old_contents: const Reducer = (state =  {
  
  }, action) => {
      switch(action.type) {
          // case 'ADD_TODO':
          //     return { ...state, nextId: state.nextId + 1,
          //         todoItems: [ ...state.todoItems,
          //             {id:state.nextId, text: action.text, done: false}]
          //     }
          default:
              return state
      }
  }
  
  export default Reducer
new_contents: const Reducer = (state =  {
      nextPage: 'LandingPage'
  }, action) => {
      switch(action.type) {
          case 'navigate':
              return Object.assign({}, state, {
                  nextPage: action.text
              })
          default:
              return state
      }
  }
  
  export default Reducer
subject: Add a reducer for navigation.