Subscribe to variable change in Angular 4 Service

If you want to subscribe to variable change in Angular 4 service or Angular 5 service, you can use something called the BehaviorSubject from the rxjs library. A behavior subject is like an observable, except that it must be created with an initial value as it is must always return a value even if the subject hasn’t been updated. A behavior subject will always emit it’s first value as soon as it’s described.