how to get value Table's or Gridvie tr td from Jquery,

Question

<script type="text/javascript">

function mainCall() {

var lb = document.getElementById('gvlblName').innerHTML;

alert(lb);

var name = $(obj).closest('tr').find('td:eq(0)').text();

alert(name);

}

</script>

===============================

<asp:GridView ID="grdRegister" runat="server" AutoGenerateColumns="false" >

<Columns>

<asp:TemplateField HeaderText="UserName">

<ItemTemplate>

<a href="#" id="na" onclick="mainCall();" >

<asp:Label ID="gvlblName" runat="server" text='<%#Eval("UserName")%>'></asp:Label>

</a>

</ItemTemplate>

</asp:TemplateField>

</Columns>

</asp:GridView>

======================================================== 

error popup : The name 'gvlblName' does not exist in the current context

error popup : undefiend