Getting the names of custom fields from Wordpress JSON API

This is the necessary Wordpress plugin modification so that the list of custom fields can be pulled from Wordpress via the JSON API.

First you need to add the JSON API plugin to your Wordpress site.
Then go to Plugins -> JSON API -> Edit
Open the json-api/controllers/core.php file and add the following code as the last functions in the class.

function get_acf_fields()
{
  global $json_api;
  extract($json_api->query->get(array('id', 'slug', 'page_id', 'page_slug', 'children')));

  // vars
  $return = array();
  $keys = get_post_custom_keys($id);

  if($keys)
  {
    foreach($keys as $key)
    {
      if(strpos($key, 'field_') !== false)
      {
        $field = $this->get_acf_field($key, $id);
        $return[$field['order_no']] = $field;
      }
    }
   
    ksort($return);
  }

  // return fields
  return $return; 
}

function get_acf_field($field_name, $post_id = false)
{
  $post_id = $post_id ? $post_id : $this->get_post_meta_post_id($field_name);
  $field = get_post_meta($post_id, $field_name, true);
  return $field; 
}