old_contents: import { TOGGLE_VISIBILITY } from './actions';
  
  function toggleVisibility(props, state = false, action) {
    return action.type === TOGGLE_VISIBILITY ? !state : state
  }
  
  export default function reducer(props, state = {}, action) {
    return {
      isVisible: toggleVisibility(props, state.isVisible, action)
    };
  }
  
new_contents: import { TOGGLE_VISIBILITY } from './actions';
  
  function toggleVisibility(props, state = props.defaultIsVisible, action) {
    return action.type === TOGGLE_VISIBILITY ? !state : state
  }
  
  export default function reducer(props, state = {}, action) {
    return {
      isVisible: toggleVisibility(props, state.isVisible, action)
    };
  }
  
subject: Use defaultIsVisible prop as default value