AngularJS Example:

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<div ng-app>
  <div ng-controller="testCtrl">
        <div ng-class="getCSSClass()">Testing ng-class using function</div>       
    </div>
</div>

CSS

.testclass { Background: lightBlue}

JavaScript

function testCtrl($scope) {
    $scope.getCSSClass = function() {
     return "testclass ";
  }     
}