function ctrl($scope) {  
  $scope.className = "selected"; 
} 

<div class="{{className}}"></div> 

function ctrl($scope) {  
  $scope.isSelected = true; 
} 

<div ng-class="{true:'selected',false:'unselected'}[isSelected]"></div> 

function ctrl($scope) {  
  $scope.isA = true; 
  $scope.isB = false; 
  $scope.isC = false; 
} 

<div ng-class="{'A':isA,'B':isB,'C':isC}"></div> 

<ion-list> 
  <ion-item ng-repeat="project in projects" ng-click="selectProject(project, $index)" ng-class="{active: activeProject == project}"> 
    {{project.title}} 
  </ion-item> 
</ion-list>