Bug 158116 – Partly implement Function.prototype.{caller,arguments} reflection proposal

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Claude Pache]

According to [ES6 6.7.1.3] Invariants of the Essential Internal Methods:


     If a property P is described as a data property with Desc.[[Value]] equal to v and Desc.[[Writable]] and Desc.[[Configurable]] are both false, then the SameValue must be returned for the Desc.[[Value]] attribute of the property on all future calls to [[GetOwnProperty]] ( P ).

[ES6 6.7.1.3]: http://www.ecma-international.org/ecma-262/6.0/#sec-invariants-of-the-essential-internal-methods


Testcase against `Function#arguments`

```js
function f() { return Object.getOwnPropertyDescriptor(f, 'arguments'); }
Object.getOwnPropertyDescriptor(f, 'arguments');
// value: null, writable: false, configurable: false
f();
// value: Arguments[], writable: false, configurable: false
```


Testcase against `Function#caller`

```js
function g() { return Object.getOwnPropertyDescriptor(g, 'caller'); }
function h() { return g(); }
Object.getOwnPropertyDescriptor(g, 'caller');
// value: null, writable: false, configurable: false
h();
// value: function h(), writable: false, configurable: false
```


Suggested fix: Use getters.


Similar bug: Bug 151348

[Leo Balter]

I've observed this bug within a non directly related code: 

```
(function() {
  'use strict';
  class Foo {}
  const a = new Proxy(function() {}, { ownKeys() { return Reflect.ownKeys(Foo) } });
  console.log(Object.getOwnPropertySymbols(a));
})(); // strict mode, ok
 
(function() {
  'use sloppy';
  class Foo {}
  const a = new Proxy(function() {}, { ownKeys() { return Reflect.ownKeys(Foo) } });
  console.log(Object.getOwnPropertySymbols(a));
})();
```

The code in sloppy mode breaks in Chrome and Safari because the function object will have non-configurable own properties arguments and caller and the Proxy trap is required to list all non-configurable properties from the Proxy target.

[Mark S. Miller]

Ping. What is the status of this?

[Alexey Shvayka]

Created attachment 430793 [details]
Patch

[Alexey Shvayka]

(In reply to Alexey Shvayka from comment #3)
> Created attachment 430793 [details]
> Patch

                                    r278563                   patch

reflect-own-keys-function       45.4040+-0.8465     ^     21.7507+-0.2064        ^ definitely 2.0875x faster
function-prototype-get          39.7061+-0.9041     ^     35.6181+-0.3754        ^ definitely 1.1148x faster

<geometric>                     42.4357+-0.6210     ^     27.8294+-0.1872        ^ definitely 1.5248x faster

[Alexey Shvayka]

Created attachment 430875 [details]
Patch

Adjust LayoutTests.

[Alexey Shvayka]

Created attachment 432853 [details]
Patch

Adjust put-to-proto-chain-overrides-put.js stress test.

[Yusuke Suzuki]

Comment on attachment 432853 [details]
Patch

r=me

[Alexey Shvayka]

Comment on attachment 432853 [details]
Patch

I appreciate the review!

[EWS]

Committed r280289 (239947@main): <https://commits.webkit.org/239947@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 432853 [details].

[Radar WebKit Bug Importer]

<rdar://problem/81083194>