AngularJS Example

HTML

<div ng-app="app">
  <div ng-controller="SomeController">
      <p ng-class="a">AAAA</p>
      <p ng-class="b">BBBB</p>
      <p ng-class="{ yellow: isImportant() }">CCCC</p>
  </div>
</div>

CSS

</style> <!-- Ugly Hack due to jsFiddle issue: http://goo.gl/BUfGZ --> 
<script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.0.5/angular.min.js"></script>
<style>
.red { background-color: red; }
.yellow { background-color: yellow; }
.green { background-color: green; }

JavaScript

angular.module('app', []).
service('Foo', function() {
    return {
        someCheck: function(x) {
            return x > 10;
        }
    };
}).
controller('SomeController', function($scope, Foo) {
    var x = 11;
    $scope.a = 'red';
    $scope.b = { green: Foo.someCheck(x) };
    $scope.isImportant = function() {
        return Foo.someCheck(x);
    };
});