Partial postback with Javascript

I used @MatthewJacobs' answer but I found that the call to __doPostBack caused a runtime exception on IE11. I found that the call to __doPostBack can be replaced by the following JavaScript call, which worked on both browsers I tested (IE11 and Chrome 52).

Sys.WebForms.PageRequestManager.getInstance().beginAsyncPostBack(
    [ '<%=UpdatePanel1.ClientID %>' ], '<%=UpdatePanel1.ClientID %>', null
);

See Sys.WebForms.PageRequestManager.beginAsyncPostBack Method for more details.

You can use an AsyncPostBackTrigger with the UpdatePanel to do this. Because you need something that can fire an event, using a button is fairly simple and when hidden works nicely.

If this is your markup:

<asp:UpdatePanel ID="UpdatePanel1" runat="server" OnLoad="UpdatePanel1_Load">
    <ContentTemplate>
        <!-- Contents... -->
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ReloadThePanel" EventName="Click" />
    </Triggers>
</asp:UpdatePanel>
<asp:Button ID="ReloadThePanel" runat="server" style="display:none;" />

When you want the panel to be updated, you just need to call:

__doPostBack('<%=ReloadThePanel.ClientID %>', null);

This will make ASP.NET think that ReloadThePanel was clicked and the JavaScript auto-generated due to the trigger will handle the rest.

EDIT

You can do a pure JavaScript update of the UpdatePanel without any triggers or hidden buttons. You just need to invoke __doPostBack with the client-side ID as the first argument.

__doPostBack('<%=UpdatePanel1.ClientID %>', null);