title: How do I get the selected index of an ExtJS Combobox
body: <p>What is the certified way to determine the index of the currently selected item in a ComboBox in ExtJS?</p>
  
  <p>Is there a difference on how to do this between ExtJS 3.x and 4?</p>
  
  <pre><code>var combo = new Ext.form.ComboBox(config);
  var selectedIndex = combo.selectedIndex; // TODO: Implement
  if(selectedIndex &gt; 2) {
      // Do something
  }
  </code></pre>
  
  <p>Bonus-points for how to add it as a property to the ComboBox-object.</p>