If you’ve built React applications that communicate with APIs, you’ve probably managed server state using tools like useState, useEffect, and sometimes Redux. But as your app grows, handling loading states, caching, background refetching, pagination, and error recovery can become complicated.

This is where React Query shines.

React Query is a powerful data-fetching and state management library designed specifically for server state. It simplifies fetching, caching, synchronizing, and updating data in your UI — without the boilerplate of Redux or manual useEffect calls.

Why React Query?

Before React Query, typical data-fetching logic looked something like this:

const [data, setData] = useState(null);
const [loading, setLoading] = useState(false);
const [error, setError] = useState(null);

useEffect(() => {
  setLoading(true);
  fetch('/api/users')
    .then(res => res.json())
    .then(setData)
    .catch(setError)
    .finally(() => setLoading(false));
}, []);

This approach works — but you’ll need to repeat similar logic across components and maintain state manually. It’s also tricky to handle things like:

• Refetching in the background
• Data caching and invalidation
• Real-time updates
• Retrying failed requests

React Query solves all of this with a clean, declarative API.

✅ Core Concepts

🟦 Query Functions (useQuery)

Used for fetching data from the server.

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

const { data, isLoading, isError, refetch } = useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
});

Useful Options

Example:

useQuery({
  queryKey: ['pokemon'],
  queryFn: fetchPokemon,
  staleTime: 1000 * 60 * 5, // 5 minutes
  refetchOnWindowFocus: false,
});

🟥 Mutation Functions (useMutation)

Used to modify data — create, update, delete.

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

const queryClient = useQueryClient();

const mutation = useMutation({
  mutationFn: addTodo,
  onSuccess: () => {
    queryClient.invalidateQueries(['todos']); // refresh list
  },
});

🟩 Query Client API

React Query stores all server state in a central cache using a Query Client.

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

const queryClient = new QueryClient();

Useful Query Client Methods

Example (Optimistic UI update):

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

🟨 Infinite Queries (useInfiniteQuery)

Used for pagination or infinite scroll.

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

const query = useInfiniteQuery({
  queryKey: ['pokemon'],
  queryFn: fetchPokemonPage,
  getNextPageParam: (lastPage) => lastPage.nextPageCursor,
});

🟪 DevTools (Highly Recommended)

npm install @tanstack/react-query-devtools

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

<QueryClientProvider client={queryClient}>
  <App />
  <ReactQueryDevtools initialIsOpen={false} />
</QueryClientProvider>

This allows you to:

• See queries in cache
• Inspect stale vs fresh status
• Debug fetching and mutations visually

🧠 Quick Summary

🎯 Final Thoughts

React Query replaces manual API handling with a predictable, maintainable pattern. Once you understand useQuery, useMutation, and cache operations, you can build complex, scalable data flows easily.

If you’d like, I can now provide:

✅ Visual diagrams ✅ Example for Mutations (Create / Update / Delete Pokémon) ✅ Infinite Scroll Pokémon List ✅ Detail Page with useQuery + dynamic params

Just say: “Continue with Pokémon mutation example” 🧩