Todo library with derived state

TodoLib.js

import { createSelector } from 'reselect';

import createBase from './StatedLibBase';

export default function createTodoLib() {
  
  const getCompletedTodos = createSelector(
    state => state.todos,
    todos => todos.filter(todo => todo.completed),
  );

  const getActiveTodos = createSelector(
    state => state.todos,
    todos => todos.filter(todo => !todo.completed),
  );

  function deriveState(state) {
    return {
      ...state,
      get completedTodos() { return getCompletedTodos(state) },
      get activeTodos() { return getActiveTodos(state) },
    };
  }
  
  const base = createBase({ todos: [], isFetching: false }, deriveState);
  
  const {updateState, state$} = base;
  
  return {
    get state: {return base.state},
    state$,
    addTodo(title) {
      updateState({ todos: base.state.todos.concat({ title, completed: false }) });
    },
    async fetchTodos() {
      updateState({ isFetching: true });
      const newTodos = await fetchFromCloud();
      updateState({
        isFetching: false,
        todos: base.state.todos.concat(newTodos),
      });
    },
  };

}