how to use reducer with index code example

Example: react/redux reducers sintaxe

const reduxSinaxe = (state = [], action) => {
  switch (action.type) {
    case "@smthg/ACTION":
      const { anyConst } = action;
      return [...state, anyConst];

    default:
      return state;
  }
};

export default reduxSinaxe;