Fullscreen API

const toggler = document.getElementById('toggler');

function toggleFullscreen(elem) {
  elem = elem || document.documentElement;
  if (!document.fullscreenElement && !document.mozFullScreenElement &&
    !document.webkitFullscreenElement && !document.msFullscreenElement) {
    if (elem.requestFullscreen) {
      elem.requestFullscreen();
    } else if (elem.msRequestFullscreen) {
      elem.msRequestFullscreen();
    } else if (elem.mozRequestFullScreen) {
      elem.mozRequestFullScreen();
    } else if (elem.webkitRequestFullscreen) {
      elem.webkitRequestFullscreen(Element.ALLOW_KEYBOARD_INPUT);
    }
    toggler.textContent = 'toggle off';
  } else {
    if (document.exitFullscreen) {
      document.exitFullscreen();
    } else if (document.msExitFullscreen) {
      document.msExitFullscreen();
    } else if (document.mozCancelFullScreen) {
      document.mozCancelFullScreen();
    } else if (document.webkitExitFullscreen) {
      document.webkitExitFullscreen();
    }
    toggler.textContent = 'toggle on';
  }
}

toggler.addEventListener('click', function() {
  toggleFullscreen();
});