form fix(form-core): fix deleteField method

🎯 Changes

Fixes: https://github.com/TanStack/form/issues/1808

If this idea make sense, I will add tests

After calling form.deleteField('fieldName'), the field is removed from form.state.values, but after a moment it returns with defaultValue and appears at the end of the values object.

The problem occurred due to React lifecycle and the logic in FieldApi.mount()

Sequence of events:

User clicks delete -> form.deleteField('field') executes -> value is remo ved from form.state.values -> field metadata is removed - cool, but
React detects state change -> component re-renders
During re-render the field is still in the DOM (not unmounted)
React calls FieldApi.update() or lifecycle hooks -> this calls setFieldValue
In FieldApi.mount() there was logic:

   if ((this.options.defaultValue as unknown) !== undefined) {
     this.form.setFieldValue(this.name, this.options.defaultValue as never, {
       dontUpdateMeta: true,
     })
   }

I think this uncoditionally restored defaultValue on every mount.update

PS: Even if we add a check in mount() - setFieldValue can be called from other places (e.g., from FieldApi.update() during React lifecycle).

I added a mechanism for tracking intentionally deleted fields to prevent their their accidental restoration.

Added _deletedFields: Set<string> in FormApi - it stores names of fields that were intentionally deleted by the user
deleteField() adds the field to _deletedFields
setFieldValue() blocks operations on deleted fields

✅ Checklist

[X] 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).

Oct 14 '25 18:10 kusiewicz

⚠️ No Changeset found

Latest commit: 3b5d96696b9e900b662bf2469aeeb265d2eff30c

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Oct 14 '25 18:10 changeset-bot[bot]

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Command	Status	Duration	Result
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☁️ Nx Cloud last updated this comment at 2025-12-15 11:31:57 UTC

Oct 14 '25 18:10 nx-cloud[bot]

More templates

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commit: 3b5d966

Oct 14 '25 18:10 pkg-pr-new[bot]

Blocked by #1729 . This seems critical to test with the linked PR.

Oct 15 '25 06:10 LeCarbonator

@kusiewicz can you fix merge conflicts and then @LeCarbonator if you could, re review for @kusiewicz since the blocker is merged?

Thanks!

Dec 01 '25 19:12 crutchcorn

@kusiewicz can you fix merge conflicts and then @LeCarbonator if you could, re review for @kusiewicz since the blocker is merged?

Thanks!

Will do today!

Dec 02 '25 08:12 kusiewicz

Codecov Report

:x: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.74%. Comparing base (6892ed0) to head (3b5d966). :warning: Report is 95 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/form-core/src/FormApi.ts	90.90%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1809      +/-   ##
==========================================
- Coverage   90.35%   89.74%   -0.62%     
==========================================
  Files          38       48      +10     
  Lines        1752     1940     +188     
  Branches      444      483      +39     
==========================================
+ Hits         1583     1741     +158     
- Misses        149      178      +29     
- Partials       20       21       +1

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