Edit in JSFiddle

const node = document.querySelector('input');
const p = document.querySelector('p');

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

Observable.fromEvent = (element, name) => {
  return new Observable((observer) => {
    const callback = (event) => observer.next(event);
    element.addEventListener(name, callback, false);
    return () => element.removeEventListener(name, callback, false);
  });
};

const input$ = Observable.fromEvent(node, 'input');

input$.subscribe({
  next: (event) => {
    p.innerHTML = event.target.value;
  }
});

<input type="text">
<p></p>