@mcollina the more I look at this the more I realize I do not actually understand one essential part. I will happily add some tests but from a usage perspective can you please assist with the following? Note I am still new to Fastify so I am also getting up to speed on some things.

In fastify-reply-from the example (https://github.com/fastify/fastify-reply-from/blob/master/test/transform-body.js#L25-L36) shows this pattern

instance.get('/', (request, reply) => {
  reply.from(`http://localhost:${target.address().port}`, {
    onResponse: (res) => {
      reply.send(res.pipe(new Transform({
        transform: function (chunk, enc, cb) {
          this.push(chunk.toString().toUpperCase())
          cb()
        }
      })))
    }
  })
})

This is very clear to me as the (request, reply) is available and I have access to reply.send(...)

My main issue is I am not sure how to do the same with fastify-http-proxy

In this case I have the follwing code:

fastify.register(proxy, {
    upstream: 'myUrl',
    prefix: '/apiProxy',
    async beforeHandler(request, reply) {
      console.log('beforeHandler from proxy called'); // works as expected
    },
    replyOptions: {
      rewriteHeaders: headers => Object.assign({ 'x-from-proxy': 'yes-indeed' }, headers), // works as expected
      onResponse: (res) => {
        // At this point in the code how to I send the reformatted/modified response? What has access to the reply object?
      },
    },
  });

As far as i can tell reply-from just streams the res back to me (https://github.com/fastify/fastify-reply-from/blob/master/index.js#L103) I just need to know where I can get access to a reply object or how I would actually send the res after I do my transformation.

Thanks again for the help.

You'll need to wrap that function so that req and reply are available.

fastify.register(proxy, {
    upstream: 'myUrl',
    prefix: '/apiProxy',
    async beforeHandler(request, reply) {
      console.log('beforeHandler from proxy called'); // works as expected
    },
    replyOptions: {
      rewriteHeaders: headers => Object.assign({ 'x-from-proxy': 'yes-indeed' }, headers), // works as expected
      onResponse: (req, reply, stream) => {
        reply.send(stream)
      },
    },
  });

@mcollina apologies on this. I tried that out and tried hacking in a few things here and there to fastify-reply-from but I did not have any luck. If you have any other tips I would be very grateful.

Something like this should do the trick:

  test('passes onResponse option to reply.from()', async (t) => {
    const proxyServer = Fastify()

    proxyServer.register(proxy, {
      upstream: `http://localhost:${origin.server.address().port}`,
      prefix: '/api',
      replyOptions: {
        onResponse (reply) {
             reply.send({ something: 'else' })
          }
      }
    })

    await proxyServer.listen(0)

    t.tearDown(() => {
      proxyServer.close()
    })

    const { body } = await got(`http://localhost:${proxyServer.server.address().port}/api`)
    t.match(body, { something: 'else' })
  })

fastify-reply-from was updated in fastify/fastify-reply-from#43

@mcollina I think this is updated with all of the latest and the new test. Running it through Standard on my machine made a bunch of changes that I can change back if needed. 🤷‍♂️

Thanks again.