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Reducers as Pure State Transitions

A reducer is a pure function `(state, action) => newState`: no mutation, no I/O, no randomness, same output for same inputs. Purity is what makes Redux state reproducible — replaying the same action sequence always yields the same state.

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A reducer is a pure function `(state, action) => newState`: no mutation, no I/O, no randomness, same output for same inputs. Purity is what makes Redux state reproducible — replaying the same action sequence always yields the same state.

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