What is a reducer function?

Difficulty: 5/10

reducers, RTK createSlice, immutability

A reducer function in Redux Toolkit is a pure function that determines how the application's state changes in response to dispatched actions. It takes the current state and an action as arguments and returns a new state based on the action type and payload.

Key Characteristics of a Reducer Function

1
Reducers are pure functions — they do not mutate the original state or cause side effects.
2
Each reducer listens for specific action types to decide how to update the state.
3
Reducers return the next state based solely on the current state and the dispatched action.

Example: Basic Reducer Function in Redux Toolkit

In this example, each function inside the reducers field is a reducer that modifies the state based on a specific action. Redux Toolkit uses the Immer library internally, allowing you to write reducers that 'mutate' state safely.

Scenario Questions

0-2 years experience

2-5 years experience

1During a sprint you notice that after adding a new async thunk, the UI sometimes shows stale data. Walk me through how you would debug the reducer logic to ensure the state updates correctly.

Follow-up Questions