title: React native null value checking
body: <p>how can I check this markerPosition value is null or not, I need to assign it a variable laying if its null.</p>
  
  <pre><code>  onSendPress() {
      const { message, markerPosition } = this.props;
      this.props.dispatch({
        type: 'Message/messageSend',
        payload: { message, markerPosition }
      });
    }
  </code></pre>