runtime error.
  What am I doing wrong?</p>