How can reducers be combined in Redux Toolkit?

Combining Reducers in Redux Toolkit

In Redux Toolkit, multiple reducers can be combined to manage different parts of the application's state tree. Each slice created with createSlice() generates its own reducer, which can then be combined using the combineReducers function or automatically merged in configureStore().

Ways to Combine Reducers

1
Use combineReducers() from Redux to manually merge multiple slice reducers into a root reducer.
2
Pass an object of slice reducers directly to configureStore(), which internally uses combineReducers().
3
Each key in the combined reducer object corresponds to a specific state field managed by its respective slice.

Example: Combining Multiple Slice Reducers

By combining reducers, each slice manages its own piece of state while contributing to a unified global store. This modular approach keeps the Redux architecture scalable and maintainable.

Difficulty: 5/10

Topics: combineReducers, slice integration, dynamic reducer injection

Scenario Questions

0-2 years experience

1
We have a todoSlice and a userSlice. How would you combine their reducers using Redux Toolkit so the store contains both pieces of state?
2
If you accidentally export a slice reducer directly instead of the combined root reducer, what error or symptom would you see when dispatching an action?
3
Show me the configureStore call that brings together three slice reducers in a small project.

2-5 years experience

1
During a sprint you notice an action meant for the orders slice is also mutating the inventory slice. Walk me through how you'd debug the reducer composition to find the root cause.
2
Your team wants to split a large feature into multiple slices but keep a single root reducer. Explain the trade‑offs between using combineReducers versus nesting reducers inside a slice.
3
A teammate added a new feature slice but forgot to add it to the store configuration, so the UI never updates. How would you detect and fix that problem?

5-8 years experience

1
Our app loads feature modules lazily and needs to inject reducers at runtime. How would you design a dynamic reducer injection mechanism with Redux Toolkit, and what pitfalls must you watch for?
2
We have several micro‑frontends sharing a global store in a monorepo. Discuss how you would structure combined reducers to avoid namespace collisions and keep bundle size low.
3
Explain the performance implications of deeply nested combineReducers structures and how you would mitigate any slowdown.

8+ years experience

1
We are migrating a legacy Redux codebase that uses manual combineReducers to Redux Toolkit. Outline a migration strategy that minimizes risk and keeps backward compatibility across multiple teams.
2
Across multiple product lines we want a shared core store with pluggable feature reducers. How would you architect the root reducer composition to support independent versioning and independent deployment?
3
From a long‑term maintenance perspective, what conventions and tooling would you put in place to enforce consistent reducer composition across dozens of teams?

Follow-up Questions

❓What happens if two slices use the same state key?
❓How would you test that a newly injected reducer works correctly?
❓Can you describe any impact on TypeScript typings when combining reducers?