function reducer(state, action) {

  switch(action.type) {

    case 'TODO_ADD' : {

      return applyAddTodo(state, action);

    }

    case 'TODO_TOGGLE' : {

      return applyToggleTodo(state, action);

    }

    default : return state;

  }

}

​

function applyAddTodo(state, action) {

  return state.concat(action.todo);

}

​

function applyToggleTodo(state, action) {

  return state.map(todo =>

    todo.id === action.todo.id

      ? Object.assign({}, todo, { completed: !todo.completed })

      : todo

  );

}

​

const store = Redux.createStore(reducer, []);

​

console.log('initial state:');

console.log(store.getState());

​

const unsubscribe = store.subscribe(() => {

  console.log('store update, current state:');

  console.log(store.getState());

});

​

store.dispatch({

  type: 'TODO_ADD',

  todo: { id: '0', name: 'learn redux', completed: false },

});

​

store.dispatch({

  type: 'TODO_ADD',

  todo: { id: '1', name: 'learn mobx', completed: false },

});

​

store.dispatch({

  type: 'TODO_TOGGLE',

  todo: { id: '0' },

});

​

unsubscribe();