Load script and wait for ready

gistfile1.js

YoutubeDefer.loadAPIVideo(block) {
  if(typeof YT === 'undefined') {
    loadYoutubeScript();
    checkIfYoutubeIsReady().then(function() {
      let player = new Vimeo.Player(iframe);
      player.play();
    });
  }
  else {
    let player = new Vimeo.Player(iframe);
    player.play();
  }
}

const YoutubeDefer.checkIfYoutubeIsReady = () => {
  var wait;
  var timeout;

  var deferred = new Promise((resolve, reject) => {
    wait = setInterval(function() {
      if (!Vimeo) {
        return;
      }

      clearInterval(wait);
      clearTimeout(timeout);
      resolve();
    }, 500);

    timeout = setTimeout(function() {
      clearInterval(wait);
      reject();
    }, 4000); // subjective. test up to 8 times over 4 seconds
  });

  return deferred;
}

const YoutubeDefer.loadYoutubeScript = () => {
  let s = document.createElement("script");
  s.type = "text/javascript";
  s.src = "https://www.youtube.com/iframe_api";
  document.querySelector("head").append(s);
}