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

bzbarsky commented on this pull request.



> +    1.  Let |handler| be the <emu-val>this</emu-val> value.
+    1.  If |P| [=is an array index=], then:
+        1.  Let |oldLenDesc| be [=!=] [$OrdinaryGetOwnProperty$](|O|, "length").
+        1.  Assert: [=!=] [$IsDataDescriptor$](|oldLenDesc|) is <emu-val>true</emu-val>.
+        1.  Let |oldLen| be |oldLenDesc|.\[[Value]].
+        1.  If [=!=] [$ToUint32$](|P|) ≠ |oldLen|, return <emu-val>false</emu-val>.
+        1.  Let |existingDescriptor| be [=!=] [$OrdinaryGetOwnProperty$](|O|, |P|).
+        1.  Assert: |existingDescriptor| is not <emu-val>undefined</emu-val>.
+        1.  Let |idlValue| 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
+            conversions in
+            <a href="#es-observable-array-defineProperty">the <code>defineProperty</code> trap</a>.
+        1.  Perform the algorithm steps given by |handler|.\[[DeleteAlgorithm]], given
+            |idlValue| and |P|.
+        1.  Perform [=!=] |O|.\[[Delete]](|O|, |P|).

Would it?  Why?  It'd be doing the length assignment on the _Array_, so it would call the _Array_'s [[Delete]] as needed.

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