Customizing TinyMCE's Formats (style_select) dropdown in rich text widgets in WordPress 4.8x

tinymce-widget-styleselect.txt

<?php 
// PHP goes in functions.php or similar


// Callback function to filter the MCE settings
function my_mce_style_options( $init_array ) {  
    // Define the style_formats array
    $style_formats = my_mce_styleselect_options();
    // Insert the array, JSON ENCODED, into 'style_formats'
    $init_array['style_formats'] = json_encode( $style_formats );  
	  $init_array['preview_styles'] = false;
    return $init_array;  

} 
add_filter( 'tiny_mce_before_init', 'my_mce_style_options' );

/*
Define the styleselect ("Formats") dropdown menu options. To be used as PHP in my_mce_style_options() and as JSON in my_rich_text_widget_buttons_enqueue()
/**/

function my_mce_styleselect_options() {
	return array(
        // Each array child is a format with its own settings
        array(  
            'title' => __( 'Button Link' ),
            'selector' => 'a',  
            'classes' => 'button'             
        ),
		array(  
            'title' => __( 'Uppercase' ),
            'selector' => 'h1, h2, h3, h4, h5, h6, p, span, li, dt',
			      'inline' => 'span',
            'classes' => 'uppercase'             
        ),
		array(  
            'title' => __( 'Red' ),
            'selector' => 'h1, h2, h3, h4, h5, h6, p, blockquote, span, li, dt',
			      'inline' => 'span',
            'classes' => 'red'             
        )
    );
}


// Declare script for rich text widget buttons
function my_rich_text_widget_buttons_enqueue( $hook ) {
	if ( 'widgets.php' !== $hook )
    	return;

    wp_enqueue_script( 'my_rich_text_widget_buttons', get_stylesheet_directory_uri() . '/js/rich-text-widget-buttons.js', array( 'jquery' ), '1.0', true  );
	wp_localize_script(
		'my_rich_text_widget_buttons',
		'TinyMCE_formats',
		my_mce_styleselect_options()
	);
}
add_action( 'admin_enqueue_scripts', 'my_rich_text_widget_buttons_enqueue' );
?>

// JS file: rich-text-widget-buttons.js

jQuery( document ).on( 'tinymce-editor-setup', function( event, editor ) {
    if ( editor.settings.toolbar1 && -1 === editor.settings.toolbar1.indexOf( 'styleselect' ) ) {
        editor.settings.toolbar1 += ',styleselect';
    }
});

jQuery( document ).on( 'wp-before-tinymce-init', function( event, settings ) {
	settings.style_formats_merge = false;
	settings.style_formats = TinyMCE_formats;
	settings.preview_styles = false;
});