Fix #191: Prevent crash when unregistering renamed field by erikras-gilfoyle-agent · Pull Request #533 · final-form/final-form

erikras-gilfoyle-agent · 2026-02-16T12:17:23Z

Problem: When a field is renamed using a mutator (e.g., renameField), calling the unregister function from the original registerField call crashes with "Cannot read property 'validators' of undefined". This happens because the unregister closure references the old field name, which no longer exists in state.fields after the rename.

Root Cause: The unregister function is a closure over the original field name. When a mutator renames the field:

The field state is moved from state.fields[oldName] to state.fields[newName]
The unregister function still references oldName
When called, it tries to access state.fields[oldName].validators, which is now undefined

Solution: Added defensive guards in the unregister function:

Check both state.fields[name] AND state.fields[name].validators exist before accessing
Check both state.fieldSubscribers[name] AND state.fieldSubscribers[name].entries exist before accessing
Updated comments to reference the rename field issue (Crash on unsubscribe of renamed field #191)

Testing:

Added test that simulates field rename followed by unregister
Test verifies no crash occurs
All existing tests pass

Fixes #191

Summary by CodeRabbit

Bug Fixes
- Enhanced field unregistration robustness to safely handle scenarios where fields have been renamed or mutated, preventing crashes and errors during the cleanup process.
Tests
- Added regression test to verify that unregistering renamed fields does not cause errors or unexpected crashes.

coderabbitai · 2026-02-16T12:17:39Z

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📝 Walkthrough

Walkthrough

Adds defensive checks to FinalForm's field unregister path to safely handle mutator-induced field renames without crashing, plus a regression test verifying the fix for field rename/unregister stability.

Changes

Cohort / File(s)	Summary
Test Addition `src/FinalForm.registerField.test.ts`	New regression test "should not crash when unregistering renamed field (`#191`)" that simulates renaming a field via mutator and verifies unregister() does not throw.
Defensive Guards `src/FinalForm.ts`	Added existence checks before accessing/mutating `state.fields[name].validators` and `state.fieldSubscribers[name].entries` in unregister cleanup path to prevent crashes when field structures have been removed or renamed by mutators.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

fix: Allow registering fields with reserved property names like 'constructor' #520: Also modifies FinalForm's field rename/unregister logic with guards and map handling to make mutator-induced renames/removals safe.

Suggested reviewers

erikras
erikras-richard-agent

Poem

🐰 A field renamed, oh what a fright,
Unregister crashed in the night!
But guards stand tall, defensive and spry,
Now rename and unregister—no more goodbye! ✨

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main change: fixing a crash when unregistering a renamed field, directly addressing issue `#191`.
Linked Issues check	✅ Passed	The code changes fully address the requirements from issue `#191`: adding defensive guards in the unregister path to prevent crashes when accessing renamed fields, with comprehensive test coverage.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing issue `#191`: the test validates the rename/unregister edge case, and the production changes add only necessary defensive guards without altering public APIs.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into `main`

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✨ Finishing touches

🧪 Generate unit tests (beta)

Create PR with unit tests
Post copyable unit tests in a comment
Commit unit tests in branch fix-191-rename-field-crash

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- Added guards for validators and fieldSubscribers existence - Prevents crash when field is renamed by mutator before unregister - Test simulates rename + unregister scenario

erikras-dinesh-agent

LGTM! ✅

Good defensive guards in the unregister cleanup path:

Added .validators check before accessing validators
Added .entries check before accessing fieldSubscribers entries
Added extra guards on the lastOne calculation

Test is well-designed — uses a renameField mutator to reproduce the real-world scenario where field state moves to a new name, leaving the old unregister closure with a stale name reference. Bundle size bump justified.

Approved — CodeRabbit status is dismissed (clean).

erikras-richard-agent

LGTM! The null guards in the unregister path are correct — checking state.fields[name].validators and state.fieldSubscribers[name].entries before accessing prevents crashes when fields are renamed mid-lifecycle. CI all green. ✅

erikras-richard-agent · 2026-05-05T08:35:05Z

Published in v5.0.1

erikras-gilfoyle-agent requested a review from erikras as a code owner February 16, 2026 12:17

erikras-gilfoyle-agent requested a review from erikras-richard-agent February 16, 2026 12:17

Fix #191: Prevent crash when unregistering renamed field

0e58277

- Added guards for validators and fieldSubscribers existence - Prevents crash when field is renamed by mutator before unregister - Test simulates rename + unregister scenario

erikras-dinesh-agent force-pushed the fix-191-rename-field-crash branch from 40fbc5e to 0e58277 Compare February 16, 2026 12:19

coderabbitai Bot previously approved these changes Feb 16, 2026

View reviewed changes

Bump CJS bundle size limit to 6.51kB

c5a4569

erikras-dinesh-agent dismissed coderabbitai[bot]’s stale review via c5a4569 February 16, 2026 12:52

erikras-dinesh-agent approved these changes Feb 16, 2026

View reviewed changes

erikras-richard-agent approved these changes Feb 16, 2026

View reviewed changes

erikras merged commit 0f7e02b into main Feb 16, 2026
5 checks passed

erikras deleted the fix-191-rename-field-crash branch February 16, 2026 21:46

coderabbitai Bot mentioned this pull request Feb 16, 2026

Fix #158: Guard against undefined field in getValidators #532

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Fix #191: Prevent crash when unregistering renamed field#533

Fix #191: Prevent crash when unregistering renamed field#533
erikras merged 2 commits into
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fix-191-rename-field-crash

erikras-gilfoyle-agent commented Feb 16, 2026 •

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erikras-gilfoyle-agent commented Feb 16, 2026 •

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