03_sameroomDispatcher.ts

    const onDispatch = (action: Event) => {
      if (action instanceof AddGreetingEvent) {
        const {payload} = action;
        this._state.newGreeting = '';
        this._state.greetings = this._state.greetings.concat(payload);
        this.emitChange();
      } else if (action instanceof RemoveGreeting) {
        const {payload} = action;
        this._state.greetings = this._state.greetings.filter(g => g !== payload);
        this.emitChange();
      } else if (action instanceof NewGreetingChanged) {
        const {payload} = action;
        this._state.newGreeting = payload;
        this.emitChange();
      }
    }