title: Value binding on span not working
body: <p>For some reason, I can't return the value of the <code>idUser</code> binding:</p>
  
  <p>HTML:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;span data-bind="value: idUser"&gt;1&lt;/span&gt;
  </code></pre>
  
  <p>At the end of HTML:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;script&gt;
      $(document).ready(function () {
          startJs();
      }); 
  &lt;/script&gt;
  </code></pre>
  
  <p>JS File:</p>
  
  <pre><code>function startJs() {
      var vm = new vmUser();
      ko.applyBindings(vm);
  }
  
  var vmUser = function () {
      self = this;
      self.idUser = ko.observable();
  }
  </code></pre>
  
  <p>When I try to use <code>self.idUser();</code> the value is always undefined.</p>