service.ts

interface User {
  fullName: string;
  age: number;
  id: number;
}

@Component({
  selector: 'some-component',
  template: `<p-dropDown [options]="users"></p-dropDown>`
})
class SomeComponent implements OnInit {
  users: SelectItem[];

  constructor(private userService: UserService){}

  ngOnInit(){
    this.userService.getAllUsers().then((users: User[]) => this.users = users) //service returns a Promise
    //won't work, because types don't match
  }
}