fix(core): fix deleteField method

[kusiewicz]

During implementing this and testing deleteField I found out that this method does not work.

When a form field has onChange validation and the deleteField method is called on it, an error appears.

FormApi.ts:1586 Uncaught TypeError: Cannot read properties of undefined (reading 'errorMap')
    at FormApi.ts:1586:27
    at functionalUpdate (utils.ts:26:8)
    at FormApi.ts:2058:20

error: https://bolt.new/~/vitejs-vite-ceu2fl73

I think flow look like this:

1. Remove button click => form.deleteField('firstName').
2. Delete function changes values so onChange validator runs.
3. This validator runs on the form level and still tries to set errors.fields.email = 'error message' - so validateSync tries to update metadata of deleted field.
4. setFieldMeta calls functionalUpdate with undefined (since field does not exists in fieldMetaBase).
5. Updater tries to do ...prev where prev now is undefined -> error.

So validateSync iterates through field list that already semes to be outdated. There is a check if (!fieldMeta) in this function but I guess we have to add as well if (!fieldMetaBase).

[nx-cloud]

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---

☁️ Nx Cloud last updated this comment at 2025-08-19 18:20:30 UTC

[pkg-pr-new]

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commit: dfd06c5

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.48%. Comparing base (d3a1b59) to head (dfd06c5).

Files with missing lines	Patch %	Lines
packages/form-core/src/FormApi.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1706      +/-   ##
==========================================
- Coverage   90.52%   90.48%   -0.04%     
==========================================
  Files          37       37              
  Lines        1688     1692       +4     
  Branches      421      423       +2     
==========================================
+ Hits         1528     1531       +3     
- Misses        143      144       +1     
  Partials       17       17              

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[kusiewicz]

Please, verify the idea. If I'm thinking in a proper direction I will proceed and add unit tests.