popup message after click server control

[jasonli]

I want to popup message after a server control is changed. I put a htmlinputhidden control on webpage and set "runat=server". When value of it changes, how to run javascript to popup message?

thank you in advance.

[Rohit Kukreti]

Either use RegisterStartupScript or RegisterClientScriptBlock.
Or else use Response.Write("<script>alert('hi');</script>");

[jasonli]

Thanks.

I used the easiest way: Response.Write("<script>alert('hi');</script>"). After the message, the web page refreshed. Font size of content changed. Why? any solution?