Bug 235880 – WeakRef deref can return null instead of undefined

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Stef Busking]

Seen in Safari 15.1, 15.2 (macOS) and Epiphany 41.3 (Linux)

According to the ECMAScript specification (https://tc39.es/ecma262/multipage/managing-memory.html#sec-weakrefderef), a WeakRef's deref method should either return the referenced object or `undefined`. This behaves as expected in Chrome, Firefox and Edge, but it seems that in WebKit browsers this method may return `null` instead.

I've confirmed this behavior using the following in the dev tools console:

```
a = new WeakRef(new ArrayBuffer(10000000))
// wait a while to allow GC to clean up the ArrayBuffer
a.deref()
```

I'm not sure if Webkit browsers have a way to force garbage collection, but waiting a while or using other tabs seems to work for triggering this issue most of the time. Firefox allows forcing GC on about:memory, Chrome and Edge provide a button for this on the Memory tab of their dev tools. 

Expected behavior: a.deref() returns undefined (happens in Chrome, Firefox and Edge)
Actual behavior: a.deref() returns null in Safari and Epiphany

[Radar WebKit Bug Importer]

<rdar://problem/88513359>

[Yusuke Suzuki]

Created attachment 450955 [details]
Patch

[Yusuke Suzuki]

Created attachment 450956 [details]
Patch

[Alexey Shvayka]

Comment on attachment 450956 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=450956&action=review

> Source/JavaScriptCore/ChangeLog:8
> +        WeakRef#deref should return undefined when a wrapped object is collected.

nit: a spec link perhaps? https://tc39.es/ecma262/#sec-weakrefderef

[Yusuke Suzuki]

Thanks Sam and Alexey!

[Yusuke Suzuki]

Comment on attachment 450956 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=450956&action=review

>> Source/JavaScriptCore/ChangeLog:8
>> +        WeakRef#deref should return undefined when a wrapped object is collected.
> 
> nit: a spec link perhaps? https://tc39.es/ecma262/#sec-weakrefderef

Nice, done.

[Yusuke Suzuki]

Committed r289152 (246848@trunk): <https://commits.webkit.org/246848@trunk>