throwError

throwError creates an Observable, that will immediately emit an error
In the example below a value on timer will be substituted with a throwError stream at 10ms:

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

timer(10).pipe(
    switchMap(()=>
      throwError('Err!')
    )
  )
  .subscribe(rxObserver());

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