From 429d060f32aceaceaca8d27db45e21061c2896be Mon Sep 17 00:00:00 2001
From: KooYS <17160263+KooYS@users.noreply.github.com>
Date: Thu, 12 Nov 2026 11:35:20 +0900
Subject: [PATCH] fix(form-core): set array values at the given index past the
 end

setBy only padded the array when the slice before the index was empty,
so writing past the end of a non-empty array placed the value at
`length` instead of at the index. Pad the gap with undefined, which is
what already happens for empty arrays.
---
 .changeset/quiet-slots-stay.md                |  5 +++
 packages/form-core/src/utils.lib.ts           |  3 +-
 .../tests/FormApi/FormApi.lib.spec.ts         | 42 +++++++++++++++++++
 packages/form-core/tests/utils.test.ts        |  6 ++-
 4 files changed, 54 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/quiet-slots-stay.md

diff --git a/.changeset/quiet-slots-stay.md b/.changeset/quiet-slots-stay.md
new file mode 100644
index 0000000000..5640ec3a80
--- /dev/null
+++ b/.changeset/quiet-slots-stay.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/form-core': patch
+---
+
+Fix setting an array value at an index past the end of a non-empty array writing to the wrong index
diff --git a/packages/form-core/src/utils.lib.ts b/packages/form-core/src/utils.lib.ts
index d1e0005cb0..e1325be667 100644
--- a/packages/form-core/src/utils.lib.ts
+++ b/packages/form-core/src/utils.lib.ts
@@ -181,7 +181,8 @@ export function setBy(obj: any, _path: any, updater: Updater<any>) {
     if (Array.isArray(parent) && typeof key === 'number') {
       const prefix = parent.slice(0, key)
       return [
-        ...(prefix.length ? prefix : new Array(key)),
+        ...prefix,
+        ...new Array(key - prefix.length),
         doSet(parent[key]),
         ...parent.slice(key + 1),
       ]
diff --git a/packages/form-core/tests/FormApi/FormApi.lib.spec.ts b/packages/form-core/tests/FormApi/FormApi.lib.spec.ts
index 3dc9393ff7..2aa39932f1 100644
--- a/packages/form-core/tests/FormApi/FormApi.lib.spec.ts
+++ b/packages/form-core/tests/FormApi/FormApi.lib.spec.ts
@@ -742,6 +742,33 @@ describe('InternalFormApi - setFieldValue', () => {
     expect(field.value).toEqual(['B'])
     expect(form.defaultValues).toEqual({ items: ['a'] })
   })
+  it('sets a value at an index past the end of a non-empty array', () => {
+    const form = new InternalFormApi({
+      defaultValues: { phones: ['555-0000'] },
+    })
+    const fields = [0, 1, 2, 3].map((i) =>
+      form._getOrCreateFieldApi({ name: `phones[${i}]` as const }),
+    )
+
+    fields[3]!.handleChange('555-0003')
+
+    expect(form.getFieldValue('phones[3]')).toBe('555-0003')
+    expect(form.state.values.phones).toStrictEqual([
+      '555-0000',
+      undefined,
+      undefined,
+      '555-0003',
+    ])
+
+    fields[1]!.handleChange('555-0001')
+
+    expect(fields.map((field) => field.value)).toStrictEqual([
+      '555-0000',
+      '555-0001',
+      undefined,
+      '555-0003',
+    ])
+  })
 })
 
 describe('InternalFormApi - reset', () => {
@@ -1053,6 +1080,21 @@ describe('InternalFormApi - resetField', () => {
     expect(form.state.values).toEqual({ person: { name: 'Carol' } })
     expect(form.state.isTouched).toBe(true)
   })
+  it('restores the default at its own index when the array is shorter than the defaults', () => {
+    const form = new InternalFormApi({
+      defaultValues: { phones: ['a', 'b', 'c', 'd'] },
+    })
+
+    form.setFieldValue('phones', ['a'])
+    form.resetField('phones[3]')
+
+    expect(form.state.values.phones).toStrictEqual([
+      'a',
+      undefined,
+      undefined,
+      'd',
+    ])
+  })
 })
 
 describe('InternalFormApi - deleteField', () => {
diff --git a/packages/form-core/tests/utils.test.ts b/packages/form-core/tests/utils.test.ts
index be555831bf..3451d66e14 100644
--- a/packages/form-core/tests/utils.test.ts
+++ b/packages/form-core/tests/utils.test.ts
@@ -99,7 +99,11 @@ describe('utils', () => {
       items: [undefined, undefined, { name: 'Ada' }],
     })
 
-    expect(setBy(['first'], '[2]', 'third')).toEqual(['first', 'third'])
+    expect(setBy(['first'], '[2]', 'third')).toStrictEqual([
+      'first',
+      undefined,
+      'third',
+    ])
     expect(setBy([], '[2]', 'third')).toEqual([undefined, undefined, 'third'])
     expect(setBy(undefined, [undefined], 'third')).toEqual([undefined, 'third'])
   })