fastify-http-proxy (fastify/reply-from) replyOptions.onResponse getting the original response body example

example.ts

'use strict';

import fastify, { FastifyReply, FastifyRequest, RawReplyDefaultExpression, RawServerBase, RequestGenericInterface } from 'fastify';
import proxy from '@fastify/http-proxy';

const server = fastify();

server.register(proxy, {
    upstream: 'http://proxied.tld',
    prefix: '/proxied',
    replyOptions: {
        onResponse: onResponse
    }
});

async function onResponse(request: FastifyRequest<RequestGenericInterface, RawServerBase>, reply: FastifyReply<RawServerBase>, res: RawReplyDefaultExpression<RawServerBase>): Promise<void> {

    /**
     * Get the original response from the proxied server.
     */
    const result = await new Promise((resolve, reject) => {

        let originalResponse: string = '';

        // We assume response is a buffer that can be converted to a string.
        res.on('data', (args: Buffer) => { originalResponse += args.toString(); });

        res.on('end', () => {
            /**
             * Then assume that string is JSON-parseable.
             * Here we get the response body (type {statusCode: number, uuid: string}) from original response `res`
             */
            const json = JSON.parse(originalResponse);

            // Assume the 200 status code is the only we want to process. For others return early.
            if (json.statusCode !== 200) { return resolve(200); }

            // At last call the domain function.
            createUser(json.uuid)
                .then(() => { return resolve(201); })
                .catch((error) => { return reject(error); });
        });

        res.on('error', (error: Error) => {
            return reject(error);
        });

    });

    /**
     * Finally respond from the local server with something appropriate.
     * Here result should be 200 or 201 status code from the `resolve`'s above. 
     * 
     */
    reply.send(result);

}

/**
 * The domain function doing something meaningful.
 */
async function createUser(uuid: string): Promise<void> {
    console.log(uuid);
}