customizer-dropdown.php

<?php
/**
 * Enqueue scripts.
 */
function prefix_scripts() {
	
	// Enqueue responsive navigation.
	wp_enqueue_script( 'prefix-navigation', get_template_directory_uri() . '/js/responsive-nav.js', array(), '1.38', true );
	
	// Enqueue JS functions.
	wp_enqueue_script( 'prefix-script', get_template_directory_uri() . '/js/functions.js', array( 'prefix-navigation' ), '1.38', true );
	wp_localize_script( 'prefix-script', 'navSettings', array(
		'expand'    => '<span class="screen-reader-text">' . esc_html__( 'Expand child menu', 'textdomain' ) . '</span>',
		'collapse'  => '<span class="screen-reader-text">' . esc_html__( 'Collapse child menu', 'textdomain' ) . '</span>',
		'closeMenu' => esc_html__( 'Close menu', 'textdomain' ),
		'openMenu'  => esc_html__( 'Open menu', 'textdomain' ),
		'dropdown'  => get_theme_mod( 'disable_dropdown' ) ? false : true,
	) );
	
}
add_action( 'wp_enqueue_scripts', 'prefix_scripts' );

customizer.php

<?php

	/* == Navigation section == */
	
	// Add the navigation section.
	$wp_customize->add_section(
		'prefix-navigation',
		array(
			'title'    => esc_html__( 'Navigation settings', 'textdomain' ),
			'priority' => 10,
			'panel'    => 'theme'
		)
	);
	
	$wp_customize->add_setting(
		'disable_dropdown',
		array(
			'default'           => '',
			'sanitize_callback' => 'prefix_sanitize_checkbox'
		)
	);
	
	$wp_customize->add_control(
		'disable_dropdown',
		array(
			'label'       => esc_html__( 'Disable multi-level menu', 'textdomain' ),
			'description' => esc_html__( 'Check this if you want to disable multi-level dropdown in Primary menu.', 'textdomain' ),
			'section'     => 'prefix-navigation',
			'priority'    => 10,
			'type'        => 'checkbox'
		)
	);

dropdown-functions.js

/**
 * Enable responsive nav for dropdown menus.
 */
( function() {
	var container;
	
	// Primary menu
	container = document.getElementById( 'menu-primary' );

	/**
	 * Enable dropdown menu.
	 */
	 
	if ( container ) {
		
		var buttonMain = document.getElementById( 'nav-toggle' );
		var navMain = responsiveNav(".main-navigation", {                   // Selector
			transition: 350,                                                // Integer: Speed of the transition, in milliseconds
			customToggle: "#nav-toggle",                                    // Selector: Specify the ID of a custom toggle
			enableFocus: true,                                              // Boolean: Do we use use 'focus' class in our nav
			enableDropdown: navSettings.dropdown,                           // Boolean: Do we use multi level dropdown
			openDropdown: navSettings.expand,                               // String: Label for opening sub menu
			closeDropdown: navSettings.collapse,                            // String: Label for closing sub menu
			resizeMobile: function () {                                     // Set ARIA for menu toggle button
				buttonMain.setAttribute( 'aria-controls', 'menu-primary' );
			},
			resizeDesktop: function () {                                    // Remove ARIA from menu toggle button
				buttonMain.removeAttribute( 'aria-controls' );
			},
		});
		
	}
	
} )();

functions.js

/**
 * Enable responsive nav for dropdown menus.
 */
( function() {
	var container;
	
	// Primary menu
	container = document.getElementById( 'menu-primary' );

	/**
	 * Enable dropdown menu.
	 */
	 
	if ( container ) {
		
		var buttonMain = document.getElementById( 'nav-toggle' );
		var navMain = responsiveNav(".main-navigation", {                   // Selector
			transition: 350,                                                // Integer: Speed of the transition, in milliseconds
			customToggle: "#nav-toggle",                                    // Selector: Specify the ID of a custom toggle
			enableFocus: true,                                              // Boolean: Do we use use 'focus' class in our nav
			enableDropdown: true,                                           // Boolean: Do we use multi level dropdown
			openDropdown: navSettings.expand,                               // String: Label for opening sub menu
			closeDropdown: navSettings.collapse,                            // String: Label for closing sub menu
			resizeMobile: function () {                                     // Set ARIA for menu toggle button
				buttonMain.setAttribute( 'aria-controls', 'menu-primary' );
			},
			resizeDesktop: function () {                                           // Remove ARIA from menu toggle button
				buttonMain.removeAttribute( 'aria-controls' );
			},
		});
		
	}
	
} )();

multi-level-dropdown.php

<?php
/**
 * Enqueue scripts.
 */
function prefix_scripts() {
	
	// Enqueue responsive navigation.
	wp_enqueue_script( 'prefix-navigation', get_template_directory_uri() . '/js/responsive-nav.js', array(), '1.38', true );
	
	// Enqueue JS functions.
	wp_enqueue_script( 'prefix-script', get_template_directory_uri() . '/js/functions.js', array( 'prefix-navigation' ), '1.38', true );
	wp_localize_script( 'prefix-script', 'navSettings', array(
		'expand'    => '<span class="screen-reader-text">' . esc_html__( 'Expand child menu', 'textdomain' ) . '</span>',
		'collapse'  => '<span class="screen-reader-text">' . esc_html__( 'Collapse child menu', 'textdomain' ) . '</span>',
		'closeMenu' => esc_html__( 'Close menu', 'textdomain' ),
		'openMenu'  => esc_html__( 'Open menu', 'textdomain' ),
	) );
	
}
add_action( 'wp_enqueue_scripts', 'prefix_scripts' );