Q: Objects retrieved from observable JSON don't have methods defined in class Service:
getGroups():Observable<Group[]> {

    return this._http.get(this.url).map((res: Response) => res.json());
}

Class:
export class Group {
    id: number;
    name: string;
    vat: string;

    items: Item[];

    addItem = function(item: Item): void{
        this.items.push(item);
    }
}

Component:
groups:Group[];
selectedGroup:Group;

onSelectGroup(currentGroup:Group):void {
    this.selectedGroup = currentGroup;
}

ngOnInit():void {

    this.groupService.getGroups().subscribe(groups => this.groups = groups);

}

Template:
    <div *ngFor="let g of groups" [class.selected]="g === selectedGroup">

                    <button (click)="onSelectGroup(g)">Select group</button>

</div>

When I try to use the method (on a single array element - group of course) addItem(ItemObject) it says "this.selectedGroup.addItem is not a function".
Help please
