old_contents: import types from '../constants/actionTypes'
  
  export default (state, action) => {
    switch(action.type) {
      case types.ON_CLICK_MENU_BUTTON:
        return Object.assign({}, state, {
          isExpanded: action.isExpanded
        })
    }
  }
new_contents: import types from '../constants/actionTypes'
  
  const initialState = {
    isExpanded: false
  }
  
  export default (state = initialState, action) => {
    switch(action.type) {
      case types.ON_CLICK_MENU_BUTTON: {
        return Object.assign({}, state, {
          isExpanded: action.isExpanded
        })
      }
      
      default: {
        return state
      }
    }
  }
subject: Update nav reducer