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

bzbarsky approved this pull request.

Thank you, this looks great!

> +    1.  If |P| [=is an array index=], then return the result of
+        [=observable array exotic object/setting the indexed value=] given |handler|, |P|, and |V|.
+    1.  Return [=?=] |O|.\[[Set]](|P|, |V|, |Receiver|).
+</div>
+
+<h4 id="es-observable-array-abstract-operations">Abstract operations</h4>
+
+<div algorithm>
+    To <dfn for="observable array exotic object" lt="set the length|setting the length">set the length</dfn>
+    of an observable array exotic object given |handler| and |newLen|:
+
+    1.  Let |uint32Len| be [=?=] [$ToUint32$](|newLen|).
+    1.  Let |numberLen| be [=?=] [$ToNumber$](|newLen|).
+    1.  If |uint32Len| ≠ |numberLen|, then throw a {{RangeError}} exception.
+    1.  Let |oldLen| be |handler|.\[[BackingList]]'s [=list/size=].
+    1.  If |numberLen| > |oldLen|, then return <emu-val>false</emu-val>.

Might be closer to the ES version to use uint32Len here and in the while loop condition.

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