fix: deepkeyandvalue behavior in union types by mdm317 · Pull Request #2057 · TanStack/form

mdm317 · 2026-03-03T07:07:26Z

🎯 Changes

Fix DeepKeysAndValues to return nullish values instead of never when the value type is nullish

main

type Test = DeepKeysAndValues<{ id : undefined }>
//        ^? never

ts playground

this branch

type Test = DeepKeysAndValues<{ id : undefined }>
//        ^? { key : 'id', value: undefined}

Due to the above change, I replaced DeepKeysOfType with DeepKeysOfNonNullableType in the group API.

#2057 (comment)

✅ Checklist

I have followed the steps in the Contributing guide.
I have tested this code locally with pnpm test:pr.

🚀 Release Impact

This change affects published code, and I have generated a changeset.
This change is docs/CI/dev-only (no release).

Change

Summary by CodeRabbit

Bug Fixes
- Improved type handling so field groups preserve nullish members in unions and reject field-group paths that are nullish-only.
Tests
- Added type-level tests covering nullish preservation, tuple/array index cases, and rejection of nullish-only field-group paths.
Chores
- Bumped form-related packages with a patch changeset.

changeset-bot · 2026-03-03T07:07:30Z

🦋 Changeset detected

Latest commit: 81806f8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@tanstack/react-form	Patch
@tanstack/solid-form	Patch
@tanstack/form-core	Patch
@tanstack/react-form-nextjs	Patch
@tanstack/react-form-remix	Patch
@tanstack/react-form-start	Patch
@tanstack/angular-form	Patch
@tanstack/form-devtools	Patch
@tanstack/lit-form	Patch
@tanstack/svelte-form	Patch
@tanstack/vue-form	Patch
@tanstack/react-form-devtools	Patch
@tanstack/solid-form-devtools	Patch

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mdm317 · 2026-03-03T07:21:56Z

+  ? [NonNullable<V>] extends [never]
+    ? never


Since the previous behavior inferred never when the value was nullish,
the changes were updated to continue inferring never for nullish value types.

mdm317 · 2026-03-03T07:36:19Z

+export type DeepKeysOfNonNullableType<TData, TValue> =
+  ExtractByNonNullableValue<DeepKeysAndValues<TData>, TValue>['key']


After changing DeepKeysAndValues to include nullish values, a bug was introduced in the group API when using DeepKeysOfType.

When the value type is nullish, it previously evaluated as:

Extract<never, nullish | TValue>

However, after the change, it became:

Extract<nullish, nullish | TValue>

which now returns nullish.
To resolve this issue, I implemented a new utility type to correctly infer TFields in the group API.

crutchcorn · 2026-05-10T19:54:36Z

The code looks good to me and makes sense, but given that @LeCarbonator has to remind me every week how FieldGroup actually works, I'm going to pass it to him 😅

Pinged internally so response time is usually quicker that way.

LeCarbonator

Looks interesting! Reported cause and the reasoning for the change make sense to me! As far as the changes go, there's only some regression tests missing as well as an instantiation check. Refer to the two open threads for more details.

Thanks for tackling this!

LeCarbonator · 2026-05-15T08:29:53Z

+            ? TAcc | UnknownDeepKeyAndValue<TParent>
+            : T extends object
+              ? DeepKeyAndValueObject<TParent, T, TAcc>
+              : TAcc


This particular change addresses not just DeepKeysOfType, but also DeepKeys and DeepValue.

I see one test was adjusted to acknowledge this, but some additional LOC for DeepKeys and DeepValue would be fantastic. Not merely for this PR, but also for future typings to follow the spec.

Thanks for review!
I added case in this commit 2ee891c

LeCarbonator · 2026-05-15T08:31:15Z

How's type instantiation treating this type change? Have you tested them before / after? Did they explode or is it only a bit more?

I would be surprised if it did, but you never know with some of those seemingly simple changes.

If you haven't already, I can help out with testing it. Just let me know.

Measured with the following command.
Instantiations increased slightly on this branch's HEAD c0d762dd compared to base 3886dcc5.

npx tsc --extendedDiagnostics

packages/form-core

metric base HEAD

Types 84,335 86,803

Instantiations 436,577 449,014

packages/react-form

metric base HEAD

Types 58,924 60,824

Instantiations 435,282 447,536

packages/preact-form

metric base HEAD

Types 51,525 52,567

Instantiations 322,435 326,877

packages/vue-form

metric base HEAD

Types 25,531 26,099

Instantiations 113,263 114,659

packages/solid-form

metric base HEAD

Types 37,855 39,301

Instantiations 203,009 210,621

coderabbitai · 2026-05-16T13:01:27Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6e3c11d3-1f55-4f93-bcc4-70892ac83b80

📥 Commits

Reviewing files that changed from the base of the PR and between 2ee891c and c0d762d.

📒 Files selected for processing (2)

packages/preact-form/src/createFormHook.tsx
packages/preact-form/src/useFieldGroup.tsx

📝 Walkthrough

Walkthrough

Adds a never-base case to deep-key recursion, introduces DeepKeysOfNonNullableType to filter deep-key paths by non-nullable accessor values, updates field-group generic constraints across form-core/react/solid/preact to use it, and adds type tests plus a changeset documenting the patch releases.

Changes

Union Type Handling in Field Groups

Layer / File(s)	Summary
Core deep-key type utilities `packages/form-core/src/util-types.ts`	`DeepKeysAndValuesImpl` short-circuits on `never`; new `DeepKeysOfNonNullableType` exported type filters deep key paths by matching `NonNullable` accessor values.
Field group core API changes `packages/form-core/src/FieldGroupApi.ts`	`FieldGroupApi` and `FieldGroupOptions` switch `TFields` constraints from `DeepKeysOfType<..., TFieldGroupData
Form hook imports and withFieldGroup adapters `packages/react-form/src/createFormHook.tsx`, `packages/solid-form/src/createFormHook.tsx`, `packages/preact-form/src/createFormHook.tsx`	Adapter files now import `DeepKeysOfNonNullableType` and use it in `withFieldGroup` return typings.
useFieldGroup and field-group signature updates `packages/react-form/src/useFieldGroup.tsx`, `packages/solid-form/src/createFieldGroup.tsx`, `packages/preact-form/src/useFieldGroup.tsx`	`useFieldGroup` and AppFieldExtended field-group `TFields` constraints updated to `DeepKeysOfNonNullableType<TFormData, TFieldGroupData>`.
Solid createFieldGroup updates `packages/solid-form/src/createFieldGroup.tsx`	Solid adapter replaces `DeepKeysOfType` imports and updates `createFieldGroup`/AppFieldExtendedSolidFieldGroupApi constraints to `DeepKeysOfNonNullableType`.
Type-level validation tests `packages/form-core/tests/util-types.test-d.ts`, `packages/form-core/tests/FieldGroupApi.test-d.ts`, `packages/react-form/tests/createFormHook.test-d.tsx`, `packages/solid-form/tests/createFormHook.test-d.tsx`	New tests validate nullish-preservation in DeepKeys/DeepValue and assert that nullish-only field-group paths are rejected using `@ts-expect-error`.
Release metadata `.changeset/blue-geese-kneel.md`	Adds a changeset declaring patch bumps for form packages and notes the DeepKeysAndValues fix preserving nullish types in union cases.

🎯 3 (Moderate) | ⏱️ ~25 minutes

"🐰 I hopped through keys and unions wide,
I nudged out never from the type's inside.
NonNullable paths now pass the test,
Nullish friends are kept, not suppressed.
Hooray — your field groups type their best!"

🚥 Pre-merge checks | ✅ 4 | ❌ 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 (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly matches the main change: fixing DeepKeysAndValues behavior to handle union types correctly.
Description check	✅ Passed	The description covers all required sections with detailed context, includes linked issue reference, checklist items completed, and changeset confirmation.
Linked Issues check	✅ Passed	All coding requirements from issue `#1813` are addressed: DeepKeysAndValues now preserves nullish types, union types are handled correctly, and related APIs updated accordingly.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing union type handling in DeepKeysAndValues and propagating that fix through affected APIs.

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mdm317 · 2026-05-16T17:26:10Z

  TFieldGroupData,
  TFields extends
-    | DeepKeysOfType<TFormData, TFieldGroupData | null | undefined>
+    | DeepKeysOfNonNullableType<TFormData, TFieldGroupData>


Apply the same non-nullable field group fix to preact-form.

mdm317 added 4 commits March 3, 2026 14:25

fix: preserve nullable types in DeepValue

bc15ad2

test: verify field groups reject nullish-only paths

9e4fc52

test : add testcase from issue

e57ffb3

changeset

81806f8

mdm317 commented Mar 3, 2026

View reviewed changes

LeCarbonator requested changes May 15, 2026

View reviewed changes

Merge branch 'main' into fix/union-types-in-deepkeysandvalues

58376b1

mdm317 added 2 commits May 17, 2026 00:51

test: add test cases

2ee891c

fix(preact-form): mirror non-nullable field group constraint

c0d762d

mdm317 commented May 16, 2026

View reviewed changes

		export type DeepKeysOfNonNullableType<TData, TValue> =
		ExtractByNonNullableValue<DeepKeysAndValues<TData>, TValue>['key']

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packages/form-core

packages/react-form

packages/preact-form

packages/vue-form

packages/solid-form

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coderabbitai Bot commented May 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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mdm317 commented Mar 3, 2026 •

edited by coderabbitai Bot

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`packages/form-core`

`packages/react-form`

`packages/preact-form`

`packages/vue-form`

`packages/solid-form`

coderabbitai Bot commented May 16, 2026 •

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