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spec.emu
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<!doctype html>
<meta charset="utf8">
<link rel="stylesheet" href="./spec.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/styles/github.min.css">
<script src="./spec.js"></script>
<pre class="metadata">
title: Array Grouping
status: proposal
stage: 2
contributors: Justin Ridgewell, Jordan Harband
location: https://tc39.es/proposal-array-grouping/
</pre>
<emu-clause id="sec-scope">
<h1>Scope</h1>
<p>
This is the spec text of the <a href="https://github.com/tc39/proposal-array-grouping/">Array Grouping proposal</a> in ECMAScript.
</p>
</emu-clause>
<emu-clause id="sec-properties-of-the-object-constructor">
<h1>Properties of the Object Constructor (<a href="https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-properties-of-the-object-constructor">20.1.2</a>)</h1>
<ins class="block">
<emu-clause id="sec-object.groupby">
<h1>Object.groupBy ( _items_, _callbackfn_ )</h1>
<emu-note>
<p>_callbackfn_ should be a function that accepts two arguments. `groupBy` calls _callbackfn_ once for each element in _items_, in ascending order, and constructs a new Object of arrays. Each value returned by _callbackfn_ is coerced to a property key, and the associated element is included in the array in the constructed object according to this property key.</p>
<p>_callbackfn_ is called with two arguments: the value of the element and the index of the element.</p>
<p>The return value of `groupBy` is an object that does not inherit from %Object.prototype%.</p>
</emu-note>
<p>When the `groupBy` method is called with two arguments, the following steps are taken:</p>
<emu-alg>
1. Let _groups_ be ? GroupBy(_items_, _callbackfn_, ~property~).
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-clause>
</ins>
</emu-clause>
<emu-clause id="sec-properties-of-the-map-constructor">
<h1>Properties of the Map Constructor (<a href="https://tc39.es/ecma262/multipage/keyed-collections.html#sec-properties-of-the-map-constructor">24.1.2</a>)</h1>
<ins class="block">
<emu-clause id="sec-map.groupby">
<h1>Map.groupBy ( _items_, _callbackfn_ )</h1>
<emu-note>
<p>_callbackfn_ should be a function that accepts two arguments. `groupBy` calls _callbackfn_ once for each element in _items_, in ascending order, and constructs a new Map of arrays. Each value returned by _callbackfn_ is used as a key in the Map, and the associated element is included in the array in the constructed Map according to this key.</p>
<p>_callbackfn_ is called with two arguments: the value of the element and the index of the element.</p>
<p>The return value of `groupBy` is a Map.</p>
</emu-note>
<p>When the `groupBy` method is called with two arguments, the following steps are taken:</p>
<emu-alg>
1. Let _groups_ be ? GroupBy(_items_, _callbackfn_, ~zero~).
1. Let _map_ be ! Construct(%Map%).
1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
1. Let _elements_ be CreateArrayFromList(_g_.[[Elements]]).
1. Let _entry_ be the Record { [[Key]]: _g_.[[Key]], [[Value]]: _elements_ }.
1. Append _entry_ as the last element of _map_.[[MapData]].
1. Return _map_.
</emu-alg>
</emu-clause>
</ins>
</emu-clause>
<emu-clause id="sec-group-by-helpers">
<h1>Group By Helpers</h1>
<ins class="block">
<emu-clause id="sec-group-by" type="abstract operation">
<h1>
GroupBy (
_items_: an ECMAScript language value,
_callbackfn_: an ECMAScript language value,
_coercion_: ~property~ or ~zero~,
): either a normal completion containing a List of Records with fields [[Key]] (an ECMAScript language value) and [[Elements]] (a List of ECMAScript language values), or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Perform ? RequireObjectCoercible(_items_).
1. If IsCallable(_callbackfn_) is *false*, throw a *TypeError* exception.
1. Let _groups_ be a new empty List.
1. Let _iteratorRecord_ be ? GetIterator(_items_).
1. Let _k_ be 0.
1. Repeat,
1. If _k_ ≥ 2<sup>53</sup> - 1, then
1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
1. Return ? IteratorClose(_iteratorRecord_, _error_).
1. Let _Pk_ be ! ToString(𝔽(_k_)).
1. Let _next_ be ? IteratorStep(_iteratorRecord_).
1. If _next_ is *false*, then
1. Return _groups_.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _key_ be Completion(Call(_callbackfn_, *undefined*, « _value_, 𝔽(_k_) »)).
1. IfAbruptCloseIterator(_key_, _iteratorRecord_).
1. If _coercion_ is ~property~, then
1. Set _key_ to Completion(ToPropertyKey(_key_)).
1. IfAbruptCloseIterator(_key_, _iteratorRecord_).
1. Else,
1. Assert: _coercion_ is ~zero~.
1. If _key_ is *-0*<sub>𝔽</sub>, set _key_ to *+0*<sub>𝔽</sub>.
1. Perform AddValueToKeyedGroup(_groups_, _key_, _value_).
1. Set _k_ to _k_ + 1.
</emu-alg>
</emu-clause>
</ins>
<ins class="block">
<emu-clause id="sec-add-value-to-keyed-group" type="abstract operation">
<h1>
AddValueToKeyedGroup (
_groups_: a List of Records that have [[Key]] and [[Elements]] fields,
_key_: an ECMAScript language value,
_value_: an ECMAScript language value,
): ~unused~
</h1>
<dl class="header">
</dl>
<emu-alg>
1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
1. If SameValue(_g_.[[Key]], _key_) is *true*, then
1. Assert: exactly one element of _groups_ meets this criteria.
1. Append _value_ as the last element of _g_.[[Elements]].
1. Return ~unused~.
1. Let _group_ be the Record { [[Key]]: _key_, [[Elements]]: « _value_ » }.
1. Append _group_ as the last element of _groups_.
1. Return ~unused~.
</emu-alg>
</emu-clause>
</ins>
</emu-clause>