Would be happy to modify the tests. But I'm not sure what to modify. Also, running the tests hang on my laptop, after the following output:

% npm test

> proxy-chain@2.5.5 test
> nyc cross-env NODE_OPTIONS=--insecure-http-parser mocha --bail



(node:232095) Warning: Using insecure HTTP parsing
(Use `node --trace-warnings ...` to show where the warning was created)
  ✔ supports localAddress
  ✔ supports custom CONNECT server handler
  ✔ supports pre-response CONNECT payload
  utils.anonymizeProxy
    ✔ throws for invalid args
    ✔ throws for unsupported https: protocol
    ✔ throws for invalid ports
    ✔ throws for invalid URLs
    ✔ keeps already anonymous proxies (both with callbacks and promises)
    1) anonymizes authenticated upstream proxy (both with callbacks and promises)


  8 passing (63ms)
  1 failing

  1) utils.anonymizeProxy
       anonymizes authenticated upstream proxy (both with callbacks and promises):
     AssertionError: expected 'socket hang up' to include 'ECONNREFUSED'
      at /home/arno/src/proxy-chain/test/anonymize_proxy.js:236:48
      at processTicksAndRejections (node:internal/process/task_queues:95:5)

Would be happy to modify the tests. But I'm not sure what to modify. Also, running the tests hang on my laptop, after the following output:

% npm test

> proxy-chain@2.5.5 test
> nyc cross-env NODE_OPTIONS=--insecure-http-parser mocha --bail



(node:232095) Warning: Using insecure HTTP parsing
(Use `node --trace-warnings ...` to show where the warning was created)
  ✔ supports localAddress
  ✔ supports custom CONNECT server handler
  ✔ supports pre-response CONNECT payload
  utils.anonymizeProxy
    ✔ throws for invalid args
    ✔ throws for unsupported https: protocol
    ✔ throws for invalid ports
    ✔ throws for invalid URLs
    ✔ keeps already anonymous proxies (both with callbacks and promises)
    1) anonymizes authenticated upstream proxy (both with callbacks and promises)


  8 passing (63ms)
  1 failing

  1) utils.anonymizeProxy
       anonymizes authenticated upstream proxy (both with callbacks and promises):
     AssertionError: expected 'socket hang up' to include 'ECONNREFUSED'
      at /home/arno/src/proxy-chain/test/anonymize_proxy.js:236:48
      at processTicksAndRejections (node:internal/process/task_queues:95:5)

Hey, for the tests, you can just see them run here in GitHub actions.
If you're adding a test locally, you can just run the test you've added and will (hopefully) work in CI as well.

I added a test. I was not able to connect to a https server in the test. But in the test, I can check that proxyServer does not consider the relay url as invalid. The code snippet will fail without the PR, and pass with it.