.getSetCookie() and .get('set-cookie') methods are both broken on headers()

[controversial]

Verify canary release

- [X] I verified that the issue exists in the latest Next.js canary release

Provide environment information

```bash Operating System: Platform: darwin Arch: arm64 Version: Darwin Kernel Version 23.0.0: Tue Aug 1 03:25:51 PDT 2023; root:xnu-10002.0.242.0.6~31/RELEASE_ARM64_T6000 Binaries: Node: 20.2.0 npm: 9.8.1 Yarn: N/A pnpm: N/A Relevant Packages: next: 13.4.15 eslint-config-next: 13.4.15 react: 18.2.0 react-dom: 18.2.0 typescript: 5.1.6 Next.js Config: output: N/A ```

Which area(s) of Next.js are affected? (leave empty if unsure)

App Router, Middleware / Edge (API routes, runtime), TypeScript (plugin, built-in types)

Link to the code that reproduces this issue or a replay of the bug

https://codesandbox.io/p/sandbox/fervent-bird-ylrc3x?file=/app/page.tsx

To Reproduce

1. Set multiple cookies on the response, e.g. in middleware
2. Read headers().get('set-cookie') in a React Server Component.
   • Observe that the two Set-Cookie header values are concatenated into a single comma-separated string
   • This is broken, spec-incompliant behavior, as pointed out in https://github.com/vercel/next.js/issues/38302, https://github.com/vercel/next.js/issues/46579, and https://github.com/vercel/next.js/issues/46355
3. Call headers().getSetCookie() (which can be called, even though its TypeScript definition is missing from Next.js)
   • Observe that an empty array is returned, even though headers.get('set-cookie') returns a value (albeit incorrect) for the Set-Cookie header.

Describe the Bug

• The headers().get('set-cookie') method concatenates multiple values into a single comma-separated string, which is incorrect behavior and explicitly violates the HTTP specification:

  Origin servers SHOULD NOT fold multiple Set-Cookie header fields into a single header field. The usual mechanism for folding HTTP headers fields (i.e., as defined in [RFC2616]) might change the semantics of the Set-Cookie header field because the %x2C (",") character is used by Set-Cookie in a way that conflicts with such folding.

• The headers().getSetCookie() method returns an empty array when headers().get('set-cookie') does not

• TypeScript definitions for headers().getSetCookie() are missing

This blocks applications from reading cookies that are updated by middleware as part of the same request, which is a common flow for use cases such as token refreshing (see discussions like https://github.com/vercel/next.js/discussions/50374, https://github.com/vercel/next.js/discussions/38650)

Expected Behavior

• The headers().get('set-cookie') method should not concatenate multiple values into a single comma-separated string
• The headers().getSetCookie() method (spec) should return a non-empty array, containing the separate values of the multiple Set-Cookie headers

Which browser are you using? (if relevant)

N/A

How are you deploying your application? (if relevant)

N/A

_NEXT-1523

[viktorlarsson]

Great find, currently blocking a major release for us. Related: https://github.com/nextauthjs/next-auth/pull/7443#issuecomment-1677261203

[balazsorban44]

The headers().get('set-cookie') method should not concatenate multiple values into a single comma-separated string

Although the HTTP spec says so, the Headers spec says the opposite for .get():

https://fetch.spec.whatwg.org/#dom-headers-get

Hence the introduction of .getSetCookie() in the first place. The bug here is that the implementation for it might be wrong.

As for TypeScript, it's still missing from the official types https://github.com/microsoft/TypeScript/issues/55270 but we can probably extend it via edge-runtime (what Next.js is using) where we can guarantee the existence of this property.

Related: https://github.com/vercel/edge-runtime/issues/536

Also note, Headers.getAll is non-spec and should not be relied on. It was a platform-specific implementation before the getSetCookie spec existed (https://github.com/whatwg/fetch/issues/973)

To sum it up, it looks like we have to make sure .getSetCookie() works as expected. The rest looks good to me.

[balazsorban44]

Upon some further investigation, it turns out that the only (new) issue here is the missing typings for getSetCookie. The fact that headers().get('set-cookie') returns something, in this case, is a bug.

You are setting response headers in your middleware, and headers() is supposed to read request headers. There is already an open issue for it at #52564

[balazsorban44]

So the type is actually out in typescript@5.2.1-rc https://github.com/microsoft/TypeScript/issues/55270#issuecomment-1683706360 just have to wait until it gets to latest. For now, the workaround is @ts-expect-error

[controversial]

Thanks for looking into it @balazsorban44!

[github-actions[bot]]

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