4. What are the default middlewares included in Redux Toolkit?

Default Middlewares in Redux Toolkit

Redux Toolkit’s configureStore() automatically adds a set of default middlewares designed to improve developer experience, catch common errors, and streamline asynchronous logic handling.

Default Middlewares Included

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1. redux-thunk: Enables writing async logic that interacts with the Redux store, such as fetching data from APIs.
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2. serializableCheck: Ensures that all actions and state values are serializable, preventing bugs caused by non-serializable data (like Dates or Promises).
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3. immutableCheck: Detects accidental state mutations in reducers, helping maintain Redux’s immutability principles.
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4. actionCreatorCheck: Warns if non-function action creators are mistakenly used (in development mode).

Example: Viewing or Customizing Default Middleware

You can modify or disable these checks based on performance needs or specific project constraints using the getDefaultMiddleware() configuration options.

Key Benefits of Default Middleware

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- Safety: Prevents accidental mutations and serialization issues during development.
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- Ease of Use: Async operations work out of the box with redux-thunk.
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- Debugging Support: Helpful warnings and checks during development improve reliability.
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- Extensibility: Developers can easily add or remove middleware as needed.

These default middlewares make Redux Toolkit’s store configuration safer and easier, allowing developers to focus on application logic rather than setup complexity.

Difficulty: 3/10

Topics: default middleware, RTK setup, thunk middleware

Scenario Questions

0-2 years experience

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You're setting up a new Redux store with RTK and your async action isn't firing — what's the most likely reason and how would you fix it?
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If you call dispatch({ type: 'FETCH_DATA' }) without a thunk, what happens? How does RTK change that?
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Your teammate says they removed the logger middleware because it's 'too noisy' — is that safe in development? Why or why not?

2-5 years experience

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Your app works fine in dev but crashes in production with a 'non-serializable value' error — how do you track down which middleware is catching it and what to fix?
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You inherited a codebase where RTK's default middleware was replaced with custom ones, and now some async actions are timing out — what would you check first?
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A feature uses async thunks and logs state changes, but the logs are missing in staging — what could be misconfigured and how would you verify it?

5-8 years experience

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You're optimizing a high-traffic app and considering removing serializableCheck to reduce overhead — what risks do you weigh, and how would you mitigate them?
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Your team wants to replace thunk with redux-saga for complex workflows — what are the tradeoffs in terms of debugging, bundle size, and team onboarding when removing RTK's default middleware?
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How would you design a custom middleware pipeline in RTK that preserves the safety of serializableCheck but skips it for specific high-performance slices?

8+ years experience

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You're leading a migration from vanilla Redux to RTK across 15 micro-frontends — how do you ensure consistent middleware behavior without forcing everyone to use the same config?
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A legacy system uses custom middleware that conflicts with RTK's serializableCheck — what’s your strategy to phase out the legacy middleware without breaking existing state or introducing silent bugs?
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How would you architect a company-wide Redux standard that allows teams to opt into or out of default RTK middleware while maintaining auditability and debugging consistency across services?

Follow-up Questions

❓What happens if you disable serializableCheck and accidentally put a Date object in state?
❓Why is thunk included by default instead of something like saga?
❓How would you know if someone removed the logger middleware in development?