i.ts

import { Observable } from 'rxjs/Observable';

import 'rxjs/add/observable/interval';

import 'rxjs/add/operator/switchMap';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mapTo';

// This observable is subscribed to and emits observables
Observable.interval(1000)
.switchMap(() =>
  // This inner observable is 'flattened' by `.switchMap` and folded into the outer observable stream
  Observable.interval(1000).mapTo('a')
        .map(a => {
          // This will be logged because it is part of the same observable stream
          console.log(a);

          // Nothing subscribes to this observable
          return Observable.interval(1000).mapTo('b')
            .map(b => {
              console.log(b);
              return 'called';
            })
        })
// This logs out the observable returned from the inner Observable
).subscribe(obs => console.log(obs));