Proposal: Multiple Snapshot Formats

[avivkeller]

As discussed in today's meeting, I feel that we should apply the same logic that we do to finding tests that we do to finding snapshots.

As for generating them, I think we should generate the snapshot either:

• Always CJS (Status Quo)
• Always ESM
• Use module from package.json
• Use --experimental-snapshot-format=[cjs|mjs|json]
• Use type of test (e.g. .test.cjs -> .snapshot.cjs)

There are plenty of times where it would be beneficial to support JSON snapshots (and ESM). For instance, if a non-JS runtime needs to access these files, they may not have the parsing capability to deconstruct a CJS file, but they will have the ability to read a JSON fiel.

[Copilot]

Pull request overview

Adds a written proposal to extend the Node.js test runner’s snapshot discovery to support multiple snapshot file extensions, analogous to how test files are discovered.

Changes:

• Introduces a new proposal document describing multi-extension snapshot resolution behavior.
• Defines an example resolution order for snapshot extensions.
• Provides an example error when multiple candidate snapshot files are found.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.