reducer.js

export default function (state = INITIAL_STATE, action = {}) {
  const {
    type,
    wishlistItem,
  } = action;
  switch (type) {
    case ADD_TO_WISHLIST_SUCCESS:
      return {
        ...state,
        index: [...state.index, {
          wishlistItem,
        }],
      };
    default:
      return state;
  }
}