Demystifying RxJS, Part I: fromEvent()

05-demystifying-rxjs-fromEvent.ts

function fromEvent<T extends Event = Event>(
  target: EventTarget,
  eventName: string
): Observable<T> {
  return new Observable(obs => {
    const listener: EventListener = (evt: T) => {
      obs.next(evt);
    };
    target.addEventListener(eventName, listener);
    return () => target.removeEventListener(eventName, listener);
  });
}

// Every time `btn` is clicked, logs "click"
fromEvent(btn, "click").subscribe(() => console.log("click"));