Re: [heycam/webidl] Introduce the observable array type (proxy-based) (#840)

bzbarsky commented on this pull request.



> +        1.  If [=!=] [$IsAccessorDescriptor$](|descriptor|) is <emu-val>true</emu-val>, then return
+            <emu-val>false</emu-val>.
+        1.  If |descriptor| has a \[[Configurable]] field and |descriptor|.\[[Configurable]] is
+            <emu-val>false</emu-val>, return <emu-val>false</emu-val>.
+        1.  If |descriptor| has a \[[Enumerable]] field and |descriptor|.\[[Enumerable]] is
+            <emu-val>false</emu-val>, return <emu-val>false</emu-val>.
+        1.  If |descriptor| has a \[[Writable]] field and |descriptor|.\[[Writable]] is
+            <emu-val>false</emu-val>, return <emu-val>false</emu-val>.
+        1.  If [=!=] [$ToUint32$](|P|) > |oldLen|, return <emu-val>false</emu-val>.
+        1.  Let |idlValue| be the result of [=converted to an IDL value|converting=]
+            |descriptor|.\[[Value]] to the type given by |handler|.\[[Type]].
+        1.  Let |existingDescriptor| be [=!=] [$OrdinaryGetOwnProperty$](|O|, |P|).
+        1.  Assert: |existingDescriptor| is not <emu-val>undefined</emu-val>.
+        1.  Let |existingIDLValue| be the result of [=converted to an IDL value|converting=]
+            |existingDescriptor|.\[[Value]] to the type given by |handler|.\[[Type]].
+        1.  Assert: the above step never throws an exception, since we already went through the

The record case is interesting.  _If_ our canonical storage here is ES values, then records are fine in the sense that indexed gets just keep getting the same ES object, and the to-idl steps have no side-effects.  Indexed sets convert the incoming object to an IDL record and then to a new object, which might be somewhat surprising behavior.

In practice, excluding records here is probably fine.  Maybe we should have some sort of general "non-primitives which are passed by value" definition that we can refer to here and have a hope of updating if we add a new one.

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