Why object is not proxied like an argument in function

Question

i have an interesting example of code that's not working like i'm expected.

I really dont understand why my obj wouldnt proxy. I'm expect that obj ill proxy via link, but it's not. Can anyone explain how it's works and what I don't understand? Thank you!

let obj = {
  foo: "123"
};

function test(fn, object) {
  object = new Proxy(object, {
    get(target, key) {
      console.log('get');
      return target[key];
    },
    set(target, key, value) {
      console.log('set');
      target[key] = value;
      return true;
    }
  });
  fn();
}

test(() => {
  obj.foo = "helloworld";
  console.log(obj.foo);
}, obj);

UPD: i want to assign object to proxy variable ONLY through arguments can i?

What is object did you mean obj = new Proxy?

Konrad
– Konrad

2023-02-02 17:39:09 +00:00
Commented Feb 2, 2023 at 17:39 — Konrad
– Konrad, Commented Feb 2, 2023 at 17:39
@Konrad object is the argument to test()

Barmar
– Barmar

2023-02-02 17:39:54 +00:00
Commented Feb 2, 2023 at 17:39 — Barmar
– Barmar, Commented Feb 2, 2023 at 17:39

Konrad · Accepted Answer · 2023-02-02 17:41:17Z

1

Either assign the proxy to obj

let obj = {
  foo: "123"
};

function test(fn, object) {
  obj = new Proxy(object, {
    get(target, key) {
      console.log('get');
      return target[key];
    },
    set(target, key, value) {
      console.log('set');
      target[key] = value;
      return true;
    }
  });
  fn();
}

test(() => {
  obj.foo = "helloworld";
  console.log(obj.foo);
}, obj);

Or pass the proxy as an argument

let obj = {
  foo: "123"
};

function test(fn, object) {
  const o = new Proxy(object, {
    get(target, key) {
      console.log('get');
      return target[key];
    },
    set(target, key, value) {
      console.log('set');
      target[key] = value;
      return true;
    }
  });
  fn(o);
}

test((o) => {
  o.foo = "helloworld";
  console.log(o.foo);
}, obj);

answered Feb 2, 2023 at 17:41

Konrad

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9 Comments

user469485 Over a year ago

Thanks for the answer, but I don’t want to do this, can you explain why this happens, because everything is logical in my opinion and the object should become proxied by reference

Barmar Over a year ago

The first solution modifies obj permanently, I think they only want the proxy to be in effect during the call to test().

Barmar Over a year ago

@user469485 Creating a proxy doesn't modify the object itself. You have to access the properties through the proxy.

Barmar Over a year ago

See the examples in MDN -- you have to access proxy2.propertyname

Konrad Over a year ago

The code in the question doesn't work, because an assignment to a parameter doesn't change the original argument.

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