title: How to count tr elements with jquery?
body: <pre><code>&lt;table&gt;
     &lt;tr&gt;
       &lt;td&gt;data&lt;/td&gt;
       &lt;td&gt;data&lt;/td&gt;
     &lt;/tr&gt;
     &lt;tr class='product_details'&gt;
       &lt;td&gt;product details&lt;/td&gt;
       &lt;td&gt;product details&lt;/td&gt;
     &lt;/tr&gt;
     &lt;tr class='product_details'&gt;
       &lt;td&gt;product details&lt;/td&gt;
       &lt;td&gt;product details&lt;/td&gt;
     &lt;/tr&gt;
  &lt;/table&gt;
  </code></pre>
  
  <p>I want to count table rows whose class is product_details. How I do that with jQuery?</p>