Understanding the dependency injection pattern

In object-oriented software development, it is good practice to prioritize composition over inheritance, meaning that a class should be composed of other classes (preferably interfaces).

In our previous example, we can see that the service class comprises the DiaryComponent component. Another way to use this service would be as follows:

. . .
export class DiaryComponent {
  private exerciseSetsService: ExerciseSetsService;
  exerciseList: ExerciseSetList;
  constructor() {
    this.exerciseSetsService = new ExerciseSetsService();
    this.exerciseList = this.exerciseSetsService.getInitialList();
  }
. . .
}

Here we modify our code, leaving the creation of the service class object expressly in the component’s constructor method. Running our code again, we can see that the interface remains the same.

This approach, although functional, has some...