Nodejs express mitm proxy stub

index.js

const express = require('express');
var proxy = require('express-http-proxy');
const cheerio = require('cheerio');
const nunjucks = require('nunjucks');

const host = 'http://hostname-or-ip/';

var app = express();
// some engine to serve own pages, not strictly necessary
nunjucks.configure('templates', {
    express: app
});

app.use('/', proxy(host, {
    userResDecorator: (proxyRes, proxyResData, userReq, userRes)=>{
        if (/url_matcher/.test(userReq.url)){
            console.log('patching');
            let $ = cheerio.load(proxyResData.toString('utf8'));
            $('head').append(`<script src="/inject/injected.js"></script>`);
            return $.html();
        }
        else
            return proxyResData;
    },
    filter: function(req, res) {
        //return true to proxy, false to not proxy
        return !/inject\/.+/.test(req.url);
    }
}));
app.use(express.json());
// render a nunjucks template if necessary
app.get('/inject/placeholder', (req, res)=>{
    res.render('placeholder.html', {templatevar: 'value'});
});
app.use('/inject', express.static('static', {setHeaders: (res, path)=>{
    //allow service worker installations for server paths
    if (/service_worker\.js/.test(path))
        res.setHeader('Service-Worker-Allowed', '/worker_scope/path');
}}));
// run
app.listen(5000);

package.json

{
    "name": "proxy stub",
    "version": "1.0.0",
    "description": "man in the middle http proxy stub",
    "license": "MIT",
    "author": "e-neko",
    "dependencies": {
        "body-parser": "^1.19.0",
        "cheerio": "^1.0.0-rc.3",
        "cookie-parser": "^1.4.5",
        "express": "^4.17.1",
        "express-http-proxy": "^1.6.0",
        "http-proxy": "^1.18.1",
        "nunjucks": "^3.2.2"
    }
}