JavaScript to validate controls in ASP.NET

 function fnValidate()
{
            var bool = true;
-----------------for textbox ----------------------
            if ($('#txtbox').val().trim() = = '') {
                $("#divError").css("display", "block");
                $("#divError").text('Please Enter value');
                bool = false;
            }


----------------for DDL---------------------------
if ($("#<%= ddl1.ClientID %>").val() == '') {
                $("#error1").css("display", "block");
                bool = false;
            }
else{
  $("#error1").css("display", "none");
}
           if (bool == true) {
                $("#<%= btnSubmit.ClientID %>").click();---------- this is aspcontrol
            }
}
           <asp:TextBox runat="server" ID="txtbox" ></asp:TextBox>
             <div id='divError' style="display: none; color: red"></div>                             

<input type="button" value="Save" runat="server"  id="btnSub" onclick="javascript:fnValidate();" />
 <asp:Button ID="btnSubmit" runat="server" Style="display: none" OnClick="btnSubmit_Click" />