Stop Onclick Execution when middle click is pressed - javascript / jquery

This is something like my code:

<li class="dropdown" id="dropdown1"><a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown<b class="caret"></b></a>
  <ul class="dropdown-menu">
       <li id="li-1"><a href="#" onclick="Alert("li-1")">List1</a></li>
       <li id="li-2"><a href="#" onclick="Alert("li-2")">List</a></li>
  </ul>
</li>

but when middle click is pressed i want to stop execution of my onclick event