Add ACF custom fields to the Wordpress JSON API

functions.php

<?php

// Add ACF data to the WP JSON REST API

  add_action('rest_api_init', 'create_api_posts_meta_field');

  function create_api_posts_meta_field() {

    function get_acf($post) {

      $acf = get_fields($post['ID']);

      if (isset($post)) {
        $post['acf'] = $acf;
      }

      return $post;

    }

    register_rest_field('post', 'acf', array(
      'get_callback'    => 'get_acf'
    ));

  }