/*
function onYouTubePlayerReady(playerId) {
  console.log(playerId);
  ytplayer = document.getElementById("ytplayer");
  console.log(ytplayer);
}

function stop() {
  if (ytplayer) {
    ytplayer.stopVideo();
  }
}
*/

// Yotubeの再生・停止などのイベントを監視・制御する


// 2. This code loads the IFrame Player API code asynchronously.
var tag = document.createElement('script');
tag.src = "https://www.youtube.com/player_api";
var firstScriptTag = document.getElementsByTagName('script')[0];
firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);

// 3. This function creates an <iframe> (and YouTube player)
//    after the API code downloads.
var player;
function onYouTubePlayerAPIReady() {
  player = new YT.Player('player', {
    height: document.body.clientWidth * 9/16,
    width: document.body.clientWidth,
    videoId: 'yKwalYCVJZg',
    events: {
      'onReady': onPlayerReady,
      'onStateChange': onPlayerStateChange
    }
  });
}

// 4. The API will call this function when the video player is ready.
function onPlayerReady(event) {
  event.target.playVideo();
}

// 5. The API calls this function when the player's state changes.
//    The function indicates that when playing a video (state=1),
//    the player should play for six seconds and then stop.
var done = false;
function onPlayerStateChange(event) {
  if (event.data == YT.PlayerState.PLAYING && !done) {
    setTimeout(stopVideo, 6000);
    done = true;
  }
}
function stopVideo() {
  player.stopVideo();
  return false;
}

ページのソースを表示 : Ctrl+U , DevTools : F12

view-source:https://hi0a.com/demo/-js/js-youtube-api/