title: Knockout Mapping - fromJS - failing on a simple example
body: <p>I'm trying to figure out what I'm misunderstanding with Knockout's mapping library. I've stripped it down to a simple example, and still can get it to fail (rather, not update with the mapped variables) with the <code>fromJS</code> call. </p>
  
  <p>What am I getting fundamentally wrong in this example?</p>
  
  <pre><code>// Here's my view model
  var ViewModel = function() {
      this.firstName = ko.observable('first');
      this.lastName = ko.observable('last');
  };
  
  var myVM = new ViewModel();
  
  ko.applyBindings(myVM); // Apply to Knockout (works)
  
  myVM.lastName('maiden name'); // Test an update (works)
  
  var newData = {firstName: 'new', lastName: 'person'};
  
  // Try update the ViewModel
  ko.mapping.fromJS(newData, myVM); //(No update, or error)
  
  // Intended result - UI updates to 'new person'
  </code></pre>
  
  <p>And the corresponding view:</p>
  
  <pre><code>&lt;div class='liveExample' &gt;   
      &lt;p&gt;First name: &lt;input data-bind='value: firstName' /&gt;&lt;/p&gt; 
      &lt;p&gt;Last name: &lt;input data-bind='value: lastName' /&gt;&lt;/p&gt; 
  &lt;/div&gt;
  </code></pre>
  
  <p>My <a href="http://jsfiddle.net/QJNTY/4/">JS Fiddle example</a>.</p>