XMLHttpRequest and render :js not working

Hi,

So I uploaded a file to my server using an XMLHttpRequest and I wanted
to notify the client of the url of the uploaded file.

So in my app on the server side, I use for testing purposes:

render :js => "alert('File URL: blabla');"

but even though the client receives the successful response (traced in
Firebug), the alert doesn't show up. What could prevent that?

Thanks in advance for your insights,