Promise.all and promise.then(...).then(...) behavior for Observables

obsall.ts

const create = (val) => {
  return (o: Observer<any>) => {
    setTimeout(() => {
      console.log(val);
      o.next(val);
      o.complete();
    }, 5000)
  }
};

const f1 = () => {
  return Observable.create(create(1));
};

const f2 = () => {
  return Observable.create(create(2));
};
const f3 = () => {
  return Observable.create(create(3));
};

// promise.then(...).then(...)
f1().flatMap(f2).flatMap(f3).subscribe((vals) => console.log("Subscribe:", vals));

// Promise.all
Observable.forkJoin(...[f1(), f2(), f3()]).subscribe((vals) => console.log("Subscribe:", vals));