Object.defineProperty

const object1 = {};
Object.defineProperty(object1, 'property1', {
  value: 42,
  writable: false
});
object1.property1 = 77;
// throws an error in strict mode
console.log(object1.property1);
// expected output: 42

为现有对象增加属性，还可以配置读写权限

let data2 = {}
data2._n = 0
Object.defineProperty(data2, 'n', {
  get(){
    return _n
  }
  set(value){
      data2. _n = value
    }
})

定义的属性 如果存在了，那么就干掉以前的重新创建一个