gistfile1.txt

class Observable {

  constructor(gen) {
    this.gen = gen;
  }

  subscribe(o) {
    const observer = (o instanceof Observer) ? o : new Observer(o);
    const cancellationFn = this.gen(observer);
    return cancellationFn;
  }

  fmap(fn) {
    return new Observable(observer => {
      const uns = this.subscribe(x => observer.next(x));
      return uns;
    });
  }

  static of(x) {
    return new Observable(observer => {
      setTimeout(() => observer.next(x));
    });
  }

}

class Observer {

  constructor(observerFn) {
    this.observerFn = observerFn;
  }

  next(evt) {
    this.observerFn(evt);
  }

}

class SecondsEvent {

   constructor(d) {
     this.d = d;
     this.preventDefaultCalled = false;
   }

   preventDefault() {
     this.preventDefaultCalled = true;
   }
}

const IntervalObservable = () => new Observable(function(observer) {
  const intId = setInterval(() => {
    const evt = new SecondsEvent(new Date());
    observer.next(evt);
    console.log(evt.preventDefaultCalled ? "Gonna spare you this time" : "Destroying world!");
  }, 1000);
  return () => clearInterval(intId);
});


const t0 = new Date();
const o1 = IntervalObservable();
const o2 = o1.fmap(x => x.getTime() - t0.getTime());

let counter = 0;
const unsub = o2.subscribe(x => {
  x.preventDefault();
  console.log("tick: " + x.d);
  if (++counter === 3) unsub();
});

const o3 = Observable.of(42);
o3.subscribe(x => console.log(x));
console.log(41);