Master Pages and JavaScript document.getElementById

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <script language="javascript" src="JScript.js"></script>
    <table>
        <tr>
            <td colspan="2">
                <h2>This is the contentpage</h2>
            </td>            
        </tr>
        <tr>
            <td>
                Fill a text here and click on "Show Text"
            </td>
            <td>
                <asp:TextBox ID="txtTest" runat="server"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                <asp:Button ID="btnShowText" Text="Show Text" runat="server" OnClientClick="ShowText()" />
            </td>
        </tr>
    </table>
</asp:Content>