title: Get argument count (or even names) of javascript function object
body: <p>I want to evaluate callback function before accepting it. This means I need to know at least the count of accepted argumens - if count doesn't match, I'll leave warning in console. 
  But I can't find out, whether a javascript function object has a property that would help me get that information.<br>
  So can this be achieved <strong>without</strong> parsing function as string (not worth it)?</p>