Nested reducer pattern for deeply nested state

reducer.js

const defaultState = {
  person: {
    name: ''
    age: 20,
    friends: []
    pets: []
  }
}

const actionMap = {
  'UPDATE_NAME': function(state, action){
    return {...state, name: action.name};
  },
  'UPDATE_PET': function(state, action){
    const pets = PetsReducer(state.pets, action)
    return(...state, pets); // Or use the update(state, {pets: {$set: pets}});
  }
}

export const PersonReducer = function (state = defaultState, action) {
    if (actionMap[action.type]) {
        return actionMap[action.type](state, action);
    } else {
        return state;
    }
};

export default PersonReducer;

const petsActionMap = {
    'UPDATE_PET': function(state, action){
        return state.map((pet) => {
            if(pet.id !== action.petId){return pet;}
            const updated pet = {}
            // Do something with the pet or call a deeper Reducer if required
            return (updatedPet);
        });
    },
};

const PetReducer = function(state=[], action){
    if (questionsActionMap[action.type]) {
        return petActionMap[action.type](state, action);
    } else {
        return state;
    }
};