title: Why counting is(':checked') or prop('checked') radio buttons by .length returning undefined?
body: <p>I want to check is any radiobuton located in one of  element is checked, but when I use my code:</p>
  
  <pre><code>var activeQst= jQuery('.mainTab:eq(0) tbody').children('tr:visible');
  var whichQst = +activeQst.index();
  var radioActive = jQuery('.mainTab:eq(0) tr:eq('+whichQst+') .RadioButton');
  var checker = radioActive.is(':checked').length;
  </code></pre>
  
  <p>or</p>
  
  <pre><code>var checker = radioActive.prop('checked').length;
  </code></pre>
  
  <p><code>alert(checker);</code> - returning <code>undefined</code>, even when checkbox is physically checked.
  <code>alert(radioActive.length)</code> - return proper length of elements.
  I couldn't find what is wrong.</p>