Property 'indexOf does not exist on type' FirebaseListObservable <any []>

I'm trying to iterate and find the index of items in Firebase firewall and scroll right and left through them:

 export class articleSwiperComponent implements OnInit {

      articles: FirebaseListObservable<any[]>;

      public orientation: Orientation;
      public selectedArticle: article;

      private changeDetectorRef: ChangeDetectorRef;
      constructor(private af: AngularFire, changeDetectorRef: ChangeDetectorRef) {
        this.articles = af.database.list('/articles');

        this.changeDetectorRef = changeDetectorRef;
        this.orientation = "none";

      this.selectedArticle = this.articles[ Math.floor( Math.random() * this.articles.length ) ];


      }

      

        
        
        
      

    

I have methods for scrolling through database articles that will randomly move the article the first time you navigate to that page.

   public showNextArticle() : void {

        this.orientation = "next";
        this.changeDetectorRef.detectChanges();

        var index = this.articles.indexOf( this.selectedArticle );

        this.selectedArticle = this.articles[ index + 1 ]
            ? this.articles[ index + 1 ]
            : this.articles[ 0 ]
        ;

    }
    public showPrevarticle() : void {

        this.orientation = "prev";

        this.changeDetectorRef.detectChanges();

        // Find the currently selected index.
        var index = this.articles.indexOf( this.selectedArticle );
        this.selectedArticle = this.articles[ index - 1 ]
            ? this.articles[ index - 1 ]
            : this.articles[ this.articles.length - 1 ]
        ;

    }
}

      

        
        
        
      

    

However, I am getting errors Property ‘indexOf’ does not exist on type 'FirebaseListObservable<any[]>

Property 'length' does not exist on type 'FirebaseListObservable<any[]>

. What is the equivalent of these properties in Firebase?