Checkboxlist in Javascript

Hello,

I have got a checkboxlist in asp.net 2.0 with items populating from Database.

After populating items from Database. I'm insert a new item "Select All"  into to the checkboxlist.

I need actually, a javascript help. When I'm checking the "Select All" checkbox from the checkboxlist it is selecting all the checkboxes inside the checkboxlist and when I'm unchecking "Select All" checkbox javascript function is unchecking all checkboxes in the checkboxlist.  But, when I'm trying to check or uncheck  any checkbox other than "Select All", it is not allowing to check or uncheck.

Here is the javascript function that I'm using.
function CheckUncheckAllItems(checkBoxListId,numOfItems)
{
        var objCtrl;
       
        objCtrl = document.getElementById(checkBoxListId.id + '_' + 0);
         
         
        if (objCtrl != null && objCtrl.checked == true)
        {
             for(i = 0; i < numOfItems; i++)
            {
               
                objItem = document.getElementById(checkBoxListId.id + '_' + i);
             
                if(objItem == null)
                {
                    continue;
                }
                objItem.checked = true;
               
            }      
         }
         
         if (objCtrl != null && objCtrl.checked == false)
         {
             for(i = 0; i < numOfItems; i++)
            {
               
                objItem = document.getElementById(checkBoxListId.id + '_' + i);
               
                if(objItem == null)
                {
                    continue;
                }
                objItem.checked = false;
               
            }
         }
 
}

Please give me a solution as this one of my project requirment. I need it urgently.
Quick response will be highly appriciated