Headers is undefined when not handling request

[kentcdodds]

Environment

Name	Version
msw	0.38.1
node	16
OS	macOS

Request handlers

import { setupServer } from 'msw/node'

const server = setupServer()

server.listen()

Actual request

I'm not making the request, the AWS SDK is. The line that's breaking is this one though:

https://github.com/aws/aws-sdk-js/blob/6cf8513d4012e3bc07330c523cf10f45ce807ff5/lib/event_listeners.js#L412

Current behavior

When I install msw and use interception without providing any handlers, the headers object is undefined. If I add a console.log of the resp.httpResponse in the AWS SDK code, I get this:

Downgrading to MSW@0.36.8 resolves this issue

Expected behavior

I expect the headers to exist. I really expect the entire request to happen without any change when I don't handle the request.

[kettanaito]

Hey, @kentcdodds. Thanks for reporting this!

The issue seems related to mswjs/interceptors#212. Node 12 is the latest officially supported version for MSW. We will upgrade to newer versions soon, as 12 is a rather old one at this point.

I'm curious to know what changed in 16 that headers are not present anymore. I suspect that PassThrough that we use to clone IncomingMessage now behaves differently. I'd appreciate any input and help on this from anybody interested.

[trevoro]

I think the issue is that in Node 16 the headers are on the prototype and in Node 12 they're on the instance. When a node interceptor clones a message, it only clones the object instance values and not the prototype values.

https://github.com/nodejs/node/blob/v12.x/lib/_http_incoming.js#L61
https://github.com/nodejs/node/blob/v16.x/lib/_http_incoming.js#L107-L124

[kettanaito]

Yeap, @trevoro, that's the case. I like your initiative to update the supported Node.js version. That will include updating how we handle headers as well.

[github-actions]

🎉 This issue has been resolved in version 0.39.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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