Add ACF Fields to WP API responses

functions.php

<?php
/*
 * Add ACF Fields to REST responses
 * Modified from: https://support.advancedcustomfields.com/forums/topic/rest-api/#post-32890
 */
add_action( 'rest_api_init', 'slug_register_acf' );
function slug_register_acf() {
    $post_types = get_post_types(['public'=>true], 'names');
    foreach ($post_types as $type) {
        register_rest_field( $type,
            'acf',
            array(
                'get_callback'    => 'slug_get_acf',
                'update_callback' => null,
                'schema'          => null,
            )
        );
    }
}
function slug_get_acf( $object, $field_name, $request ) {
    return get_fields($object[ 'id' ]);
}