Object.freezing proxies should freeze or throw?

doodad-js Admin doodadjs at gmail.com
Tue Aug 9 17:58:02 UTC 2016

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

"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'

-----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 [], 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.

[]: 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:

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.


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