Clear ASP.Net TextBox on Button click using JavaScript

I want to clear an asp textbox when click to a button (html).

so basically the JS function is:

<script type="text/javascript">
    function clearTextBox() {
        document.getElementByID('<%=txtFName.ClientID%>').value = "";
    }
</script>

an asp.net TextBox & a HTML button

<asp:TextBox ID="txtFName" runat="server" class="form-control" placeholder="First name" AutoCompleteType="Disabled" MaxLength="30" />

<span class="input-group-btn">
<button class="btn default" id="clearButton" type="button" onclick="clearTextBox()"><i class="fa fa-times"></i></button>
</span>

All buttons and other controls are in an asp update-panel (don't know if it makes different)

Nothing happend when I clicked to the HTML clearButton, error on the web console:

Uncaught TypeError: undefined is not a function
clearTextBox 
onclick

Any solution to make it work (using JQuery is fine), i dont know much about JS.