WP REST API - SNIPPETS

allow-cors-origin.php

// Access-Control-Allow-Origin
add_action( 'rest_api_init', function() {
	remove_filter( 'rest_pre_serve_request', 'rest_send_cors_headers' );
	add_filter( 'rest_pre_serve_request', function( $value ) {
		$origin = get_http_origin();
		if ( $origin && in_array( $origin, array(
				'http://localhost'
			) ) ) {
			header( 'Access-Control-Allow-Origin: ' . esc_url_raw( $origin ) );
			header( 'Access-Control-Allow-Methods: POST, GET, OPTIONS, PUT, DELETE' );
			header( 'Access-Control-Allow-Credentials: true' );
		}
		return $value;
	});
}, 15 );

modifying-response.php

// REST API - register and set parent category
add_action( 'rest_api_init', 'register_parent_category' );
function register_parent_category() {
    register_rest_field( 'post',
        'parent_category',
        array(
            'get_callback'    => 'set_parent_category',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}
function set_parent_category( $object, $field_name, $request ) {
    if ( empty( $object['categories'] ) ) return false;
	foreach ( $object['categories'] as $key => $category ) {
		$parentCategory = get_ancestors( $category, 'category' );
		if ( !empty ( $parentCategory ) ) {
			return get_term( reset( $parentCategory ), 'category' );
		}
		else {
			return get_term( $category, 'category' );
		}
	}
}