"render :js => '...' " cannot working

Can you provide some failing test case, patch or at least a failing application.
Thank you

thanks
this problem is my error.
in controller i write:
respond_to do |format|

format.html { render :js => "...." }

end in client js i send a ajax request with "dataType: 'script'", so the response header include "Content-Type text/html; charset=utf-8", i wanna get script content type, so ...

Now, I change "format.html" to "format.js", the problem is ok.