redux.example2.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: !state.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 todoApp = ( state={}, action ) =>{
  return {
    todos: todos( state.todos, action),
    visibilityFilter: visibilityFilter(state.visibilityFilter, action)
  };
};


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


console.log('Current State');
console.log( store.getState() );

console.log('Dispatching ADD_TODO');
store.dispatch({
  type:'ADD_TODO',
  id:0,
  text:'Go Shopping'
});

console.log('Current State');
console.log( store.getState() );

console.log('Dispatching ADD_TODO');
store.dispatch({
  type:'SET_VISIBILITY_FILTER',
  filter:'SHOW_COMPLETED'
});

console.log('Current State');
console.log( store.getState() );