Validation of Controls Using JavaScript

<script type="text/javascript">
    function validation() {
        try {
       
        //for textbox

            if (document.getElementById("<%= textboxID.ClientID %>").value == "") {
                alert('Please Enter Invoice Number');
                return false;
            }
            //for dropdown box

            if (document.getElementById("<%= ddlID.ClientID %>").selectedIndex == 0) { alert('Please Select Nature Of Service'); return false; }
           
        //for upload file control

            var File = document.getElementById("<%= fuID.ClientID %>");
            if (File.value == "") { alert('Please Select File For Upload'); return false; }

            var flag = false;
            var inputElements = document.getElementsByTagName('input');
            for (var i = 0; i < inputElements.length; i++) {
                var myElement = inputElements[i];
                if (myElement.type === "checkbox") {
                    if (myElement.checked) {
                        flag = true;
                        return true;
                    }
                }
            }
            if (flag == false) {
                alert('Please select option'); return false;
            }
           
        }
        catch (ex) { alert(ex.get_Message()); }
    }

</script>


In Design mode add this code:

<asp:Button ID="btnSubmit" runat="server" CssClass="btn" Text="Submit" OnClick="btnSubmit_Click"
                            OnClientClick="return validation();" />