reducers.js

import {ADD_TODO, COMPLETE_TODO, SET_VISIBILITY_FILTER, VisibilityFilters} from './actions';
import { combineReducers } from 'redux';

const { SHOW_ALL } = VisibilityFilters;

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

function todos(state=[], action) {
    switch (action.type) {
        case ADD_TODO:
            return [
                ...state,
                {
                    text: action.text,
                    completed: false
                }
            ];
        case COMPLETE_TODO:
            return [
                ...state.slice(0, action.index),
                Object.assign({}, state[action.index], {completed: true}),
                ...state.slice(action.index + 1)
            ];
        default:
            return state
    }
}

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

export default todoApp;