JS+后台验证数据是否存在于数据库中

<asp:TemplateField HeaderText="所屬年月">
            <ItemTemplate>
                        <asp:TextBox ID="txtDate" runat="server" Width="90%" CssClass="TextBox" MaxLength="5"
                                   Text='<%# Eval("Var_Date") %>' onblur="this.value=this.value.replace(/D/g,'');ChkVatInvoice(this,'vatDate');"></asp:TextBox>
            </ItemTemplate>
            <HeaderStyle Width="6%" />
</asp:TemplateField>

function ChkVatInvoice(obj, val) {

 var txtVal = "";
            var vatType = "";
            var vatDate = "";
            if (val == "txt") {
                txtVal = $(obj).val();
                vatType = $(obj).parent().prev().children().first().val();
                vatDate = $(obj).parent().prev().prev().prev().prev().children().first().val();
            }
            else if (val == "select") {
                txtVal = $(obj).parent().next().children().first().val();
                vatType = obj.value;
                vatDate = $(obj).parent().prev().prev().prev().children().first().val();
            }
            else {
                txtVal = $(obj).parent().next().next().next().next().children().first().val();
                vatType = $(obj).parent().next().next().next().children().first().val();
                vatDate = $(obj).val();
            }

            if (txtVal != "") {
                if (txtVal.length < 2) {
                    alert("<%= localize.UnifiedNo + localize.IsNotValid %>"); //统编格式无效。
                }
                else {
                    //格式代码
                    var char1 = txtVal.substr(0, 1);
                    var char2 = txtVal.substr(1, 1);

                    if (vatType == "21" || vatType == "23" || vatType == "25") {
                        //前两位必须是英文且要存在于发票字轨

                        if (char1.charCodeAt() >= 65 &&
                      char1.charCodeAt() <= 90 &&
                      char2.charCodeAt() >= 65 &&
                      char2.charCodeAt() <= 90) {

  //检测是否存在发票字轨
                            var xmlURL = "JsonData.ashx?t=IT&val=" + txtVal.substr(0, 2) + "&vatType=" + vatType + "&vatDate=" + vatDate;

                            $.get(xmlURL, {}, function(result) {
                                if (result == "N") {
                                    alert("<%=localize.lblVatInvoice %>"" + txtVal.substr(0, 2) + ""<%= localize.IsNotExistsInInvoice%>"); //发票号码 不存在於发票字轨。
                                    if (val == "txt") {
                                        obj.value = '';
                                        obj.focus();
                                    }

}