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// iframe
var youtubePlayer = document.getElementsByClassName('youtube-player')[0];

// play
youtubePlayer.contentWindow.postMessage('{"event":"command","func":"playVideo","args":""}', '*');

// pause
youtubePlayer.contentWindow.postMessage('{"event":"command","func":"pauseVideo","args":""}', '*');

// stop
youtubePlayer.contentWindow.postMessage('{"event":"command","func":"stopVideo","args":""}', '*');