2 type of reducer.

reducers.js

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;
    }
}