title: What is the default value of React.PropTypes.func
body: <p>What's the correct value to replace the question marks with and why?</p>
  
  <pre><code>RecentProjectsSection.propTypes = {
    onClose: React.PropTypes.func.isRequired,
    projects: React.PropTypes.array.isRequired,
  };
  
  RecentProjectsSection.defaultProps = {
    onClose: ?????
    projects: [],
  };
  </code></pre>