definePropertyDemo

function Product(name, price) {
  this.name = name;
  this.price = price;
}

// Object.defineProperty(Product.prototype, "discount", {
//   get() {
//     return 1;
//   },
//   set(value) {
//     this.discount = value;
//   }
// });
Object.defineProperty(Product.prototype, "discount", {
  value: 1,
  configurable: true, //是否可以被删除
  enumberable: true, //能否通过for..in取到
});

var shoes = new Product("Shoes", 20);
var tshirt = new Product("T-shirt", 30);
console.log(shoes.discount);
console.log(tshirt);