fix(form-core): fix form.resetField() ignoring nested fields

packages/form-core/src/FormApi.ts

@@ -2136,12 +2136,9 @@ export class FormApi<
           ...prev.fieldMetaBase,
           [field]: defaultFieldMeta,
         },
-        values: {
-          ...prev.values,
-          [field]:
-            this.options.defaultValues &&
-            this.options.defaultValues[field as keyof TFormData],
-        },
+        values: this.options.defaultValues
+          ? setBy(prev.values, field, getBy(this.options.defaultValues, field))
+          : prev.values,
       }
     })
   }

packages/form-core/tests/FormApi.spec.ts

@@ -3639,3 +3639,88 @@ it('should mark sourceMap as undefined when async field error is resolved', asyn
 
   expect(field.getMeta().errorSourceMap.onChange).toBeUndefined()
 })
+
+it('should reset nested object fields', () => {
+  const defaultValues = {
+    shallow: '',
+    nested: {
+      field: {
+        name: '',
+      },
+    },
+  }
+
+  const form = new FormApi({
+    defaultValues,
+  })
+  form.mount()
+
+  form.setFieldValue('shallow', 'Shallow')
+  form.setFieldValue('nested.field.name', 'Nested')
+
+  expect(form.state.values.shallow).toEqual('Shallow')
+  expect(form.state.values.nested.field.name).toEqual('Nested')
+
+  form.resetField('shallow')
+  expect(form.state.values.shallow).toEqual('')
+
+  form.resetField('nested.field.name')
+  expect(form.state.values.nested.field.name).toEqual('')
+})
+
+it('should reset nested array fields', () => {
+  const defaultValues = {
+    shallow: '',
+    nested: {
+      arr: [{ name: '' }, { test: 'array-test' }],
+    },
+  }
+
+  const form = new FormApi({
+    defaultValues,
+  })
+  form.mount()
+
+  form.setFieldValue('shallow', 'Shallow')
+  form.setFieldValue('nested.arr[0].name', 'nested-arr')
+  form.setFieldValue('nested.arr[1].test', 'array-test-changed')
+
+  expect(form.state.values.shallow).toEqual('Shallow')
+  expect(form.state.values.nested.arr[0]?.name).toEqual('nested-arr')
+  expect(form.state.values.nested.arr[1]?.test).toEqual('array-test-changed')
+
+  form.resetField('shallow')
+  expect(form.state.values.shallow).toEqual('')
+
+  form.resetField('nested.arr[0].name')
+  expect(form.state.values.nested.arr[0]?.name).toEqual('')
+  expect(form.state.values.nested.arr[1]?.test).toEqual('array-test-changed')
+})
+
+it('should preserve nested fields on resetField if defaultValues is not provided', () => {
+  const state = {
+    shallow: '',
+    nested: {
+      field: {
+        name: '',
+      },
+    },
+  }
+
+  const form = new FormApi({
+    defaultState: { values: state },
+  })
+  form.mount()
+
+  form.setFieldValue('shallow', 'Shallow')
+  form.setFieldValue('nested.field.name', 'Nested')
+
+  expect(form.state.values.shallow).toEqual('Shallow')
+  expect(form.state.values.nested.field.name).toEqual('Nested')
+
+  form.resetField('shallow')
+  expect(form.state.values.shallow).toEqual('Shallow')
+
+  form.resetField('nested.field.name')
+  expect(form.state.values.nested.field.name).toEqual('Nested')
+})

packages/form-core/tests/FormApi.test-d.ts

@@ -273,3 +273,33 @@ it('should only allow array fields for array-specific methods', () => {
   // @ts-expect-error too wide!
   const validate3 = form.validateArrayFieldsStartingFrom<RandomKeys>
 })
+
+it('should infer full field name union for form.resetField parameters', () => {
+  type FormData = {
+    shallow: string
+    nested: {
+      field: {
+        name: string
+      }
+    }
+  }
+
+  const defaultValue = {
+    shallow: '',
+    nested: {
+      field: {
+        name: '',
+      },
+    },
+  }
+
+  const form = new FormApi({
+    defaultValues: defaultValue as FormData,
+  })
+
+  expectTypeOf(form.resetField)
+    .parameter(0)
+    .toEqualTypeOf<
+      'shallow' | 'nested' | 'nested.field' | 'nested.field.name'
+    >()
+})