[Webkit-unassigned] [Bug 253814] Concat with a CSSStyleSheet and shadowRoot.adoptedStyleSheets returns array in array

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=253814

--- Comment #7 from Chris Dumez <cdumez at apple.com> ---
(In reply to Chris Dumez from comment #6)
> (In reply to Chris Dumez from comment #5)
> > Ecmascript for concat [1] says to rely on IsConcatSpreadable() [2].
> > IsConcatSpreadable() checks the @@isConcatSpreadable symbol and then
> > fallback to calling IsArray() [3].
> > 
> > ObservableArray is defined here [4].
> > 
> > [1]
> > https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.
> > prototype.concat
> > [2]
> > https://tc39.es/ecma262/multipage/indexed-collections.html#sec-
> > isconcatspreadable
> > [3] https://tc39.es/ecma262/multipage/abstract-operations.html#sec-isarray
> > [4] https://webidl.spec.whatwg.org/#es-observable-array
> 
> From https://webidl.spec.whatwg.org/#observable-array-exotic-object:
> ```
> An observable array exotic object is a specific type of ECMAScript Proxy
> exotic object which is created using the proxy traps defined in this
> section. They are defined in this manner because the ECMAScript
> specification includes special treatment for Proxy exotic objects that have
> Array instances as their proxy target, and we want to ensure that observable
> array types are exposed to ECMAScript code with this special treatment
> intact.
> ```
> 
> So I believe concat should indeed spread the observable array here. However,
> our implementation currently relies on isJSArray(), not isArray() (which
> would be what the spec says).

I have tried several approaches here but I keep hitting assertions. Seems existing code is not happy dealing with a JSObservableArray, even though it is a JSArray subclass.

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