How can we select all check box in one click using javascript

<HeaderTemplate>
  <input id="chkAll"  

onclick="javascript:SelectchooseAllCheckboxes(this);" 
              runat="server" type="checkbox" />
</HeaderTemplate> 

<script language="javascript"> 

 function chooseAllCheckboxes(spanChk){

   // Added as ASPX uses SPAN for checkbox
   var oItem = spanChk.children;
   var theBox= (spanChk.type=="checkbox") ? 
        spanChk : spanChk.children.item[0];
   xState=theBox.checked;
   elm=theBox.form.elements;

   for(i=0;i<elm.length;i++)
     if(elm[i].type=="checkbox" && 
              elm[i].id!=theBox.id)
     {
       //elm[i].click();
       if(elm[i].checked!=xState)
         elm[i].click();
       //elm[i].checked=xState;
     }
 }
</script>