Why must Redux reducers be pure?

Answer

A pure reducer returns the same next state for the same inputs and has no side effects. That makes replay, testing, time-travel debugging, and predictable updates possible.

Context

Redux centralizes state transitions as explicit actions reduced into immutable state. Keep reducers pure, derive views with selectors, and isolate I/O in middleware or async workflows.

Example

const todosSlice = createSlice({
  name: "todos", initialState: [],
  reducers: { added: (state, action) => { state.push(action.payload) } }
})

Redux Toolkit uses Immer, so this reducer syntax produces an immutable update.

Practical considerations

1. Choose the approach from the requirement and constraints, not from habit.
2. Include validation, error handling, and cleanup where the boundary requires them.
3. Verify the observable result with focused tests or measurement.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

In practice

For this medium-level topic, make assumptions explicit, choose the smallest safe implementation, and verify the behavior at the relevant boundary.