How to Access Asp:textbox With Runat=server Attribute in Javascript

For example you have a textbox which has a default value, and you want to clear that value when the user click on the textbox:

<asp:TextBox ID="txtBox" runat="server" onclick="clear()" Text="Enter Keyword" />

<script type="text/javascript">
        function clear() {
            document.getElementById("<%=txtBox.ClientID %>").value = "";
        }
</script>