What are the key properties inside createSlice()?

createSlice() is a function provided by Redux Toolkit that simplifies creating Redux logic for a specific feature. It accepts an object with key properties that define the slice's behavior — including its name, initial state, and reducers.

Key Properties of createSlice()

1
name: A string that identifies the slice and prefixes generated action types.
2
initialState: Defines the default state for that slice of the store.
3
reducers: An object containing reducer functions that define how the state changes in response to actions. Action creators are automatically generated from these.
4
extraReducers (optional): Handles actions defined outside the slice, such as those created by createAsyncThunk or other slices.

Example: createSlice() with Main Properties

Difficulty: 4/10

Topics: slice configuration, reducers vs extraReducers, payload handling

Scenario Questions

0-2 years experience

1
We need a simple counter slice with an increment action and a loading flag. Which properties would you include in createSlice and how would you set them up?
2
If you forget to provide a 'name' when calling createSlice, what error or behavior do you see at runtime?
3
What happens if you place a non‑serializable object (like a Date) in the initialState of a slice?

2-5 years experience

1
You have a thunk that fetches user data and you want the user slice to react to its pending/fulfilled/rejected actions. How would you configure createSlice to handle those external actions?
2
During debugging you notice that an action defined in the slice's reducers isn’t updating the state. What could be wrong with how the reducers were declared inside createSlice?
3
Explain the trade‑offs between putting async‑related case handling in reducers versus extraReducers.

5-8 years experience

1
Your app now has dozens of slices and the bundle size is growing. How would you organize your createSlice definitions to reduce duplication and keep the codebase maintainable?
2
When integrating RTK Query with existing slices, you need to share loading/error state. How would you design the slice’s extraReducers to efficiently handle the pending/fulfilled/rejected actions from the query?
3
Describe how you’d use the prepare callback in createSlice to enforce type‑safe payloads across many slices.

8+ years experience

1
We’re migrating a large legacy Redux codebase to Redux Toolkit. What systematic approach would you take to refactor existing reducers into createSlice calls, and how would you handle naming collisions and shared logic?
2
In a micro‑frontend setup, multiple teams own slices that are loaded dynamically at runtime. How would you design the slice creation (name, extraReducers, etc.) to avoid action‑type conflicts and support hot‑loading?
3
Consider a scenario where several independent teams need to extend a core slice with additional case reducers without modifying the original file. How would you structure createSlice and extraReducers to enable this extensibility?

Follow-up Questions

❓How do you test that the generated action creators work as expected?
❓What would you change if you needed to add type‑safe payload validation?
❓Can you describe a situation where you’d prefer extraReducers over reducers?