Reducer: A pure function that returns the next state.?

Short answer: function counterReducer(state = 0, action) { switch (action.type) { case 'INCREMENT': return state + 1; default: return state; } }

Example code

function counterReducer(state = 0, action) { switch (action.type) { case 'INCREMENT': return state + 1; default: return state;
}
}

Real-world example (ShopNest)

ProductCard receives price via props. The parent Catalog owns selected filters in state and passes them down.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.