2. How would you migrate an old Redux app to Redux Toolkit?

Migrating a Legacy Redux Application to Redux Toolkit

Migrating from classic Redux to Redux Toolkit (RTK) helps simplify boilerplate, improve maintainability, and modernize state management using opinionated utilities. The migration process can be done incrementally, feature by feature, without breaking existing logic.

Steps to Migrate to Redux Toolkit

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1. Replace createStore with configureStore: RTK’s configureStore automatically sets up the Redux DevTools, middleware, and thunk integration.
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2. Convert Reducers to Slices: Use createSlice to combine action creators and reducers in a single file, reducing boilerplate.
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3. Migrate Thunks to createAsyncThunk: Simplify async logic using createAsyncThunk to manage pending, fulfilled, and rejected actions automatically.
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4. Integrate RTK Query (Optional): Replace custom API calls and manual loading/error handling with RTK Query endpoints for automatic caching and fetching.
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5. Clean Up Boilerplate: Remove manually defined constants, action creators, and switch-case reducers that are replaced by RTK utilities.

Before: Classic Redux Setup

After: Redux Toolkit Setup

By gradually introducing Redux Toolkit’s utilities, you can modernize your Redux codebase while retaining existing functionality. Start with configureStore, then migrate slices and async logic step-by-step to achieve a clean, maintainable architecture.

Difficulty: 6/10

Topics: store configuration, createSlice, migration strategy

Scenario Questions

0-2 years experience

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We have a small component that uses a classic Redux reducer and action creators. How would you refactor it to use Redux Toolkit's createSlice?
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If you replace a hand‑written action type string with a slice‑generated action, what changes do you need to make in the component's mapDispatchToProps?
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The old store combines reducers manually. How would you transition that to configureStore without breaking existing selectors?

2-5 years experience

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Your team needs to migrate a feature module that relies on thunk middleware and custom logging middleware. How would you incorporate those into Redux Toolkit's configureStore, and what pitfalls might you encounter?
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After switching a reducer to createSlice, a component starts receiving undefined state. Walk me through how you'd debug the issue.
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You have legacy reducers that mutate state directly. How does Redux Toolkit handle this, and what steps would you take to ensure correctness during migration?

5-8 years experience

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The app uses dynamic reducer injection for code‑splitting. How would you adapt that pattern when moving to Redux Toolkit, and what performance considerations arise?
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Some selectors are memoized with reselect and depend on the state tree shape. How would you preserve selector correctness while refactoring slices?
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If you need to keep part of the app on the old Redux implementation while gradually migrating, how would you design a coexistence strategy to avoid breaking existing features?

8+ years experience

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Your organization plans a phased migration of a large legacy Redux codebase to Redux Toolkit across multiple squads. What governance, tooling, and CI/CD changes would you propose to ensure a smooth transition?
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How would you evaluate the trade‑offs between a full rewrite using Redux Toolkit versus incremental migration, considering long‑term maintainability and developer onboarding?
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The legacy app uses custom store enhancers that interact with non‑serializable data. How would you redesign those enhancers to align with Redux Toolkit's best practices while preserving functionality?

Follow-up Questions

❓What would you watch out for regarding state immutability during the migration?
❓How would you verify that existing behavior hasn't changed after refactoring?
❓How would you coordinate the migration effort across multiple teams?