http-proxy-middleware blocking web page - node.js

Question

Do you have any idea why when opening the following web page on the browser it keeps loading (blocking)?
http://localhost:3002/
If you place a return; just before the line: res.writeHead = (...args) => { writeHeadArgs = args; };, the page works normal but the logic of the code is not executed. If you place the return after that line, it keeps loading forever.
Run
$ node server.js
server.js
var express = require('express');
var http = require('http');
var proxy = require('http-proxy-middleware');
function bodyTransform(body) {
return body.replace('World', 'People');
}
var onProxyRes = (proxyRes, req, res) => {
const end = res.end;
const writeHead = res.writeHead;
let writeHeadArgs;
let body;
var buffer = new Buffer('');
proxyRes.on('data', (chunk) => {
console.log("inside: .on('data', ...)");
buffer = Buffer.concat([buffer, chunk]);
});
proxyRes.on('end', () => {
console.log("inside: .on('end', ...)");
body = buffer.toString('utf8');
});
res.write = () => { };
res.writeHead = (...args) => { writeHeadArgs = args; };
res.end = () => {
const output = bodyTransform(buffer.toString());
res.setHeader('content-length', output.length);
if (writeHeadArgs)
writeHead.apply(res, writeHeadArgs);
end.apply(res, [output]);
console.log('after: end.apply(res, [output]);');
};
}
const portApp01 = 3001;
const app01 = express()
app01.get('/', (req, res) => {
res.send('Hello World!');
});
app01.listen(portApp01, () => {
console.log('app01 is listening on port: ' + portApp01);
});
const portProxy = 3002;
const appProxy = express()
appProxy.use(proxy({ target:'http://localhost:3001', onProxyRes: onProxyRes, changeOrigin: true }));
http.createServer(appProxy).listen(portProxy, function () {
console.log('appProxy is listening on port: ' + portProxy);
});
This is the output I get on the terminal:
[HPM] Proxy created: / -> http://localhost:3001
app01 is listening on port: 3001
appProxy is listening on port: 3002
inside: .on('data', ...)
inside: .on('end', ...)
after: end.apply(res, [output]);
Any idea on how to solve this?
Thanks!