title: How to add two classes to an existing class with jquery?
body: <p>I have this element:</p>
  
  <pre><code>&lt;div class="btn-group"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>And I have an click event in jquery and I have selected above div and now after selecting the element I want to add two classes named <code>dropdown</code> and <code>open</code> to its btn-group class.</p>
  
  <p>It means I want to have this:</p>
  
  <pre><code>&lt;div class="btn-group dropdown open"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>If the element already contains the <code>btn-group</code> class.</p>
  
  <p>How can I do that?</p>