How do I find out and change the value of <input type = "text"> in asp.net? : <p>I want to find the value of  and want to mask the first 4th of the text field to another character when the length of the text field is 4 or greater. </p>

<p>but I don't know well asp.net skill. help me plz.</p>

<p>here is some html in body tag</p>

<pre class="lang-html prettyprint-override"><code>&lt;body&gt;
    &lt;form id="form1" runat="server"&gt;
        &lt;div&gt;
            &lt;input type="text" runat="server" id="input1" name="input1" value="TestText" /&gt;
            &lt;input type="button" runat="server" class="btnCnvt" name="btnCnvt" id="btnCnvt" value="convert" onclick="GetValue()" /&gt;
        &lt;/div&gt;
    &lt;/form&gt;
&lt;/body&gt;
</code></pre>

<p>here is javascript :(</p>

<pre><code>&lt;script type="text/javascript"&gt;
        function GetValue()
        {
              // Don't Work!!!(not fount 'value' attribute)
              var str = document.getElementById("input1").value;
        }
&lt;/script&gt;
</code></pre>
