snapshot-testing

name	snapshot-testing
description	Snapshot testing — when to use, maintenance, CI integration.

When to Use

Use snapshots for	Do NOT use snapshots for
UI components with stable, complex render output	Business logic with frequently changing return values
Serialized data structures (API response shape)	Anything that changes on every run (timestamps, random IDs)
Error message strings that must stay stable	Core algorithm correctness — use explicit assertions
CLI command output format	Database query results in integration tests

Rule of thumb: if a snapshot update is expected more than once per sprint for a given component, snapshots are the wrong tool for that component.

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Snapshot Rules

• Always review snapshot diffs in PR. A snapshot diff is a behavioral change — treat it like a code change, not noise.
• Never auto-accept snapshots with --updateSnapshot without visually confirming the new output is correct.
• Delete stale snapshots immediately. A snapshot for a deleted component is dead weight and can mask regressions.
• Snapshots are not a substitute for behavioral assertions. They verify structure; they do not verify correctness. Pair snapshots with explicit expect(value).toBe(...) assertions for critical logic.

Size limit

If a snapshot file exceeds 50 lines, the component under test is too complex. Options:

1. Break the component into smaller units and snapshot each independently
2. Test the component's behavior with targeted assertions instead of a full render snapshot
3. Extract sub-components and snapshot them separately

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Updating Snapshots

# Jest
jest --updateSnapshot

# Vitest
vitest --update

# Run only tests for a specific component
jest src/components/Button --updateSnapshot

Before running update:

1. Confirm the visual/structural change was intentional (design change, new feature)
2. Run the app locally and visually verify the component
3. After updating, re-run tests without --updateSnapshot to confirm they pass clean

Commit message when updating snapshots:

test(Button): update snapshot after adding disabled state variant

Always explain WHY the snapshot changed — not just "update snapshots".

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CI Integration

• Snapshots must be committed to the repository — never .gitignore snapshot directories
• CI runs tests without --updateSnapshot; if a snapshot changed without an update commit, the build fails
• A failing snapshot in CI means either:
  1. A developer forgot to commit the updated snapshot, OR
  2. An unintended regression — investigate before accepting

Jest config example

// jest.config.js
module.exports = {
  snapshotSerializers: ['@emotion/jest/serializer'],  // if using CSS-in-JS
  // CI will fail automatically if snapshots are outdated — no extra config needed
};

Snapshot file location

• Keep snapshot files co-located with tests: src/components/__snapshots__/Button.test.tsx.snap
• Do not move snapshots to a central directory — co-location makes ownership clear

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Common Mistakes

Mistake	Fix
Snapshotting the entire page/route	Snapshot only the component under test
Not seeding deterministic test data	Mock Date.now(), UUIDs, and random values
Leaving obsolete snapshots after component rename	Run jest --ci with --verbose to find orphaned snapshots
Committing --updateSnapshot in CI script	CI must never auto-update; only developers update locally

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Checklist

• Snapshots reviewed in every PR that touches UI components
• No snapshot file exceeds 50 lines
• Snapshots committed alongside test files
• CI does not pass --updateSnapshot
• All snapshot updates have a commit message explaining the change