feat(examples): add Next.js App Router optimistic updates example

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 } 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.prefetchQuery({
+    queryKey: ['todos'],
+    queryFn: getTodos,
+  })
+
+  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>
+  )
+}