Question

I want to assign a value to an asp.net hiddenfield via javascript prior to post back.

But in the code behind the hidden fieldvalue is null. The code I am using is:

<asp:UpdatePanel ID="UpdatePanel1" runat="server" ChildrenAsTriggers="True" Visible="True">
        <Triggers>
            <asp:AsyncPostBackTrigger   ControlID="save1"    EventName="Click">         
            </asp:AsyncPostBackTrigger>
        </Triggers>
        <ContentTemplate>
            <asp:HiddenField ID="HiddenField1" runat="server" />
       </div>

        </ContentTemplate>
   </asp:UpdatePanel>

 <asp:Button ID="save1" runat="server" Text="Send"  OnClientClick="return SaveFase();"  />

       function SaveFase() {     
           var UP = jQuery.get('<% = HiddenField1.ClientID %>');
           UP.Value= "xxxxxxxxxxxxxxxxxxxx"
          return true             
       }

Protected Sub PassBackImage(sender As Object, e As EventArgs) Handles save1.Click

      dim Value = HiddenField1.Value      
End Sub

Answer 1

You probably meant to write:

$('#<% = HiddenField1.ClientID %>').val('xxxxxxxxxxxxxxxxxxxx');

Also, put your javascript function in a sctipt block:

<script type="text/javascript">
       function SaveFase() {     
           return $('#<% = HiddenField1.ClientID %>').val('xxxxxxxxxxxxxxxxxxxx');
       }
<script>