Book Offline Service for displaying data only

book-offline.service.ts

import { Injectable } from '@angular/core';
import Dexie from 'dexie';
import { Book } from '../books/books.component';
import { OnlineOfflineService } from './online-offline.service';
import { BackendService } from '../services/backend.service';
import { MatSnackBar } from '@angular/material/snack-bar';


@Injectable({
  providedIn: 'root'
})
export class BookOfflineService {
  private rDb: any; // this database is for caching data from the MongoDB

  constructor(
    private onlineOfflineService: OnlineOfflineService,
    private backendService: BackendService,
    private snackBar: MatSnackBar
  ) {
    // Listen to network status events (i.e. online and offline)
    this.registerToEvents(onlineOfflineService);
    this.createDatabases();
  }

  // Observe network status (i.e. online or offline)
  private registerToEvents(onlineOfflineService: OnlineOfflineService) {
    onlineOfflineService.connectionChanged.subscribe(online => {
      if (online) {
        this.snackBar.open("You are back online", 'Close', { duration: 2000 });
      } else {
        this.snackBar.open("You are working offline", 'Close', { duration: 2000 });
      }
    });
  }

  private createDatabases() {
    this.rDb = new Dexie('RBooks');
    this.rDb.version(1).stores({
      books: '_id,title,isbn,author,price,picture'
    });
  }

  public bulkAddToRDb(books: Array<Book>) {
    return this.rDb.books
      .bulkAdd(books)
      .catch(e => {
        alert('Error: ' + (e.stack || e));
      });
  }

  public clearRDb() {
    return this.rDb.books.clear();
  }

  public async fecthAllItemsFromRDb() {
    const books: Book[] = await this.rDb.books.toArray();
    return books;
  }
}