Here is the code i am using now,and its working fine...
function filterSearch()
{
var combo = $find('<%=RadComboBox1.ClientID %>');
var textbox = document.getElementById("txtSearch");
if (combo.get_selectedIndex() == null)
{
alert("You must select a category before processing search!!");
return false;
}
}
and for button same as before--
<asp:Button ID="btnSearch" runat="server" Style="float: left; color: black; font-weight: bold" Text="Search" OnClick="btnSearch_Click" OnClientClick="javascript: return filterSearch();" />