how to use onclientclick for both validation and alert message

<asp:Button ID="btn_sub" runat="server" Text="SUBMIT" CssClass="btn" OnClick="btn_sub_Click" OnClientClick="javascript:return valid();" />

<script type="text/javascript">
        function valid() {
            var t1 = document.getElementById('text_box1').value;
            var t2 = document.getElementById('text_box2').value;
            if (t1 == "") {
                alert("Enter Text 1");
                return false;
            }
            else if (t2 == "") {
                alert("Enter Text 2");
                return false;
            }
            else {
                return true;
            }
        }
    </script>