Add an operator alike Promise.then with two arguments

[yastanotheruser]

When migrating from Promises to RxJS observables it's recommended that .then chains be replaced with .flatMap operators or any others depending on the case. However, there's currently no operator that does what would be equivalent to .then with both onFulfilled and onRejected callbacks. I guess that would be equivalent to a .map operator that also takes two callbacks, one for next and other for error.

A pipe with both a sequence of .map operators and a .catchError operator would not behave the same, because errors thrown from the project function would be caught, whereas this does not happen when using a single .then with Promises.

[martinsik]

You can use tap() or handle the error in subscribe() handler. Evetually, you can "map" the error to a different value catchError(error => of(whatever)) (even non-error value) or ignore it completely with catchError(error => EMPTY).

[yastanotheruser]

tap() doesn't map the values or error, and as I said, chaining map() and catchError() is different because catchError() could catch an error thrown from the map() callback function. Here's an example:

function getDataItems(): Promise<Data> {
  return firstValueFrom(this.http.get<Data>('/api/data'))
    .then((data) => {
      if (!responseIsValid(data)) {
        throw new InvalidResponseError('/api/data returned a bad response');
      }
      return data.items;
    }, (err) => {
      if (err instanceof HttpErrorResponse) {
        if (err.status === 404) {
          return [];
        }
      }
      throw err;
    });
}

For this trivial example I guess rethrowing in catchError() is the RxJS solution. Still it's currently not possible with operators to prevent catching the InvalidResponseError.

[martinsik]

tap() and map() are two different operators. You can use map() for error notifications. You want to have an operator that will catch only one specific type of error (like in your case InvalidResponseError)?

[yastanotheruser]

Not specifically, the example just illustrates how .then() with both fulfill and catch callbacks differs from .then().catch(), which maps to .pipe(flatMap(), catchError()) for RxJS operators, but there's no equivalent of the former.