get the checked/unchecked state of checkboxlist when clicking a checkbox

function myfn() {
            var dlist = document.getElementById("CheckBoxList1");
            var count = dlist.cells.length;
            for (var i = 0; i < count; i++) {
                var cId = "CheckBoxList1_" + i;
//cId is the dynamic id created for checkboxes in the list
                var cbox = document.getElementById(cId);
                if (cbox.checked == true) {
                    alert("ok");
                }
            }
        }

<asp:CheckBoxList ID="CheckBoxList1" runat="server"
            onclick="myfn()">
            <asp:ListItem>one</asp:ListItem>
            <asp:ListItem>two</asp:ListItem>
            <asp:ListItem>three</asp:ListItem>
            <asp:ListItem>four</asp:ListItem>
        </asp:CheckBoxList>