Variable declared in $scope suddenly losing its definition

Within my directive controller, I've implemented a $watch function as follows:

$scope.$watch(function () {
    return service.abc;
}, function(newVal, oldVal) {
  $scope.abc = {abc: newVal};
});

However, I've encountered issues with the variable becoming undefined when utilized in other controllers. To address this problem, I created another watch:

 $scope.$watch('abc', function () {
     console.log($scope.abc);
 });

Observing this second watch, I noticed that the abc object is initially defined with the value from the service, but then it becomes undefined afterward. Despite taking precautions by using:

$scope.abc = {abc: angular.copy(newVal)}; 

To prevent any modifications to the object, it still ends up being undefined. Even after experimenting with different variable names and utilizing the vm controllerAs syntax, the issue persists. What could be causing this unexpected behavior? It's crucial for me to pass this object into a directive, make changes in the parent controller, and have those modifications reflected in the inner directive controller.

Answer №1

One crucial point to keep in mind is that scope variables, whether in controllers or directives, must be initialized each time the controller or directive is initiated.

If you intend to utilize variables set in services, it is recommended since they are singletons, ensuring that the values persist from one page to another throughout the application's lifespan.

The $watch function is specifically designed for monitoring scope variables within controllers and directives. For instance:

$scope.$watch('abc', function () {
    console.log($scope.abc);
});

This code snippet monitors the scope variable named abc in the respective controller/directive. Without initializing abc in your controller/directive, the watch will not trigger.

Furthermore, it is important to employ this particular syntax for defining watches in controllers/directives:

$scope.$on('$destroy', $scope.$watch('abc', function (newVal, oldVal) {
    if (newVal != oldVal) {
      // perform actions here
    }
}));

This approach ensures that the watch does not execute multiple times when navigating away from and back to the page. The watches may be defined repeatedly but are not destroyed.

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