redux-devtools/packages/redux-slider-monitor/examples/todomvc/reducers/todos.js

import { ADD_TODO, DELETE_TODO, EDIT_TODO, MARK_TODO, MARK_ALL, CLEAR_MARKED } from '../constants/ActionTypes';

const initialState = [{
  text: 'Use Redux',
  marked: false,
  id: 0
}];

export default function todos(state = initialState, action) {
  switch (action.type) {
    case ADD_TODO:
      return [{
        id: (state.length === 0) ? 0 : state[0].id + 1,
        marked: false,
        text: action.text
      }, ...state];

    case DELETE_TODO:
      return state.filter(todo =>
        todo.id !== action.id
      );

    case EDIT_TODO:
      return state.map(todo => (
        todo.id === action.id ?
          { ...todo, text: action.text } :
          todo
      ));

    case MARK_TODO:
      return state.map(todo => (
        todo.id === action.id ?
          { ...todo, marked: !todo.marked } :
          todo
      ));

    case MARK_ALL: {
      const areAllMarked = state.every(todo => todo.marked);
      return state.map(todo => ({
        ...todo,
        marked: !areAllMarked
      }));
    }
    case CLEAR_MARKED:
      return state.filter(todo => todo.marked === false);

    default:
      return state;
  }
}