Running serverside code form client side javascript

If you are using asp button, then the javaScript need to be called on the event OnClientClick[^].

Then on that call javaScript function to validate like below...

<asp:Button ID="Button1"
       text="Open Web site"
       onclientclick="return Validate()"
       runat="server" onclick="Button1_Click" />

Now the javaScript function will validate and return appropriate boolean value like below...

function Validate(){
    if validated
        return true;
    else
        return false;
}

So, when it is not validated it will return false and the service side event Button1_Click will not fire and if it returns true, then the server side event will get fired.