JavaScript getElementId Ans alert

 function validateForm2()
     { 
     if(document.getElementById('<%=SpellTextBox1.ClientID%>').value.length>50)
         {
               $find('<%=MPE10.ClientID%>').hide();
               alert("Please edit title it is more than 50 characters.");
               return false;
         }
         else if(document.getElementById('<%=SpellTextBox2.ClientID%>').value.length>500)
         {
             $find('<%=MPE10.ClientID%>').hide();
             alert("Please edit Description it is more than 500 characters.");
             return false;
         }
         else
         {
            $find('<%=MPE10.ClientID%>').show();
            return false;
         }
     }


<asp:Button ID="Btn_EventContinue" runat="server" CausesValidation="true" OnClick="Btn_EventContinue_Click"
                                OnClientClick="return validateForm2()" Text="Continue" ValidationGroup="Event"
                                TabIndex="9" /> 



 <cc1:ModalPopupExtender ID="MPE10" runat="server" BackgroundCssClass="modalBackground"
                                        DropShadow="false" PopupControlID="Panel1" TargetControlID="lblconfirm">
                                    </cc1:ModalPopupExtender>