Object.freezing proxies should freeze or throw?

[Mark Miller]

On Tue, Aug 9, 2016 at 10:58 AM, doodad-js Admin <doodadjs at gmail.com> wrote:

> I think too much validation is not a good idea. Let the proxy lie. If you
> don't, what is the purpose of Proxy?


http://research.google.com/pubs/pub40736.html

Of course, we can still agree that too much, by definition, is not a good
idea ;).




> I have a case where I wanted to force every new property to be read-only
> through a Proxy so that, once created, they can no longer change. But I get
> "TypeError" because of such a validation:
>
> ```js
> "use strict";
> const proxy = new Proxy({}, {
>         set(target, property, value, receiver) {
>                 if (Object.prototype.hasOwnProperty.call(target,
> property)) return false;
>                 Object.defineProperty(target, property, {configurable:
> false, enumerable: true, writable: false, value: value});
>                 return true;
>         },
>         defineProperty(target, property, descriptor) {
>                 if (Object.prototype.hasOwnProperty.call(target,
> property)) return false;
>                 descriptor = Object.assign({}, descriptor, {configurable:
> false, enumerable: true});
>                 if (!descriptor.get && !descriptor.set)
> descriptor.writable = false;
>                 Object.defineProperty(target, property, descriptor);
>                 return true;
>         },
> });
>
> proxy.a = 1;
> proxy.a = 2; // TypeError: 'set' on proxy: trap returned falsish for
> property 'a'
>
> Object.defineProperty(proxy, 'b', {value: 3});
> Object.defineProperty(proxy, 'b', {value: 4}); // TypeError:
> 'defineProperty' on proxy: trap returned falsish for property 'b'
>

I don't understand the purpose of this code. From your description, don't
you want these TypeErrors? Could you show a hypothetical test case that, if
passed, demonstrates what you are actually trying to accomplish?




> ```
>
> -----Original Message-----
> From: Claude Pache [mailto:claude.pache at gmail.com]
> Sent: Tuesday, August 09, 2016 8:44 AM
> To: es-discuss <es-discuss at mozilla.org>
> Cc: Mark S. Miller <erights at google.com>; Raul-Sebastian Mihăilă <
> raul.mihaila at gmail.com>
> Subject: Re: Object.freezing proxies should freeze or throw?
>
> Given a Proxy that pretends to be in state A while its target is
> observably in state B, and assuming that the target satisfies the
> Invariants of the Essential Internal Methods [6.1.7.3], I claim that, in
> order to force the Proxy to satisfy those Invariants, it is necessary and
> sufficient to check that the two following conditions hold:
>
> * it is legal for an object to pass from state A to state B; and,
> * it is legal for an object to pass from state B to state A.
>
> [6.1.7.3]: https://tc39.github.io/ecma262/#sec-invariants-of-
> the-essential-internal-methods
>
>
> Because I am too lazy to write the proof just now, I cowardly leave it as
> an exercice to the reader. Meanwhile, that principle may be used to audit
> the robustness of the Proxy specification. I have found the following bug
> in Proxy.[[Delete]]() by applying the above principle to:
>
> * state A: nonexistent property on a nonextensible object;
> * state B: existent own property on a nonextensible object.
>
> Resurrection of a successfully deleted property on a nonextensible object:
>
> ```js
> var target = Object.preventExtensions({ x: 1 }) var proxy = new
> Proxy(target, {
>     deleteProperty() { return true }
> })
>
> Object.isExtensible(proxy) // false
> delete proxy.x // true
> proxy.hasOwnProperty('x') // true
> ```
>
> After a first scan, I haven't found other bugs in the essential methods of
> Proxy, than that one and the missing nonconfigurable-but-writable check in
> [[GetOwnPropertyDescriptor]] and [[DefineOwnProperty]] already mentioned in
> that thread.
>
> I plan to propose a minimal patch (i.e., just adding the missing checks)
> in a few days.
>
> —Claude
>
>
>
>
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>



-- 
  Cheers,
  --MarkM
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