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Jim Wharton
Jim Wharton

Reputation: 1415

Unit testing NestJS Observable Http Retry

I'm making a request to a 3rd party API via NestJS's built in HttpService. I'm trying to simulate a scenario where the initial call to one of this api's endpoints might return an empty array on the first try. I'd like to use RxJS's retryWhen to hit the api again after a delay of 1 second. I'm currently unable to get the unit test to mock the second response however:

it('Retries view account status if needed', (done) => {
    jest.spyOn(httpService, 'post')
      .mockReturnValueOnce(of(failView)) // mock gets stuck on returning this value
      .mockReturnValueOnce(of(successfulView));
    const accountId = '0812081208';
    const batchNo = '39cba402-bfa9-424c-b265-1c98204df7ea';
    const response =client.viewAccountStatus(accountId, batchNo);
    response.subscribe(
      data => {
        expect(data[0].accountNo)
          .toBe('0812081208');
        expect(data[0].companyName)
          .toBe('Some company name');
        done();
      },
    )
  });

My implementation is:

viewAccountStatus(accountId: string, batchNo: string): Observable<any> {
    const verificationRequest = new VerificationRequest();
    verificationRequest.accountNo = accountId;
    verificationRequest.batchNo = batchNo;


    this.logger.debug(`Calling 3rd party service with batchNo: ${batchNo}`);

    const config = {
      headers: {
        'Content-Type': 'application/json',
      },
    };

    const response = this.httpService.post(url, verificationRequest, config)
      .pipe(
        map(res => {
          console.log(res.data); // always empty
          if (res.status >= 400) {
            throw new HttpException(res.statusText, res.status);
          }

          if (!res.data.length) {
            this.logger.debug('Response was empty');
            throw new HttpException('Account not found', 404);
          }

          return res.data;
        }),
        retryWhen(errors => {
          this.logger.debug(`Retrying accountId: ${accountId}`);
          // It's entirely possible the first call will return an empty array
          // So we retry with a backoff
          return errors.pipe(
            delayWhen(() => timer(1000)),
            take(1),
          );
        }),
      );

    return response;
  }

When logging from inside the initial map, I can see that the array is always empty. It's as if the second mocked value never happens. Perhaps I also have a solid misunderstanding of how observables work and I should somehow be trying to assert against the SECOND value that gets emitted? Regardless, when the observable retries, we should be seeing that second mocked value, right?

I'm also getting 

: Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Error:

On each run... so I'm guessing I'm not calling done() in the right place.

Upvotes: 1

Views: 2468

Answers (1)

Andrei Gătej
Andrei Gătej

Reputation: 11979

I think the problem is that retryWhen(notifier) will resubscribe to the same source when its notifier emits.

Meaning that if you have

new Observable(s => {
  s.next(1);
  s.next(2);

  s.error(new Error('err!'));
}).pipe(
  retryWhen(/* ... */)
)

The callback will be invoked every time the source is re-subscribed. In your example, it will call the logic which is responsible for sending the request, but it won't call the post method again.

The source could be thought of as the Observable's callback: s => { ... }.

What I think you'll have to do is to conditionally choose the source, based on whether the error took place or not.

Maybe you could use mockImplementation:

let hasErr = false;

jest.spyOn(httpService, 'post')
  .mockImplementation(
    () => hasErr ? of(successView) : (hasErr = true, of(failView))
  )

Edit

I think the above does not do anything different, where's what I think mockImplementation should look like:

let err = false;

mockImplementation(
 () => new Observable(s => {
   if (err) { 
     s.next(success) 
   } 
   else { 
    err = true;
    s.next(fail) 
   } 
 })
)

Upvotes: 1

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