Javascript Function to get the Checkbox Selected

<script type="text/javascript">

        function SelectAll() {
            var totalcheck = 0;
            var frm = document.forms[0];
            for (i = 0; i < frm.elements.length; i++) {
                if (frm.elements[i].type == "checkbox")
                 {
                     if (frm.elements[i].checked) {
                     
                         document.getElementById('<%=h1.ClientID %>').value=document.getElementById('<%=h1.ClientID %>').value+','+ frm.elements[i].id;
                     }

                 }

             }
             alert(document.getElementById('<%=h1.ClientID %>').value);
        }

</script>

//h1 is the HiddenField