normal-reducer.js

const initialState = {
  todos: {
     'a': { id: 'a', description: '...', isComplete: false },
     'b': { id" 'b', description: '...', isComplete: false }
  }
}
  
const reducer = (state, action) => ({
  ...state,
  todos: {
    ...state.todos,
    [action.todoId]: {
      ...state.todos[action.todoId],
      isComplete: action.payload,
    }
  }
})