Redux: Implementing combineReducers() from Scratch

redux.js

const todo = (state, action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return {
        id: action.id,
          text: action.text,
          completed: false
      };
    
    case 'TOGGLE_TODO':
      if (state.id != action.id) {
        return state;
      }
      
      return {
        ...state,
        completed: !todo.completed
      };
    
    default:
      return state;
  }
};

const todos = (state=[], action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return [
        ...state,
        todo(undefined, action)
      ];
      
    case 'TOGGLE_TODO':
      return state.map(
        t => todo(t, action)
      );
      
    default:
      return state;
  }
};

const visibilityFilter = (
  state = 'SHOW_ALL',
  action
) => {
  switch (action.type){
    case 'SET_VISIBILITY_FILTER':
      return action.filter;
    default:
      return state;
  }
};

const combineReducers = (reducers) => {
  return ( state = {}, action) => {
    return Object.keys(reducers).reduce(
      (nextState, key) => {
        nextState[key] = reducers[key](
          state[key],
          action
        );      
        return nextState;
      },      
      {}      
    );    
  };
};

const todoApp = combineReducers({
  todos,
  visibilityFilter
});

// const todoApp = (state = {}, action) =>{
//   return {
//     todos: todos(
//       state.todos,
//       action
//     ),
//     visibilityFilter: visibilityFilter(
//       state.visibilityFilter,
//       action
//     )
//   };
// };

const { createStore } = Redux;
const store = createStore(todoApp);

console.log('Initial state');
console.log(store.getState());
console.log('------------------------');

console.log('Dispatching ADD_TODO.');
store.dispatch({
  type: 'ADD_TODO',
  id: 0,
  text: 'Learn Redux'
});
console.log('Current state:');
console.log(store.getState());
console.log('------------------------');

console.log('Dispatching ADD_TODO');
store.dispatch({
  type: 'ADD_TODO',
  id: 1,
  text: 'Go shopping'
});
console.log('Current state:');
console.log(store.getState());
console.log('------------------------');

console.log('Dispatch TOGGLE_TODO.');
store.dispatch({
  type: 'TOGGLE_TODO',
  id: 0
});
console.log('Current state:');
console.log(store.getState());
console.log('------------------------');

console.log('Dispatch SET_VISIBILITY_FILTER');
store.dispatch({
  type: 'SET_VISIBILITY_FILTER',
  filter: 'SHOW_COMPLETED'
});
console.log('Current state:');
console.log(store.getState());
console.log('------------------------');