Including external javascript on async postbacks

Update: it has come to my attention that these kind of errors appear only when debug="true" in the web config. In production sites it might work regardless.

You want to programatically load a js in a user control. The user control becomes visible in the page only during async postbacks and it is hidden at page load. So you want to use some method of loading the external javascript file during that asynchronous UpdatePanel magic.

Use ScriptManager.RegisterClientScriptInclude. There is a catch, though. For each script that you want to include asynchronously, you must end it with

if (typeof (Sys) !== 'undefined') Sys.Application.notifyScriptLoaded();

else the Ajax.Net script loader will throw an error like: Microsoft JScript runtime error: Sys.ScriptLoadFailedException:
The script '...' failed to load. Check for:
Inaccessible path. Script errors. (IE) Enable 'Display a notification about every script error' under advanced settings.
Missing call to Sys.Application.notifyScriptLoaded(). .

For the aspx version, use the ScriptManagerProxy control:

    <asp:ScriptManagerProxy runat="server">
        <Scripts>
            <asp:ScriptReference Path="~/js/myScript.js" NotifyScriptLoaded="true" />
        </Scripts>
    </asp:ScriptManagerProxy>