wp_nav_menu-functions.php

/**
 * Place this code in your theme's functions.php file to make the HTML 
 * it produces is more BEM friendly.
 * This works out the menu_class value from wp_nav_menu() and uses that to create
 * the class values.
 */

// Add a class to wp_nav_menu <li> tags so you get <li class="{menu_class}_item">
add_filter('nav_menu_css_class' , 'my_nav_special_class' , 10 , 3);
function my_nav_special_class($classes, $item, $args){
  $classes[] = $args->menu_class . '__item';
  return $classes;
}

// Add a class to wp_nav_menu's <a> tags so you get <a class="{menu_class}__link">
add_filter('nav_menu_link_attributes' , 'my_nav_special_attribute' , 10 , 3);
function my_nav_special_attribute($attributes, $item, $args){
  $attributes['class'] = $args->menu_class . '__link';
  return $attributes;
}