Custom Wordpress Nav Walker - Add your own classnames to WP_Nav_Menu

functions.php

// Updated the Walker_Nav_Menu class to accept an array of classnames via argument
// Comments below show what has been changed / updated
// Original Walker_Nav_Menu copied from here: https://core.trac.wordpress.org/browser/tags/4.5.3/src//wp-includes/nav-menu-template.php#L0

<?php
  class Nav_Walker extends Walker_Nav_Menu {
    // Setup empty $data array
    // Can be used to store any other data
    private $data = [];
    
    // Add __construct function to pull in array via $class argument
    function __construct($class) {
      // Assign $class array to $data['classes']
      $this->data['classes'] = $class;
    }

    function start_lvl( &$output, $depth = 0, $args = array() ) {
      $indent = str_repeat("\t", $depth);
  		$output .= "\n$indent<ul class=\"sub-menu\">\n";
  	}

  	function end_lvl( &$output, $depth = 0, $args = array() ) {
      $indent = str_repeat("\t", $depth);
  		$output .= "$indent</ul>\n";
  	}

  	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
      $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
      
      // Implode $data['classes'] array and assign to $custom_classes
      // " / " is added to the end to separate your classes from built in WP classes
      $custom_classes = implode(" ", $this->data['classes']) . " / ";

      $classes = empty( $item->classes ) ? array() : (array) $item->classes;
  		$classes[] = 'menu-item-' . $item->ID;

  		$args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );

  		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
      
      //Added esc_attr($custom_classes) to output your classes
  		$class_names = $class_names ? ' class="' . esc_attr($custom_classes) . esc_attr( $class_names ) . '"' : '';
      
  		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
  		$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

  		$output .= $indent . '<li' . $id . $class_names .'>';

  		$atts = array();
  		$atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
  		$atts['target'] = ! empty( $item->target )     ? $item->target     : '';
  		$atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
  		$atts['href']   = ! empty( $item->url )        ? $item->url        : '';
  		$atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

  		$attributes = '';
  		foreach ( $atts as $attr => $value ) {
  			if ( ! empty( $value ) ) {
  				$value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
  				$attributes .= ' ' . $attr . '="' . $value . '"';
  			}
  		}

  		$title = apply_filters( 'the_title', $item->title, $item->ID );
  		$title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

  		$item_output = $args->before;
  		$item_output .= '<a'. $attributes .'>';
  		$item_output .= $args->link_before . $title . $args->link_after;
  		$item_output .= '</a>';
  		$item_output .= $args->after;

  		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
  	}

  	function end_el( &$output, $item, $depth = 0, $args = array() ) {
  		$output .= "</li>\n";
  	}
  }
?>

header.php

// This allows you to add BEM style classnames ( or any other css methodology ) to your nav items
// Args:
//    menu - Name of your wordpress menu as setup in WP Admin > Appearance > Menus
//    menu_class - set the class of the <ul></ul> wrapper
//    walker - An array of classes should be added as an argument

<?php wp_nav_menu(array('menu' => 'Wp Menu Name', 'container' => '', 'menu_class' => 'class-name', 'walker' => new Nav_Walker(['class-one', 'class-two', 'class-three']) )); ?>