How to get Textbox Id in javascript

<script type="text/javascript">
       function CheckValidation() {
           var strMachineCode = document.getElementById('<%= txtMachineCode.ClientID %>').value;
           if (strMachineCode == "") {
               alert("Please provide Machine code");
           }

       }
   </script>

Call to server side button
<asp:Button ID="btnSave" runat="server" CssClass="button" Text="Save" 
            Width="80px" onclick="btnSave_Click" OnClientClick="CheckValidation();" />