modified startCall() - create a new room only if we aren't passed a callUrl argument

start-call-2.js

const createRoomEndpoint = 'https://create-a-room--kwindla.repl.co';

async function startCall(callUrl) {
  if (!callUrl) {
    let response = await fetch(createRoomEndpoint),
        roomData = await response.json();
    callUrl = roomData.url;
  }

  let iframeEl = document.createElement('iframe');
  iframeEl.width = 350;
  iframeEl.height = 425;
  iframeEl.style.position = 'absolute';
  iframeEl.style.right = '1em';
  iframeEl.style.bottom = '3em';
  iframeEl.src = callUrl;
  iframeEl.allow="camera; microphone; autoplay";
  document.body.appendChild(iframeEl);
  window.location = '#' + callUrl
}