Converting event to Observable

gistfile1.txt

function fromEvent(dom, eventName) {
    return {
        forEach: function(observer) {
            var handler = (e) => {
                observer.onNext(e);
            };
            dom.addEventListener(eventName, handler);

            // Subscription
            return {
                dispose: function() {
                    dom.removeEventListener(eventName, handler);
                }
            }
        }
    };
}