vue中为啥能this 直接获取到data中的值

[dcharlie123]

new Vue({
   .....
   data(){
     return {
       a:1
     }
   }
})

// 4762行
proxy(vm, "_data", key);

vm为全局Vue

 // sourceKey-> '_data'
  function proxy (target, sourceKey, key) {
    sharedPropertyDefinition.get = function proxyGetter () {
      return this[sourceKey][key]
    };
    sharedPropertyDefinition.set = function proxySetter (val) {
      this[sourceKey][key] = val;
    };
    Object.defineProperty(target, key, sharedPropertyDefinition);// target->vm,  key->'a'
  }

当我们访问this.a时程序会走Object.defineProperty(target, key, sharedPropertyDefinition)中的get返回this[sourceKey][key]，即this['_data']['a'],设置值也同理