wpgraphql-menu-locations.php

<?php

namespace WPGraphQL\Menu\Locations;

/**
 * Enable all menus in WPGraphQL
 *
 * This code adds menu support, loops through all available menus 
 * and programatically syncs them to menu locations to make them 
 * all accessible through WPGraphQL.
 *
 * https://www.wpgraphql.com/docs/menus/
 */

add_action( 'after_setup_theme', __NAMESPACE__ . '\\after_setup_theme' );

function after_setup_theme () {
  // Enable support for menus
  add_theme_support( 'menus' );
  // Get all of the menus
  $nav_menu = get_terms( 'nav_menu' );
  // Create an array of locations
  $nav_menus = array_combine( wp_list_pluck( $nav_menu, 'slug' ), wp_list_pluck( $nav_menu, 'name' ) );
  // Register the locations
  register_nav_menus( $nav_menus );
  // Assign the menus to the locations
  $nav_menu_locations = array_combine( wp_list_pluck( $nav_menu, 'slug' ), wp_list_pluck( $nav_menu, 'term_id' ) );
  // Update the locations
  set_theme_mod( 'nav_menu_locations', $nav_menu_locations );
}

Turns out there's a much better way of doing this:

https://www.wpgraphql.com/recipes/making-menus-and-menu-items-public/