console.clear();

​

const source$ = Rx.Observable.timer(1, 1000);

source$

  .switchMap(val => {

    if (val > 3) {

      return Rx.Observable.timer(1, 1000).mapTo('This is an error!');

    }

    return Rx.Observable.of(val);

  })

  .catch(err => Rx.Observable.of(`I caught: ${err}`))

  .subscribe(

    val => console.log(val),

    null,

    val => console.log('complete')

  );