Code behind, javascript, code behind?

I'd appreciate any help on the matter...
Until now, on a click of the button I've been executing:
1) Javascript function that writes a message into a text box on the screen ("Executing..." (returns true) or "No data for filters." (returns false)).

2) if returns true, code behind executes
<asp:Button ID="btnGo" runat="server" Text="Go" Width="300px" onclick="btnGo_Click" onClientClick="message();" UseSubmitBehavior="True"/>

Now I need to do another check before executing the Javascript function. I have to read some data from the database, if some conditions are true, I need to go ahead and do what the JS function message() does.
How can I do that? To me it seems like I have to execute some codebehind first, then javascript and t ...