feat: add support for nullable objects.

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

@@ -923,6 +923,45 @@ describe('form api', () => {
     ])
   })
 
+  it('should validate optional object fields during submit', async () => {
+    const form = new FormApi({
+      defaultValues: {
+        person: null,
+      } as { person: { firstName: string; lastName: string } | null },
+    })
+
+    const field = new FieldApi({
+      form,
+      name: 'person.firstName',
+      validators: {
+        onChange: ({ value }) =>
+          value && value.length > 0 ? undefined : 'first name is required',
+      },
+    })
+
+    const lastNameField = new FieldApi({
+      form,
+      name: 'person.lastName',
+      validators: {
+        onChange: ({ value }) =>
+          value && value.length > 0 ? undefined : 'last name is required',
+      },
+    })
+
+    field.mount()
+    lastNameField.mount()
+
+    await form.handleSubmit()
+    expect(form.state.isFieldsValid).toEqual(false)
+    expect(form.state.canSubmit).toEqual(false)
+    expect(form.state.fieldMeta['person.firstName'].errors).toEqual([
+      'first name is required',
+    ])
+    expect(form.state.fieldMeta['person.lastName'].errors).toEqual([
+      'last name is required',
+    ])
+  })
+
   it('should run all types of validation on fields during submit', async () => {
     const form = new FormApi({
       defaultValues: {

packages/form-core/src/tests/util-types.test-d.ts

@@ -75,6 +75,14 @@ type NestedKeysExample = DeepValue<
 >
 assertType<number>(0 as never as NestedKeysExample)
 
+type NestedNullableKeys = DeepValue<
+  {
+    meta: { mainUser: 'hello' } | null
+  },
+  'meta.mainUser'
+>
+assertType<'hello' | null>(0 as never as NestedNullableKeys)
+
 type NestedArrayExample = DeepValue<{ users: User[] }, 'users[0].age'>
 assertType<number>(0 as never as NestedArrayExample)
 

packages/form-core/src/util-types.ts

@@ -1,3 +1,6 @@
+type Nullable<T> = T | null
+type IsNullable<T> = [null] extends [T] ? true : false
+
 export type RequiredByKey<T, K extends keyof T> = Omit<T, K> &
   Required<Pick<T, K>>
 
@@ -96,6 +99,7 @@ export type DeepValue<
   TValue,
   // A string representing the path of the property we're trying to access
   TAccessor,
+  TNullable extends boolean = IsNullable<TValue>,
 > =
   // If TValue is any it will recurse forever, this terminates the recursion
   unknown extends TValue
@@ -122,7 +126,9 @@ export type DeepValue<
             : TAccessor extends `${infer TBefore}.${infer TAfter}`
               ? DeepValue<DeepValue<TValue, TBefore>, TAfter>
               : TAccessor extends string
-                ? TValue[TAccessor]
+                ? TNullable extends true
+                  ? Nullable<TValue[TAccessor]>
+                  : TValue[TAccessor]
                 : never
         : // Do not allow `TValue` to be anything else
           never

packages/form-core/src/utils.ts

@@ -23,6 +23,7 @@ export function functionalUpdate<TInput, TOutput = TInput>(
 export function getBy(obj: any, path: any) {
   const pathObj = makePathArray(path)
   return pathObj.reduce((current: any, pathPart: any) => {
+    if (current === null) return null
     if (typeof current !== 'undefined') {
       return current[pathPart]
     }