npm install @tanstack/react-query

// 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 * 5, // 5 minutes
      retry: 3,
      refetchOnWindowFocus: false,
    },
  },
});

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

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

interface User {
  id: string;
  name: string;
  email: string;
}

async function fetchUser(userId: string): Promise<User> {
  const response = await fetch(`/api/users/${userId}`);
  if (!response.ok) throw new Error('Failed to fetch user');
  return response.json();
}

function UserProfile({ userId }: { userId: string }) {
  const {
    data: user,
    isLoading,
    isError,
    error,
    refetch,
  } = useQuery({
    queryKey: ['user', userId],
    queryFn: () => fetchUser(userId),
  });

  if (isLoading) return <Skeleton />;
  if (isError) return <Error message={error.message} onRetry={refetch} />;

  return (
    <div>
      <h1>{user.name}</h1>
      <p>{user.email}</p>
    </div>
  );
}

const { data } = useQuery({
  queryKey: ['todos', { status: 'active' }],
  queryFn: fetchActiveTodos,
  staleTime: 1000 * 60 * 10,     // Data fresh for 10 minutes
  gcTime: 1000 * 60 * 30,        // Cache for 30 minutes (formerly cacheTime)
  refetchInterval: 1000 * 30,    // Refetch every 30 seconds
  refetchOnMount: true,
  refetchOnWindowFocus: true,
  enabled: !!userId,             // Only run if userId exists
  placeholderData: [],           // Show while loading
  select: (data) => data.filter(todo => !todo.completed), // Transform data
});

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

interface CreateTodoInput {
  title: string;
  completed: boolean;
}

function TodoForm() {
  const queryClient = useQueryClient();

  const mutation = useMutation({
    mutationFn: (newTodo: CreateTodoInput) =>
      fetch('/api/todos', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify(newTodo),
      }).then(res => res.json()),

    onSuccess: () => {
      // Invalidate and refetch
      queryClient.invalidateQueries({ queryKey: ['todos'] });
    },

    onError: (error) => {
      toast.error(`Failed to create: ${error.message}`);
    },
  });

  return (
    <form onSubmit={(e) => {
      e.preventDefault();
      const formData = new FormData(e.currentTarget);
      mutation.mutate({
        title: formData.get('title') as string,
        completed: false,
      });
    }}>
      <input name="title" required />
      <button type="submit" disabled={mutation.isPending}>
        {mutation.isPending ? 'Adding...' : 'Add Todo'}
      </button>
    </form>
  );
}

const mutation = useMutation({
  mutationFn: updateTodo,

  onMutate: async (newTodo) => {
    // Cancel outgoing refetches
    await queryClient.cancelQueries({ queryKey: ['todos'] });

    // Snapshot previous value
    const previousTodos = queryClient.getQueryData(['todos']);

    // Optimistically update
    queryClient.setQueryData(['todos'], (old: Todo[]) =>
      old.map(todo =>
        todo.id === newTodo.id ? { ...todo, ...newTodo } : todo
      )
    );

    // Return snapshot for rollback
    return { previousTodos };
  },

  onError: (err, newTodo, context) => {
    // Rollback on error
    queryClient.setQueryData(['todos'], context?.previousTodos);
  },

  onSettled: () => {
    // Refetch after error or success
    queryClient.invalidateQueries({ queryKey: ['todos'] });
  },
});

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

interface PostsPage {
  posts: Post[];
  nextCursor: string | null;
}

function PostsList() {
  const {
    data,
    fetchNextPage,
    hasNextPage,
    isFetchingNextPage,
  } = useInfiniteQuery({
    queryKey: ['posts'],
    queryFn: async ({ pageParam }): Promise<PostsPage> => {
      const res = await fetch(`/api/posts?cursor=${pageParam}`);
      return res.json();
    },
    initialPageParam: '',
    getNextPageParam: (lastPage) => lastPage.nextCursor,
  });

  const allPosts = data?.pages.flatMap(page => page.posts) ?? [];

  return (
    <div>
      {allPosts.map(post => (
        <PostCard key={post.id} post={post} />
      ))}

      {hasNextPage && (
        <button
          onClick={() => fetchNextPage()}
          disabled={isFetchingNextPage}
        >
          {isFetchingNextPage ? 'Loading more...' : 'Load More'}
        </button>
      )}
    </div>
  );
}

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

function PostsList() {
  const queryClient = useQueryClient();

  const prefetchPost = (postId: string) => {
    queryClient.prefetchQuery({
      queryKey: ['post', postId],
      queryFn: () => fetchPost(postId),
      staleTime: 1000 * 60 * 5,
    });
  };

  return (
    <ul>
      {posts.map(post => (
        <li
          key={post.id}
          onMouseEnter={() => prefetchPost(post.id)}
        >
          <Link to={`/posts/${post.id}`}>{post.title}</Link>
        </li>
      ))}
    </ul>
  );
}

// Structure keys for easy invalidation
const queryKeys = {
  all: ['todos'] as const,
  lists: () => [...queryKeys.all, 'list'] as const,
  list: (filters: Filters) => [...queryKeys.lists(), filters] as const,
  details: () => [...queryKeys.all, 'detail'] as const,
  detail: (id: string) => [...queryKeys.details(), id] as const,
};

// Usage
useQuery({
  queryKey: queryKeys.detail(todoId),
  queryFn: () => fetchTodo(todoId),
});

// Invalidate all todos
queryClient.invalidateQueries({ queryKey: queryKeys.all });

// Invalidate only lists
queryClient.invalidateQueries({ queryKey: queryKeys.lists() });

function Dashboard() {
  const usersQuery = useQuery({
    queryKey: ['users'],
    queryFn: fetchUsers,
  });

  const projectsQuery = useQuery({
    queryKey: ['projects'],
    queryFn: fetchProjects,
  });

  if (usersQuery.isLoading || projectsQuery.isLoading) {
    return <Loading />;
  }

  return (
    <>
      <UsersList users={usersQuery.data} />
      <ProjectsList projects={projectsQuery.data} />
    </>
  );
}

function UserPosts({ userId }: { userId: string }) {
  // First query
  const { data: user } = useQuery({
    queryKey: ['user', userId],
    queryFn: () => fetchUser(userId),
  });

  // Dependent query - only runs when user exists
  const { data: posts } = useQuery({
    queryKey: ['posts', user?.id],
    queryFn: () => fetchUserPosts(user!.id),
    enabled: !!user?.id, // Only run when user.id exists
  });

  return <PostsList posts={posts} />;
}

function ErrorBoundaryWithRetry({
  error,
  resetErrorBoundary,
}: {
  error: Error;
  resetErrorBoundary: () => void;
}) {
  return (
    <div className="error-container">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
      <button onClick={resetErrorBoundary}>Try again</button>
    </div>
  );
}

// Global error handling
const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      throwOnError: true, // Propagate to error boundary
    },
    mutations: {
      onError: (error) => {
        toast.error(`Error: ${error.message}`);
      },
    },
  },
});