Source: misc/dialog/dialog.jquery-ui.es6.js

/**
 * @file
 * Adds default classes to buttons for styling purposes.
 */

(function ($) {
  $.widget('ui.dialog', $.ui.dialog, {
    options: {
      buttonClass: 'button',
      buttonPrimaryClass: 'button--primary',
    },
    _createButtons() {
      const opts = this.options;
      let primaryIndex;
      let index;
      const il = opts.buttons.length;
      for (index = 0; index < il; index++) {
        if (
          opts.buttons[index].primary &&
          opts.buttons[index].primary === true
        ) {
          primaryIndex = index;
          delete opts.buttons[index].primary;
          break;
        }
      }
      this._super();
      const $buttons = this.uiButtonSet.children().addClass(opts.buttonClass);
      if (typeof primaryIndex !== 'undefined') {
        $buttons.eq(index).addClass(opts.buttonPrimaryClass);
      }
    },
  });
})(jQuery);