How to set class for links in navigation?

Question

WordPress

There is a menu

<ul class="navbar-nav mr-auto">
      <li class="nav-item active">	
        <a class="nav-link" href="index.html">Главная <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Sony Wegas Pro</a>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          After Efects
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">
          <a class="dropdown-item" href="#">Action</a>
          <a class="dropdown-item" href="#">Another action</a>
          <a class="dropdown-item" href="#">Something else here</a>
        </div>
      </li>
      <li class="nav-item">
        <a class="nav-link " href="articles.html">Статьи</a>
      </li>
    </ul>

I need to put it on WP, but I don’t know how to set a class for links. I display the menu like this:
<a class="nav-link ">Статьи</a>

wp_nav_menu( array(
    'theme_location'  => 'menu', 
    'menu'            => '',
    'container'       => 'false',
    'container_class' => ' ',
    'container_id'    => '',
    'menu_class'      => 'navbar-nav mr-auto ', 
    'menu_id'         => '', 
    'echo'            => true,
    'fallback_cb'     => 'wp_page_menu',
    'before'          => '',
    'after'           => '',
    'link_before'     => '',
    'link_after'      => '',
    'items_wrap'      => '<ul id = "%1$s" class = "%2$s">%3$s</ul>',
    'depth'           => 2, 
    'walker'          => '',
  ) );

Answer 1

You need the walker property, you can customize it in functions.php. I found an example here.
To change the appearance of ul, add the code to functions.php

class My_Walker_Nav_Menu extends Walker_Nav_Menu {

    /**
     * Starts the element output.
     *
     * @since 3.0.0
     * @since 4.4.0 The {@see 'nav_menu_item_args'} filter was added.
     *
     * @see Walker::start_el()
     *
     * @param string   $output Passed by reference. Used to append additional content.
     * @param WP_Post  $item   Menu item data object.
     * @param int      $depth  Depth of menu item. Used for padding.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     * @param int      $id     Current item ID.
     */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
            $t = '';
            $n = '';
        } else {
            $t = "\t";
            $n = "\n";
        }
        $indent = ( $depth ) ? str_repeat( $t, $depth ) : '';

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

        $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 ) );
        $class_names = $class_names ? ' class="' . 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' . $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['class']   = 'header__menu-link nav-link';

        $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 );
    }

}

I have already set the right class in the right place, you just need to paste the code in functions.php and connect in the property

wp_nav_menu( array(
    'theme_location'  => 'menu', 
    'menu'            => '',
    'container'       => 'false',
    'container_class' => ' ',
    'container_id'    => '',
    'menu_class'      => 'navbar-nav mr-auto ', 
    'menu_id'         => '', 
    'echo'            => true,
    'fallback_cb'     => 'wp_page_menu',
    'before'          => '',
    'after'           => '',
    'link_before'     => '',
    'link_after'      => '',
    'items_wrap'      => '<ul id = "%1$s" class = "%2$s">%3$s</ul>',
    'depth'           => 2, 
    'walker'         => new My_Walker_Nav_Menu(),
  ) );

PS didn't test it, let me know if it doesn't work