Unraveling the Mystery of the Undefined Parameter in Angular Observables

Question

Unraveling the Mystery of the Undefined Parameter in Angular Observables

I am facing an issue with the Observable parameter in my code. I have a consultService that contains the functions consult() and response as a parameter. The function sendConsultRequest() is called before using the response parameter. Although the sendConsultRequest function works perfectly fine and shows results on the page, I encounter an error when trying to export the content using the response parameter:

ERROR TypeError: Cannot read property 'dataList' of undefined

In my opinion, the response parameter should be initialized before the export action since the consultPage is working correctly. Could you please take a look at the service code below and help me identify why it's not functioning properly?

@Injectable()
export class ConsultService {
  private url = 'api/consultApplication';
  response: Application;

sendConsultRequest(id: string) {
  console.log('sendConsultRequest');
  this.http.get<Application[]>(this.url)
    .subscribe(response => {
      response.map(res => {
        this.response = res;
        this.updateApplication(res);
      }),
        pipe(catchError(this.handleError('sendConsultRequest', [])));
    });
}

Below is the component for exporting data:

ngOnInit() {
  console.log('ExportComponent init');
  console.log(this.consultService.response);
  this.application = this.consultService.response;

  this.appID= this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field)
      ? this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field).value : null;
}

Any assistance will be greatly appreciated.

angular typescript observable

Answer 1

Answer №1

When you encounter the error message

Cannot read property 'foo' of undefined

, it indicates that the object you are trying to access data from has not been assigned a value.

In JavaScript, if a variable is not explicitly set, it defaults to undefined.

let foo; // foo will be undefined

Similarly, if a function parameter is expected but not provided a value, it will also be undefined.

function bar(arg) {
   console.log(foo);
}

bar(); // prints 'undefined' to the console

Therefore, when facing such errors, it suggests that the variable was never given a value in your code. You should review where you believe the assignment occurs and ensure it functions as intended.

For example, the following line:

this.appID = this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field)
  ? this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field).value : null;

Will fail if this.consultService.response is undefined. The logic can only execute after assigning a value to this.consultService.response.

The asynchronous HTTP call results in this.consultService.response being undefined. To obtain the value from the API call, return the observable from the service and subscribe within the component.

Service:

sendConsultRequest(id: string): Observable<any> {
  return this.http.get<Application[]>(this.url).pipe(
    tap(response => {
      return response.map(res => {
        this.response = res;
        this.updateApplication(res);
      })
    ),
    catchError(this.handleError('sendConsultRequest', []))
  );
}

Component:

ngOnInit() {
    this.consuleService
        .sendConsuleRequestId(id)
        .subscribe(response => {
            this.appID = response.dataList.find(pair => 'PROCEDURE' === pair.field) ? response.dataList.find(pair => 'PROCEDURE' === pair.field).value : null;
        });
}

UPDATE (demonstrating ReplaySubject usage) - untested:

Service:

consultStream: ReplaySubject = new ReplaySubject();
consult$: Observable<Consult> = this.consultStream.asObservable();

constructor() {
     this.sendConsultRequest(1);
}

sendConsultRequest(id: string): Observable<any> {
  return this.http.get<Application[]>(this.url)
      .subscribe((consult: Consult) => this.consultStream.next(consult));
}

Component:

ngOnInit() {
    this.consult$
        .subscribe((consult: Consult) => {
            // ...
        });
}

Answer 2

When you encounter the error message

Cannot read property 'foo' of undefined

, it indicates that the object you are trying to access data from has not been assigned a value.

In JavaScript, if a variable is not explicitly set, it defaults to undefined.

let foo; // foo will be undefined

Similarly, if a function parameter is expected but not provided a value, it will also be undefined.

function bar(arg) {
   console.log(foo);
}

bar(); // prints 'undefined' to the console

Therefore, when facing such errors, it suggests that the variable was never given a value in your code. You should review where you believe the assignment occurs and ensure it functions as intended.

For example, the following line:

this.appID = this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field)
  ? this.consultService.response.dataList.find(pair => 'PROCEDURE' === pair.field).value : null;

Will fail if this.consultService.response is undefined. The logic can only execute after assigning a value to this.consultService.response.

The asynchronous HTTP call results in this.consultService.response being undefined. To obtain the value from the API call, return the observable from the service and subscribe within the component.

Service:

sendConsultRequest(id: string): Observable<any> {
  return this.http.get<Application[]>(this.url).pipe(
    tap(response => {
      return response.map(res => {
        this.response = res;
        this.updateApplication(res);
      })
    ),
    catchError(this.handleError('sendConsultRequest', []))
  );
}

Component:

ngOnInit() {
    this.consuleService
        .sendConsuleRequestId(id)
        .subscribe(response => {
            this.appID = response.dataList.find(pair => 'PROCEDURE' === pair.field) ? response.dataList.find(pair => 'PROCEDURE' === pair.field).value : null;
        });
}

UPDATE (demonstrating ReplaySubject usage) - untested:

Service:

consultStream: ReplaySubject = new ReplaySubject();
consult$: Observable<Consult> = this.consultStream.asObservable();

constructor() {
     this.sendConsultRequest(1);
}

sendConsultRequest(id: string): Observable<any> {
  return this.http.get<Application[]>(this.url)
      .subscribe((consult: Consult) => this.consultStream.next(consult));
}

Component:

ngOnInit() {
    this.consult$
        .subscribe((consult: Consult) => {
            // ...
        });
}

Answer 3

Answer №2

That's not the correct process.

Let me explain how it actually works:

@Injectable()
export class ConsultService {
  private apiURL = 'api/consultApplication';
  response: Application;

sendConsultRequest() {
  console.log('Sending consult request');
  return this.http.get<Application[]>(this.apiURL);
}

In your component:

ngOnInit() {
  console.log('Initializing ExportComponent');
  console.log(this.consultService.response);

  this.consultService.sendConsultRequest().subscribe(
    response => {
      response.map(res => {
        this.consultService.updateApplication(res);
        this.appID = response.dataList.find(pair => 'PROCEDURE' === pair.field); 
      });
    }, error => this.consultService.handleError('sendConsultRequest', []);
  );
}

The issue with your current approach is that you are not triggering the service call correctly (or maybe you didn't include that part of the code). Since the service call is asynchronous, the component should wait for it to complete rather than the service itself.

As it stands, the component is not waiting for the service to finish the call. It simply assumes a value without confirmation, which may result in null or undefined data being retrieved.

Answer 4

That's not the correct process.

Let me explain how it actually works:

@Injectable()
export class ConsultService {
  private apiURL = 'api/consultApplication';
  response: Application;

sendConsultRequest() {
  console.log('Sending consult request');
  return this.http.get<Application[]>(this.apiURL);
}

In your component:

ngOnInit() {
  console.log('Initializing ExportComponent');
  console.log(this.consultService.response);

  this.consultService.sendConsultRequest().subscribe(
    response => {
      response.map(res => {
        this.consultService.updateApplication(res);
        this.appID = response.dataList.find(pair => 'PROCEDURE' === pair.field); 
      });
    }, error => this.consultService.handleError('sendConsultRequest', []);
  );
}

The issue with your current approach is that you are not triggering the service call correctly (or maybe you didn't include that part of the code). Since the service call is asynchronous, the component should wait for it to complete rather than the service itself.

As it stands, the component is not waiting for the service to finish the call. It simply assumes a value without confirmation, which may result in null or undefined data being retrieved.

Unraveling the Mystery of the Undefined Parameter in Angular Observables

Answer №1

Answer №2

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