checkbox全选功能

<script language="javascript" type="text/javascript">
    function ckall(sender) {
        var rep = document.getElementById("Rep_System");
        var ckall = document.getElementsByTagName("input");
        for (var i = 0; i < ckall.length; i++) {
            if (ckall[i].type == "checkbox" && ckall[i].id.indexOf("ckinsert") > -1) {
                ckall[i].checked = sender.checked;
             }
        }
    }
    function ckitem(sender) {
        var rep = document.getElementById("Rep_System");
        var ckarr = document.getElementsByTagName("input");
        var ckall;
        for (var i = 0; i < ckarr.length; i++) {
            if (ckarr[i].type == "checkbox" && ckarr[i].id.indexOf("ckall") > -1) {
                ckall = ckarr[i];
                break;
             }

        }
        for (var i = 0; i < ckarr.length; i++) {
            if (ckarr[i].type == "checkbox" && ckarr[i].id.indexOf("ckinsert") > -1 && !ckarr[i].checked) {
                ckall.checked = false;
                return;
                
             }

        }
        ckall.checked = true;
    }
</script>