title: How to add a dynamic class in knockoutjs?
body: <p>Let's say i have,</p>
  
  <pre><code>&lt;span class="cls1 cls2" data-bind="title: title" &gt;&lt;/span&gt;
  </code></pre>
  
  <p>I want to add another class via JSON object,</p>
  
  <pre><code>{ title: 'a', clas: 'cls3' }
  </code></pre>
  
  <p>This work's,</p>
  
  <pre><code>    &lt;span class="cls1 cls2" data-bind="attr:{title: title,'class': 'cls1 cls2'+clas}" &gt;&lt;/span&gt;
  </code></pre>
  
  <p>But the problem is that it will add two class attributes. I need the cls1 and cls2 class on beginning. But need cls3 class after some event. </p>