title: set default selected index value to -1 jquery easy ui combobox
body: <p>I have a <a href="http://www.jeasyui.com/documentation/combobox.php" rel="nofollow">jQuery EasyUI combobox</a> on a page like this:</p>
  
  <pre><code>&lt;select id="subject_selection" class="easyui-combobox"&gt;
      &lt;option value="math"&gt;Math&lt;/option&gt;
      &lt;option value="physics"&gt;Physics&lt;/option&gt;
      &lt;option value="chemistry"&gt;Chemistry&lt;/option&gt; 
      &lt;option value="biology"&gt;Biology&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I'm trying to set the index of the default selected value to -1 if there is no value set, I tried to set the attribute <code>selectedIndex</code> to -1 like <code>selectedIndex="-1"</code> for the <code>select</code> element, but that still does not work. I don't want to give the user the ability to chose and empty value in the combobox, therefore I don't want to add this:</p>
  
  <pre><code>&lt;option value=""&gt;&lt;/option&gt;
  </code></pre>
  
  <p>In my combobox. Does anybody have any idea how I can achieve this?</p>
  
  <p>Thank you</p>