A tiny Mockttp script that creates a response-rewriting proxy using Mockttp.

mockttp-response-rewriting.js

const mockttp = require('mockttp');

const server = mockttp.getLocal();

server.start().then(async () => {
    // This creates a proxy rule, which will forward all traffic to the real server:
    await server.anyRequest().thenPassThrough({
        // Every time the real server gets a response, this callback will be called:
        beforeResponse: (response) => {
            console.log(`Got ${response.statusCode} response with body: ${response.body.text}`);

            return {
                // TODO: Any values set here replace parts of the response:
                body: "replacement body"
            };
        }
    });

    console.log(`Server running on port ${server.port}`);
});

• Save this as index.js
• In the same folder, run npm install mockttp
• Run node index.js to start the server
• It will print the port it uses (8000 by default, if it's available). If you use that as a proxy, every response will go through the callback above.
  • You can test this manually with curl http://localhost:8000 -H"Host: example.com" (this simulates transparent proxying: it sends a request to localhost on port 8000 whose headers say it's intended for example.com). It should print the HTML from example.com in the console, and return "replacement body" in the CURL response.
• To rewrite HTTP Toolkit traffic, create a mock rule that redirects the traffic you're interested in to this server, preserving the Host header, like so:
  
• Edit the code inside beforeResponse to change how the response is logged or rewritten.
  • The full docs for the callback are here: https://httptoolkit.github.io/mockttp/interfaces/_rules_requests_request_handlers_.passthroughhandleroptions.html#beforeresponse.
  • Ctrl-C, node index.js to restart the server.