docs: function buttonStyleClasses( buttonConfiguration ) {
        const classes = {};
        classes[ BUTTON_CLASS_ACTIVE ] = false;
        classes[ BUTTON_CLASS_HIDDEN ] = false;
        classes[ BUTTON_CLASS_DISABLED ] = false;
        classes[ BUTTON_CLASS_OMITTED ] = false;
        classes[ BUTTON_CLASS_BUSY ] = false;
  
        const buttonClass = buttonConfiguration[ 'class' ];
        if( buttonClass ) {
           const typePart =
              CONFIG_TO_BOOTSTRAP_STYLE_MAP[ buttonClass ] ||
              CONFIG_TO_BOOTSTRAP_STYLE_MAP.NORMAL;
           const styleClass = BUTTON_CLASS_PREFIX + typePart;
           classes[ styleClass ] = true;
        }
  
        const { size = 'DEFAULT' } = buttonConfiguration;
        if( size !== 'DEFAULT' ) {
           const sizeClass = BUTTON_CLASS_PREFIX + CONFIG_TO_BOOTSTRAP_SIZE_MAP[ size ];
           classes[ sizeClass ] = true;
        }
  
        return classes;
     }