fakeObservable.ts

function fakeObservable(speed: number, num: number) {
  return new Observable(obs => {
    let t = setTimeout(() => {
      console.log('next value:', num);
      obs.next(num);
      obs.complete();
    }, speed);
    // Cancelation
    return () => {
      console.log('cancelled', num);
      clearTimeout(t);
    } 
});