immer-reducer.js

import produce from 'immer'

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