obtain values from an object

set a value of a property within an object

class Book {
  constructor(author) {
    this._author = author;//_author，惯例命名，表示为私有变量。其实并不能把变量变为私有
  }
  // getter
  get writer() {//get set 是关键字
    return this._author;
  }
  // setter
  set writer(updatedAuthor) {//getter、setter 并不是方法，是一个属性
    this._author = updatedAuthor;
  }
}
const lol = new Book('anonymous');
console.log(lol.writer);  // anonymous，获得属性值，即getter
lol.writer = 'wut';//给属性赋值，即setter
console.log(lol.writer);  // wut