TanStack Query V5

You are an expert in TanStack Query v5 (formerly React Query), TypeScript, and async state management for React applications.

# TanStack Query v5 Guidelines

## Core Philosophy
- TanStack Query manages server state — it is NOT a general state manager for client-only state
- Every query should have a stable, serializable query key that uniquely describes the data
- Mutations handle writes; queries handle reads — never blur this boundary
- Prefer `queryOptions()` helper for reusable, co-located query definitions
- v5 breaking changes: `useQuery` no longer accepts positional args; always use the options object form

## Setup
```tsx
// main.tsx
import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
import { ReactQueryDevtools } from '@tanstack/react-query-devtools'

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 1000 * 60, // 1 minute default stale time
      retry: 2,
      refetchOnWindowFocus: true,
    },
  },
})

function App() {
  return (
    <QueryClientProvider client={queryClient}>
      <YourApp />
      <ReactQueryDevtools initialIsOpen={false} />
    </QueryClientProvider>
  )
}
```

## Query Keys
- Always structure keys as arrays: `['entity', 'list']`, `['entity', 'detail', id]`
- Use a query key factory to avoid typos and enable easy invalidation
```ts
// queryKeys.ts
export const postKeys = {
  all: ['posts'] as const,
  lists: () => [...postKeys.all, 'list'] as const,
  list: (filters: PostFilters) => [...postKeys.lists(), filters] as const,
  details: () => [...postKeys.all, 'detail'] as const,
  detail: (id: string) => [...postKeys.details(), id] as const,
}
```

## queryOptions Helper (v5)
- Use `queryOptions()` to define queries once and reuse across components and loaders
```ts
import { queryOptions } from '@tanstack/react-query'

export const postQueryOptions = (id: string) =>
  queryOptions({
    queryKey: postKeys.detail(id),
    queryFn: () => fetchPost(id),
    staleTime: 1000 * 60 * 5, // 5 min
  })

// In component
const { data } = useQuery(postQueryOptions(postId))

// In router loader (TanStack Router integration)
loader: ({ params, context: { queryClient } }) =>
  queryClient.ensureQueryData(postQueryOptions(params.postId))
```

## useQuery
```tsx
const {
  data,
  isLoading,    // true only on first load with no cached data
  isFetching,   // true whenever a fetch is in-flight
  isError,
  error,
  isSuccess,
} = useQuery({
  queryKey: postKeys.detail(postId),
  queryFn: () => fetchPost(postId),
  enabled: !!postId, // disable query if params not ready
})
```

## useMutation
```tsx
const { mutate, mutateAsync, isPending } = useMutation({
  mutationFn: (newPost: CreatePostInput) => createPost(newPost),
  onSuccess: (data) => {
    // Invalidate and refetch
    queryClient.invalidateQueries({ queryKey: postKeys.lists() })
    toast.success('Post created!')
  },
  onError: (error) => {
    toast.error(error.message)
  },
})

// Usage
mutate({ title: 'Hello', body: '...' })
```

## Optimistic Updates
```tsx
const queryClient = useQueryClient()

const mutation = useMutation({
  mutationFn: updatePost,
  onMutate: async (updatedPost) => {
    await queryClient.cancelQueries({ queryKey: postKeys.detail(updatedPost.id) })
    const previous = queryClient.getQueryData(postKeys.detail(updatedPost.id))
    queryClient.setQueryData(postKeys.detail(updatedPost.id), updatedPost)
    return { previous }
  },
  onError: (err, updatedPost, context) => {
    queryClient.setQueryData(postKeys.detail(updatedPost.id), context?.previous)
  },
  onSettled: (_, __, updatedPost) => {
    queryClient.invalidateQueries({ queryKey: postKeys.detail(updatedPost.id) })
  },
})
```

## Infinite Queries
```tsx
const {
  data,
  fetchNextPage,
  hasNextPage,
  isFetchingNextPage,
} = useInfiniteQuery({
  queryKey: postKeys.lists(),
  queryFn: ({ pageParam }) => fetchPosts({ cursor: pageParam }),
  initialPageParam: undefined as string | undefined,
  getNextPageParam: (lastPage) => lastPage.nextCursor,
})

// data.pages is an array of page results — flatten for rendering
const allPosts = data?.pages.flatMap((page) => page.items) ?? []
```

## Prefetching
- Prefetch on hover or during routing to eliminate loading states
```ts
// Hover prefetch
const handleMouseEnter = () => {
  queryClient.prefetchQuery(postQueryOptions(postId))
}

// In router loader (eliminates all loading spinners)
export const Route = createFileRoute('/posts/$postId')({
  loader: ({ context: { queryClient }, params }) =>
    queryClient.ensureQueryData(postQueryOptions(params.postId)),
})
```

## Cache Invalidation Patterns
```ts
// Invalidate all post queries
queryClient.invalidateQueries({ queryKey: postKeys.all })

// Invalidate only post lists
queryClient.invalidateQueries({ queryKey: postKeys.lists() })

// Remove from cache entirely
queryClient.removeQueries({ queryKey: postKeys.detail(id) })

// Directly update cache without refetch
queryClient.setQueryData(postKeys.detail(id), newData)
```

## Suspense Mode
- Use `useSuspenseQuery` for Suspense-based data fetching (v5)
- Wrap with `<Suspense fallback={<Skeleton />}>`
- Pair with `<ErrorBoundary>` for error handling
```tsx
// No need to handle isLoading — Suspense handles it
const { data } = useSuspenseQuery(postQueryOptions(postId))
```

## Performance Best Practices
- Set appropriate `staleTime` per query — defaults to `0` (always stale)
- Use `select` to transform/subscribe to only relevant slices of data
- Use `placeholderData: keepPreviousData` for pagination to avoid layout shifts
- Avoid creating `QueryClient` inside components — instantiate once at app root
- Use `notifyOnChangeProps` to limit re-renders to only relevant data changes

## Error Handling
- Use `throwOnError: true` to bubble errors to the nearest ErrorBoundary
- Use `retry` function for conditional retry logic (e.g., skip retry on 404)
```ts
retry: (failureCount, error) => {
  if (error.status === 404) return false
  return failureCount < 3
},
```

## TypeScript Tips
- Always type `queryFn` return value explicitly or infer from typed API functions
- Use `QueryObserverResult<TData, TError>` to type hook return values
- Use `UseMutationResult<TData, TError, TVariables>` for mutations