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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: [Proxy] No way to hook property creation?
Date: Mon, 22 Aug 2022 01:08:06 +0200
Organization: PointedEars Software (PES)
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JJ wrote:
^^
Please post here using your real name.

> I'm trying to use Proxy and the `with` statement to redirect access to the
> global object.
> 
> The problem is that, Proxy doesn't seem to have any handler for property
> creation which is done without object accessor or
> `Object.defineProperty()`. e.g. `prop = 123` instead of `obj.prop = 123`

The (by-design) inconsistent and unpredictable behavior of the *legacy* 
“with” statement that *never* a property on the target object is being 
created with implicit access, but either

  a) the property of an object higher up in the scope chain is modified –
     or not if it is read-only or has a dismissive object value getter
     (host object) or property setter,

or

  b) a property of the global object is created (or not)

is one of the reasons why it is *deprecated* since ECMAScript Edition 5,
and *code containing it cannot even be executed* in Strict Mode.

This has nothing to do with the capabilities of the Proxy object because its 
instance does not even "see" that write access.

<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/with>

You can determine whether a property is being created in the “set” handler 
of the Proxy instance:

  var target = {};

  var proxy = new Proxy(target, {
    set (target, propertyName, value) {
      var propertyExisted = (propertyName in target);
  
      target[propertyName] = value;

      if (!propertyExisted) {
        console.log(`Created property "${propertyName}" on target with value 
"${value}".`);
      }
    }
  });

  /*
   * displays
   * "Created property "foo" on target with value ""bar""
   */
  proxy.foo = "bar";


  /* does not display anything */
  proxy.foo = "baz";

Tested in Chromium 90 on GNU/Linux,

navigator.userAgent == "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 
(KHTML, like Gecko) Chrome/90.0.4430.212 Safari/537.36".

The usual caveats for the “in” statement and ES 6+ features apply.

See also:

<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy>
 
-- 
PointedEars
<https://github.com/PointedEars> | <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2
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