TanStack Query for Next.js: Server State, Optimistic Updates, and Server Prefetching

TanStack Query (React Query) is the gold standard for server state management in React. It handles caching, deduplication, background refetching, and optimistic updates so you don't have to.

Why Not Just useEffect?

// The useEffect pattern you should stop writing
'use client'
import { useState, useEffect } from 'react'

function UserProfile({ userId }) {
  const [user, setUser] = useState(null)
  const [loading, setLoading] = useState(true)
  const [error, setError] = useState(null)

  useEffect(() => {
    fetch(`/api/users/${userId}`)
      .then(r => r.json())
      .then(setUser)
      .catch(setError)
      .finally(() => setLoading(false))
  }, [userId])

  // No caching -- refetches on every mount
  // No deduplication -- two components make two requests
  // No background refresh
  // No stale-while-revalidate
}

Setup

npm install @tanstack/react-query @tanstack/react-query-devtools

// app/providers.tsx
'use client'
import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
import { ReactQueryDevtools } from '@tanstack/react-query-devtools'
import { useState } from 'react'

export function Providers({ children }: { children: React.ReactNode }) {
  const [queryClient] = useState(() => new QueryClient({
    defaultOptions: {
      queries: {
        staleTime: 60 * 1000,    // 1 minute before re-fetching
        gcTime: 10 * 60 * 1000,  // 10 minutes in cache
        retry: 1,
        refetchOnWindowFocus: false
      }
    }
  }))

  return (
    <QueryClientProvider client={queryClient}>
      {children}
      <ReactQueryDevtools initialIsOpen={false} />
    </QueryClientProvider>
  )
}

Basic Query

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

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

function UserProfile({ userId }: { userId: string }) {
  const { data: user, isLoading, error } = useQuery({
    queryKey: ['user', userId],
    queryFn: () => fetchUser(userId),
    enabled: !!userId // Only run if userId exists
  })

  if (isLoading) return <Skeleton />
  if (error) return <ErrorMessage error={error} />

  return <div>{user.name}</div>
}
// Automatically caches by queryKey
// Second component with same userId gets cached result instantly
// Background refetch after staleTime

Mutations

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

function UpdateProfileForm({ user }) {
  const queryClient = useQueryClient()

  const mutation = useMutation({
    mutationFn: async (data: { name: string }) => {
      const r = await fetch('/api/profile', {
        method: 'PATCH',
        body: JSON.stringify(data),
        headers: { 'Content-Type': 'application/json' }
      })
      if (!r.ok) throw new Error('Update failed')
      return r.json()
    },
    onSuccess: (updatedUser) => {
      // Update cached data
      queryClient.setQueryData(['user', user.id], updatedUser)
      // Or invalidate to refetch
      queryClient.invalidateQueries({ queryKey: ['user', user.id] })
    }
  })

  return (
    <form onSubmit={(e) => {
      e.preventDefault()
      mutation.mutate({ name: e.currentTarget.name.value })
    }}>
      <input name='name' defaultValue={user.name} />
      <button disabled={mutation.isPending}>
        {mutation.isPending ? 'Saving...' : 'Save'}
      </button>
      {mutation.isError && <p>Error: {mutation.error.message}</p>}
    </form>
  )
}

Optimistic Updates

const mutation = useMutation({
  mutationFn: (variables) => toggleLikeApi(variables.postId),
  onMutate: async (variables) => {
    // Cancel any in-flight queries
    await queryClient.cancelQueries({ queryKey: ['posts'] })

    // Snapshot current value
    const previous = queryClient.getQueryData(['posts'])

    // Optimistically update
    queryClient.setQueryData(['posts'], (old: Post[]) =>
      old.map(post =>
        post.id === variables.postId
          ? { ...post, likedByUser: !post.likedByUser, likes: post.likes + (post.likedByUser ? -1 : 1) }
          : post
      )
    )

    return { previous } // Context for rollback
  },
  onError: (err, variables, context) => {
    // Rollback on error
    queryClient.setQueryData(['posts'], context?.previous)
  },
  onSettled: () => {
    queryClient.invalidateQueries({ queryKey: ['posts'] })
  }
})

Parallel Queries

function Dashboard() {
  const [userQuery, postsQuery, statsQuery] = useQueries({
    queries: [
      { queryKey: ['user'], queryFn: fetchUser },
      { queryKey: ['posts'], queryFn: fetchPosts },
      { queryKey: ['stats'], queryFn: fetchStats }
    ]
  })
  // All three fetch in parallel
}

With Next.js Server Components

Prefetch on the server, use on the client:

// app/dashboard/page.tsx (Server Component)
import { HydrationBoundary, QueryClient, dehydrate } from '@tanstack/react-query'
import { DashboardClient } from './dashboard-client'

export default async function DashboardPage() {
  const queryClient = new QueryClient()

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

  return (
    <HydrationBoundary state={dehydrate(queryClient)}>
      <DashboardClient /> {/* Has data immediately, no loading state */}
    </HydrationBoundary>
  )
}

Pre-Wired in the Starter

The AI SaaS Starter uses React Query for all client-side data fetching with the prefetch pattern for Server Components.

AI SaaS Starter Kit -- $99 one-time -- React Query configured and ready. Clone and ship.

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