title: Add a dynamic class name
body: <p>In <a href="http://facebook.github.io/react/index.html" rel="nofollow noreferrer">react.js</a> I need to add a dynamic class name to a <code>div</code>.<br>
  Using <a href="http://facebook.github.io/react/docs/addons.html" rel="nofollow noreferrer">react-addons</a>, I tried it the following way but in vain:</p>
  
  <pre><code>var addons = require('react-addons');
  var cx = addons.classSet;   
  
  var Overlay = React.createClass({
  
      render: function() {
          var prod_id = this.props.prop_id;
          var large_prod_class = 'large_prod_modal_' + prod_id;
  
          var modal_classes = cx({
              'large_prod_modal': true,
              large_prod_class: true,
              'hidden': true
          });
  
          return (&lt;div className={modal_classes}&gt;lorem ipsum&lt;/div&gt;);
      }
  });
  </code></pre>
  
  <p>And <code>Overlay</code> component is used in the following way:</p>
  
  <pre><code>&lt;Overlay prod_id="9" /&gt;
  </code></pre>
  
  <p>The prop (i.e: <code>prod_id</code>) value is random however. I do not get the <code>large_prod_modal_9</code> class for the <code>div</code>. All the classes I get are <code>large_prod_modal</code>,<code>large_prod_class</code> and <code>hidden</code></p>
  
  <p>How to get the <code>large_prod_modal_9</code> class for the <code>div</code>?</p>