Reducers

Reducers are pure functions `(state, action) => nextState`. No network, no `Date.now()`, no mutating arguments. Combined reducers partition state by slice; each slice owns its subtree.

					function todosReducer(state = [], action) {
  switch (action.type) {
    case 'todos/added':
      return [...state, action.payload];
    default:
      return state;
  }
}
				

With Immer inside `createSlice`, you write mutable-looking updates that produce immutable snapshots. Return `state` unchanged for unknown actions to preserve reference stability.

The trade-off is pure, testable transitions versus verbosity; Immer trades some purity visibility for ergonomics.

On interviews: prove purity — same inputs always same output; explain default branch returning previous state.

Common pitfalls: side effects inside reducers, shared mutable initial state objects, and cross-slice writes that break modularity.

Checklist:

• Pure transitions only.
• One reducer per slice domain.
• Default case returns previous state.
• Immer for nested updates in RTK.