为什么我无法访问ngOnInit上设置的对象

Question

Ran into a few problems lately and wonder why I can't access an object, that was set in the ngOnInit function for later purposes, like in another function?

I want to access and use this.appointmentDetailId in my cancelAppointment() function but it is undefined . Hope someone can help. Thanks.

Here's my code:

export class AppointmentDetailComponent implements OnInit {
  id: any;
  appointmentDetailId: any;
  appointmentDetail$: Observable<AppointmentDetails>;
  appointmentDetail: AppointmentDetails;
  pageTitle = 'Some Default Title Maybe';

  constructor(
    private route: ActivatedRoute,
    private title: Title,
    private apiService: APIService
  ) {
    this.appointmentDetailId = this.id;
    console.log(this.appointmentDetailId);
  }

  ngOnInit() {
    this.route.paramMap
      .pipe(
        tap((params: ParamMap) => {
          this.id = params.get('id');
          // Or this.id = +params.get('id'); to coerce to type number maybe
          this.pageTitle = 'Termin Details: ' + this.id;
          this.title.setTitle(this.pageTitle);
        }),
        switchMap(() => this.apiService.getAppointmentDetailsById(this.id))
      )
      .subscribe((data: AppointmentDetails) => {
        this.appointmentDetail = data;
        console.log(this.appointmentDetail);
      });
  }

  cancelAppointment() {
    console.log(this.appointmentDetailId);
    this.apiService.cancelUserAppointment(this.appointmentDetailId);
  }
}

Answer 1

In the constructor you set this.appointmentDetailId = this.id which sets the value to the initial value of this.id (probably undefined );

Later you set this.id = params.get('id') , but this will not change this.appointmentDetailId , because they're two different objects.

---

If you want this.appointmentDetailId to always match this.id you should make it a simple wrapper, rather than its own object.

export class AppointmentDetailComponent implements OnInit {
  id: any;
  get appointmentDetailId() {
    return this.id;
  }
  set appointmentDetailId(value: any) {
    this.id = value;
  }

  // Other code
}

Using a custom get and set method, you can still access this.appointmentDetailId as if it were it's own field - but it will actually be the same as this.id . Now, any changes to either field will always be in sync.

// Always true
this.appointmentDetailId === this.id

this.appointmentDetailId = 123;
this.appointmentDetailId === 123; // True
this.id === 123; // True;

this.id = "Test";
this.appointmentDetailId === "Test"; // True
this.id === "Test"; // True

---

Alternatively, if you can simply omit the set method and then you'll just be able to access the same value, but you cannot change it.

// Always true
this.appointmentDetailId === this.id

this.appointmentDetailId = 123; // Error

this.id = "Test";
this.appointmentDetailId === "Test"; // True
this.id === "Test"; // True