Bootstrap Nav walker class for wp_nav_menu

functions.php

<?php 

class Bootstrap_Nav_Walker extends Walker_Nav_Menu {
  public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
    
    $active = in_array( 'current-menu-item', $item->classes ) ? 'active' : '';

    if ( in_array( 'menu-item-has-children', $item->classes ) ) {
      $output .= "
        <li class='nav-item dropdown'>
          <a 
            href='{$item->url}' 
            class='nav-link dropdown-toggle' 
            role='button' 
            data-bs-toggle='dropdown' 
            aria-expanded='false'
          >
            {$item->title}
          </a>
      ";
    } 
    else {
      if ( (int) $item->menu_item_parent === 0 ) {
        $output .= "
          <li class='nav-item {$active}'>
            <a class='nav-link' href='{$item->url}'>{$item->title}</a>
        ";
      }
      else {
        $output .= "
          <li>
            <a class='dropdown-item' href='{$item->url}'>{$item->title}</a>
        ";
      }
    }
  }

  public function start_lvl( &$output, $depth = 0, $args = null ) {
    $output .= "<ul class='dropdown-menu'>";
  }
}

index.html

<?php 
  wp_nav_menu([
    'theme_location'  =>  'primary',
    'menu_class'      =>  'navbar-nav',
    'walker'          =>  new Bootstrap_Nav_Walker
  ]);
?>