catchError (formerly .catch)

// Throw an error on the 5th emission. Then, create a new observable
// and keep going. This also switches letters to numbers.
preCatch = timer(0, 1000).pipe(
  map(i => {
    if (4 === i) {
      throw Error;
    }
    return i;
  }),
  mapNumberToChar(),
);
postCatch = new Subject();
caught = this.preCatch.pipe(
  catchError(() =>
    timer(0, 1000).pipe(
      take(15),
      map(value => value + 1),
      tap(value => this.postCatch.next(value)),
      finalize(() => this.postCatch.complete()),
    ),
  ),
);