Validate list of checkbox using javascript

 var atLeast = 1
        function Validate()
        {
           var CHK = document.getElementById("<%=chkOpenReq.ClientID%>");
           var checkbox = CHK.getElementsByTagName("input");
           var counter=0;
           for (var i=0;i
           {
               if (checkbox[i].checked)
               {
                    counter++;
               }
           }
           if(atLeast>counter)
           {
                alert("Please select atleast " + atLeast +  " item(s)");
               return false;
           }
          return true;
        }