Question

I am using json to run a function in code behind of my app. everything works fine except i cannot figure out how to pass hidden field to it here is my jquery code:

  $(document).ready(function () {
             $("input[id$='btnP']").click(function (e) {
 var hiddenfield= $("#<%=hidden.UniqueID%>"); //This Does not work!!!!!
                 $.ajax({
                     type: "POST",
                     url: "MyDoc.aspx/BtnOpen",
                     data: "{'message': '" + hidden.val() + "'}",
                     contentType: "application/json; charset=utf-8",
                     dataType: "json",

                     success: function (msg) {
                         if (msg.d == 'Sent') {

                         }
                         else {

                         }
                     }

                 });

                 e.preventDefault();
             });

EDIT:::

My hidden field is in gridview as so:

 <asp:TemplateField HeaderText="View">                      
         <ItemTemplate>
    <input type="submit" value="Send" id="btnP"  runat="server" />
    <asp:HiddenField runat="server" ID="hidden"  Value='<%# Eval("ID" )%>'  />
           </ItemTemplate>
             </asp:TemplateField>

Answer 1

there are a couple of options to take hidden field value:

1) you can set up ClientIDMode attribute of your hidden field to Static

<asp:HiddenField runat="server" ID="hidden" ClientIDMode="Static" Value="yourValue"/>

and then you can use the following code:

var hiddenField = document.getElementById('hidden');
....
hiddenField.value

2) use ClientID property:

var hiddenField = $("#<%=hidden.ClientID %>");
...
hiddenField.val();

read about the difference between ClientID and UniqueID here

UPD.

If you need to pass some parameter from GridView to a function, you may not use HiddenField:

Just remove “server” tag from your input and add onclick event.

<asp:TemplateField HeaderText="View">                      
    <ItemTemplate>
       <input type="submit" value="Send" id="btnP" onclick="BtnOpen('<%# Eval("ID" ) %>');" />
    </ItemTemplate>
</asp:TemplateField>

And than add your function:

<script>
    function BtnOpen(id) {
        $.ajax({
            type: "POST",
            url: "MyDoc.aspx/BtnOpen",
            data: "{'message': '" + id + "'}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",

            success: function (msg) {
                if (msg.d == 'Sent') {

                }
                else {

                }
            }
        });
    }
</script>