manipulate checkboxs

<!--        Script by hscripts.com          -->
<!--        copyright of HIOX INDIA         -->
<!-- Free javascripts @ http://www.hscripts.com -->
<script type="text/javascript">
checked=false;
function checkedAll (frm1) {
  var aa= document.getElementById('frm1');
	 if (checked == false)
          {
           checked = true
          }
        else
          {
          checked = false
          }
	for (var i =0; i < aa.elements.length; i++) 
	{
	 aa.elements[i].checked = checked;
	}
      }
</script>
<!-- Script by hscripts.com -->


<form id ="frm1">
<input type="checkbox" name="chk1">
<input type="checkbox" name="chk2">
</form>

<input type='checkbox' name='checkall' onclick='checkedAll(frm1);'>