snapshot of https://github.com/TanStack/query.git (main, 2026-09-16; blobs > 131072B omitted)

examples/react/nextjs-app-optimistic-updates/.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env*.local
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo

examples/react/nextjs-app-optimistic-updates/README.md

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+# TanStack Query — Next.js App Router Optimistic Updates
+
+This example demonstrates **optimistic updates** with TanStack Query v5 in a Next.js 14 App Router project.
+
+## What it shows
+
+A todo list where items appear in the UI immediately after submission — before the server confirms. The server randomly fails ~30% of the time so you can observe automatic rollback behaviour.
+
+Two approaches are shown side by side via a tab toggle:
+
+### Approach 1 — Via UI Variables (simpler)
+
+Render the pending item directly from `mutation.variables`. No cache touching required. On error, the pending item simply disappears and an error message is shown.
+
+```ts
+const mutation = useMutation({ mutationFn: addTodo, onSettled: invalidate })
+
+// In JSX:
+{mutation.isPending && <li style={{ opacity: 0.5 }}>{mutation.variables}</li>}
+```
+
+**Best when:** the mutation input maps 1-to-1 to what you'd show while pending.
+
+### Approach 2 — Via Cache Manipulation (`onMutate` + rollback)
+
+`onMutate` cancels in-flight refetches, snapshots the current cache, and writes an optimistic item directly into the cache. `onError` restores the snapshot.
+
+```ts
+const mutation = useMutation({
+  mutationFn: addTodo,
+  onMutate: async (text) => {
+    await queryClient.cancelQueries({ queryKey: ['todos'] })
+    const previousTodos = queryClient.getQueryData<Todo[]>(['todos'])
+    queryClient.setQueryData<Todo[]>(['todos'], (old = []) => [
+      ...old,
+      optimistic,
+    ])
+    return { previousTodos }
+  },
+  onError: (_err, _vars, context) => {
+    queryClient.setQueryData(['todos'], context?.previousTodos)
+  },
+  onSettled: () => queryClient.invalidateQueries({ queryKey: ['todos'] }),
+})
+```
+
+**Best when:** you need fine-grained control or the optimistic shape differs from `mutation.variables`.
+
+## Running the example
+
+```bash
+npm install
+npm run dev
+```
+
+Open [http://localhost:3000](http://localhost:3000).
+
+## Learn more
+
+- [TanStack Query — Optimistic Updates](https://tanstack.com/query/latest/docs/framework/react/guides/optimistic-updates)
+- [Next.js App Router](https://nextjs.org/docs/app)

examples/react/nextjs-app-optimistic-updates/app/api/todos/data.ts

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+export interface Todo {
+  id: string
+  text: string
+  createdAt: number
+}
+
+export const todos: Array<Todo> = [
+  {
+    id: crypto.randomUUID(),
+    text: 'Buy groceries',
+    createdAt: Date.now() - 3000,
+  },
+  {
+    id: crypto.randomUUID(),
+    text: 'Walk the dog',
+    createdAt: Date.now() - 2000,
+  },
+  {
+    id: crypto.randomUUID(),
+    text: 'Read a book',
+    createdAt: Date.now() - 1000,
+  },
+]
+
+export async function getTodos(): Promise<Array<Todo>> {
+  await new Promise((resolve) => setTimeout(resolve, 200))
+  return todos
+}

examples/react/nextjs-app-optimistic-updates/app/api/todos/route.ts

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+import { NextResponse } from 'next/server'
+import { todos, getTodos, type Todo } from './data'
+
+export async function GET() {
+  return NextResponse.json(await getTodos())
+}
+
+export async function POST(request: Request) {
+  let body: unknown
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'text is required' }, { status: 400 })
+  }
+
+  const text =
+    body !== null &&
+    typeof body === 'object' &&
+    'text' in body &&
+    typeof (body as { text: unknown }).text === 'string'
+      ? (body as { text: string }).text.trim()
+      : ''
+
+  if (!text) {
+    return NextResponse.json({ error: 'text is required' }, { status: 400 })
+  }
+
+  await new Promise((resolve) => setTimeout(resolve, 500))
+
+  if (Math.random() < 0.3) {
+    return NextResponse.json(
+      { error: 'Server error — please try again' },
+      { status: 500 },
+    )
+  }
+
+  const newTodo: Todo = {
+    id: crypto.randomUUID(),
+    text,
+    createdAt: Date.now(),
+  }
+
+  todos.push(newTodo)
+
+  return NextResponse.json(newTodo, { status: 201 })
+}

examples/react/nextjs-app-optimistic-updates/app/get-query-client.ts

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+import {
+  QueryClient,
+  defaultShouldDehydrateQuery,
+  environmentManager,
+} from '@tanstack/react-query'
+
+function makeQueryClient() {
+  return new QueryClient({
+    defaultOptions: {
+      queries: {
+        staleTime: 60 * 1000,
+      },
+      dehydrate: {
+        shouldDehydrateQuery: (query) =>
+          defaultShouldDehydrateQuery(query) ||
+          query.state.status === 'pending',
+      },
+    },
+  })
+}
+
+let browserQueryClient: QueryClient | undefined = undefined
+
+export function getQueryClient() {
+  if (environmentManager.isServer()) {
+    return makeQueryClient()
+  } else {
+    if (!browserQueryClient) browserQueryClient = makeQueryClient()
+    return browserQueryClient
+  }
+}

examples/react/nextjs-app-optimistic-updates/app/layout.tsx

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+import Providers from './providers'
+import type React from 'react'
+import type { Metadata } from 'next'
+
+export const metadata: Metadata = {
+  title: 'TanStack Query — Optimistic Updates',
+  description: 'Next.js App Router example with optimistic updates',
+}
+
+export default function RootLayout({
+  children,
+}: {
+  children: React.ReactNode
+}) {
+  return (
+    <html lang="en">
+      <body>
+        <Providers>{children}</Providers>
+      </body>
+    </html>
+  )
+}

examples/react/nextjs-app-optimistic-updates/app/page.tsx

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+import React from 'react'
+import { HydrationBoundary, dehydrate, noop } from '@tanstack/react-query'
+import { getQueryClient } from '@/app/get-query-client'
+import { getTodos } from '@/app/api/todos/data'
+import ApproachTabs from '@/components/ApproachTabs'
+
+export default function Home() {
+  const queryClient = getQueryClient()
+
+  void queryClient
+    .query({
+      queryKey: ['todos'],
+      queryFn: getTodos,
+    })
+    .catch(noop)
+
+  return (
+    <main style={{ maxWidth: 600, margin: '0 auto', padding: '2rem 1rem' }}>
+      <h1>Optimistic Updates with TanStack Query</h1>
+      <p style={{ color: '#666', marginBottom: '1.5rem' }}>
+        Add todos to see optimistic updates in action. The server randomly fails
+        ~30% of the time so you can observe automatic rollback.
+      </p>
+      <HydrationBoundary state={dehydrate(queryClient)}>
+        <ApproachTabs />
+      </HydrationBoundary>
+    </main>
+  )
+}

examples/react/nextjs-app-optimistic-updates/app/providers.tsx

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+'use client'
+import { QueryClientProvider } from '@tanstack/react-query'
+import { ReactQueryDevtools } from '@tanstack/react-query-devtools'
+import { getQueryClient } from '@/app/get-query-client'
+import type * as React from 'react'
+
+export default function Providers({ children }: { children: React.ReactNode }) {
+  const queryClient = getQueryClient()
+
+  return (
+    <QueryClientProvider client={queryClient}>
+      {children}
+      <ReactQueryDevtools />
+    </QueryClientProvider>
+  )
+}

examples/react/nextjs-app-optimistic-updates/components/ApproachTabs.tsx

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+'use client'
+
+import React, { useState } from 'react'
+import TodoListUI from './TodoListUI'
+import TodoListCache from './TodoListCache'
+
+type Tab = 'ui-variables' | 'cache'
+
+export default function ApproachTabs() {
+  const [activeTab, setActiveTab] = useState<Tab>('ui-variables')
+
+  const tabStyle = (tab: Tab): React.CSSProperties => ({
+    padding: '0.5rem 1rem',
+    border: 'none',
+    borderBottom:
+      activeTab === tab ? '2px solid #0070f3' : '2px solid transparent',
+    background: 'none',
+    cursor: 'pointer',
+    fontWeight: activeTab === tab ? 600 : 400,
+    color: activeTab === tab ? '#0070f3' : '#555',
+  })
+
+  return (
+    <div>
+      <div
+        style={{
+          display: 'flex',
+          borderBottom: '1px solid #ddd',
+          marginBottom: '1.5rem',
+        }}
+      >
+        <button
+          style={tabStyle('ui-variables')}
+          onClick={() => setActiveTab('ui-variables')}
+        >
+          Via UI Variables
+        </button>
+        <button style={tabStyle('cache')} onClick={() => setActiveTab('cache')}>
+          Via Cache Manipulation
+        </button>
+      </div>
+
+      {activeTab === 'ui-variables' ? <TodoListUI /> : <TodoListCache />}
+    </div>
+  )
+}

examples/react/nextjs-app-optimistic-updates/components/TodoListCache.tsx

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+'use client'
+
+import React, { useState } from 'react'
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import type { Todo } from '@/app/api/todos/data'
+
+interface MutationContext {
+  previousTodos: Array<Todo> | undefined
+  optimisticId: string
+}
+
+async function fetchTodos(): Promise<Array<Todo>> {
+  const res = await fetch('/api/todos')
+  if (!res.ok) throw new Error('Failed to fetch todos')
+  return res.json()
+}
+
+async function addTodo(text: string): Promise<Todo> {
+  const res = await fetch('/api/todos', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ text }),
+  })
+  if (!res.ok) {
+    const err = (await res.json()) as { error: string }
+    throw new Error(err.error)
+  }
+  return res.json()
+}
+
+export default function TodoListCache() {
+  const queryClient = useQueryClient()
+  const [inputValue, setInputValue] = useState('')
+  const [lastError, setLastError] = useState<string | null>(null)
+
+  const { data: todos = [] } = useQuery({
+    queryKey: ['todos'],
+    queryFn: fetchTodos,
+  })
+
+  const addTodoMutation = useMutation<Todo, Error, string, MutationContext>({
+    mutationFn: addTodo,
+    onMutate: async (text) => {
+      setLastError(null)
+      await queryClient.cancelQueries({ queryKey: ['todos'] })
+
+      const previousTodos = queryClient.getQueryData<Array<Todo>>(['todos'])
+
+      const optimisticId = `optimistic-${Date.now()}`
+      const optimisticTodo: Todo = {
+        id: optimisticId,
+        text,
+        createdAt: Date.now(),
+      }
+
+      queryClient.setQueryData<Array<Todo>>(['todos'], (old = []) => [
+        ...old,
+        optimisticTodo,
+      ])
+
+      return { previousTodos, optimisticId }
+    },
+    onError: (_err, _text, context) => {
+      setLastError(_err.message)
+      if (context?.previousTodos !== undefined) {
+        queryClient.setQueryData(['todos'], context.previousTodos)
+      } else if (context?.optimisticId) {
+        queryClient.setQueryData<Array<Todo>>(['todos'], (old = []) =>
+          old.filter((todo) => todo.id !== context.optimisticId),
+        )
+      }
+    },
+    onSettled: () => {
+      void queryClient.invalidateQueries({ queryKey: ['todos'] })
+    },
+  })
+
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault()
+    const text = inputValue.trim()
+    if (!text) return
+    addTodoMutation.mutate(text)
+    setInputValue('')
+  }
+
+  return (
+    <div>
+      <p style={{ fontSize: '0.875rem', color: '#555', marginBottom: '1rem' }}>
+        <strong>Approach 2 — via cache manipulation:</strong>{' '}
+        <code>onMutate</code> snapshots the cache and writes the optimistic item
+        in. <code>onError</code> restores the snapshot on failure.
+      </p>
+
+      <form
+        onSubmit={handleSubmit}
+        style={{ display: 'flex', gap: 8, marginBottom: '1rem' }}
+      >
+        <input
+          value={inputValue}
+          onChange={(e) => setInputValue(e.target.value)}
+          placeholder="New todo…"
+          style={{ flex: 1, padding: '0.4rem 0.6rem', fontSize: '1rem' }}
+        />
+        <button type="submit" disabled={addTodoMutation.isPending}>
+          Add
+        </button>
+      </form>
+
+      {lastError && (
+        <p style={{ color: 'red', marginBottom: '0.5rem' }}>{lastError}</p>
+      )}
+
+      <ul style={{ listStyle: 'none', padding: 0 }}>
+        {todos.map((todo) => (
+          <li
+            key={todo.id}
+            style={{
+              padding: '0.4rem 0',
+              borderBottom: '1px solid #eee',
+              opacity: todo.id.startsWith('optimistic-') ? 0.5 : 1,
+            }}
+          >
+            {todo.text}
+            {todo.id.startsWith('optimistic-') && <em> (saving…)</em>}
+          </li>
+        ))}
+      </ul>
+    </div>
+  )
+}

examples/react/nextjs-app-optimistic-updates/components/TodoListUI.tsx

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+'use client'
+
+import React, { useState } from 'react'
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import type { Todo } from '@/app/api/todos/data'
+
+async function fetchTodos(): Promise<Array<Todo>> {
+  const res = await fetch('/api/todos')
+  if (!res.ok) throw new Error('Failed to fetch todos')
+  return res.json()
+}
+
+async function addTodo(text: string): Promise<Todo> {
+  const res = await fetch('/api/todos', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ text }),
+  })
+  if (!res.ok) {
+    const err = (await res.json()) as { error: string }
+    throw new Error(err.error)
+  }
+  return res.json()
+}
+
+export default function TodoListUI() {
+  const queryClient = useQueryClient()
+  const [inputValue, setInputValue] = useState('')
+
+  const { data: todos = [] } = useQuery({
+    queryKey: ['todos'],
+    queryFn: fetchTodos,
+  })
+
+  const addTodoMutation = useMutation({
+    mutationFn: addTodo,
+    onSettled: () => {
+      void queryClient.invalidateQueries({ queryKey: ['todos'] })
+    },
+  })
+
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault()
+    const text = inputValue.trim()
+    if (!text) return
+    addTodoMutation.mutate(text)
+    setInputValue('')
+  }
+
+  return (
+    <div>
+      <p style={{ fontSize: '0.875rem', color: '#555', marginBottom: '1rem' }}>
+        <strong>Approach 1 — via UI variables:</strong> The pending item is
+        rendered directly from <code>mutation.variables</code>. No cache
+        manipulation needed. On error the pending item simply disappears.
+      </p>
+
+      <form
+        onSubmit={handleSubmit}
+        style={{ display: 'flex', gap: 8, marginBottom: '1rem' }}
+      >
+        <input
+          value={inputValue}
+          onChange={(e) => setInputValue(e.target.value)}
+          placeholder="New todo…"
+          style={{ flex: 1, padding: '0.4rem 0.6rem', fontSize: '1rem' }}
+        />
+        <button type="submit" disabled={addTodoMutation.isPending}>
+          Add
+        </button>
+      </form>
+
+      {addTodoMutation.isError && (
+        <p style={{ color: 'red', marginBottom: '0.5rem' }}>
+          {addTodoMutation.error.message}
+        </p>
+      )}
+
+      <ul style={{ listStyle: 'none', padding: 0 }}>
+        {todos.map((todo) => (
+          <li
+            key={todo.id}
+            style={{ padding: '0.4rem 0', borderBottom: '1px solid #eee' }}
+          >
+            {todo.text}
+          </li>
+        ))}
+        {addTodoMutation.isPending && (
+          <li
+            style={{
+              padding: '0.4rem 0',
+              borderBottom: '1px solid #eee',
+              opacity: 0.5,
+            }}
+          >
+            {addTodoMutation.variables} <em>(saving…)</em>
+          </li>
+        )}
+      </ul>
+    </div>
+  )
+}

examples/react/nextjs-app-optimistic-updates/next.config.js

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+// @ts-check
+
+/** @type {import('next').NextConfig} */
+const nextConfig = {}
+
+export default nextConfig

examples/react/nextjs-app-optimistic-updates/package.json

@@ -0,0 +1,23 @@
+{
+  "name": "@tanstack/query-example-react-nextjs-app-optimistic-updates",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "next dev --webpack",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "@tanstack/react-query": "^5.102.8",
+    "@tanstack/react-query-devtools": "^5.102.8",
+    "next": "^16.0.7",
+    "react": "^19.2.1",
+    "react-dom": "^19.2.1"
+  },
+  "devDependencies": {
+    "@types/node": "26.0.1",
+    "@types/react": "^19.2.7",
+    "@types/react-dom": "^19.2.3",
+    "typescript": "5.8.3"
+  }
+}

examples/react/nextjs-app-optimistic-updates/tsconfig.json

@@ -0,0 +1,35 @@
+{
+  "compilerOptions": {
+    "target": "ES5",
+    "lib": ["dom", "dom.iterable", "esnext"],
+    "allowJs": true,
+    "skipLibCheck": true,
+    "strict": true,
+    "forceConsistentCasingInFileNames": true,
+    "noEmit": true,
+    "esModuleInterop": true,
+    "module": "ESNext",
+    "moduleResolution": "Bundler",
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "jsx": "react-jsx",
+    "incremental": true,
+    "plugins": [
+      {
+        "name": "next"
+      }
+    ],
+    "paths": {
+      "@/*": ["./*"]
+    }
+  },
+  "include": [
+    "next-env.d.ts",
+    "**/*.ts",
+    "**/*.tsx",
+    ".next/types/**/*.ts",
+    ".eslintrc.cjs",
+    ".next/dev/types/**/*.ts"
+  ],
+  "exclude": ["node_modules"]
+}