title: Add a class to a DIV with javascript
body: <p>Example HTML:</p>
  
  <pre><code>&lt;div id="foo" class="class_one"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>How can I add the class <code>class_two</code> without replacing <code>class_one</code>?</p>
  
  <p>end result:</p>
  
  <pre><code>&lt;div id="foo" class="class_one class_two"&gt;&lt;/div&gt;
  </code></pre>