function proxyObject(obj, key, val, enume){
  Object.defineProperty(obj, key, {
    value: val,
    enumerable: !!enume,
    writable: true,
    configurable: true
  });  
};

proxyObject(extendObj, '$set', function(key, val){
  //this指向extendObj
  if(this.hasOwnProperty(key)){
    return;
  }else{
    /*
     TODO:在extendObj中监听key属性，
     且，若key属性值为对象，再次监听key属性值
    */     
  }  
});

//observer.js
p.walk = function(data){
  let keys = Object.keys(data);
  keys.forEach( key => {
    let val = data[key];
    if(typeof val === 'object'){
      new Observer(val);
    }
    this.convert(key, val);
  });
  //$Observer属性指向Observer自身实例
  data.$Observer = this;
}
//新增一个observe方法
p.observe = function(data){
  if(typeof data === 'object'){
    new Observer(data);  
  }  
}

proxyObject(extendObj, '$set', function(key, val){
  if(this.hasOwnProperty(key)){
    return;
  }else{
    proxyObject(this, key, val, true);
    let ob = this.$Observer;
    ob.observe(val);
    ob.convert(key, val);  
  }  
});

//observer.js
function Observer(data){
  if(!(this instanceof Observer)){
    return new Observer(data);
  }
  //将监听对象的隐指针指向我们的extendObj对象
  data.__proto__ = extendObj;
  this.data = data;
  this.walk(data);  
}

<script src="./extendObj.js"></script>
<script src="./observer.js"></script>
<script>
  let data = {
    user: {
      name: 'Monkey',
      age: 24
    },
    lover: {
      name: 'Dorie',
      age: 23
    }
  };
  Observer(data);
</script>