title: Form Validation in external JavaScript File using document.getElementById
body: <p>My document.getElementById function is returning null on a value I know exists. Specifically I try:</p>
  
  <pre><code>//in an external .js file
  var UserName = document.getElementById('&lt;%=_username.ClientID%&gt;');
  </code></pre>
  
  <p>And test it using some conditions, which never happen due to the element id value being null. I define _username in a user control file here:</p>
  
  <pre><code>...
  &lt;fieldset class="login"&gt; 
      &lt;legend class="login"&gt;Login&lt;/legend&gt;
      &lt;label for="&lt;%=_username.ClientID%&gt;"&gt;
          User Name:&lt;/label&gt;
      &lt;asp:TextBox ID="_username" runat="server" CssClass="login" /&gt;
      &lt;label for="&lt;%=_password.ClientID%&gt;"&gt;
          Password:&lt;/label&gt;
      &lt;asp:TextBox ID="_password" runat="server" CssClass="login" TextMode="Password" /&gt;
      &lt;br /&gt;
      &lt;span class="response"&gt;
          &lt;asp:Literal ID="_response" runat="server" /&gt;&lt;/span&gt;        
      &lt;asp:Button ID="_login" runat="server" Text="Login"/&gt;
  &lt;/fieldset&gt;
  </code></pre>
  
  <p>I verified my page is listening to the javascript file by testing a simpler function that doesn't require use of getElementById().</p>