How do you add custom fields defined in posts to the rest API responses in wordpress

How do you add custom fields defined in posts to the rest API responses in wordpress.md

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 RESPONSEs
array(
    '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 here
return $customfieldvalue;
}

Tested on local site