createQualityButton() {
    const player = this.player;

    this._qualityButton = new ConcreteButton(player);

    const placementIndex = player.controlBar.children().length - 2;
    const concreteButtonInstance = player.controlBar.addChild(
      this._qualityButton,
      { componentClass: 'qualitySelector' },
      this.config.placementIndex || placementIndex
    );

    concreteButtonInstance.addClass('vjs-quality-selector');
    if (!this.config.displayCurrentQuality) {
      const icon = ` ${this.config.vjsIconClass || 'vjs-icon-hd'}`;

      concreteButtonInstance.menuButton_.$('.vjs-icon-placeholder').className += icon;
    } else {
      this.setButtonInnerText('auto');
    }
    concreteButtonInstance.removeClass('vjs-hidden');
  }
Adds the quality menu button to the player control bar.