How to create a server that is only internal to the app in node / npm or modify any response body from outgoing requests

I am trying to develop a node app and require a server that can only be used by the app internally.

I have tried instantiating a server without listening to the port, but can't do anything with it from that point forwards:

let http = require("http");
http.createServer(function (req, res) {
    // custom code
})

This app is being built with NWJS and I need to intercept any outgoing requests (including file resources; CSS, JS, images, etc.) and modify the response, but I am not having any success with it except if I use a server for this purpose.

Problem is it becomes possible to open that server on any browser and I just want it to be used only inside the app or another way to intercept outgoing requests from the app so that the response body can be modified.

Solution

I have tried Keith's suggestion of using a service worker to intercept requests, but in my case I could not load a service worker from local environment into live environment (for example, run a local sw file in stackoverflow page), so that suggestion ended there.

Stdob's suggestion of using a proxy ended up being redundant and more troublesome than my original attempt.

In the end I went with my original attempt as follows:

Using chrome.webRequest.onBeforeRequest (Chrome API) and a local node server.
The server is created with an arbitrary port to reduce the risk of hitting an already used port.
The chrome API redirects all connections to the local server (ex. url: http://127.0.0.1:5050) and then the server will handle the requests as needed, returning the requested files modified or intact.
Last step, add a unique header with a unique value that only the app knows, so that no server access can be made from outside the app.

It is not the best solution, ideally I would prefer to have something like Firefox's webRequest.filterResponseData, but until Chrome implements that, this will have to do.