message-service.ts

  // maintains messages across channels
  private messageDB = {1: [], 2: [], 3: []};

  // Tracks messages for the currently selected channel
  private messageSubject$ = new BehaviorSubject<string[]>([]);
  public readonly messages$ = this.messageSubject$.asObservable();

  private selectedChannelId: number;

  constructor() { 

  }

  // the getter will return the last value emitted in message subject
  private get messages(): string[] {
    return this.messageSubject$.getValue();
  }

  private set messages(val: string[]) {
    this.messageSubject$.next(val);
    this.messageDB[this.selectedChannelId] = val;
  }

  getInitialMessagesForChannel(channelId: number) {
     this.selectedChannelId = channelId;
     this.messages = this.messageDB[channelId];
  }