Javascript - Proxy on a nested object triggers GET instead of SET

Question

Good morning everyone,

I'm an enthusiastic ES6 amateur and I've recently approached the "Proxy object" subject for the first time. From the online material I've came across so far my understanding is that a Proxy is acting as a wrapper, on a generic object, that intercept actions on it in order to change its behavior. With this in mind I've tried to run the following snippet and the outcome is surprising (at least for me):

 "use strict";
 
  let user = {
    id: {
      name: "Johnny",
      surname: "Potts",
    },

    age: 44,
  };

  let userProxy = new Proxy(user, {
    get(target, key, receiver) {
      alert("GET triggered");
      return Reflect.get(...arguments);
    },

    set(target, key, value, receiver) {
      alert("SET triggered");
      return Reflect.set(...arguments);
    },
  });

  userProxy.id.name = "Pete"; //change an existing property in the nested object.

  userProxy.id.gender = "male"; //introduce a new property in the nested object.

As you'll easily notice by running this code, the "GET" trap triggers every time instead of "SET" and the new property doesn't get written inside the "user" object however the existing one gets updated.

Can anyone explain me why? Thanks in advance for your time and support.

Antonio · Accepted Answer · 2021-03-10 07:21:33Z

0

The answer is that PROXY only replace the "direct" object. Any other object linked to the proxied object, such as the nested one in this case, is not trapped. That explains also why "GET" is always invoked; nested object needs to be retrived first from parent one...'Elementary, my dear Watson!'

answered Mar 10, 2021 at 7:21

Antonio

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