Modify server response using node http proxy

I am a beginner to node js and this is my first post in here so apologies if this is a stupid question.

The problem I need to solve is to change information in an HTTPS server response before it hits the client. The reason is that my my client (Im developing a GARMIN sports watch application) cannot read the server response as the body data format differs from the content_type as defined in the response header. I have no means of impacting the server code and on the client I am stuck as well, Garmin has acknowledged it should be fixed but have not put it on priority.

I guess that leaves me with having to implement my own http proxy that I can connect in between the client and server to change either the header or the data format in the server response.

I have browsed around a bit and also played around a bit with node-http-proxy but I am not sure what is the best solution approach. Basically, the SERVER expects HTTPS comms, and preferably I also want secured comms between the PROXY and the CLIENT (since they will exchange user credentials).

I guess my first question is if this is a possible use case:

CLIENT -> (HTTPS_POST_req) -> PROXY -> (HTTPS_POST_req) -> SERVER SERVER -> (HTTPS_resp) -> PROXY -> (modified_HTTPS_resp) -> CLIENT

I took a quick shot at it using node-http-proxy sample code but got stuck. What I did was to create an http server on localhost and a proxy server that would proxy the request to SERVER.

const http      = require('http');
const httpProxy = require('http-proxy');
const proxy     = httpProxy.createProxyServer({});

http.createServer(function(req, res) {
    console.log('incoming request');
    console.log('URL:' + req.url);
    console.log('Method:' + req.method);
    console.log('Headers:' + JSON.stringify(req.headers));
    proxy.web(req, res, { target:'SERVER ADDRESS HERE' });
}).listen(8888, () => {
  console.log("Waiting for requests...");
});

From the console I can see the incoming request and also read the url, headers and method but then after a short while I get this:

Error: connect ECONNREFUSED 127.0.0.1:80
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16) {
  errno: 'ECONNREFUSED',
  code: 'ECONNREFUSED',
  syscall: 'connect',
  address: '127.0.0.1',
  port: 80
}

I have no idea why something want to connect to localhost on port 80.

Perhaps this approach will never work for HTTPS/SSL traffic? For instance, do I have to setup my own server certificate to do this or can node-http-proxy handle it "under the hood"? Need a bit of help, please.

//Fredrik