Add Custom Fields to Rest API

functions.php

<?php
// =============================================================
// Add Custom Fields to Rest API
// =============================================================
function add_rest_api_custom_fields() {
  register_rest_field('post', 'authorName', array(
    'get_callback' => function() {return get_the_author();}
  ));
  register_rest_field('post', 'yourCustomField', array(
    'get_callback' => function() {return get_field('custom-field-name');}
  ));
}

add_action('rest_api_init', 'add_rest_api_custom_fields');