const toggleButton = document.querySelector('.toggleButton')  

const toggleablePanel = document.querySelector('.panel');  

const buttonInitialState = false;

const clicks = Rx.Observable.fromEvent(toggleButton, 'click');

​

const determinePanelVisibility = () => {  

  return toggleablePanel.style.display === 'none'

}

​

const toggle = clicks  

                .map(determinePanelVisibility)

​

toggle.subscribe ((show) => {  

  toggleablePanel.style.display = show ? 'block' : 'none';

});