responseInterceptor changes headers

Is this a bug report?

Yes

Steps to reproduce

I have a (nasty) legacy app that I’m trying to migrate away from, and I’m using http-proxy-middleware to wrap endpoints of the new and old backends. Some of the old app serves a horrible mashup of PHP, JS, and HTML, often with db queries embedded directly in the HTML/PHP files. I need to route to these, but I also need to make some changes to the HTML, pending a real overhaul of the backend. The responseInterceptor should be the perfect tool for this, but for some reason the response I get never loads: i.e. when I try to log in to our web app (legacy API), rather than serving me the homepage, I get redirected back to the login page. I recognize this may be a problem on my end, but the same process works fine if I use proxyRes.on("data", (data) => {};

This breaks:

router.use("/", proxy({
  target: <my app url>,
  changeOrigin: true,
  ws: true,
  selfHandleResponse: true,
  onProxyRes: responseInterceptor(
    async (responseBuffer, proxyRes, req, res) => {
      return responseBuffer;
    }
  ),
}));

This returns headers:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
content-type: text/html; charset=UTF-8
Connection: close
content-length: 120251
Date: Tue, 20 Apr 2021 12:28:42 GMT

This works fine:

router.use("/", proxy({
  target: <my app url>,
  changeOrigin: true,
  ws: true,
  onProxyRes: (proxyRes, req, res) => {
    proxyRes.on("data", (data) => {
      const bufferAsString = data.toString("utf-8");
      console.log(bufferAsString);
    });
  },
}));

Which returns the desired headers:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
server: nginx/1.18.0 (Ubuntu)
date: Tue, 20 Apr 2021 12:18:51 GMT
content-type: image/svg+xml
content-length: 2188
connection: close
last-modified: Thu, 28 Jan 2021 09:44:48 GMT
etag: "60128790-88c"
expires: Tue, 20 Apr 2021 13:18:51 GMT
cache-control: max-age=3600, public
accept-ranges: bytes

Expected behavior

I would expect responseInterceptor, when used in such a way that it does not modify the response buffer, to be side-effect free.

Actual behavior

Instead, it breaks my app ☹️

Setup

• http-proxy-middleware: 1.1.2
• http-proxy-middleware configuration: vanilla; see snippets above
• server: express 4.17.1

client info

Firefox/Chrome

[chimurai]

Thanks for your report.

I’ll have to investigate this. Think when server responds with cookies it might cause issues too.

Did you identify which header was needed/missing?

If it is: Access-Control-Allow-Origin: *, you can try to set it manually:

router.use("/", proxy({
  target: <my app url>,
  changeOrigin: true,
  ws: true,
  selfHandleResponse: true,
  onProxyRes: responseInterceptor(
    async (responseBuffer, proxyRes, req, res) => {

      // set headers manually
      res.setHeader('Access-Control-Allow-Origin', '*);

      return responseBuffer;
    }
  ),
}));

[chimurai]

Thanks for the kind words!

Published the fix in v1.2.1