From dddeeba88f499400eb14d2b59b0d8376d3c5edf9 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <jridgewell@google.com>
Date: Thu, 15 Jul 2021 01:09:36 -0400
Subject: [PATCH 1/7] Rewrite spec to use generic List/Records

This should pave the way for TypedArray support.
---
 index.html |  4 ++--
 spec.emu   | 22 +++++++++++++---------
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/index.html b/index.html
index d96dd1d..6b64bde 100644
--- a/index.html
+++ b/index.html
@@ -1095,7 +1095,7 @@
 });
 
 let sdoMap = JSON.parse(`{}`);
-let biblio = JSON.parse(`{"refsByClause":{"sec-array.prototype.groupby":["_ref_0","_ref_1","_ref_2","_ref_3","_ref_4","_ref_5","_ref_6","_ref_7","_ref_8","_ref_9","_ref_10","_ref_11"]},"entries":[{"type":"clause","id":"sec-scope","aoid":null,"titleHTML":"Scope","number":"1","referencingIds":[],"key":"Scope"},{"type":"clause","id":"sec-array.prototype.groupby","aoid":null,"titleHTML":"Array.prototype.groupBy ( <var>callbackfn</var> [ , <var>thisArg</var> ] )","number":"2.1","referencingIds":[],"key":"Array.prototype.groupBy ( callbackfn [ , thisArg ] )"},{"type":"clause","id":"sec-properties-of-the-array-prototype-object","aoid":null,"titleHTML":"Properties of the Array Prototype Object (<a href=\\"https://tc39.es/ecma262/#sec-properties-of-the-array-prototype-object\\">22.1.3</a>)","number":"2","referencingIds":[],"key":"Properties of the Array Prototype Object (22.1.3)"},{"type":"clause","id":"sec-copyright-and-software-license","aoid":null,"titleHTML":"Copyright &amp; Software License","number":"A","referencingIds":[],"key":"Copyright & Software License"}]}`);
+let biblio = JSON.parse(`{"refsByClause":{"sec-array.prototype.groupby":["_ref_0","_ref_1","_ref_2","_ref_3","_ref_4","_ref_5","_ref_6","_ref_7","_ref_8","_ref_9"]},"entries":[{"type":"clause","id":"sec-scope","aoid":null,"titleHTML":"Scope","number":"1","referencingIds":[],"key":"Scope"},{"type":"clause","id":"sec-array.prototype.groupby","aoid":null,"titleHTML":"Array.prototype.groupBy ( <var>callbackfn</var> [ , <var>thisArg</var> ] )","number":"2.1","referencingIds":[],"key":"Array.prototype.groupBy ( callbackfn [ , thisArg ] )"},{"type":"clause","id":"sec-properties-of-the-array-prototype-object","aoid":null,"titleHTML":"Properties of the Array Prototype Object (<a href=\\"https://tc39.es/ecma262/#sec-properties-of-the-array-prototype-object\\">22.1.3</a>)","number":"2","referencingIds":[],"key":"Properties of the Array Prototype Object (22.1.3)"},{"type":"clause","id":"sec-copyright-and-software-license","aoid":null,"titleHTML":"Copyright &amp; Software License","number":"A","referencingIds":[],"key":"Copyright & Software License"}]}`);
 ;let usesMultipage = false</script><style>body {
   display: flex;
   font-size: 18px;
@@ -2264,7 +2264,7 @@ <h1><span class="secnum">2.1</span> Array.prototype.groupBy ( <var>callbackfn</v
       <p>The return value of <code>groupBy</code> is an object that does not inherit from <var>Object.prototype</var>.</p>
     </div></emu-note>
     <p>When the <code>groupBy</code> method is called with one or two arguments, the following steps are taken:</p>
-    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(null).</li><li><emu-xref href="#assert"><a href="https://tc39.es/ecma262/#assert">Assert</a></emu-xref>: <var>obj</var> is an extensible ordinary object with no own properties.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>seen</var> be ?&nbsp;<emu-xref aoid="HasOwnProperty" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-hasownproperty">HasOwnProperty</a></emu-xref>(<var>obj</var>, <var>propertyKey</var>).</li><li>If <var>seen</var> is <emu-val>true</emu-val>, then<ol><li>Let <var>arr</var> be !&nbsp;<emu-xref aoid="Get" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>obj</var>, <var>propertyKey</var>).</li></ol></li><li>Else<ol><li>Let <var>arr</var> be ?&nbsp;<emu-xref aoid="ArraySpeciesCreate" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-arrayspeciescreate">ArraySpeciesCreate</a></emu-xref>(O, 0).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_10"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>propertyKey</var>, <var>arr</var>).</li></ol></li><li>Perform ?&nbsp;<emu-xref aoid="Call" id="_ref_11"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(%Array.prototype.push%, <var>arr</var>, <var>kValue</var>).</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
+    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If <emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Let <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Let <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
     <emu-note><span class="note">Note 2</span><div class="note-contents">
       <p>The <code>groupBy</code> function is intentionally generic; it does not require that its <emu-val>this</emu-val> value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.</p>
     </div></emu-note>
diff --git a/spec.emu b/spec.emu
index 1cf3a80..6d5d1af 100644
--- a/spec.emu
+++ b/spec.emu
@@ -38,20 +38,24 @@ location: https://tc39.es/proposal-array-grouping/
       1. Let _len_ be ? LengthOfArrayLike(_O_).
       1. If IsCallable(_callbackfn_) is *false*, throw a *TypeError* exception.
       1. Let _k_ be 0.
-      1. Let _obj_ be ! OrdinaryObjectCreate(null).
-      1. Assert: _obj_ is an extensible ordinary object with no own properties.
+      1. Let _groups_ be a new empty List.
       1. Repeat, while _k_ &lt; _len_
         1. Let _Pk_ be ! ToString(_k_).
         1. Let _kValue_ be ? Get(_O_, _Pk_).
         1. Let _propertyKey_ be ? ToPropertyKey(? Call(_callbackfn_, _thisArg_, &laquo; _kValue_, _k_, _O_ &raquo;)).
-        1. Let _seen_ be ? HasOwnProperty(_obj_, _propertyKey_).
-        1. If _seen_ is *true*, then
-          1. Let _arr_ be ! Get(_obj_, _propertyKey_).
-        1. Else
-          1. Let _arr_ be ? ArraySpeciesCreate(O, 0).
-          1. Perform ! CreateDataPropertyOrThrow(_obj_, _propertyKey_, _arr_).
-        1. Perform ? Call(%Array.prototype.push%, _arr_, _kValue_).
+        1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
+          1. If SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
+            1. Let _group_ be _g_.
+        1. If _group_ is ~empty~, then
+          1. Let _elements_ be a new empty List.
+          1. Let _group_ be the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
+          1. Append _group_ as the last element of _groups_.
+        1. Append _kValue_ as the last element of _group_.[[Elements]].
         1. Set _k_ to _k_ + 1.
+      1. Let _obj_ be ! OrdinaryObjectCreate(*null*).
+      1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
+        1. Let _elements_ be ! CreateArrayFromList(_g_.[[Elements]]).
+        1. Perform ! CreateDataPropertyOrThrow(_obj_, _g_.[[Key]], _elements_).
       1. Return _obj_.
     </emu-alg>
     <emu-note>

From 07e108a6a67c9a0053d938a0a456af2d0d6cda40 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Thu, 15 Jul 2021 01:39:04 -0400
Subject: [PATCH 2/7] Update spec.emu

Co-authored-by: Jordan Harband <ljharb@gmail.com>
---
 spec.emu | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/spec.emu b/spec.emu
index 6d5d1af..a5a0683 100644
--- a/spec.emu
+++ b/spec.emu
@@ -44,7 +44,7 @@ location: https://tc39.es/proposal-array-grouping/
         1. Let _kValue_ be ? Get(_O_, _Pk_).
         1. Let _propertyKey_ be ? ToPropertyKey(? Call(_callbackfn_, _thisArg_, &laquo; _kValue_, _k_, _O_ &raquo;)).
         1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
-          1. If SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
+          1. If ! SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
             1. Let _group_ be _g_.
         1. If _group_ is ~empty~, then
           1. Let _elements_ be a new empty List.

From c11802c949bb369a79b444966f056399080246e3 Mon Sep 17 00:00:00 2001
From: jridgewell <jridgewell@users.noreply.github.com>
Date: Thu, 15 Jul 2021 05:39:37 +0000
Subject: [PATCH 3/7] fixup: [spec] `npm run build`

---
 index.html | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/index.html b/index.html
index 6b64bde..4593a40 100644
--- a/index.html
+++ b/index.html
@@ -2264,7 +2264,7 @@ <h1><span class="secnum">2.1</span> Array.prototype.groupBy ( <var>callbackfn</v
       <p>The return value of <code>groupBy</code> is an object that does not inherit from <var>Object.prototype</var>.</p>
     </div></emu-note>
     <p>When the <code>groupBy</code> method is called with one or two arguments, the following steps are taken:</p>
-    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If <emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Let <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Let <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
+    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Let <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Let <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
     <emu-note><span class="note">Note 2</span><div class="note-contents">
       <p>The <code>groupBy</code> function is intentionally generic; it does not require that its <emu-val>this</emu-val> value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.</p>
     </div></emu-note>

From 8901b5d711eeb224a840fc4eadc192b7e8ec368a Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <jridgewell@google.com>
Date: Thu, 15 Jul 2021 01:42:40 -0400
Subject: [PATCH 4/7] Define group before looping

---
 index.html | 2 +-
 spec.emu   | 5 +++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/index.html b/index.html
index 4593a40..a548700 100644
--- a/index.html
+++ b/index.html
@@ -2264,7 +2264,7 @@ <h1><span class="secnum">2.1</span> Array.prototype.groupBy ( <var>callbackfn</v
       <p>The return value of <code>groupBy</code> is an object that does not inherit from <var>Object.prototype</var>.</p>
     </div></emu-note>
     <p>When the <code>groupBy</code> method is called with one or two arguments, the following steps are taken:</p>
-    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Let <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Let <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
+    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>group</var> be <emu-const>empty</emu-const>.</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Set <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Set <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
     <emu-note><span class="note">Note 2</span><div class="note-contents">
       <p>The <code>groupBy</code> function is intentionally generic; it does not require that its <emu-val>this</emu-val> value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.</p>
     </div></emu-note>
diff --git a/spec.emu b/spec.emu
index a5a0683..3a21702 100644
--- a/spec.emu
+++ b/spec.emu
@@ -43,12 +43,13 @@ location: https://tc39.es/proposal-array-grouping/
         1. Let _Pk_ be ! ToString(_k_).
         1. Let _kValue_ be ? Get(_O_, _Pk_).
         1. Let _propertyKey_ be ? ToPropertyKey(? Call(_callbackfn_, _thisArg_, &laquo; _kValue_, _k_, _O_ &raquo;)).
+        1. Let _group_ be ~empty~.
         1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
           1. If ! SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
-            1. Let _group_ be _g_.
+            1. Set _group_ be _g_.
         1. If _group_ is ~empty~, then
           1. Let _elements_ be a new empty List.
-          1. Let _group_ be the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
+          1. Set _group_ be the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
           1. Append _group_ as the last element of _groups_.
         1. Append _kValue_ as the last element of _group_.[[Elements]].
         1. Set _k_ to _k_ + 1.

From a53904f37d40df6f21058487d5311d7fe4086578 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Thu, 15 Jul 2021 01:51:33 -0400
Subject: [PATCH 5/7] Apply suggestions from code review

Co-authored-by: Jordan Harband <ljharb@gmail.com>
---
 spec.emu | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/spec.emu b/spec.emu
index 3a21702..c4a21b7 100644
--- a/spec.emu
+++ b/spec.emu
@@ -46,10 +46,10 @@ location: https://tc39.es/proposal-array-grouping/
         1. Let _group_ be ~empty~.
         1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
           1. If ! SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
-            1. Set _group_ be _g_.
+            1. Set _group_ to _g_.
         1. If _group_ is ~empty~, then
           1. Let _elements_ be a new empty List.
-          1. Set _group_ be the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
+          1. Set _group_ to the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
           1. Append _group_ as the last element of _groups_.
         1. Append _kValue_ as the last element of _group_.[[Elements]].
         1. Set _k_ to _k_ + 1.

From 1669dfaf937758058c7967b090a771a3b77f3eeb Mon Sep 17 00:00:00 2001
From: jridgewell <jridgewell@users.noreply.github.com>
Date: Thu, 15 Jul 2021 05:52:11 +0000
Subject: [PATCH 6/7] fixup: [spec] `npm run build`

---
 index.html | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/index.html b/index.html
index a548700..90fd43a 100644
--- a/index.html
+++ b/index.html
@@ -2264,7 +2264,7 @@ <h1><span class="secnum">2.1</span> Array.prototype.groupBy ( <var>callbackfn</v
       <p>The return value of <code>groupBy</code> is an object that does not inherit from <var>Object.prototype</var>.</p>
     </div></emu-note>
     <p>When the <code>groupBy</code> method is called with one or two arguments, the following steps are taken:</p>
-    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>group</var> be <emu-const>empty</emu-const>.</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Set <var>group</var> be <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Set <var>group</var> be the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
+    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>group</var> be <emu-const>empty</emu-const>.</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Set <var>group</var> to <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Set <var>group</var> to the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
     <emu-note><span class="note">Note 2</span><div class="note-contents">
       <p>The <code>groupBy</code> function is intentionally generic; it does not require that its <emu-val>this</emu-val> value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.</p>
     </div></emu-note>

From 689ea5ccd2f336bf92b58b64f9f46381503dd1c5 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <jridgewell@google.com>
Date: Thu, 15 Jul 2021 02:01:17 -0400
Subject: [PATCH 7/7] Use ArraySpeciesCreate

---
 index.html | 4 ++--
 spec.emu   | 7 ++++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/index.html b/index.html
index 90fd43a..d886de3 100644
--- a/index.html
+++ b/index.html
@@ -1095,7 +1095,7 @@
 });
 
 let sdoMap = JSON.parse(`{}`);
-let biblio = JSON.parse(`{"refsByClause":{"sec-array.prototype.groupby":["_ref_0","_ref_1","_ref_2","_ref_3","_ref_4","_ref_5","_ref_6","_ref_7","_ref_8","_ref_9"]},"entries":[{"type":"clause","id":"sec-scope","aoid":null,"titleHTML":"Scope","number":"1","referencingIds":[],"key":"Scope"},{"type":"clause","id":"sec-array.prototype.groupby","aoid":null,"titleHTML":"Array.prototype.groupBy ( <var>callbackfn</var> [ , <var>thisArg</var> ] )","number":"2.1","referencingIds":[],"key":"Array.prototype.groupBy ( callbackfn [ , thisArg ] )"},{"type":"clause","id":"sec-properties-of-the-array-prototype-object","aoid":null,"titleHTML":"Properties of the Array Prototype Object (<a href=\\"https://tc39.es/ecma262/#sec-properties-of-the-array-prototype-object\\">22.1.3</a>)","number":"2","referencingIds":[],"key":"Properties of the Array Prototype Object (22.1.3)"},{"type":"clause","id":"sec-copyright-and-software-license","aoid":null,"titleHTML":"Copyright &amp; Software License","number":"A","referencingIds":[],"key":"Copyright & Software License"}]}`);
+let biblio = JSON.parse(`{"refsByClause":{"sec-array.prototype.groupby":["_ref_0","_ref_1","_ref_2","_ref_3","_ref_4","_ref_5","_ref_6","_ref_7","_ref_8","_ref_9","_ref_10","_ref_11"]},"entries":[{"type":"clause","id":"sec-scope","aoid":null,"titleHTML":"Scope","number":"1","referencingIds":[],"key":"Scope"},{"type":"clause","id":"sec-array.prototype.groupby","aoid":null,"titleHTML":"Array.prototype.groupBy ( <var>callbackfn</var> [ , <var>thisArg</var> ] )","number":"2.1","referencingIds":[],"key":"Array.prototype.groupBy ( callbackfn [ , thisArg ] )"},{"type":"clause","id":"sec-properties-of-the-array-prototype-object","aoid":null,"titleHTML":"Properties of the Array Prototype Object (<a href=\\"https://tc39.es/ecma262/#sec-properties-of-the-array-prototype-object\\">22.1.3</a>)","number":"2","referencingIds":[],"key":"Properties of the Array Prototype Object (22.1.3)"},{"type":"clause","id":"sec-copyright-and-software-license","aoid":null,"titleHTML":"Copyright &amp; Software License","number":"A","referencingIds":[],"key":"Copyright & Software License"}]}`);
 ;let usesMultipage = false</script><style>body {
   display: flex;
   font-size: 18px;
@@ -2264,7 +2264,7 @@ <h1><span class="secnum">2.1</span> Array.prototype.groupBy ( <var>callbackfn</v
       <p>The return value of <code>groupBy</code> is an object that does not inherit from <var>Object.prototype</var>.</p>
     </div></emu-note>
     <p>When the <code>groupBy</code> method is called with one or two arguments, the following steps are taken:</p>
-    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>group</var> be <emu-const>empty</emu-const>.</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Set <var>group</var> to <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Set <var>group</var> to the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>elements</var> be !&nbsp;<emu-xref aoid="CreateArrayFromList" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-createarrayfromlist">CreateArrayFromList</a></emu-xref>(<var>g</var>.[[Elements]]).</li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
+    <emu-alg><ol><li>Let <var>O</var> be ?&nbsp;<emu-xref aoid="ToObject" id="_ref_0"><a href="https://tc39.es/ecma262/#sec-toobject">ToObject</a></emu-xref>(<emu-val>this</emu-val> value).</li><li>Let <var>len</var> be ?&nbsp;<emu-xref aoid="LengthOfArrayLike" id="_ref_1"><a href="https://tc39.es/ecma262/#sec-lengthofarraylike">LengthOfArrayLike</a></emu-xref>(<var>O</var>).</li><li>If <emu-xref aoid="IsCallable" id="_ref_2"><a href="https://tc39.es/ecma262/#sec-iscallable">IsCallable</a></emu-xref>(<var>callbackfn</var>) is <emu-val>false</emu-val>, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>k</var> be 0.</li><li>Let <var>groups</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Repeat, while <var>k</var> &lt; <var>len</var><ol><li>Let <var>Pk</var> be !&nbsp;<emu-xref aoid="ToString" id="_ref_3"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<var>k</var>).</li><li>Let <var>kValue</var> be ?&nbsp;<emu-xref aoid="Get" id="_ref_4"><a href="https://tc39.es/ecma262/#sec-get-o-p">Get</a></emu-xref>(<var>O</var>, <var>Pk</var>).</li><li>Let <var>propertyKey</var> be ?&nbsp;<emu-xref aoid="ToPropertyKey" id="_ref_5"><a href="https://tc39.es/ecma262/#sec-topropertykey">ToPropertyKey</a></emu-xref>(? <emu-xref aoid="Call" id="_ref_6"><a href="https://tc39.es/ecma262/#sec-call">Call</a></emu-xref>(<var>callbackfn</var>, <var>thisArg</var>, « <var>kValue</var>, <var>k</var>, <var>O</var> »)).</li><li>Let <var>group</var> be <emu-const>empty</emu-const>.</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>If !&nbsp;<emu-xref aoid="SameValueNonNumeric" id="_ref_7"><a href="https://tc39.es/ecma262/#sec-samevaluenonnumeric">SameValueNonNumeric</a></emu-xref>(<var>g</var>.[[Key]], <var>propertyKey</var>) is <emu-val>true</emu-val>, then<ol><li>Set <var>group</var> to <var>g</var>.</li></ol></li></ol></li><li>If <var>group</var> is <emu-const>empty</emu-const>, then<ol><li>Let <var>elements</var> be a new empty <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">List</a></emu-xref>.</li><li>Set <var>group</var> to the <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]]: <var>propertyKey</var>, [[Elements]]: <var>elements</var> }.</li><li>Append <var>group</var> as the last element of <var>groups</var>.</li></ol></li><li>Append <var>kValue</var> as the last element of <var>group</var>.[[Elements]].</li><li>Set <var>k</var> to <var>k</var> + 1.</li></ol></li><li>Let <var>obj</var> be !&nbsp;OrdinaryObjectCreate(<emu-val>null</emu-val>).</li><li>For each <emu-xref href="#sec-list-and-record-specification-type"><a href="https://tc39.es/ecma262/#sec-list-and-record-specification-type">Record</a></emu-xref> { [[Key]], [[Elements]] } <var>g</var> of <var>groups</var>, do<ol><li>Let <var>len</var> be the number of elements in <var>g</var>.[[Elements]].</li><li>Let <var>elements</var> be ?&nbsp;<emu-xref aoid="ArraySpeciesCreate" id="_ref_8"><a href="https://tc39.es/ecma262/#sec-arrayspeciescreate">ArraySpeciesCreate</a></emu-xref>(<var>O</var>, <var>len</var>).</li><li>Let <var>i</var> be 0.</li><li>For each element <var>e</var> of <var>g</var>.[[Elements]], do<ol><li>Perform ?&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_9"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>elements</var>, !&nbsp;<emu-xref aoid="ToString" id="_ref_10"><a href="https://tc39.es/ecma262/#sec-tostring">ToString</a></emu-xref>(<emu-xref href="#𝔽"><a href="https://tc39.es/ecma262/#𝔽">𝔽</a></emu-xref>(<var>i</var>)), <var>e</var>).</li><li>Set <var>i</var> to <var>i</var> + 1.</li></ol></li><li>Perform !&nbsp;<emu-xref aoid="CreateDataPropertyOrThrow" id="_ref_11"><a href="https://tc39.es/ecma262/#sec-createdatapropertyorthrow">CreateDataPropertyOrThrow</a></emu-xref>(<var>obj</var>, <var>g</var>.[[Key]], <var>elements</var>).</li></ol></li><li>Return <var>obj</var>.</li></ol></emu-alg>
     <emu-note><span class="note">Note 2</span><div class="note-contents">
       <p>The <code>groupBy</code> function is intentionally generic; it does not require that its <emu-val>this</emu-val> value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.</p>
     </div></emu-note>
diff --git a/spec.emu b/spec.emu
index c4a21b7..810c816 100644
--- a/spec.emu
+++ b/spec.emu
@@ -55,7 +55,12 @@ location: https://tc39.es/proposal-array-grouping/
         1. Set _k_ to _k_ + 1.
       1. Let _obj_ be ! OrdinaryObjectCreate(*null*).
       1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
-        1. Let _elements_ be ! CreateArrayFromList(_g_.[[Elements]]).
+        1. Let _len_ be the number of elements in _g_.[[Elements]].
+        1. Let _elements_ be ? ArraySpeciesCreate(_O_, _len_).
+        1. Let _i_ be 0.
+        1. For each element _e_ of _g_.[[Elements]], do
+          1. Perform ? CreateDataPropertyOrThrow(_elements_, ! ToString(𝔽(_i_)), _e_).
+          1. Set _i_ to _i_ + 1.
         1. Perform ! CreateDataPropertyOrThrow(_obj_, _g_.[[Key]], _elements_).
       1. Return _obj_.
     </emu-alg>