TanStack Query Full Course 2026 | useQuery, useMutation, Caching, Infinite Queries, Pagination...

Key Concepts

• TanStack Query (React Query): A library for managing server state, including fetching, caching, synchronizing, and updating data.
• Query Client: The central instance that manages the cache and configuration for all queries and mutations.
• Query Key: A unique identifier (array) used to cache and track specific data requests.
• Stale Time: The duration (in milliseconds) before data is considered "stale" and requires a refetch.
• GC Time (Garbage Collection Time): The duration unused cached data remains in memory before being deleted.
• Mutation: An operation to create, update, or delete data on the server.
• Optimistic Updates: Updating the UI immediately before the server confirms the request, rolling back if an error occurs.
• Invalidation: Forcing a query to become stale, triggering an automatic refetch.

1. Setup and Initialization

To integrate TanStack Query, install @tanstack/react-query and @tanstack/react-query-devtools.

• Provider Pattern: Wrap the application in a QueryClientProvider.
• QueryClient: Instantiate a QueryClient object and pass it to the provider. This enables global access to data management functions and devtools.

2. Data Fetching with useQuery

The useQuery hook replaces manual useEffect and useState logic for fetching data.

• Parameters: Requires a queryKey (for caching) and a queryFn (the function that performs the fetch).
• Return Values: Provides data, isPending, error, isError, isSuccess, and a refetch function.
• Lazy Queries: By using the enabled property, developers can prevent automatic fetching, triggering it only when specific conditions (e.g., a button click) are met.

3. Mutations with useMutation

Used for server-side data modifications (POST, PUT, DELETE).

• Process: Unlike useQuery, mutations are triggered manually via the mutate function.
• Lifecycle: Developers can hook into the mutation process using onMutate (before the request), onError (if it fails), and onSettled (after completion, regardless of outcome).

4. Caching and Data Freshness

TanStack Query manages the cache to reduce unnecessary network requests.

• Stale vs. Fresh: Data is "fresh" until the staleTime expires. Once stale, the library may trigger a background refetch.
• Refetching Strategies:
  • refetchOnWindowFocus: Automatically refetches when the user returns to the browser tab.
  • refetchOnReconnect: Refetches when the internet connection is restored.
  • refetchInterval: Polls the server at a set interval (e.g., for live dashboards).
• Invalidation: Use queryClient.invalidateQueries({ queryKey: [...] }) to manually force a refetch. This is highly effective for updating UI state after a mutation (e.g., refreshing a post list after adding a new post).

5. Advanced Patterns

• Optimistic Updates:
  1. onMutate: Cancel existing queries, capture current data, and update the cache with the expected result.
  2. onError: If the request fails, roll back the cache to the captured previous data.
  3. onSettled: Invalidate the query to ensure the UI reflects the true server state.
• Pagination: Use placeholderData and keepPreviousData to maintain a smooth UI while fetching new pages, preventing the screen from flickering or showing loading states during navigation.
• Infinite Queries: Use useInfiniteQuery to handle "load more" functionality. It provides fetchNextPage and hasNextPage to manage growing datasets efficiently.

Synthesis

TanStack Query significantly reduces boilerplate code by abstracting complex state management tasks like caching, loading, and error handling. By shifting from manual useEffect fetching to a declarative hook-based approach, developers can build more performant and scalable applications. The core takeaway is the power of the Query Key—by uniquely identifying data, developers gain granular control over when to fetch, cache, and invalidate data, leading to a superior user experience.