Extracting a value from a subscription in Angular 4

Question

Extracting a value from a subscription in Angular 4

I'm a beginner with observables in Angular and I encountered an issue where I need to return a value from inside a subscribe method. This is my method (

getFirebaseData(idForm:string):observable <any[]>

):

getTotalQuestions(idForm:string){
let totalQuestions:number;
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => 
  {
    items.map(item => {
      totalQuestions=item.Total;
      console.log(totalQuestions);
    });
  }
);
console.log(totalQuestions);
return totalQuestions;
}

The first console.log(totalQuestions) shows 4, but the second console.log(totalQuestions) shows undefined. I am aware that subscribe is asynchronous and that's why the second console.log(totalQuestions) returns undefined. I can't figure out how to return the variable after the subscription has completed. If I change the subscribe to map:

getTotalQuestions(idForm:string){
let totalQuestions:number;
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => 
  {
    items.map(item => {
      totalQuestions=item.Total;
      console.log(totalQuestions);
    });
  }
);
console.log(totalQuestions);
return totalQuestions;
}

The first console.log(totalQuestions) doesn't show anything, while the second console.log(totalQuestions) displays undefined. This is confusing me.

I hope someone can help clarify this concept for me. Thank you!

angular typescript rxjs observable

Answer 1

Answer №1

If you want to retrieve the totalQuestions directly, you cannot do it in that manner. You must utilize a subject to accomplish this.

getTotalQuestions(idForm:string): Observable<string> {
let totalQuestions:number;
var subject = new Subject<string>();
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => {
    items.map(item => {

      totalQuestions=item.Total;
      console.log(totalQuestions);
      subject.next(totalQuestions);
    });
  }
);
  return subject.asObservable();
}

Example:

getTotalQuestion(idForm).subscribe((r)=>console.log(r))

Answer 2

If you want to retrieve the totalQuestions directly, you cannot do it in that manner. You must utilize a subject to accomplish this.

getTotalQuestions(idForm:string): Observable<string> {
let totalQuestions:number;
var subject = new Subject<string>();
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => {
    items.map(item => {

      totalQuestions=item.Total;
      console.log(totalQuestions);
      subject.next(totalQuestions);
    });
  }
);
  return subject.asObservable();
}

Example:

getTotalQuestion(idForm).subscribe((r)=>console.log(r))

Answer 3

Answer №2

To tackle this issue, I would establish a property called totalQuestions$ as an observable.

In the TypeScript file:

totalQuestions$ = this.getFirebaseData(idForm + "/Metadatos").pipe(
  map(items => items.reduce((prev, {Total}) => prev + Total), 0)))
)

Now, in your template, you can utilize the async pipe:

<span>{{ totalQuestions$ | async }}</span>

If you wish to access the variable in your TypeScript file, you can employ combination operators like forkJoin or combineLatest:

newVariable$ = combineLatest([totalQuestions$, ...]).pipe(
  map(([totalQuestions, ... ]) => {
    // Perform calculations here and return a result
  })
)

Answer 4

To tackle this issue, I would establish a property called totalQuestions$ as an observable.

In the TypeScript file:

totalQuestions$ = this.getFirebaseData(idForm + "/Metadatos").pipe(
  map(items => items.reduce((prev, {Total}) => prev + Total), 0)))
)

Now, in your template, you can utilize the async pipe:

<span>{{ totalQuestions$ | async }}</span>

If you wish to access the variable in your TypeScript file, you can employ combination operators like forkJoin or combineLatest:

newVariable$ = combineLatest([totalQuestions$, ...]).pipe(
  map(([totalQuestions, ... ]) => {
    // Perform calculations here and return a result
  })
)

Answer 5

Answer №3

Executing Observable occurs upon subscription, and the result returned is always a subscription object similar to setInterval. In this case: due to the asynchronous nature of the call, the second console.log will not wait for the subscription to complete before running.

getTotalQuestions(idForm: string) {
  let totalQuestions: number;
  return this.getFirebaseData(idForm + "/Metadatos").pipe(
     map(items =>
      items.map(item => {
        totalQuestions = item.Total;
        console.log(totalQuestions);
      });
    ));
}

getTotalQuestions('231').subscribe(console.log)

Answer 6

Executing Observable occurs upon subscription, and the result returned is always a subscription object similar to setInterval. In this case: due to the asynchronous nature of the call, the second console.log will not wait for the subscription to complete before running.

getTotalQuestions(idForm: string) {
  let totalQuestions: number;
  return this.getFirebaseData(idForm + "/Metadatos").pipe(
     map(items =>
      items.map(item => {
        totalQuestions = item.Total;
        console.log(totalQuestions);
      });
    ));
}

getTotalQuestions('231').subscribe(console.log)

Answer 7

Answer №4

Check out my interactive demo. You can manipulate arrays and reduce objects without activating the subscription. Here's how I approach it. The service logic

getTotalQuestions(idForm: string): Observable<any> {
    return this.getFirebaseData(`${idForm}/Metadatos`)
        .pipe(map(items => items
            .map(item => item.Total)
            .reduce((a, b) => a+b), 0));
}

The component integration

this.service.getTotalQuestions('id')
    .subscribe(totalQuestions => console.log(totalQuestions));

Answer 8

Check out my interactive demo. You can manipulate arrays and reduce objects without activating the subscription. Here's how I approach it. The service logic

getTotalQuestions(idForm: string): Observable<any> {
    return this.getFirebaseData(`${idForm}/Metadatos`)
        .pipe(map(items => items
            .map(item => item.Total)
            .reduce((a, b) => a+b), 0));
}

The component integration

this.service.getTotalQuestions('id')
    .subscribe(totalQuestions => console.log(totalQuestions));

Extracting a value from a subscription in Angular 4

Answer №1

Answer №2

Answer №3

Answer №4

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