RxJS retryWhen() Error Handling Operator

by Online Tutorials Library July 22, 2022

RxJS retryWhen() Error Handling Operator

RxJS retryWhen() operator is an error-handling operator used to return an observable that mirrors the source observable except an error. If the source observable calls an error, this method will emit the Throwable that caused the error to the observable returned from the notifier. If that observable call is completed or got an error, this method will call complete or error on the child’s subscription. Otherwise, this method will resubscribe to the source observable.

In other words, we can say that the RxJS retryWhen() operator enables to retry the request instead of giving up after the first failure. It facilitates you to retry an observable sequence of errors based on custom criteria.

The function provided to the operator receives a stream of errors, and we need to return an observable, which will trigger the requested retry after every value emission.

Syntax:

Following is the syntax of the RxJS retryWhen() error handling operator:

  retryWhen(receives: (errors: Observable) => Observable, the: scheduler): Observable   

  retryWhen<T>(notifier: (errors: Observable<any>) => Observable<any>): MonoTypeOperatorFunction<T>  

Parameter Explanation

notifier: It receives an observable of notifications which facilitates the user to complete or getting error, aborting the retry.

Return value

The RxJS retryWhen() operator returns an observable that retry count logic and is modified with the retry logic.

Let us see some examples of the RxJS retryWhen() operator to understand it clearly.

Example 1 (Trigger retry after specified duration)

  import { timer, interval } from ‘rxjs’;  import { map, tap, retryWhen, delayWhen } from ‘rxjs/operators’;  //emit value every 1s  const source = interval(1000);  const example = source.pipe(    map(val => {      if (val > 5) {        //error will be picked up by retryWhen        throw val;      }      return val;    }),    retryWhen(errors =>      errors.pipe(        //log error message        tap(val => console.log(`Value ${val} was too high!`)),        //restart in 6 seconds        delayWhen(val => timer(val * 1000))      )    )  );  const subscribe = example.subscribe(val => console.log(val));  

Output:

After executing the above example, you will see the following result:

In the above example, you can see that it has emitted the specified values only and then wait for 6 seconds and then repeat.

Example 2 (Customizable retry with increased duration)

  import { Observable } from ‘rxjs/Observable’;  import { _throw } from ‘rxjs/observable/throw’;  import { timer } from ‘rxjs/observable/timer’;  import { mergeMap, finalize } from ‘rxjs/operators’;  export const genericRetryStrategy = (    {      maxRetryAttempts = 3,      scalingDuration = 1000,      excludedStatusCodes = []    }: {      maxRetryAttempts?: number;      scalingDuration?: number;      excludedStatusCodes?: number[];    } = {}  ) => (attempts: Observable) => {    return attempts.pipe(      mergeMap((error, i) => {        const retryAttempt = i + 1;        // if maximum number of retries have been met        // or response is a status code we don’t wish to retry, throw error        if (retryAttempt > maxRetryAttempts || excludedStatusCodes.find(e => e === error.status)) {          return _throw(error);        }        console.log(`Attempt ${retryAttempt}: retrying in ${retryAttempt * scalingDuration}ms`);        // retry after 1s, 2s, etc…        return timer(retryAttempt * scalingDuration);      }),      finalize(() => console.log(‘We are done!’))    );  };  

Output:

After executing the above example, you will see the following result:

Next TopicRxJS Operators

RxJS retryWhen() Error Handling Operator

RxJS retryWhen() Error Handling Operator

Syntax:

Parameter Explanation

Return value

Example 1 (Trigger retry after specified duration)

Example 2 (Customizable retry with increased duration)

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