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spec.emu
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<!doctype html>
<meta charset="utf8">
<pre class="metadata">
title: Pattern Matching
stage: 1
contributors:
- Daniel Rosenwasser
- Jack Works
- Jordan Harband
- Mark Cohen
- Ross Kirsling
- Tab Atkins
</pre>
<style>
#welcome .secnum, [title="Welcome"] .secnum {
display: none;
}
emu-note[code] > .note, #welcome > h1 {
font-size: 0;
}
emu-note[code] > .note::before {
content: "Example";
font-size: 18px;
}
emu-note[code] pre {
margin: 0;
}
pre.inline {
display: inline;
}
pre.inline code {
display: inline;
font-style: italic;
text-decoration: underline;
}
body.folded .fold, body.folded .todo {
display: none;
}
.show-ao-annotations a.e-user-code::before, .show-ao-annotations span.e-user-code::before {
display: inline-block;
}
#sec-todos {
border-left: 5px solid #ff6600;
padding: 0.5em;
background: #ffeedd;
}
emu-intro {
margin-top: 1em !important;
}
[role="button"] {
cursor: pointer;
}
</style>
<emu-intro id="welcome">
<h1>Welcome</h1>
<emu-intro id="sec-todos">
<h1 class="attributes-tag">TODOs</h1>
<ul>
<li>
Scope and bindings
<ul>
<li>Basic case.</li>
<li>Work with `for` loop (CreatePerIterationEnvironment).</li>
<li>Work with function parameters.</li>
</ul>
</li>
</ul>
</emu-intro>
<emu-intro id="sec-nav">
<h1>Introduction</h1>
<p>This specification consists of the following parts:</p>
<ul>
<li><emu-xref href="#sec-pattern-matching">Patterns in pattern matching</emu-xref></li>
<li>Interesting AOs:
<ul>
<li><emu-xref href="#sec-invoke-custom-matcher" title></emu-xref></li>
<li><emu-xref href="#sec-pattern-match-cache-note">Cache semantics</emu-xref></li>
</ul>
</li>
<li><emu-xref href="#sec-relational-operators" title>The `is` expression</emu-xref></li>
<li><emu-xref href="#sec-match-expression" title></emu-xref></li>
<li>
Non-trivial Built-in %Symbol.customMatcher% methods:
<ul>
<li><emu-xref href="#sec-function.prototype-%symbol.custommatcher%" title></emu-xref></li>
<li><emu-xref href="#sec-regexp.prototype-%symbol.custommatcher%" title></emu-xref></li>
</ul>
</li>
<li>(TODO) Scope analysis changes: <emu-xref href="#sec-syntax-directed-operations" title></emu-xref></li>
<li>`new` semantics changes:
<emu-xref href="#sec-initializeinstance"></emu-xref>,
<emu-xref href="#sec-weakly-hold-targets-processing-model"></emu-xref>, and
<emu-xref href="#sec-runtime-semantics-classdefinitionevaluation"></emu-xref>
</li>
<li>
Possible extensions:
<emu-xref href="#sec-for-in-and-for-of-statements" title></emu-xref> and
<emu-xref href="#sec-try-statement" title></emu-xref>
</li>
</ul>
<p>
Trivia built-in matchers are folded.
<a role="button" id="expand">Click to <span class="fold">not</span> show the trivia sections.</a>
</p>
<script defer async src="./assets/expand.js"></script>
</emu-intro>
<emu-intro id="sec-notes-layering">
<h1>Layering</h1>
<p>The pattern-matching champion group designed this proposal with a layering approach. It does not mean the proposal is an MVP. The champion group wishes to ship the proposal as a whole when possible, but we can drop some features if there is strong pushback from the committee.</p>
<p>This approach allows the champion group to consider how all features combine and also how the proposal should behave if any of the features are missing.</p>
<p>A feature will have a note if</p>
<ul>
<li>it is a convenient feature instead of a necessary feature.</li>
<li>not all champion group members represent the hope to include it.</li>
</ul>
</emu-intro>
</emu-intro>
<emu-clause class="fold" id="sec-overview" number="4">
<h1>Overview</h1>
<emu-clause id="sec-organization-of-this-specification" number="5">
<h1>Organization of This Specification</h1>
<p><ins>Clause <emu-xref href="#sec-pattern-matching"></emu-xref> describes the pattern-matching feature.</ins></p>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-ecmascript-data-types-and-values" aoid="Type" number="6">
<h1>ECMAScript Data Types and Values</h1>
<emu-clause id="sec-ecmascript-language-types">
<h1>ECMAScript Language Types</h1>
<emu-clause id="sec-ecmascript-language-types-symbol-type" number="5">
<h1>The Symbol Type</h1>
<emu-clause id="sec-well-known-symbols">
<h1>Well-Known Symbols</h1>
<emu-table id="table-1" caption="Well-known Symbols">
<table>
<tbody>
<tr>
<th>
Specification Name
</th>
<th>
[[Description]]
</th>
<th>
Value and Purpose
</th>
</tr>
<tr>
<td>
<ins><dfn>%Symbol.customMatcher%</dfn></ins>
</td>
<td>
<ins>`"Symbol.customMatcher"`</ins>
</td>
<td>
<ins>A method that performs custom pattern matching semantics. Called by the semantics of the pattern-matching features.</ins>
</td>
</tr>
</tbody>
</table>
</emu-table>
</emu-clause>
</emu-clause>
<emu-clause id="sec-object-internal-methods-and-internal-slots" number="7">
<h1>Object Internal Methods and Internal Slots</h1>
<p><ins>All objects have an internal slot named [[ConstructedBy]], which is a List of ECMAScript language values.
This List represents the origin of the object. Initially, it is an empty List.</ins></p>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-abstract-operations" number="7">
<h1>Abstract Operations</h1>
<emu-clause id="sec-operations-on-objects" number="3">
<h1>Operations on Objects</h1>
<emu-clause oldids="sec-initializeinstanceelements" id="sec-initializeinstance" type="abstract operation" number="34">
<h1>
InitializeInstanceElements (
_O_: an Object,
_constructor_: an ECMAScript function object,
): either a normal completion containing ~unused~ or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Let _methods_ be the value of _constructor_.[[PrivateMethods]].
1. For each PrivateElement _method_ of _methods_, do
1. Perform ? PrivateMethodOrAccessorAdd(_O_, _method_).
1. Let _fields_ be the value of _constructor_.[[Fields]].
1. For each element _fieldRecord_ of _fields_, do
1. Perform ? DefineField(_O_, _fieldRecord_).
1. <ins>Append _constructor_ to _O_.[[ConstructedBy]].</ins>
1. Return ~unused~.
</emu-alg>
<emu-note type="editor">Rename this abstract operation to <dfn>InitializeInstance</dfn>.</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="todo" id="sec-syntax-directed-operations">
<h1>Syntax-Directed Operations</h1>
<emu-clause id="sec-syntax-directed-operations-scope-analysis" number="2">
<h1>Scope Analysis</h1>
<emu-clause id="sec-static-semantics-boundnames" type="sdo" number="1">
<h1>Static Semantics: BoundNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-declarationpart" type="sdo" number="2">
<h1>Static Semantics: DeclarationPart ( ): a Parse Node</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-isconstantdeclaration" type="sdo" number="3">
<h1>Static Semantics: IsConstantDeclaration ( ): a Boolean</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-lexicallydeclarednames" type="sdo" number="4">
<h1>Static Semantics: LexicallyDeclaredNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-lexicallyscopeddeclarations" type="sdo" number="5">
<h1>Static Semantics: LexicallyScopedDeclarations ( ): a List of Parse Nodes</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-vardeclarednames" type="sdo" number="6">
<h1>Static Semantics: VarDeclaredNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-varscopeddeclarations" type="sdo" number="7">
<h1>Static Semantics: VarScopedDeclarations ( ): a List of Parse Nodes</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-toplevellexicallydeclarednames" type="sdo" number="8">
<h1>Static Semantics: TopLevelLexicallyDeclaredNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-toplevellexicallyscopeddeclarations" type="sdo" number="9">
<h1>Static Semantics: TopLevelLexicallyScopedDeclarations ( ): a List of Parse Nodes</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-toplevelvardeclarednames" type="sdo" number="10">
<h1>Static Semantics: TopLevelVarDeclaredNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-static-semantics-toplevelvarscopeddeclarations" type="sdo" number="11">
<h1>Static Semantics: TopLevelVarScopedDeclarations ( ): a List of Parse Nodes</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-syntax-directed-operations-miscellaneous" number="6">
<h1>Miscellaneous</h1>
<emu-clause id="sec-runtime-semantics-bindinginitialization" type="sdo" number="2">
<h1>
Runtime Semantics: BindingInitialization (
_value_: an ECMAScript language value,
_environment_: an Environment Record or *undefined*,
): either a normal completion containing ~unused~ or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
<emu-clause id="sec-runtime-semantics-iteratorbindinginitialization" type="sdo" number="3">
<h1>
Runtime Semantics: IteratorBindingInitialization (
_iteratorRecord_: an Iterator Record,
_environment_: an Environment Record or *undefined*,
): either a normal completion containing ~unused~ or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-note type="editor">
TODO: Scope Analysis.
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-executable-code-and-execution-contexts" number="9">
<h1>Executable Code and Execution Contexts</h1>
<emu-clause id="sec-weakly-hold-targets-processing-model" oldids="sec-weakref-processing-model" number="10">
<h1>Processing Model of <del>WeakRef and FinalizationRegistry</del><ins>weakly hold</ins> Targets</h1>
<emu-clause id="sec-weakly-hold-execution" oldids="sec-weakref-execution" number="3">
<h1>Execution</h1>
<p>At any time, if a set of objects and/or symbols _S_ is not live, an ECMAScript implementation may perform the following steps atomically:</p>
<emu-alg>
1. For each element _value_ of _S_, do
1. For each WeakRef _ref_ such that _ref_.[[WeakRefTarget]] is _value_, do
1. Set _ref_.[[WeakRefTarget]] to ~empty~.
1. For each FinalizationRegistry _fg_ such that _fg_.[[Cells]] contains a Record _cell_ such that _cell_.[[WeakRefTarget]] is _value_, do
1. Set _cell_.[[WeakRefTarget]] to ~empty~.
1. Optionally, perform HostEnqueueFinalizationRegistryCleanupJob(_fg_).
1. For each WeakMap _map_ such that _map_.[[WeakMapData]] contains a Record _r_ such that _r_.[[Key]] is _value_, do
1. Set _r_.[[Key]] to ~empty~.
1. Set _r_.[[Value]] to ~empty~.
1. For each WeakSet _set_ such that _set_.[[WeakSetData]] contains _value_, do
1. Replace the element of _set_.[[WeakSetData]] whose value is _value_ with an element whose value is ~empty~.
1. <ins>For each Object _o_ such that _o_.[[ConstructedBy]] contains _value_, do</ins>
1. <ins>Remove _value_ from _o_.[[ConstructedBy]].</ins>
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-ecmascript-language-lexical-grammar" number="12">
<h1>ECMAScript Language: Lexical Grammar</h1>
<emu-clause id="sec-automatic-semicolon-insertion" number="10">
<h1>Automatic Semicolon Insertion</h1>
<emu-clause id="sec-rules-of-automatic-semicolon-insertion" number="1">
<h1>Rules of Automatic Semicolon Insertion</h1>
<emu-note>
<p>The following are the additions of the restricted productions in the grammar:</p>
<emu-grammar>
RelationalExpression[In, Yield, Await] :
RelationalExpression [no LineTerminator here] `is` MatchPattern
MatchExpression[Yield, Await] :
CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] `{` MatchExpressionClauses[?Yield, ?Await] `;` `}`
MatchHead :
`match` [no LineTerminator here] `(` Expression `)`
</emu-grammar>
</emu-note>
</emu-clause>
<emu-clause id="sec-interesting-cases-of-automatic-semicolon-insertion" number="3">
<h1>Interesting Cases of Automatic Semicolon Insertion</h1>
<emu-clause id="sec-asi-cases-with-no-lineterminator-here" number="2">
<h1>Cases of Automatic Semicolon Insertion and “[no |LineTerminator| here]”</h1>
<emu-clause id="sec-no-lineterminator-here-automatic-semicolon-insertion-list" number="1">
<h1>List of Grammar Productions with Optional Operands and “[no |LineTerminator| here]”</h1>
<ul>
<li><ins>|MatchExpression|.</ins></li>
</ul>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-expressions" number="13">
<h1>ECMAScript Language: Expressions</h1>
<emu-clause id="sec-primary-expression" number="2">
<h1>Primary Expression</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
PrimaryExpression[Yield, Await] :
RegularExpressionLiteral
<ins>MatchExpression[?Yield, ?Await]</ins>
</emu-grammar>
<emu-clause id="sec-primary-expression-match-expression" number="10">
<h1>Match Expression</h1>
<p>See <emu-xref href="#sec-match-expression" title></emu-xref> for <emu-grammar>PrimaryExpression : MatchExpression</emu-grammar></p>
</emu-clause>
</emu-clause>
<emu-clause id="sec-relational-operators" number="10">
<h1>Relational Operators</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
RelationalExpression[In, Yield, Await] :
<ins>RelationalExpression[?In, ?Yield, ?Await] [no LineTerminator here] `is` MatchPattern[?Yield, ?Await]</ins>
</emu-grammar>
<emu-note code>
<pre><code class="javascript">
const isOk = response is { ok: true, status: > 200 and < 400 };
</code></pre>
</emu-note>
<emu-note type="editor">
This feature can be replaced by <emu-xref href="#sec-match-expression" title></emu-xref>.
The code example above can be written as:
<pre><code class="javascript">
const isOk = match (response) {
{ ok: true, status: > 200 and < 400 }: true,
default: false
};
</code></pre>
</emu-note>
<emu-note type="editor">
We may need to use a non-contextual keyword like `~=` instead of a contextual keyword like `is`. See <a
href="https://github.com/waldemarhorwat/syntax/blob/main/contextual-keywords.md?rgh-link-date=2024-09-06T16%3A43%3A25Z">waldemarhorwat/syntax@main/contextual-keywords.md</a>
and <a href="https://github.com/tc39/proposal-pattern-matching/issues/323">Syntax effects on rest of the language</a>.
</emu-note>
<emu-clause id="sec-relational-operators-runtime-semantics-evaluation" number="1">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>
<ins>RelationalExpression : RelationalExpression `is` MatchPattern</ins>
</emu-grammar>
<emu-alg>
1. Let _lref_ be ? Evaluation of |RelationalExpression|.
1. Let _lval_ be ? GetValue(_lref_).
1. Let _cacheGroup_ be CreateMatchCache().
1. Let _matchCompletion_ be ? MatchPatternMatches of |MatchPattern| with argument _lval_ and _cacheGroup_.
1. If _matchCompletion_ is a normal completion, then
1. If _matchCompletion_.[[Value]] is ~not-matched~, set _matchCompletion_ to NormalCompletion(*false*).
1. Else, set _matchCompletion_ to NormalCompletion(*true*).
1. Let _result_ be Completion(FinishMatch(_matchCompletion_, _cacheGroup_)).
1. Assert: _result_ is a normal completion or an abrupt completion.
1. Return _result_.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-statements-and-declarations" number="14">
<h1>ECMAScript Language: Statements and Declarations</h1>
<h2>Syntax</h2>
<emu-clause id="sec-iteration-statements" number="7">
<h1>Iteration Statements</h1>
<emu-clause id="sec-for-in-and-for-of-statements">
<h1>The `for`-`in`, `for`-`of`, and `for`-`await`-`of` Statements</h1>
<emu-note type="editor">
It is possible to add pattern-matching to the `for` iteration statements. It might look like this:
<pre><code class="javascript">
for (const response of responses) {
if (item is { ok: true, let body }) {
}
}
// can be written as
for (const response is { ok: true, let body } of responses) {
}
// or
for (const response of responses matches { ok: true, let body }) {
}
</code></pre>
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-try-statement" number="14">
<h1>The `try` Statement</h1>
<emu-note type="editor">
It is possible to add pattern-matching to the `try` statement. It might look like this:
<pre><code class="javascript">
try { }
catch (error) {
if (error is { message: /JSON/ }) { return null; }
throw error;
}
// can be written as
try { }
catch (error is { message: /JSON/ }) { return null; }
// unmatched error will be re-thrown.
</code></pre>
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-functions-and-classes" number="15">
<h1>ECMAScript Language: Functions and Classes</h1>
<emu-clause id="sec-class-definitions" number="7">
<h1>Class Definitions</h1>
<emu-clause id="sec-runtime-semantics-classdefinitionevaluation" type="sdo" number="14">
<h1>
Runtime Semantics: ClassDefinitionEvaluation (
_classBinding_: a String or *undefined*,
_className_: a property key or a Private Name,
): either a normal completion containing a function object or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-note type="editor">
See <a href="https://github.com/tc39/ecma262/pull/3212/files" target="_blank">Editorial: call MakeClassConstructor on default class constructor</a>.
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-tail-position-calls" number="10">
<h1>Tail Position Calls</h1>
<emu-clause id="sec-static-semantics-hascallintailposition" number="2" type="sdo">
<h1>
Static Semantics: HasCallInTailPosition (
_call_: a |CallExpression| Parse Node, a |MemberExpression| Parse Node, or an |OptionalChain| Parse Node,
): a Boolean
</h1>
<dl class="header">
</dl>
<emu-grammar>
<ins>RelationalExpression : RelationalExpression `is` MatchPattern</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>
<ins>PrimaryExpression : MatchExpression</ins>
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |MatchExpression| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpression : CoverCallExpressionAndAsyncArrowHead `{` MatchExpressionClauses `;` `}`</ins>
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |MatchExpressionClauses| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpressionClauses : MatchExpressionClause</ins>
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |MatchExpressionClause| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpressionClauses : MatchExpressionClauses `;` MatchExpressionClause</ins>
</emu-grammar>
<emu-alg>
1. Let _result_ be HasCallInTailPosition of |MatchExpressionClauses| with argument _call_.
1. If _result_ is *true*, return *true*.
1. Return HasCallInTailPosition of |MatchExpressionClause| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpressionClauses : MatchExpressionClauses `;` `default` `:` Expression</ins>
</emu-grammar>
<emu-alg>
1. Let _result_ be HasCallInTailPosition of |MatchExpressionClauses| with argument _call_.
1. If _result_ is *true*, return *true*.
1. Return HasCallInTailPosition of |Expression| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpressionClauses : `default` `:` Expression</ins>
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |Expression| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>MatchExpressionClause : MatchPattern `:` Expression</ins>
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |Expression| with argument _call_.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-fundamental-objects" number="20">
<h1>Fundamental Objects</h1>
<emu-clause class="fold" id="sec-object-objects" number="1">
<h1>Object Objects</h1>
<emu-clause id="sec-properties-of-the-object-constructor" number="2">
<h1>Properties of the Object Constructor</h1>
<emu-clause id="sec-object-%symbol.custommatcher%" number="24">
<h1>Object [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. If _subject_ is not an Object, return *false*.
1. Return *true*.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-function-objects" number="2">
<h1>Function Objects</h1>
<emu-clause id="sec-properties-of-the-function-constructor" number="2">
<h1>Properties of the Function Constructor</h1>
<emu-clause id="sec-function-%symbol.custommatcher%" number="2">
<h1>Function [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. Return IsCallable(_subject_).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-function-prototype-object" number="3">
<h1>Properties of the Function Prototype Object</h1>
<emu-clause id="sec-function.prototype-%symbol.custommatcher%" number="7">
<h1>Function.prototype [ %Symbol.customMatcher% ] ( _subject_, _hint_, _receiver_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. Let _func_ be the *this* value.
1. If IsCallable(_func_) is false, throw a *TypeError* exception.
1. If _subject_.[[ConstructedBy]] contains _func_, return *true*.
1. If _func_ does not have a [[IsClassConstructor]] internal slot or _func_.[[IsClassConstructor]] is *false*, return ? Call(_func_, _receiver_, « _subject_, _hint_ »).
1. Return *false*.
</emu-alg>
<emu-note code><pre><code class="javascript">
// For non-class functions.
[] is Array.isArray; // true, by Array.isArray(expr)
// For objects created by `new`, it uses private-field-like semantics.
class MyError extends Error {}
const myError = new MyError();
myError is MyError; // true
myError is Error; // true
Object.create(MyError.prototype) is MyError; // false
// Also works for normal functions
function ES5StyleClass() {}
new ES5StyleClass() is ES5StyleClass; // true
Object.create(ES5StyleClass.prototype) is ES5StyleClass; // false
</code></pre></emu-note>
<emu-note type="editor">
<p>This does not work with ES5 style class inherit.</p>
<pre><code class="javascript">
function MyError() {
Error.call(this);
}
MyError.prototype = Object.create(Error.prototype);
var error = new MyError();
error is MyError; // true
error is Error; // false
</code></pre>
</emu-note>
<emu-note type="editor">
<p>Not everyone in the champion group agrees with private-field-like brand check semantics.</p>
<p>
There are
<a href="https://github.com/tc39/proposal-pattern-matching/pull/293#issuecomment-1725097699" target="_blank">performance concerns</a>,
<a href="https://github.com/tc39/proposal-pattern-matching/pull/293#issuecomment-1725097699" target="_blank">"hackable" concerns</a>, and
<a href="https://github.com/tc39/proposal-pattern-matching/pull/293#issuecomment-1725097699" target="_blank">interaction
with %Symbol.hasInstance% concerns</a>.
</p>
<p>Another approach is to use the `instanceof` semantics.</p>
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-boolean-objects" number="3">
<h1>Boolean Objects</h1>
<emu-clause id="sec-properties-of-the-boolean-constructor" number="2">
<h1>Properties of the Boolean Constructor</h1>
<emu-clause id="sec-boolean-%symbol.custommatcher%" number="2">
<h1>Boolean [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. If _subject_ is not a Boolean and does not have a [[BooleanData]] internal slot, return *false*.
1. If _hint_ is *"boolean"*, return *true*.
1. If _subject_ is a Boolean, return CreateArrayFromList(« _subject_ »).
1. Return CreateArrayFromList(« _subject_.[[BooleanData]] »).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-note type="editor">
Another approach is to ignore boxed primitives and only match primitive values.
</emu-note>
</emu-clause>
<emu-clause class="fold" id="sec-symbol-objects" number="4">
<h1>Symbol Objects</h1>
<emu-clause id="sec-properties-of-the-symbol-constructor" number="2">
<h1>Properties of the Symbol Constructor</h1>
<emu-clause id="sec-symbol.custommatcher" number="17">
<h1>Symbol.customMatcher</h1>
<p>The initial value of `Symbol.customMatcher` is the well-known symbol %Symbol.customMatcher% (<emu-xref
href="#table-well-known-symbols"></emu-xref>).</p>
<p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>
</emu-clause>
<emu-clause id="sec-symbol-%symbol.custommatcher%">
<h1>Symbol [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. If _subject_ is not a Symbol and does not have a [[SymbolData]] internal slot, return *false*.
1. If _hint_ is *"boolean"*, return *true*.
1. If _subject_ is a Symbol, return CreateArrayFromList(« _subject_ »).
1. Return CreateArrayFromList(« _subject_.[[SymbolData]] »).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-note type="editor">
Another approach is to ignore boxed primitives and only match primitive values.
</emu-note>
</emu-clause>
<emu-clause class="fold" id="sec-error-objects" number="5">
<h1>Error Objects</h1>
<emu-clause id="sec-error-constructor" number="1">
<h1>The Error Constructor</h1>
<emu-clause id="sec-error-message">
<h1>Error ( _message_ [ , _options_ ] )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. If NewTarget is *undefined*, let _newTarget_ be the active function object; else let _newTarget_ be NewTarget.
1. Let _O_ be ? OrdinaryCreateFromConstructor(_newTarget_, *"%Error.prototype%"*, « [[ErrorData]] »).
1. <ins>Set _O_.[[ErrorData]] to *"Error"*.</ins>
1. If _message_ is not *undefined*, then
1. Let _msg_ be ? ToString(_message_).
1. Perform CreateNonEnumerableDataPropertyOrThrow(_O_, *"message"*, _msg_).
1. Perform ? InstallErrorCause(_O_, _options_).
1. Return _O_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-error-constructors" number="2">
<h1>Properties of the Error Constructor</h1>
<emu-clause id="sec-error-%symbol.custommatcher%" number="2">
<h1>Error [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. If _subject_ does not have a [[ErrorData]] internal slot, return *false*.
1. Return *true*.
</emu-alg>
</emu-clause>
<emu-note type="editor">
It is possible to provide extractor semantics for Error matchers.
<pre><code class="javascript">
if (expr is Error(let message, { let cause })) {}
</code></pre>
</emu-note>
</emu-clause>
<emu-clause id="sec-properties-of-error-instances" number="4">
<h1>Properties of Error Instances</h1>
<p>Error instances are ordinary objects that inherit properties from the Error prototype object and have an
[[ErrorData]] internal slot whose value is <del>*undefined*</del> <ins>a String</ins>. The only specified uses of
[[ErrorData]] is to identify Error, AggregateError, and _NativeError_ instances as Error objects within
`Object.prototype.toString` <ins>and their %Symbol.customMatcher% methods</ins>.</p>
<emu-note type="editor">
Rename this internal slot to [[ErrorKind]].
</emu-note>
</emu-clause>
<emu-clause id="sec-nativeerror-object-structure" number="6">
<h1>_NativeError_ Object Structure</h1>
<emu-clause id="sec-nativeerror-constructors" number="1">
<h1>The _NativeError_ Constructors</h1>
<emu-clause id="sec-nativeerror" number="1">
<h1>_NativeError_ ( _message_ [ , _options_ ] )</h1>
<p>Each _NativeError_ function performs the following steps when called:</p>
<emu-alg>
1. If NewTarget is *undefined*, let _newTarget_ be the active function object; else let _newTarget_ be NewTarget.
1. Let _O_ be ? OrdinaryCreateFromConstructor(_newTarget_, <code>"%<var>NativeError</var>.prototype%"</code>, « [[ErrorData]] »).
1. <ins>Set _O_.[[ErrorData]] to _NativeError_.</ins>
1. If _message_ is not *undefined*, then
1. Let _msg_ be ? ToString(_message_).
1. Perform CreateNonEnumerableDataPropertyOrThrow(_O_, *"message"*, _msg_).
1. Perform ? InstallErrorCause(_O_, _options_).
1. Return _O_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-nativeerror-constructors" number="2">
<h1>Properties of the _NativeError_ Constructors</h1>
<emu-clause id="sec-nativeerror-%symbol.custommatcher%" number="2">
<h1>_NativeError_ [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. If _subject_ does not have a [[ErrorData]] internal slot or _subject_.[[ErrorData]] is not _NativeError_, return *false*.
1. Return *true*.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-nativeerror-instances" number="4">
<h1>Properties of _NativeError_ Instances</h1>
<p>_NativeError_ instances are ordinary objects that inherit properties from their _NativeError_ prototype object and
have an [[ErrorData]] internal slot whose value is <del>*undefined*</del> <ins>a String</ins>. The only specified use
of [[ErrorData]] is by `Object.prototype.toString` (<emu-xref href="#sec-object.prototype.tostring"></emu-xref>)
<ins>and their %Symbol.customMatcher% methods</ins> to identify Error, AggregateError, or _NativeError_ instances.
</p>
</emu-clause>
</emu-clause>
<emu-clause id="sec-aggregate-error-objects" number="7">
<h1>AggregateError Objects</h1>
<emu-clause id="sec-aggregate-error-constructor" number="1">
<h1>The AggregateError Constructor</h1>
<emu-clause id="sec-aggregate-error">
<h1>AggregateError ( _errors_, _message_ [ , _options_ ] )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. If NewTarget is *undefined*, let _newTarget_ be the active function object; else let _newTarget_ be NewTarget.
1. Let _O_ be ? OrdinaryCreateFromConstructor(_newTarget_, *"%AggregateError.prototype%"*, « [[ErrorData]] »).
1. <ins>Set _O_.[[ErrorData]] to *"AggregateError"*.</ins>
1. If _message_ is not *undefined*, then
1. Let _msg_ be ? ToString(_message_).
1. Perform CreateNonEnumerableDataPropertyOrThrow(_O_, *"message"*, _msg_).
1. Perform ? InstallErrorCause(_O_, _options_).
1. Let _errorsList_ be ? IteratorToList(? GetIterator(_errors_, ~sync~)).
1. Perform ! DefinePropertyOrThrow(_O_, *"errors"*, PropertyDescriptor { [[Configurable]]: *true*, [[Enumerable]]: *false*, [[Writable]]: *true*, [[Value]]: CreateArrayFromList(_errorsList_) }).
1. Return _O_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-aggregate-error-constructors" number="2">
<h1>Properties of the AggregateError Constructor</h1>
<emu-clause id="sec-aggregate-error-%symbol.custommatcher%" number="2">
<h1>AggregateError [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. If _subject_ does not have a [[ErrorData]] internal slot or _subject_.[[ErrorData]] is not *"AggregateError"*, return *false*.
1. Return *true*.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-aggregate-error-instances" number="4">
<h1>Properties of AggregateError Instances</h1>
<p>AggregateError instances are ordinary objects that inherit properties from their AggregateError prototype object and
have an [[ErrorData]] internal slot whose value is <del>*undefined*</del> <ins>a String</ins>. The only specified use
of [[ErrorData]] is by `Object.prototype.toString` (<emu-xref href="#sec-object.prototype.tostring"></emu-xref>)
<ins>and their %Symbol.customMatcher% methods</ins> to identify Error, AggregateError, or _NativeError_ instances.
</p>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-numbers-and-dates" number="21">
<h1>Numbers and Dates</h1>
<emu-clause id="sec-number-objects" number="1">
<h1>Number Objects</h1>
<emu-clause id="sec-properties-of-the-number-constructor" number="2">
<h1>Properties of the Number Constructor</h1>
<emu-clause id="sec-number-%symbol.custommatcher%" number="16">
<h1>Number [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. If _subject_ is not a Number and does not have a [[NumberData]] internal slot, return *false*.
1. If _hint_ is *"boolean"*, return *true*.
1. If _subject_ is a Number, return CreateArrayFromList(« _subject_ »).
1. Return CreateArrayFromList(« _subject_.[[NumberData]] »).
</emu-alg>
</emu-clause>
<emu-note type="editor">
Another approach is to ignore boxed primitives and only match primitive values.
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-bigint-objects" number="2">
<h1>BigInt Objects</h1>
<emu-clause id="sec-properties-of-the-bigint-constructor" number="2">
<h1>Properties of the BigInt Constructor</h1>
<emu-clause id="sec-bigint-%symbol.custommatcher%" number="4">
<h1>BigInt [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. If _subject_ is not a BigInt and does not have a [[BigIntData]] internal slot, return *false*.
1. If _hint_ is *"boolean"*, return *true*.
1. If _subject_ is a BigInt, return CreateArrayFromList(« _subject_ »).
1. Return CreateArrayFromList(« _subject_.[[BigIntData]] »).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-note type="editor">
Another approach is to ignore boxed primitives and only match primitive values.
</emu-note>
</emu-clause>
<emu-clause id="sec-date-objects" number="4">
<h1>Date Objects</h1>
<emu-clause id="sec-properties-of-the-date-constructor" number="3">
<h1>Properties of the Date Constructor</h1>
<emu-clause id="sec-date-%symbol.custommatcher%" number="5">
<h1>Date [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_, ~boolean~).
1. If _subject_ does not have a [[DateValue]] internal slot, return *false*.
1. Return *true*.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause class="fold" id="sec-text-processing" number="22">
<h1>Text Processing</h1>
<emu-clause id="sec-string-objects" number="1">
<h1>String Objects</h1>
<emu-clause id="sec-properties-of-the-string-constructor" number="2">
<h1>Properties of the String Constructor</h1>
<emu-clause id="sec-string-%symbol.custommatcher%" number="5">
<h1>String [ %Symbol.customMatcher% ] ( _subject_, _hint_ )</h1>
<p>This function performs the following steps when called:</p>
<emu-alg>
1. Perform ? ValidateCustomMatcherHint(_hint_).
1. If _subject_ is not a String and does not have a [[StringData]] internal slot, return *false*.
1. If _hint_ is *"boolean"*, return *true*.
1. If _subject_ is a String, return CreateArrayFromList(« _subject_ »).
1. Return CreateArrayFromList(« _subject_.[[StringData]] »).
</emu-alg>