auth.component.ts

export class AuthComponent implements OnInit {
  private form: FormGroup;
  private returnUrl: string;
  constructor(private authService: AuthService, private router: Router, 
    private activated: ActivatedRoute) { }

  ngOnInit() {
    this.activated.queryParams.subscribe(params => {
        this.returnUrl = params["returnUrl"];
    });

    this.form = new FormGroup({
      username: new FormControl(null),
      password: new FormControl(null)
    });
  }

  ngSubmit() {
    // code omitted to explain later
  }
}