feat: zstandard support

[Phillip9587]

This PR introduces support for the Zstandard (Zstd) algorithm, which was released as an experimental feature in Node.js 23.8.0.

Key Changes

• Adds support for Zstandard (Zstd) as an optional compression algorithm.

Notes

• This feature is experimental and subject to changes in future Node.js releases.
• Requires Node.js 23.8.0+ to function correctly.

---

closes#560

[bjohansebas]

I would really prefer to wait until the API becomes stable. This is the same stance I’m going to take with compression. There may be no changes during the transition from experimental to stable, but it’s better to use only stable APIs

Great job anyway!

[wesleytodd]

I agree, we should wait until it is stable then just enable support without an experimental phase.

[DienoX]

Isn't it that time already?

[Phillip9587]

Zsdt is still experimental as of Node.js 25.6.1: https://nodejs.org/api/zlib.html#class-zlibzstddecompress

[Phillip9587]

As @bjohansebas mentioned in expressjs/compression#250 (comment), several ecosystem packages have already adopted zstd. Should we revisit this discussion? Zstd was backported to Node.js v22.15.0, so once Node.js 20 LTS reaches EOL in April, all active LTS lines will include zstd support.

[Phillip9587]

@UlisesGascon@jonchurch What are your opinions on this topic?

[Phillip9587]

@UlisesGascon@wesleytodd All active LTS Node.js Versions now contain the Zstandard support. It is still experimental but already adopted by fastify in https://github.com/fastify/fastify-compress and by other ecosystem packages. I think we should also support it and document in which node.js versions zstd is available.

[Phillip9587]

Hey @jonchurch@UlisesGascon, Can I please get some feedback on this?

[bjohansebas]

LGMT!

[Phillip9587]

Hi @wesleytodd, @UlisesGascon, @jonchurch, could you please review this PR? I believe it would be great to get this moving. Thanks!

[Phillip9587]

After asking for reviews since May 12 I'm just going to merge this now. We can still revert it if repo captains disagree