I am trying to get access and find text-box in GridView using javascript but i am getting an error: 'The name txt_UID' does not exist in the current context'. Everything worked fine when my text-box was outside of the GridView. Here is my text-box in the gridview and my gridview is called GridView1: 

 <asp:TemplateField ItemStyle-Width="150px" HeaderText="User Assigned">
    <ItemTemplate>  
     <asp:TextBox ID="txt_UID" runat="server" Text='<%# Eval("UID")%>'
        CssClass="AutoCompleteTextBox" Width="130px" BackColor="LightGoldenrodYellow"></asp:TextBox>
       </ItemTemplate>
         <ItemStyle Width="150px" />
         </asp:TemplateField>

Here is my JavaScript: 

  <script type ="text/javascript">
        function setAutoComplete() {
            var textBoxes = document.getElementsByClassName("AutoCompleteTextBox");
            for (var i = 0; i < textBoxes.length; i++) {
                addAutoComplete(textBoxes[i].id);
            }
        }
</script> 
<script type="text/javascript">
    function addAutoComplete(textBoxId) {
        $("#" + textBoxId).autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: '<%=ResolveUrl("~/Service.asmx/GetUserNames") %>',
                    data: "{ 'prefix': '" + request.term + "'}",
                    dataType: "json",
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    success: function (data) {
                        response($.map(data.d, function (item) {
                            return {
                                label: item.split('-')[0],
                                val: item.split('-')[1]
                            }
                        }))
                    },
                    error: function (response) {
                        alert(response.responseText);
                    },
                    failure: function (response) {
                        alert(response.responseText);
                    }
                });
            },
            select: function (e, i) {
                $("#<%=hfUserId.ClientID %>").val(i.item.val);
            },
            minLength: 1
        });
    }; 
<script type ="text/javascript">
     $(document).ready(function () { setAutoComplete(); });
    </script>