title: Cannot combine Observable Array to empty Observable array in my angular4 code
body: <p>I have written a component using angular 4 and angularfire2, which I get an error in return saying <strong>"ERROR TypeError: _this.memberEvents$.combineLatest is not a function"</strong>.</p>
  
  <p>Please refer following code and point out the way to correct it.</p>
  
  <pre><code>    import { Observable } from 'rxjs/Observable';
      import { MemberService } from './../../member.service';
      import { ClubService } from './../../club.service';
      import { ActivatedRoute } from '@angular/router';
      import { EventService } from './../../event.service';
      import { Component, OnInit, OnChanges, OnDestroy } from '@angular/core';
      import { select } from 'ng2-redux';
      import 'rxjs/add/operator/take';
      import 'rxjs/add/operator/map';
      import 'rxjs/add/operator/mergeMap';
      import 'rxjs/add/observable/empty' 
      import 'rxjs/add/operator/takeUntil';
      import 'rxjs/add/observable/combineLatest';
      import 'rxjs/add/observable/of';
      import { Subject } from 'rxjs';
  
      @Component({
        selector: 'member-favorite',
        templateUrl: './member-favorite.component.html',
        styleUrls: ['./member-favorite.component.css']
      })
      export class MemberFavoriteComponent implements OnInit, OnDestroy {
        userId;
        favoriteEvents$;
        memberEvents$ = Observable.of([]);
        id;
        private ngClubsUnsubscribe$ = new Subject();
  
        constructor(private route: ActivatedRoute,
          private eventService: EventService, 
          private clubService: ClubService,
          private memberService: MemberService) {
          this.userId = localStorage.getItem('userId');
          this.id = this.route.snapshot.paramMap.get('id');
  
        }
  
        ngOnInit() {
          this.getMemberEvents();
        }
  
        ngOnDestroy() {
          this.ngClubsUnsubscribe$.next();
          this.ngClubsUnsubscribe$.complete();
        }
  
        getMemberEvents() {
          this.clubService.getActiveClubs().takeUntil(this.ngClubsUnsubscribe$)
            .subscribe(clubs =&gt; {
              clubs.map(c =&gt; {
                this.memberService.isMember(c.id, this.userId)
                .take(1)
                .subscribe(res =&gt; {
                  if(res &amp;&amp; res === true) {
                    console.log("user is a member of club ", c.clubname);
  
                    let clubEvents = this.eventService.getForthCommingEventsOfClub(c.id);
                    this.memberEvents$.combineLatest(clubEvents); 
  </code></pre>
  
  <p><strong>How do I correct this line.
     Is there any other way to do this.</strong></p>
  
  <pre><code>                }
                });
              });
            });
        }
  
      }
  </code></pre>