Make youtube video fullscreen using iframe and javascript API

This worked perfect in my case. You can find more details on this link: demo on CodePen

var player, iframe;
var $ = document.querySelector.bind(document);

// init player
function onYouTubeIframeAPIReady() {
  player = new YT.Player('player', {
    height: '200',
    width: '300',
    videoId: 'dQw4w9WgXcQ',
    events: {
      'onReady': onPlayerReady
    }
  });
}

// when ready, wait for clicks
function onPlayerReady(event) {
  var player = event.target;
  iframe = $('#player');
  setupListener(); 
}

function setupListener (){
$('button').addEventListener('click', playFullscreen);
}

function playFullscreen (){
  player.playVideo();//won't work on mobile

  var requestFullScreen = iframe.requestFullScreen || iframe.mozRequestFullScreen || iframe.webkitRequestFullScreen;
  if (requestFullScreen) {
    requestFullScreen.bind(iframe)();
  }
}