Why OnClientClick is not calling javascript alert in sharepoint application page ?

Hello,

In aspx page I have set following code for button :

<asp:ButtonID="btnDelete"runat="server" OnClientClick="if (!ValidateSearch()) { return false;};"

                              OnClick="btnDelete_Click"     Text="Delete"></asp:Button>

In script I did as follow :

<scripttype="text/javascript">

        function ValidateSearch() {

             alert("Its working now :)");

            var flag = true;

            // JavaScript code for validation

            return flag;

         }

Same is working and give alert if I click on delete button in asp.net web project , while its not working in SharePoint application page hosted in layout folder through VS 2010 in my project !!!!

Would you please let me know what can be the issue ?

Dipti Chhatrapati