onErrorResumeNext

Switch to the next observable when current errors.
This helps you build a fallback strategy:

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const { rxObserver } = require('api/v0.3');
const { timer, onErrorResumeNext, throwError, of } = require('rxjs');
const { concat, ignoreElements } = require('rxjs/operators');


const failTimer$ = timer(5).pipe(
    ignoreElements(),
    concat(throwError('Ouch!'))
  );

const fineTimer$ = timer(5).pipe(
    ignoreElements(),
    concat(of('Ok!'))
  );

const strategicTimer$ = onErrorResumeNext(
    failTimer$,
    fineTimer$
  );

failTimer$.subscribe(rxObserver('fail timer'));
fineTimer$.subscribe(rxObserver('fine timer'));
strategicTimer$.subscribe(rxObserver('strategic'));

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Check out "Error handling in RxJS" article to get better understanding how not to fail with Observables.