How do you add custom fields defined in posts to the rest API responses in wordpress
First you need to register_rest_fields to adding custom endpoints in WP REST API JSON Response
add_action( 'rest_api_init', 'add_custom_fields' );function add_custom_fields() {register_rest_field('post', 'custom_fields', //New Field Name in JSON RESPONSEsarray( 'get_callback' => 'get_custom_fields', // custom function name 'update_callback' => null, 'schema' => null, ));}Then define your functions to get custom fields
function get_custom_fields( $object, $field_name, $request ) {//your code goes herereturn $customfieldvalue;}Tested on local site
Use this plugin for this solution:https://wordpress.org/plugins/acf-to-rest-api/ as it allows you to expose the Advanced Custom Fields into their own JSON response via new endpoints listed here https://github.com/airesvsg/acf-to-rest-api#endpoints. You can then forkJoin the two observables (if using Angular) and utilize the combined response toward your purpose.