title: Functional component renders once, class component renders twice
body: <p>So, I started this new project with create-react-app (it's running react v.16.13.1).
  I rewrote the base App component as a class, and I found that when the component is a function, like this:</p>
  
  <pre><code>function App() {
    console.log('App (function)');
    return 'App (function)';
  }
  </code></pre>
  
  <p>the browser console prints out </p>
  
  <pre><code>App (function)
  </code></pre>
  
  <p>Great, thanks! But if the same App component is written as</p>
  
  <pre><code>class App extends React.Component {
    render() {
      console.log('App (class)');
      return 'App (class)';
    }
  }
  </code></pre>
  
  <p>console writes </p>
  
  <pre><code>App (class)
  App (class)
  </code></pre>