non-namespaced route for WP REST API

non-namespaced-route.php

<?php

add_action( 'rest_api_init', function ( $server ) {
    $server->register_route( 'foo', '/foo', array(
        'methods'  => 'GET',
        'callback' => function () {
            return 'baz';
        },
    ) );
} );

alternatively

 rest_get_server()->register_route( 'foo', '/foo', array(
        'methods'  => 'GET',
        'callback' => function () {
            return 'baz';
        },
    ) );