Prepack Observable

function Observable(subscribe) {
  this._subscribe = subscribe;
}

const of = (v) => new Observable(observer => {
  return observer.next(v);
});

Observable.prototype = {
  map(f) {
    return new Observable((observer) => {
      return this.subscribe({
        next: (v) => observer.next(f(v))
      });
    });
  },
  
  filter(f) {
    return new Observable((observer) => {
      return this.subscribe({
        next: (v) => {
          if (f(v)) {
            observer.next(v);
          }
        }
      });
    });
  },
  
  flatMap(f) {
    return new Observable((observer) => {
      return this.subscribe({
        next(v) {
          const otherObs = f(v);
          otherObs.subscribe(observer);
        }
      });
    });
  },
  
  subscribe(observer) {
    return this._subscribe(observer);
  }
};

const whatever = (v) => of(v).map((v) => -v).filter((v) => v < 0);
whatever(Math.random()).
  flatMap((v) => whatever(v)).
  map((v) => v * Math.random()).
  subscribe({
    next: (v) => console.log(v)
  });