Tanstack Query

Core Concepts

Query Keys

Query keys uniquely identify cached data. They must be serializable arrays:

// Simple key
useQuery({ queryKey: ['todos'], queryFn: fetchTodos })

// With variables (dependency array pattern)
useQuery({ queryKey: ['todos', { status, page }], queryFn: fetchTodos })

// Hierarchical keys for invalidation
useQuery({ queryKey: ['todos', todoId], queryFn: () => fetchTodo(todoId) })
useQuery({
  queryKey: ['todos', todoId, 'comments'],
  queryFn: () => fetchComments(todoId),
})

// Invalidation matches prefixes:
// queryClient.invalidateQueries({ queryKey: ['todos'] })
// ^ Invalidates ALL queries starting with 'todos'

Query Functions

// Query function receives a QueryFunctionContext
useQuery({
  queryKey: ['todos', todoId],
  queryFn: async ({ queryKey, signal, meta }) => {
    const [_key, id] = queryKey
    const response = await fetch(`/api/todos/${id}`, { signal })
    if (!response.ok) throw new Error('Failed to fetch')
    return response.json()
  },
})

// Using the signal for automatic cancellation
useQuery({
  queryKey: ['todos'],
  queryFn: async ({ signal }) => {
    const response = await fetch('/api/todos', { signal })
    return response.json()
  },
})

queryOptions Helper

Create reusable, type-safe query configurations:

import { queryOptions } from '@tanstack/react-query'

export const todosQueryOptions = queryOptions({
  queryKey: ['todos'],
  queryFn: fetchTodos,
  staleTime: 5000,
})

export const todoQueryOptions = (todoId: string) =>
  queryOptions({
    queryKey: ['todos', todoId],
    queryFn: () => fetchTodo(todoId),
    enabled: !!todoId,
  })

// Usage
const { data } = useQuery(todosQueryOptions)
const { data } = useSuspenseQuery(todoQueryOptions(id))
await queryClient.prefetchQuery(todosQueryOptions)

Queries (useQuery)

Basic Usage

import { useQuery } from '@tanstack/react-query'

function Todos() {
  const {
    data,
    error,
    isLoading, // First load, no data yet
    isFetching, // Any fetch in progress (including background)
    isError,
    isSuccess,
    isPending, // No data yet (same as isLoading in most cases)
    status, // 'pending' | 'error' | 'success'
    fetchStatus, // 'fetching' | 'paused' | 'idle'
    refetch,
    isStale,
    isPlaceholderData,
    dataUpdatedAt,
    errorUpdatedAt,
  } = useQuery({
    queryKey: ['todos'],
    queryFn: fetchTodos,
  })

  if (isLoading) return <Spinner />
  if (isError) return <Error message={error.message} />
  return <TodoList todos={data} />
}

Query Options

useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,

  // Freshness
  staleTime: 5000, // ms data stays fresh (default: 0)
  gcTime: 300000, // ms unused data stays in cache (default: 5 min)

  // Refetching
  refetchInterval: 10000, // Poll every 10s
  refetchIntervalInBackground: false, // Don't poll when tab hidden
  refetchOnMount: true, // Refetch on component mount if stale
  refetchOnWindowFocus: true, // Refetch on window focus if stale
  refetchOnReconnect: true, // Refetch on network reconnect

  // Retry
  retry: 3, // Number of retries (or function)
  retryDelay: (attemptIndex) => Math.min(1000 * 2 ** attemptIndex, 30000),

  // Conditional
  enabled: !!userId, // Only run when truthy

  // Initial/placeholder data
  initialData: () => cachedData,
  initialDataUpdatedAt: Date.now() - 10000,
  placeholderData: (previousData) => previousData, // keepPreviousData pattern
  placeholderData: initialTodos,

  // Transform
  select: (data) => data.filter((todo) => !todo.done),

  // Structural sharing (default: true)
  structuralSharing: true,

  // Network mode
  networkMode: 'online', // 'online' | 'always' | 'offlineFirst'

  // Meta (accessible in query function context)
  meta: { purpose: 'user-facing' },
})

Mutations (useMutation)

Basic Usage

import { useMutation, useQueryClient } from '@tanstack/react-query'

function AddTodo() {
  const queryClient = useQueryClient()

  const mutation = useMutation({
    mutationFn: (newTodo: { title: string }) => {
      return fetch('/api/todos', {
        method: 'POST',
        body: JSON.stringify(newTodo),
      }).then((res) => res.json())
    },
    // Lifecycle callbacks
    onMutate: async (variables) => {
      // Called before mutationFn
      // Good for optimistic updates
      return { previousTodos } // context for onError
    },
    onSuccess: (data, variables, context) => {
      // Invalidate related queries
      queryClient.invalidateQueries({ queryKey: ['todos'] })
    },
    onError: (error, variables, context) => {
      // Rollback optimistic updates
      queryClient.setQueryData(['todos'], context.previousTodos)
    },
    onSettled: (data, error, variables, context) => {
      // Always runs (success or error)
      queryClient.invalidateQueries({ queryKey: ['todos'] })
    },
  })

  return (
    <button
      onClick={() => mutation.mutate({ title: 'New Todo' })}
      disabled={mutation.isPending}
    >
      {mutation.isPending ? 'Adding...' : 'Add Todo'}
    </button>
  )
}

Mutation State

const {
  mutate,         // Fire-and-forget
  mutateAsync,    // Returns promise
  isPending,      // Mutation in progress
  isError,
  isSuccess,
  isIdle,         // Not yet fired
  data,           // Success response
  error,          // Error object
  reset,          // Reset state to idle
  variables,      // Variables passed to mutate
  status,         // 'idle' | 'pending' | 'error' | 'success'
} = useMutation({ ... })

Optimistic Updates

const mutation = useMutation({
  mutationFn: updateTodo,
  onMutate: async (newTodo) => {
    // 1. Cancel outgoing refetches
    await queryClient.cancelQueries({ queryKey: ['todos', newTodo.id] })

    // 2. Snapshot previous value
    const previousTodo = queryClient.getQueryData(['todos', newTodo.id])

    // 3. Optimistically update
    queryClient.setQueryData(['todos', newTodo.id], newTodo)

    // 4. Return context for rollback
    return { previousTodo }
  },
  onError: (err, newTodo, context) => {
    // Rollback on error
    queryClient.setQueryData(['todos', newTodo.id], context.previousTodo)
  },
  onSettled: () => {
    // Always refetch to sync with server
    queryClient.invalidateQueries({ queryKey: ['todos'] })
  },
})

Optimistic Updates on Lists

onMutate: async (newTodo) => {
  await queryClient.cancelQueries({ queryKey: ['todos'] })
  const previousTodos = queryClient.getQueryData(['todos'])

  queryClient.setQueryData(['todos'], (old) => [...old, newTodo])

  return { previousTodos }
},
onError: (err, newTodo, context) => {
  queryClient.setQueryData(['todos'], context.previousTodos)
},

Query Invalidation

const queryClient = useQueryClient()

// Invalidate all queries
queryClient.invalidateQueries()

// Invalidate by prefix
queryClient.invalidateQueries({ queryKey: ['todos'] })

// Invalidate exact match
queryClient.invalidateQueries({ queryKey: ['todos', 1], exact: true })

// Invalidate with predicate
queryClient.invalidateQueries({
  predicate: (query) =>
    query.queryKey[0] === 'todos' && query.queryKey[1]?.status === 'done',
})

// Invalidate and refetch immediately
queryClient.refetchQueries({ queryKey: ['todos'] })

// Remove from cache entirely
queryClient.removeQueries({ queryKey: ['todos', 1] })

// Reset to initial state
queryClient.resetQueries({ queryKey: ['todos'] })

Infinite Queries

import { useInfiniteQuery } from '@tanstack/react-query'

function InfiniteList() {
  const {
    data,
    fetchNextPage,
    fetchPreviousPage,
    hasNextPage,
    hasPreviousPage,
    isFetchingNextPage,
    isFetchingPreviousPage,
  } = useInfiniteQuery({
    queryKey: ['projects'],
    queryFn: async ({ pageParam }) => {
      const res = await fetch(`/api/projects?cursor=${pageParam}`)
      return res.json()
    },
    initialPageParam: 0,
    getNextPageParam: (lastPage, allPages, lastPageParam) => {
      return lastPage.nextCursor ?? undefined // undefined = no more pages
    },
    getPreviousPageParam: (firstPage, allPages, firstPageParam) => {
      return firstPage.prevCursor ?? undefined
    },
    maxPages: 3, // Keep max 3 pages in cache (for performance)
  })

  return (
    <div>
      {data.pages.map((page) =>
        page.items.map((item) => <Item key={item.id} item={item} />),
      )}
      <button
        onClick={() => fetchNextPage()}
        disabled={!hasNextPage || isFetchingNextPage}
      >
        {isFetchingNextPage
          ? 'Loading...'
          : hasNextPage
            ? 'Load More'
            : 'No more'}
      </button>
    </div>
  )
}

Parallel Queries

// Multiple independent queries run in parallel automatically
function Dashboard() {
  const usersQuery = useQuery({ queryKey: ['users'], queryFn: fetchUsers })
  const projectsQuery = useQuery({
    queryKey: ['projects'],
    queryFn: fetchProjects,
  })

  // Both fetch simultaneously
}

// Dynamic parallel queries with useQueries
function UserProjects({ userIds }) {
  const queries = useQueries({
    queries: userIds.map((id) => ({
      queryKey: ['user', id],
      queryFn: () => fetchUser(id),
    })),
    combine: (results) => ({
      data: results.map((r) => r.data),
      pending: results.some((r) => r.isPending),
    }),
  })
}

Dependent Queries

// Sequential queries using enabled
function UserPosts({ userId }) {
  const userQuery = useQuery({
    queryKey: ['user', userId],
    queryFn: () => fetchUser(userId),
  })

  const postsQuery = useQuery({
    queryKey: ['posts', userId],
    queryFn: () => fetchPostsByUser(userId),
    enabled: !!userQuery.data, // Only run when user is loaded
  })
}

Paginated Queries

function PaginatedList() {
  const [page, setPage] = useState(1)

  const { data, isPlaceholderData } = useQuery({
    queryKey: ['todos', page],
    queryFn: () => fetchTodos(page),
    placeholderData: (previousData) => previousData, // Keep showing old data
  })

  return (
    <div style={{ opacity: isPlaceholderData ? 0.5 : 1 }}>
      {data.items.map((item) => (
        <Item key={item.id} item={item} />
      ))}
      <button
        onClick={() => setPage((p) => p + 1)}
        disabled={isPlaceholderData || !data.hasMore}
      >
        Next
      </button>
    </div>
  )
}

Suspense Integration

import {
  useSuspenseQuery,
  useSuspenseInfiniteQuery,
} from '@tanstack/react-query'

// Component will suspend until data is loaded
function TodoList() {
  const { data } = useSuspenseQuery({
    queryKey: ['todos'],
    queryFn: fetchTodos,
  })
  // data is guaranteed to be defined here
  return (
    <ul>
      {data.map((todo) => (
        <li key={todo.id}>{todo.title}</li>
      ))}
    </ul>
  )
}

// Wrap with Suspense boundary
function App() {
  return (
    <ErrorBoundary fallback={<Error />}>
      <Suspense fallback={<Loading />}>
        <TodoList />
      </Suspense>
    </ErrorBoundary>
  )
}

// Multiple suspense queries (fetch in parallel)
function Dashboard() {
  const [{ data: users }, { data: projects }] = useSuspenseQueries({
    queries: [
      { queryKey: ['users'], queryFn: fetchUsers },
      { queryKey: ['projects'], queryFn: fetchProjects },
    ],
  })
}

Prefetching

const queryClient = useQueryClient()

// Prefetch on hover
function TodoLink({ todoId }) {
  const prefetch = () => {
    queryClient.prefetchQuery({
      queryKey: ['todo', todoId],
      queryFn: () => fetchTodo(todoId),
      staleTime: 5000, // Only prefetch if data older than 5s
    })
  }

  return (
    <Link to={`/todos/${todoId}`} onMouseEnter={prefetch}>
      Todo {todoId}
    </Link>
  )
}

// Prefetch in route loader (TanStack Router integration)
export const Route = createFileRoute('/todos/$todoId')({
  loader: ({ context: { queryClient }, params: { todoId } }) =>
    queryClient.ensureQueryData(todoQueryOptions(todoId)),
})

// Prefetch infinite queries
queryClient.prefetchInfiniteQuery({
  queryKey: ['projects'],
  queryFn: fetchProjects,
  initialPageParam: 0,
  pages: 3, // Prefetch first 3 pages
})

SSR & Hydration

Server-Side Prefetching

// Server component or loader
import {
  dehydrate,
  HydrationBoundary,
  QueryClient,
} from '@tanstack/react-query'

async function getServerSideProps() {
  const queryClient = new QueryClient()

  await queryClient.prefetchQuery({
    queryKey: ['todos'],
    queryFn: fetchTodos,
  })

  return {
    props: {
      dehydratedState: dehydrate(queryClient),
    },
  }
}

function Page({ dehydratedState }) {
  return (
    <HydrationBoundary state={dehydratedState}>
      <Todos />
    </HydrationBoundary>
  )
}

Streaming SSR (React Server Components)

import { dehydrate, HydrationBoundary } from '@tanstack/react-query'
import { makeQueryClient } from './query-client'

export default async function Page() {
  const queryClient = makeQueryClient()

  // Prefetch on server
  await queryClient.prefetchQuery({
    queryKey: ['todos'],
    queryFn: fetchTodos,
  })

  return (
    <HydrationBoundary state={dehydrate(queryClient)}>
      <TodoList />
    </HydrationBoundary>
  )
}

QueryClient API

const queryClient = useQueryClient()

// Get cached data
queryClient.getQueryData(['todos'])

// Set cached data
queryClient.setQueryData(['todos'], updatedTodos)
queryClient.setQueryData(['todos'], (old) => [...old, newTodo])

// Get query state
queryClient.getQueryState(['todos'])

// Check if fetching
queryClient.isFetching({ queryKey: ['todos'] })
queryClient.isMutating()

// Cancel queries
queryClient.cancelQueries({ queryKey: ['todos'] })

// Invalidate (marks stale, refetches active)
queryClient.invalidateQueries({ queryKey: ['todos'] })

// Refetch (force refetch even if fresh)
queryClient.refetchQueries({ queryKey: ['todos'] })

// Remove from cache
queryClient.removeQueries({ queryKey: ['todos'] })

// Reset to initial state
queryClient.resetQueries({ queryKey: ['todos'] })

// Clear entire cache
queryClient.clear()

// Prefetch
queryClient.prefetchQuery({ queryKey: ['todos'], queryFn: fetchTodos })
queryClient.ensureQueryData({ queryKey: ['todos'], queryFn: fetchTodos })

// Get/set defaults
queryClient.setQueryDefaults(['todos'], { staleTime: 10000 })
queryClient.getQueryDefaults(['todos'])
queryClient.setMutationDefaults(['addTodo'], { mutationFn: addTodo })

Testing

import { renderHook, waitFor } from '@testing-library/react'
import { QueryClient, QueryClientProvider } from '@tanstack/react-query'

function createWrapper() {
  const queryClient = new QueryClient({
    defaultOptions: {
      queries: {
        retry: false, // Don't retry in tests
        gcTime: Infinity, // Prevent garbage collection during tests
      },
    },
  })
  return ({ children }) => (
    <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
  )
}

test('fetches todos', async () => {
  const { result } = renderHook(
    () =>
      useQuery({
        queryKey: ['todos'],
        queryFn: fetchTodos,
      }),
    { wrapper: createWrapper() },
  )

  await waitFor(() => expect(result.current.isSuccess).toBe(true))
  expect(result.current.data).toEqual(expectedTodos)
})

// Mock with setQueryData for component tests
test('renders todos', () => {
  const queryClient = new QueryClient()
  queryClient.setQueryData(['todos'], mockTodos)

  render(
    <QueryClientProvider client={queryClient}>
      <TodoList />
    </QueryClientProvider>,
  )

  expect(screen.getByText('Todo 1')).toBeInTheDocument()
})

TypeScript Patterns

Typing Query Functions

interface Todo {
  id: number
  title: string
  completed: boolean
}

// Type is inferred from queryFn return type
const { data } = useQuery({
  queryKey: ['todos'],
  queryFn: async (): Promise<Todo[]> => {
    const res = await fetch('/api/todos')
    return res.json()
  },
})
// data: Todo[] | undefined

// With select
const { data } = useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
  select: (data): string[] => data.map((t) => t.title),
})
// data: string[] | undefined

Typing Errors

// Default error type is Error
const { error } = useQuery<Todo[], AxiosError>({
  queryKey: ['todos'],
  queryFn: fetchTodos,
})

// Or register globally
declare module '@tanstack/react-query' {
  interface Register {
    defaultError: AxiosError
  }
}

Query Options Pattern (Recommended)

import { queryOptions, infiniteQueryOptions } from '@tanstack/react-query'

export const todosOptions = queryOptions({
  queryKey: ['todos'] as const,
  queryFn: fetchTodos,
  staleTime: 5000,
})

export const todoOptions = (id: string) =>
  queryOptions({
    queryKey: ['todos', id] as const,
    queryFn: () => fetchTodo(id),
    enabled: !!id,
  })

// Full type inference everywhere
const { data } = useQuery(todosOptions)
const { data } = useSuspenseQuery(todoOptions('123'))
await queryClient.ensureQueryData(todosOptions)
queryClient.invalidateQueries({ queryKey: todosOptions.queryKey })

Advanced Patterns

Window Focus Refetching

// Disable globally
const queryClient = new QueryClient({
  defaultOptions: {
    queries: { refetchOnWindowFocus: false },
  },
})

// Custom focus manager
import { focusManager } from '@tanstack/react-query'

// For React Native
focusManager.setEventListener((handleFocus) => {
  const subscription = AppState.addEventListener('change', (state) => {
    handleFocus(state === 'active')
  })
  return () => subscription.remove()
})

Network Mode

useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
  // 'online' (default): only fetch when online
  // 'always': always fetch (useful for local-first)
  // 'offlineFirst': try fetch, use cache if offline
  networkMode: 'offlineFirst',
})

Query Cancellation

useQuery({
  queryKey: ['todos'],
  queryFn: async ({ signal }) => {
    // signal is AbortSignal - automatically cancelled on unmount or key change
    const res = await fetch('/api/todos', { signal })
    return res.json()
  },
})

// Manual cancellation
queryClient.cancelQueries({ queryKey: ['todos'] })

Persistence

import { persistQueryClient } from '@tanstack/react-query-persist-client'
import { createSyncStoragePersister } from '@tanstack/query-sync-storage-persister'

const persister = createSyncStoragePersister({
  storage: window.localStorage,
})

persistQueryClient({
  queryClient,
  persister,
  maxAge: 1000 * 60 * 60 * 24, // 24 hours
})

Best Practices

1. Use queryOptions helper for type-safe, reusable query configurations
2. Structure query keys hierarchically for granular invalidation
3. Set appropriate staleTime - 0 means always refetch on mount (default), increase for less dynamic data
4. Use placeholderData (not initialData) for keeping previous page data during pagination
5. Prefer useSuspenseQuery when using Suspense boundaries for cleaner component code
6. Use enabled for dependent queries, not conditional hook calls
7. Always invalidate after mutations - don't rely solely on optimistic updates
8. Cancel queries in onMutate before optimistic updates to prevent race conditions
9. Use ensureQueryData in route loaders instead of prefetchQuery for immediate access
10. Set retry: false in tests to avoid timeout issues
11. Don't destructure the query result if you need to pass it around (breaks reactivity)
12. Use select for derived data instead of transforming in the component
13. Keep query functions pure - they should only fetch, not cause side effects
14. Use gcTime: Infinity in tests to prevent cache cleanup during assertions

Common Pitfalls

• Using initialData when you mean placeholderData (initialData counts as "fresh" data)
• Not providing initialPageParam for infinite queries (required in v5)
• Calling hooks conditionally (violates React rules)
• Not cancelling queries before optimistic updates (race conditions)
• Setting staleTime higher than gcTime (data gets garbage collected while "fresh")
• Forgetting to wrap tests with QueryClientProvider
• Using same QueryClient instance across tests (shared state)
• Not awaiting invalidateQueries in mutation callbacks when order matters