[Javascript]Data ownership,avoid accidently mutation

When implement ing the store partten,we need to be careful about mutation.

class DataStore {
  private lessons: Lesson[] = [];

  private lessonsSubject = new SubjectImplementation();

  lessonsLists$: Observable = {
    subscribe(obs) {
      this.lessonsSubject.subscribe(obs);
      obs.next(lessons);
    },
    unsubscribe(obs) {
      this.lessonsSubject.unsubscribe(obs);
    },
  };

  initializeLessonsList(newList: Lesson[]) {
    this.lessons = _.cloneDeep(newList);
    this.lessonsSubject.next(lessons);
  }

  addLessons(newLessons) {
    this.lessons.push(_.cloneDeep(newLessons)); // make a deep clone
    this.lessonsSubject.next(this.lessons);
  }
}

export const store = new DataStore();

We need to make a deep clone in order to avoid accidently mutation.