5. How do you add custom middleware to the store?
Adding Custom Middleware in Redux Toolkit
In Redux Toolkit, you can easily add custom middleware to your store using the middleware option in configureStore(). This allows you to extend the behavior of Redux by intercepting actions, logging, or performing side effects before they reach the reducer.
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1. Create a Middleware Function: Define a middleware function that takes
store,next, andactionas parameters. - 2
2. Use getDefaultMiddleware(): Retrieve Redux Toolkit’s built-in middlewares.
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3. Concatenate Custom Middleware: Add your middleware using
.concat()to ensure default middlewares are preserved.
This setup keeps Redux Toolkit’s default middlewares (like redux-thunk, serializableCheck, and immutableCheck) while adding your own logic to enhance debugging or handle custom behavior.
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- Flexibility: Add cross-cutting logic such as logging, analytics, or authentication checks.
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- Maintainability: Keeps side effects separate from reducers, improving code organization.
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- Extensibility: Multiple middlewares can be composed together for modular logic handling.
In summary, custom middleware extends Redux Toolkit’s capabilities without replacing the defaults, giving developers fine-grained control over how actions flow through the store.
Scenario Questions
0-2 years experience
- 1You need to log every dispatched action in your RTK store. Walk me through how you would add a custom middleware to achieve this.
- 2If you add a middleware that mutates the action payload before it reaches reducers, what steps do you take to integrate it with configureStore?
2-5 years experience
- 1We introduced a custom middleware to debounce rapid search actions, but the UI now sometimes never updates. How would you troubleshoot the issue?
- 2During a feature rollout, a new middleware that adds a request ID to each action caused a type error in some reducers. Explain why this happened and how you'd fix it.
5-8 years experience
- 1Our application has dozens of feature slices and we want to add a performance‑monitoring middleware that samples actions. What considerations would you make when inserting it into the store, and how would you ensure it doesn't impact critical paths?
- 2We need to share authentication state across multiple micro‑frontends, each with its own RTK store. How would you design a custom middleware strategy that works across these stores without duplicating logic?
8+ years experience
- 1Looking ahead, we plan to migrate several legacy Redux stores to RTK and want a unified logging and error‑handling middleware layer. How would you architect this middleware to be reusable, configurable per team, and safe for future extensions?
- 2If a new compliance requirement mandates that every action payload be encrypted before reaching reducers, describe how you'd implement a custom middleware at the organization level, considering testing, performance, and backward compatibility.
Follow-up Questions
- Why might you choose prepend versus append when registering middleware?
- How does getDefaultMiddleware affect serializable checks for your custom code?
- What testing strategy would you use to ensure the middleware doesn’t break existing reducers?