title: check one checkbox is selected
body: <p>I have a checkbox array </p>
  
  <pre><code>&lt;input type="checkbox" name="event-types[]" value="family" /&gt;
  &lt;input type="checkbox" name="event-types[]"  value="adult" /&gt;
  &lt;input type="checkbox" name="event-types[]" value="children"  /&gt;
  </code></pre>
  
  <p>and I want to check at least one is selected</p>
  
  <p>but this always equals 0</p>
  
  <pre><code>alert($("[name='event-types']:checked").length);
  </code></pre>