RxJS: the art of triggering and handling errors

Question

RxJS: the art of triggering and handling errors

This is more of a syntax question rather than a bug I'm facing.

The process is straightforward:

Send an HTTP request that returns a boolean value
If the boolean is true, proceed
If the boolean is false, log a warning and stop the flow.

To handle this, my current code looks like this:

Basic Setup

private _getBoolean() { return this.http.get(...); }
private _getData() { return this.http.get(...); }

Current Implementation

public getData() {
  return this._getBoolean().pipe(
    filter(bool => {
      if(!bool) {
        console.warn('Incorrect server response, stream stopped');
        return false;
      }
      return true;
    }),
    switchMap(bool => this._getData())
  );
}

However, I feel like there might be a more natural and optimized way to do this.

I was wondering if there is a simpler syntax, something like this:

public getData() {
  return this._getBoolean().pipe(
    throwError(bool => bool ? new Error('Incorrect server response, stream stopped') : null),
    catchError(err => console.warn(err)),
    switchMap(bool => this._getData())
  );
}

Is there an approach similar to this, or am I using the correct syntax?

angular typescript rxjs rxjs-pipeable-operators

Answer 1

Answer №1

When dealing with an observable that emits values from 1 to 4, it's important to consider how errors are handled. For example, if an error occurs when the value is 3, you have the option to catch that error using the catchError operator or within the subscribe method. The decision of where to handle the error depends on the specific use case and whether it should be allowed to propagate up to the subscriber or dealt with earlier in the pipeline.

of(1, 2, 3, 4).pipe(
  // Throw error when value is 3
  tap(value => { if(value === 3) throw new Error('Oops!') }),
  catchError(err => {
    console.log('Catch error in operator', err);

    // You can choose to rethrow the same error or a new one
    // return throwError(err);

    // Alternatively, handle the error and return a new observable
    return of(3)
  })
).subscribe(
  value => console.log(value),
  // If `catchError` returns a new observable, the error handler 
  // function here will not be triggered
  err => console.log('Catch error within subscribe', err),
  () => console.log('Done!')
)

It's worth noting that in this scenario, even though the error is managed, the observable still completes and the value 4 is never emitted. To keep the observable running when an error occurs, refer to this StackOverflow solution.

Answer 2

When dealing with an observable that emits values from 1 to 4, it's important to consider how errors are handled. For example, if an error occurs when the value is 3, you have the option to catch that error using the catchError operator or within the subscribe method. The decision of where to handle the error depends on the specific use case and whether it should be allowed to propagate up to the subscriber or dealt with earlier in the pipeline.

of(1, 2, 3, 4).pipe(
  // Throw error when value is 3
  tap(value => { if(value === 3) throw new Error('Oops!') }),
  catchError(err => {
    console.log('Catch error in operator', err);

    // You can choose to rethrow the same error or a new one
    // return throwError(err);

    // Alternatively, handle the error and return a new observable
    return of(3)
  })
).subscribe(
  value => console.log(value),
  // If `catchError` returns a new observable, the error handler 
  // function here will not be triggered
  err => console.log('Catch error within subscribe', err),
  () => console.log('Done!')
)

It's worth noting that in this scenario, even though the error is managed, the observable still completes and the value 4 is never emitted. To keep the observable running when an error occurs, refer to this StackOverflow solution.

Answer 3

Answer №2

rather than:

public fetchData() {
  return this._getStatusCode().pipe(
    throwError(status => status ? new Error('Incorrect server response, stream halted') : null),
    catchError(error => console.warn(error)),
    switchMap(status => this._fetchData())
  );
}

how about trying this approach instead:

public fetchData() {
  return this._getStatusCode().pipe(
    tap(response => !response && throwError('Incorrect server response, stream halted')),
    switchMap(status => this._fetchData()),
    catchError(error => console.warn(error))
  );
}

Answer 4

rather than:

public fetchData() {
  return this._getStatusCode().pipe(
    throwError(status => status ? new Error('Incorrect server response, stream halted') : null),
    catchError(error => console.warn(error)),
    switchMap(status => this._fetchData())
  );
}

how about trying this approach instead:

public fetchData() {
  return this._getStatusCode().pipe(
    tap(response => !response && throwError('Incorrect server response, stream halted')),
    switchMap(status => this._fetchData()),
    catchError(error => console.warn(error))
  );
}

Answer 5

Answer №3

It seems like there may be a misunderstanding in your problem, but you could try replacing

    console.warn('Wrong server answer, stream stopped');
    return false;

With

   Observable.throw('Some error cause')

Then catch it with the closest catch block in your stream, which allows you to: - Stop the stream if you re-throw an error - Restart it if you return the input observable - Return a completely new observable

public getData() {
  return this._getBoolean().pipe(
    filter(bool => {
      if(!bool) {
        console.warn('Wrong server answer, stream stopped');
        //return false;
        Observable.throw('I got false where I expected true')
      }
      return true;
    }),
    switchMap(bool => this._getData())
  );
}

After that:

getData()
.any()
.operator()
.you()
.wish()
.catch(e => {
  /* The stream will be terminated on a thrown error here */
})

Answer 6

It seems like there may be a misunderstanding in your problem, but you could try replacing

    console.warn('Wrong server answer, stream stopped');
    return false;

With

   Observable.throw('Some error cause')

Then catch it with the closest catch block in your stream, which allows you to: - Stop the stream if you re-throw an error - Restart it if you return the input observable - Return a completely new observable

public getData() {
  return this._getBoolean().pipe(
    filter(bool => {
      if(!bool) {
        console.warn('Wrong server answer, stream stopped');
        //return false;
        Observable.throw('I got false where I expected true')
      }
      return true;
    }),
    switchMap(bool => this._getData())
  );
}

After that:

getData()
.any()
.operator()
.you()
.wish()
.catch(e => {
  /* The stream will be terminated on a thrown error here */
})

RxJS: the art of triggering and handling errors

Answer №1

Answer №2

Answer №3

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