title: Plus operator is appending instead of doing math
body: <p>In JavaScript when I run this code:</p>
  
  <pre><code>parent.css("top") + parent.height() * 2
  </code></pre>
  
  <p>The result in Chrome console is:</p>
  
  <blockquote>
    <p>100px320</p>
  </blockquote>
  
  <p>Why is this happening?<br>
  <code>parent.height()</code> equals 160, so it seems like <code>parent.css("top")</code> is a string and JS is simply adding some characters to a string instead of doing math calculations.  </p>
  
  <p>Why is it not working and how do I do this correctly?</p>