AngularJS: Observe multiple variables with $watch()

If you want to observe a variable this can be done easily by using the $watch function.

$scope.$watch('foo', function(value) {
  // will be triggered when $ will be changed

In order to observe multiple properties all you need to do is concatenate them with a +.

$scope.$watch('foo + bar', function(value) {
  //  do something if $ or $ changes