feat(store): return callback result from batch and add subscriber test coverage

[grzdev]

🎯 Changes

• Updates �atch(fn: () => T): T in @tanstack/store (packages/store/src/atom.ts) so it returns the value produced by the batched function execution.
• Adds comprehensive test coverage in �atch.test.ts verifying that:
  1. Return values are correctly passed through �atch.
  2. Store subscribers are notified exactly once with the final value after the entire batch finishes rather than on intermediate state transitions.

✅ Checklist

• I have read the contributing guide
• Tests pass locally (pnpm --filter @tanstack/store run test:lib)
• ESLint passes locally (pnpm --filter @tanstack/store run test:eslint)
• TypeScript type checks pass locally (pnpm --filter @tanstack/store run test:types)

🚀 Release Impact

• Published package changes (requires changeset)
• Tests / CI / Dev only

Summary by CodeRabbit

• New Features

  • batch now returns the value produced by its callback.

• Tests

  • Added coverage confirming that subscribers are notified once after batched updates, with the final state, and that batch returns the callback’s value.

[changeset-bot]

⚠️ No Changeset found

Latest commit: e4821f3

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[coderabbitai]

Important

Review skipped

Review was skipped as selected files did not have any reviewable changes.

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• Configuration used: Repository: TanStack/store/.coderabbit.yaml
• Review profile: CHILL
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• Run ID: 7c0a01ba-a5d7-47be-b982-33c058fba0ea

📥 Commits

Reviewing files that changed from the base of the PR and between 51ddb30 and 924f7ff.

📒 Files selected for processing (2)

• packages/store/src/atom.ts
• packages/store/tests/batch.test.ts

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📝 Walkthrough

Walkthrough

batch now returns the value produced by its callback. Tests check that multiple updates in a batch notify subscribers once after completion, with the final state.

Changes

Batch behavior

Layer / File(s)	Summary
Batch return and flush behavior packages/store/src/atom.ts, packages/store/tests/batch.test.ts	batch returns its callback’s value and retains its existing flush behavior. Tests check one subscriber notification after three updates and verify the returned value.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Suggested reviewers: sheraff

Merge Risk: ⚪ Minimal · up to 924f7

The change exposes the batch callback’s result without altering flush behavior, and the added tests check the final subscriber notification. No actionable merge-blocking risk is evident in the reviewed changes.

Pre-merge checks | 3 | 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check		The description includes all required sections and explains the implementation and tests. However, it marks the change as tests/CI/dev-only even though packages/store/src/atom.ts changes the public …	Correct the Release Impact section. Mark the change as affecting published code and add the required changeset, or provide repository-specific evidence that this package change is not released.
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes both primary changes: returning the callback result from batch and adding subscriber test coverage.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description includes all required sections and explains the implementation and tests. However, it marks the change as tests/CI/dev-only even though packages/store/src/atom.ts changes the public batch API and may affect published code.

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