What is causing the subscriber to receive the error message "Cannot access 'variable' before initialization" while referencing itself?

Question

What is causing the subscriber to receive the error message "Cannot access 'variable' before initialization" while referencing itself?

Within my Angular 8 project, I have a component that is dependent on a service for data loading. It waits for a notification from the service signaling that it has finished loading in the following manner:

constructor(public contentService: ContractService) {
    let self = this;
    let sub = contentService.loaded.subscribe(function(loaded) {
        if (loaded) {
            self.loading = false;
            if (sub) {
                sub.unsubscribe();
            }
        }
    });
}

This setup functions correctly most of the time, but occasionally an error message surfaces:

ERROR ReferenceError: Cannot access 'sub' before initialization
    at SafeSubscriber._next (abstract-content.component.ts:51)
    at SafeSubscriber.__tryOrUnsub (Subscriber.js:185)
    at SafeSubscriber.next (Subscriber.js:124)
    at Subscriber._next (Subscriber.js:72)
    at Subscriber.next (Subscriber.js:49)
    at BehaviorSubject._subscribe (BehaviorSubject.js:14)
    at BehaviorSubject._trySubscribe (Observable.js:42)
    at BehaviorSubject._trySubscribe (Subject.js:81)
    at BehaviorSubject.subscribe (Observable.js:28)
    at Observable._subscribe (Observable.js:76)

The abstract-content-component mentioned in the error is actually a parent class with multiple implementations, being called by a child class which serves as a shell without much logic to allow for easy template switching.

The error itself seems puzzling (particularly line 51 referring to the if(sub) check), as how can a subscription not exist within its own event handler? This pattern has been successfully implemented elsewhere in my codebase, so why does this specific instance encounter issues?

angular typescript angular8

Answer 1

Answer №1

To prevent this error from occurring, ensure that sub is declared as an attribute of the component.

sub: Subscription = new Subscription();
// ...

constructor(public contentService: ContractService) {
    let self = this;
    this.sub = contentService.loaded.subscribe(function(loaded) {
          // ...
          this.sub.unsubscribe();
          // ...
    });
}

However, it's recommended to avoid making these calls directly in the constructor. Consider placing them inside the ngOnInit method instead.

In order to ensure that the subscriber does not continue receiving values after the first one is returned by the service, unsubscribe once its task is completed (as you were doing), but make sure to declare sub as a component attribute and initialize it with new Subscription();

To prevent any lingering subscriptions, remember to unsubscribe from them in the component's ngOnDestroy method:

ngOnDestroy() {
  // ...
  this.sub.unsubscribe();
  // ...
}

If you want a deeper understanding of how the Observables pattern functions, refer to the Angular Docs on Observables

Answer 2

To prevent this error from occurring, ensure that sub is declared as an attribute of the component.

sub: Subscription = new Subscription();
// ...

constructor(public contentService: ContractService) {
    let self = this;
    this.sub = contentService.loaded.subscribe(function(loaded) {
          // ...
          this.sub.unsubscribe();
          // ...
    });
}

However, it's recommended to avoid making these calls directly in the constructor. Consider placing them inside the ngOnInit method instead.

In order to ensure that the subscriber does not continue receiving values after the first one is returned by the service, unsubscribe once its task is completed (as you were doing), but make sure to declare sub as a component attribute and initialize it with new Subscription();

To prevent any lingering subscriptions, remember to unsubscribe from them in the component's ngOnDestroy method:

ngOnDestroy() {
  // ...
  this.sub.unsubscribe();
  // ...
}

If you want a deeper understanding of how the Observables pattern functions, refer to the Angular Docs on Observables

Answer 3

Answer №2

Prior to establishing the value for sub, you are double-checking its existence.

If you analyze your code methodically and step by step, you'll notice that you are initializing sub with a function, specifically in this instance, a subscription. This is acceptable on its own.

The problem arises from the fact that within the function itself, you are depending on the variable sub being already defined. It's not initialized yet because you are currently in the process of declaring it at that moment.

This results in the error message:

Cannot access 'sub' before initialization

It seems unclear what your intention is, since logically it appears as though you want to subscribe to something, but then immediately unsubscribe? However, performing an unsubscribe operation inside the subscribe block is not recommended.

If preferred, you can modify your code like so:

// Assuming you have a "loaded" variable declared above, based on the context where self is set to this
private unsubscribeStream = new Subject();

this.contentservice.loaded.pipe(
  filter(x => x !== null), // Filter out cases where x is non-existent
  takeUntil(this.unsubscribeStream)).subscribe(loaded => {
    this.loaded = loaded;
  });

// Upon component destruction, trigger the next value of unsubscribe stream to cancel any ongoing operations subscribed with takeUntil
ngOnDestroy() {
   this.unsubscribeStream.next();
}

Answer 4

Prior to establishing the value for sub, you are double-checking its existence.

If you analyze your code methodically and step by step, you'll notice that you are initializing sub with a function, specifically in this instance, a subscription. This is acceptable on its own.

The problem arises from the fact that within the function itself, you are depending on the variable sub being already defined. It's not initialized yet because you are currently in the process of declaring it at that moment.

This results in the error message:

Cannot access 'sub' before initialization

It seems unclear what your intention is, since logically it appears as though you want to subscribe to something, but then immediately unsubscribe? However, performing an unsubscribe operation inside the subscribe block is not recommended.

If preferred, you can modify your code like so:

// Assuming you have a "loaded" variable declared above, based on the context where self is set to this
private unsubscribeStream = new Subject();

this.contentservice.loaded.pipe(
  filter(x => x !== null), // Filter out cases where x is non-existent
  takeUntil(this.unsubscribeStream)).subscribe(loaded => {
    this.loaded = loaded;
  });

// Upon component destruction, trigger the next value of unsubscribe stream to cancel any ongoing operations subscribed with takeUntil
ngOnDestroy() {
   this.unsubscribeStream.next();
}

Answer 5

Answer №3

Recently, I encountered a similar issue where I realized that my code was outdated and created when my knowledge of rxjs was limited. Looking back, I now consider it an anti-pattern to write code in that manner.

The ideal scenario is to receive only one emitted value and then complete the operation. Here are some options:

My approach:

contentService.loaded .pipe( first(), ) .subscribe(...);
Alternatively, another effective method:

contentService.loaded .pipe( take(1), ) .subscribe(...);
Consider using Promises; they are still relevant. If you always require exactly ONE value, there may be no need for an Observable. (Just like how you wouldn't convert a number to a string if you only need a string because numbers have their own unique properties)

Stay vigilant!

Answer 6

Recently, I encountered a similar issue where I realized that my code was outdated and created when my knowledge of rxjs was limited. Looking back, I now consider it an anti-pattern to write code in that manner.

The ideal scenario is to receive only one emitted value and then complete the operation. Here are some options:

My approach:

contentService.loaded .pipe( first(), ) .subscribe(...);
Alternatively, another effective method:

contentService.loaded .pipe( take(1), ) .subscribe(...);
Consider using Promises; they are still relevant. If you always require exactly ONE value, there may be no need for an Observable. (Just like how you wouldn't convert a number to a string if you only need a string because numbers have their own unique properties)

Stay vigilant!

What is causing the subscriber to receive the error message "Cannot access 'variable' before initialization" while referencing itself?

Answer №1

Answer №2

Answer №3

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