transform our button into a "leave call" button

leave-call.js

let button = document.getElementById('start-call-button');
button.innerHTML = 'end call';
button.onclick = () => {
  document.body.removeChild(iframeEl);
  iframeEl.src = null;
  button.innerHTML = 'start call';
  button.onclick = startCall;
  // window.location.origin is this page's url
  // without the hash fragment
  window.location = window.location.origin
}