redux-devtools/extension/examples/todomvc/reducers/todos.js

import {
  ADD_TODO,
  DELETE_TODO,
  EDIT_TODO,
  COMPLETE_TODO,
  COMPLETE_ALL,
  CLEAR_COMPLETED,
} from '../constants/ActionTypes';

const initialState = [
  {
    text: 'Use Redux',
    completed: false,
    modified: new Date(),
    id: 0,
  },
];

export default function todos(state = initialState, action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        {
          id: state.reduce((maxId, todo) => Math.max(todo.id, maxId), -1) + 1,
          completed: false,
          modified: new Date(),
          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
          ? Object.assign({}, todo, { text: action.text, modified: new Date() })
          : todo
      );

    case COMPLETE_TODO:
      return state.map((todo) =>
        todo.id === action.id
          ? Object.assign({}, todo, {
              completed: !todo.completed,
              modified: new Date(),
            })
          : todo
      );

    case COMPLETE_ALL:
      const areAllMarked = state.every((todo) => todo.completed);
      return state.map((todo) =>
        Object.assign({}, todo, {
          completed: !areAllMarked,
          modified: new Date(),
        })
      );

    case CLEAR_COMPLETED:
      return state.filter((todo) => todo.completed === false);

    default:
      return state;
  }
}