React Redux Interview Questions 2026, 30 Q&A on Redux Toolkit and RTK Query

Redux interviews in 2026 are not about reducers boilerplate anymore. Candidates report that interviewers at product companies test Redux Toolkit, RTK Query, the data-flow model, and crucially the judgment to know when Redux is the wrong tool. This guide compiles 30 questions from candidate-reported rounds and public preparation resources, each with code and the trade-off the interviewer is probing.

The shift: Redux Toolkit (RTK) is the official standard. If you mention old-style switch reducers and connect HOCs without RTK, candidates report it reads as dated.

Related: React Interview Questions 2026 | React Hooks Interview Questions 2026 | React Coding Questions with Solutions 2026 | System Design Interview Questions 2026

Core Redux Concepts (Q1 to Q12)

Q1. What is Redux and what problem does it solve?

Q2. What are the three principles of Redux?

Q3. What is Redux Toolkit and why is it recommended?

const counter = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: (state) => { state.value += 1; }, // Immer makes this safe
  },
});
export const { increment } = counter.actions;

Q4. How does createSlice reduce boilerplate?

Q5. What is the store and how do you configure it?

const store = configureStore({
  reducer: { counter: counterReducer, users: usersReducer },
});

configureStore enables Redux DevTools, adds thunk middleware, and turns on immutability and serializability checks in development by default.

Q6. How do you read and write state in a component?

import { useSelector, useDispatch } from 'react-redux';
const value = useSelector(state => state.counter.value);
const dispatch = useDispatch();
dispatch(increment());

useSelector subscribes the component and re-renders when the selected value changes (strict-equal by default).

Q7. Why must reducers be pure?

Q8. What is immutability and why does Redux require it?

Q9. What is a thunk? How does createAsyncThunk work?

const fetchUser = createAsyncThunk('users/fetch', async (id) => {
  const res = await fetch(`/api/user/${id}`);
  return res.json();
});
// in slice:
extraReducers: (b) => {
  b.addCase(fetchUser.pending, s => { s.loading = true; })
   .addCase(fetchUser.fulfilled, (s, a) => { s.loading = false; s.data = a.payload; })
   .addCase(fetchUser.rejected, (s, a) => { s.loading = false; s.error = a.error.message; });
}

createAsyncThunk auto-dispatches pending, fulfilled, and rejected actions around an async function.

Q10. What is middleware in Redux?

Q11. What is the difference between Redux and Context plus useReducer?

Q12. When should you NOT use Redux?

RTK Query and Async (Q13 to Q22)

Q13. What is RTK Query and what problem does it solve?

const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/api' }),
  endpoints: (b) => ({
    getUser: b.query({ query: (id) => `user/${id}` }),
  }),
});
export const { useGetUserQuery } = api;

Q14. How does caching work in RTK Query?

Q15. What are tags and how do they enable cache invalidation?

getPosts: b.query({ query: () => 'posts', providesTags: ['Post'] }),
addPost:  b.mutation({ query: (body) => ({ url: 'posts', method: 'POST', body }), invalidatesTags: ['Post'] }),

Adding a post invalidates the Post tag, so the posts list refetches automatically.

Q16. What is optimistic update in RTK Query?

Q17. RTK Query vs React Query, what is the trade-off?

Q18. How do you handle loading and error states with RTK Query hooks?

const { data, isLoading, isError, error } = useGetUserQuery(id);
if (isLoading) return <Spinner />;
if (isError) return <p>{error.toString()}</p>;

Q19. What is polling in RTK Query?

useGetStatusQuery(undefined, { pollingInterval: 5000 });

It refetches on an interval for live data without manual timers.

Q20. Explain reselect and memoized selectors.

import { createSelector } from '@reduxjs/toolkit';
const selectDone = createSelector(
  state => state.todos,
  todos => todos.filter(t => t.done)
);

createSelector memoizes derived data so the filter recomputes only when todos changes, avoiding unnecessary re-renders.

Q21. Why can a selector returning a new object cause extra re-renders?

Q22. How do you persist Redux state across reloads?

Scenario and Architecture (Q23 to Q30)

Q23. Scenario: a list re-renders on every keystroke in an unrelated input. Diagnose.

Q24. How would you structure slices for a medium app?

Q25. What is the normalized state shape and why use it?

Q26. How do you test a slice?

expect(counterReducer({ value: 0 }, increment())).toEqual({ value: 1 });

Thunks are tested by dispatching against a mock store or asserting the resulting actions.

Q27. What is the role of Immer in RTK and what is its limit?

Q28. How do you handle authentication state in Redux?

Q29. Scenario: two components dispatch the same fetch on mount. How does RTK Query prevent a double request?

Q30. How do you migrate a legacy Redux app to Redux Toolkit?

Redux Mock Test, 2026 Edition

5 original questions calibrated to the 2026 Redux batch by Aditya Sharma, from candidate-reported patterns.

Question 1

What library powers RTK's mutating reducer syntax?

a) Lodash b) Immer c) RxJS d) Reselect

Solution: Immer's proxy draft produces immutable updates. Answer: (b)

Question 2

RTK Query dedupes two identical queries by:

a) random b) endpoint plus serialized args cache key c) component name d) URL only

Solution: Same cache key means one shared request. Answer: (b)

Question 3

A selector that returns todos.filter(...) causes extra renders because:

a) Redux bug b) it returns a new array reference each call c) wrong import d) missing key

Solution: Reference inequality; memoize with createSelector. Answer: (b)

Question 4

When should you avoid Redux?

a) always b) for local UI state and server cache c) for large apps d) never

Solution: Local state and server cache have better tools. Answer: (b)

Question 5

configureStore enables by default:

a) nothing b) DevTools, thunk, dev-only immutability checks c) routing d) SSR

Solution: Sane defaults out of the box. Answer: (b)

FAQ, React Redux Interview Questions

Q: Do I still need to learn classic Redux? Know the principles, but lead with Redux Toolkit. Candidate-reported rounds expect RTK; classic boilerplate appears mainly in maintenance contexts.

Q: RTK Query or React Query? Use RTK Query if you already run Redux; React Query if you do not. Interviewers reward the reasoning, not a dogmatic pick.

Q: How deep is Redux tested for freshers? Candidate-reported fresher rounds cover the core flow and createSlice. RTK Query and reselect skew mid to senior.

Q: What is the most common Redux red flag? Putting everything in Redux. Knowing when Context or local state fits better signals maturity.

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