How do you define an API slice using createApi()?

In RTK Query, an API slice is defined using the createApi() function. This slice manages how your app interacts with APIs — including fetching, caching, and updating server data — directly within the Redux store.

Key Components of an API Slice

1
reducerPath: A unique key to identify the API slice in the Redux store.
2
baseQuery: Defines how API requests are made, often using fetchBaseQuery.
3
endpoints: A function that declares all queries and mutations (data fetching and modifying endpoints).

Example: Creating an API Slice with createApi()

The createApi() function automatically generates hooks like useGetUsersQuery and useAddUserMutation, making it easy to integrate API data directly into React components.

By defining an API slice this way, you centralize API logic, reduce boilerplate, and enable features like caching, invalidation, and automatic refetching out of the box.

Difficulty: 5/10

Topics: RTK Query, API slice definition, caching

Scenario Questions

0-2 years experience

1
We need to fetch a list of products in a new component. Walk me through how you'd set up an API slice with createApi to call /api/products.
2
If you add a new endpoint to an existing API slice, what steps do you take to ensure the generated hooks are available in your component?
3
What happens if you forget to provide a baseQuery when calling createApi?

2-5 years experience

1
Your team added pagination to the /users endpoint, but the UI still shows stale data after navigating pages. How would you debug the API slice definition to fix this?
2
You need to call two different back‑ends from the same app. How would you structure your createApi calls, and what trade‑offs does that introduce?
3
During a code review you notice the API slice uses fetchBaseQuery with hard‑coded URLs. What would you change to make it environment‑agnostic, and why?

5-8 years experience

1
Our app experiences memory pressure because cached responses from several API slices aren't being cleared. How would you redesign the API slice definitions or caching policies to mitigate this at scale?
2
We want to enable server‑side rendering for a page that uses RTK Query hooks. What changes are required in the createApi configuration and why?
3
Explain how you would integrate optimistic updates for a mutation in an API slice, and what considerations you’d have for error rollback.

8+ years experience

1
The organization is migrating from a custom data‑fetching layer to RTK Query across dozens of services. What strategy would you propose for defining API slices to minimize duplication and support versioned back‑ends?
2
How would you design a shared, cross‑team API slice architecture that balances type safety, caching granularity, and the need for different authentication schemes?
3
Discuss the long‑term maintenance implications of placing all endpoints in a single createApi call versus modularizing them per domain.

Follow-up Questions

❓How would you test that the generated hooks correctly fetch and cache data?
❓What monitoring would you put in place to detect stale or over‑cached responses in production?
❓If an endpoint needs custom error handling, where would you implement that in the slice?