Another way to write JavaScript Validaton for Server Controls in SharePoint

<script language="javascript" type="text/javascript">
    function validateControls()
    {

        if (document.getElementById("<%=txtUserName.ClientID %>").value == "")
        {
            alert('UserName Required');
            document.getElementById("<%=txtUserName.ClientID %>").focus();
            return false;
        }

        if (document.getElementById("<%=txtPassWord.ClientID %>").value == "")
        {
            alert('PassWord Required');
            document.getElementById("<%=txtPassWord.ClientID %>").focus();
            return false;
        }
        return true;
    }
</script>

In Page_Load of UserControl use the button properties in this way.
 btnSave.Attributes.Add("OnClick", "return validateControls();");