code: function isFullscreen() {
      var _document = document;
      if (!_document.fullscreenElement &&
          !_document.mozFullScreenElement &&
          !_document.webkitFullscreenElement &&
          !_document.msFullscreenElement) {
          return false;
      }
      return true;
  }
docs: check if fullscreen or not.
  @returns {boolean}