tanstack-query

What this skill does

## Overview

TanStack Query (formerly React Query) manages server state - data that lives on the server and needs to be fetched, cached, synchronized, and updated. It provides automatic caching, background refetching, stale-while-revalidate patterns, pagination, infinite scrolling, and optimistic updates out of the box.

**Package:** `@tanstack/react-query`
**Devtools:** `@tanstack/react-query-devtools`
**Current Version:** v5

## Installation

```bash
npm install @tanstack/react-query
npm install -D @tanstack/react-query-devtools  # Optional
```

## Setup

```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
      gcTime: 1000 * 60 * 5, // 5 minutes (garbage collection)
      retry: 3,
      refetchOnWindowFocus: true,
      refetchOnReconnect: true,
    },
  },
})

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

## Core Concepts

### Query Keys

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

```tsx
// 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

```tsx
// 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:

```tsx
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

```tsx
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

```tsx
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

```tsx
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

```tsx
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

```tsx
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

```tsx
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.previousTod