text: // state argument is not application state, only the state
  // the reducer is responsible for.
  /*
  * state = null -> a piece of ES6 syntax, which assign null ti state if state returns undefined.
  */
  export default function (state = null, action) {
  
      switch (action.type) {
          case 'BOOK_SELECTED':
              // the return most always be a fresh object without any changes
              // to its properties.
              return action.payload;
      }
  
      return (state);
  }