title: What angularjs expression syntax is this in ng-class
body: <p>The <a href="https://github.com/angular/angular.js/wiki/Noob-Handbook">AngularJS Noob Handbook</a> has some code which reduces class manipulation to a simple expression and binding :</p>
  
  <pre><code>&lt;a ng-click="flags.open=!flags.open"&gt;...&lt;div ng-class="{active:flags.open}"&gt;
  </code></pre>
  
  <p>However, what is the expression syntax in ng-class? I understand that a vertical bar (|) would pass through a filter and that a filter can be passed parameters after a colon but the above code is doing something different. If the scope variable on the right evaluates to true then the expression on the left is included otherwise it's dropped.</p>
  
  <p>Is this specific to the ng-class directive? Is there some documentation on <a href="http://docs.angularjs.org">http://docs.angularjs.org</a> that explains this?</p>