redux-devtools/packages/redux-devtools-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;
  }
}