fix(form-core): keep undefined out of the field error array type

[CodeAndParty]

🎯 Changes

field.state.meta.errors carries a spurious undefined in its element type, so the array can't be iterated without a guard or a cast:

const form = useForm({
  defaultValues: { name: '' },
  validators: { onChange: z.object({ name: z.string().min(1, 'Required') }) },
})

// (StandardSchemaV1Issue | undefined)[]
field.state.meta.errors.map((e) => e.message)
//                                 ^ 'e' is possibly 'undefined'

Cause

FieldLikeMetaDerived['errors'] unions UnwrapOneLevelOfArray<UnwrapFieldValidateOrFn<…>> across all nine validator slots. A slot with no validator resolves to undefined, and UnwrapOneLevelOfArray<undefined> is undefined (T extends (infer U)[] ? U : T), so every unused slot contributes undefined to the element union. A single form-level schema leaves eight slots unused.

This is not specific to onDynamic or to Standard Schema — any config that leaves slots empty is affected.

Why NonNullable is the right fix

The value is already filtered at runtime, in FormApi:

fieldErrors = Object.values(currBaseMeta.errorMap ?? {}).filter(
  (val) => val !== undefined,
)

Group meta does the same (if (curr === undefined) return acc). And the form-level types already wrap their unions in NonNullable — FormState['errors'] and getAllErrors()['form']['errors'] both do. FieldLikeMetaDerived['errors'] was the only one that didn't, which is why two adjacent tests with the identical validator disagreed:

// onChange: () => '123' as const
expectTypeOf(form.state.errors).toEqualTypeOf<Array<'123'>>()                  // form
expectTypeOf(field.state.meta.errors).toEqualTypeOf<Array<'123' | undefined>>() // field

So this aligns the field-level type with both the runtime and the existing form-level convention.

Updated assertions

Five existing type tests encoded the old shape and are updated to drop | undefined — this is the behavioural crux of the PR, so worth a close look:

• form-core/tests/FieldApi.test-d.ts (4 assertions)
• form-core/tests/FormGroupApi.test-d.ts, mirrored in react-form, solid-form, preact-form

Two runtime tests had defensively written key={err?.toString()}; the optional chain is now flagged as unnecessary by @typescript-eslint/no-unnecessary-condition and has been removed — a nice confirmation that the fix reaches real call sites.

Added coverage

Two regression tests in form-core/tests/standardSchemaValidator.test-d.ts:

• form-level schema only → Array<StandardSchemaV1Issue>, and .map((i) => i.message) is Array<string>
• form-level schema + a field validator returning 'Required' | undefined → Array<StandardSchemaV1Issue | 'Required'>, confirming a validator's own return type is still preserved and only the spurious undefined is dropped

✅ Checklist

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

Verified across all 14 packages: test:types (TS 5.4–5.9), test:lib, test:eslint, and build all pass.

🚀 Release Impact

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

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Corrected form and field error array types so they contain only actual validation errors, excluding spurious undefined values.
  • Standard Schema validation errors can now be iterated and mapped without additional guards or type casts.
  • Preserved custom validator error types while improving type accuracy across React, Preact, Solid, and core form APIs.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 6953dbd0-beee-40c2-980b-911a73a300c2

📥 Commits

Reviewing files that changed from the base of the PR and between 57a855b and e82ebbc.

📒 Files selected for processing (10)

• .changeset/tidy-errors-narrow.md
• packages/form-core/src/types.ts
• packages/form-core/tests/FieldApi.test-d.ts
• packages/form-core/tests/FormGroupApi.test-d.ts
• packages/form-core/tests/standardSchemaValidator.test-d.ts
• packages/preact-form/tests/useField.test.tsx
• packages/preact-form/tests/useFormGroup.test-d.tsx
• packages/react-form/tests/useField.test.tsx
• packages/react-form/tests/useFormGroup.test-d.tsx
• packages/solid-form/tests/createFormGroup.test-d.tsx

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📝 Walkthrough

Walkthrough

The change narrows field and group error array element types by removing undefined from validator error unions. Type tests across form-core, React, Preact, Solid, and Standard Schema validate the updated behavior.

Changes

Error type narrowing

Layer / File(s)	Summary
Field error type contract packages/form-core/src/types.ts, .changeset/tidy-errors-narrow.md	FieldLikeMetaDerived.errors now excludes null and undefined from its element type. A patch release changeset documents the fix.
Error type validation packages/form-core/tests/*, packages/preact-form/tests/*, packages/react-form/tests/*, packages/solid-form/tests/*	Type tests verify definite validator error types, Standard Schema issue mapping, and direct error rendering without optional chaining.

Priority: ⬇️ Low

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

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to e82eb

The change consistently narrows error-array element types without altering runtime validation behavior. No actionable merge risk remains.

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 9 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the type fix, its cause, runtime alignment, test updates, and release impact. It includes all required template sections and completed checklist items.
Title check	✅ Passed	The title is concise and accurately identifies the main change: removing the spurious undefined from the form-core field error array type.

Full details: Docstring Coverage

Explanation

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 5 functions across 9 files. (1 skipped: 1 unsupported.)

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