let a = {'b': 1};
Object.defineProperty(a, 'b', {
  enumerable: false,
  configurable: false,
  get: function(){
    console.log('b' + '被訪問');
  },
  set: function(newVal){
    console.log('b' + '被修改，新' + 'b' + '=' + newVal);
  }
});

a.b = 2;  // b被修改，新b=2
a.b;    // b被訪問

// 觀察者構造函數(shù)
function Observer(data){
  this.data = data;
  this.walk(data);
}

let p = Observer.prototype;
p.walk = function(obj){
  let val;
  for(let key in obj){
    // 通過 hasOwnProperty 過濾掉一個對象本身擁有的屬性 
    if(obj.hasOwnProperty(key)){
      val = obj[key];
      // 遞歸調(diào)用 循環(huán)所有對象出來
      if(typeof val === 'object'){
        new Observer(val);
      }
      this.convert(key, val);
    }
  }
};

p.convert = function(key, val){
  Object.defineProperty(this.data, key, {
    enumerable: false,
    configurable: false,
    get: function(){
      console.log(key + '被訪問');
    },
    set: function(newVal){
      console.log(key + '被修改，新' + key + '=' + newVal);
      if(newVal === val) return ;
      val = newVal;
    }
  })
};

let data = {
  user: {
    name: 'zhangsan',
    age: 14
  },
  address: {
    city: 'beijing'
  }
}

let app = new Observer(data);
data.user.name;  // user被訪問