title: React.js: Set Prop only when condition is true
body: <p>I'm quite new to react.js and have some troubles passing props to children. 
  I want to do something like this:</p>
  
  <pre><code>render: function(){
     var idProp = this.props.id ? 'id={this.props.id}' : null;
     return (
       &lt;div {idProp} /&gt;
     );
  }
  </code></pre>
  
  <p>So the prop id should only be passed if a condition is true.
  But the above code is not working. 
  Is this possible? What am I doing wrong?
  Thanks! </p>