redux createStore and combineReducer

redux_practice.js

/*
 * Open the console
 * to see the state log.
 */

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

  if (action.type === 'TOGGLE_TODO') {
    if(state.id === action.id) {
      return {
        ...state,
        completed: !state.completed
      }
    } else {
      return state;
    }
  }
  
  return state;
}

const todos = (state = [], action) => {
  if (action.type === 'ADD_TODO') {
    return [
      ...state,
      todo(undefined, action)
    ]
  }

  if (action.type === 'TOGGLE_TODO') {
    return state.map(t => todo(t, action));
  }

  return state;
}

const visibilityFilter = (state = 'SHOW_ALL', action) => {
  if (action.type === 'SET_VISIBILITY_FILTER') {
    return action.filter;
  }
  return state;
}


const createStore = (reducer) => {
  let state;
  let listeners = [];
  
  const getState = () => state;
  
  const dispatch = (action) => {
    state = reducer(state, action);
    listeners.forEach(listener => listener());
  }
  
  const subscribe = (listener) => {
    listeners.push(listener);
    
    return () => {
      listeners.filter(l => l !== listener);
    }
  }
  
  return {getState, dispatch, subscribe}
}

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

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