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<!DOCTYPE html>
<meta charset="utf-8">
<pre class="metadata">
title: Iterator Helpers
status: proposal
stage: 3
location: https://tc39.es/proposal-iterator-helpers
copyright: false
contributors: Gus Caplan
</pre>
<style>
ins.block {
font-weight: bold;
font-style: italic;
}
hr {
height: 0.25em;
background: #ccc;
border: 0;
margin: 2em 0;
}
.unicode-property-table {
table-layout: fixed;
width: 100%;
font-size: 80%;
}
.unicode-property-table ul {
padding-left: 0;
list-style: none;
}
#metadata-block {
margin: 4em 0;
padding: 10px;
border: 1px solid #ee8421;
}
#metadata-block h1 {
font-size: 1.5em;
margin-top: 0;
}
#metadata-block > ul {
list-style-type: none;
margin: 0; padding: 0;
}
</style>
<div id="metadata-block">
<h1>Contributing to this Proposal</h1>
<p>This proposal is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:</p>
<ul>
<li>GitHub Repository: <a href="https://github.com/tc39/proposal-iterator-helpers">https://github.com/tc39/proposal-iterator-helpers</a></li>
<li>Issues: <a href="https://github.com/tc39/proposal-iterator-helpers/issues">All Issues</a>, <a href="https://github.com/tc39/proposal-iterator-helpers/issues/new">File a New Issue</a></li>
<li>Pull Requests: <a href="https://github.com/tc39/proposal-iterator-helpers/pulls">All Pull Requests</a>, <a href="https://github.com/tc39/proposal-iterator-helpers/pulls/new">Create a New Pull Request</a></li>
<!-- <li>Test Suite: <a href="https://github.com/tc39/test262">Test262</a></li> -->
<li>
<p>Community:</p>
<ul>
<li>Discourse: <a href="https://es.discourse.group">https://es.discourse.group</a></li>
<li>Chat: <a href="https://github.com/tc39/how-we-work/blob/HEAD/matrix-guide.md">Matrix</a></li>
</ul>
</li>
</ul>
</div>
<emu-clause id="sec-well-known-intrinsic-objects">
<h1>Well-Known Intrinsic Objects</h1>
<emu-table id="table-7" caption="Well-Known Intrinsic Objects">
<table>
<tr>
<th>
Intrinsic Name
</th>
<th>
Global Name
</th>
<th>
ECMAScript Language Association
</th>
</tr>
<tr>
<td>
<ins>%Iterator%</ins>
</td>
<td>
<ins>`Iterator`</ins>
</td>
<td>
<ins>The `Iterator` constructor (<emu-xref href="#sec-iterator-constructor"></emu-xref>)</ins>
</td>
</tr>
<tr>
<td>
%IteratorPrototype%
</td>
<td>
<ins>`Iterator.prototype`</ins>
</td>
<td>
An object that all standard built-in iterator objects indirectly inherit from
<p><ins>The initial value of the *"prototype"* data property of %Iterator%; i.e., %Iterator.prototype%</ins></p>
</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-abstract-operations">
<h1>Abstract Operations</h1>
<emu-clause id="sec-SetterThatIgnoresPrototypeProperties" type="abstract operation">
<h1>
SetterThatIgnoresPrototypeProperties (
_this_: an ECMAScript language value,
_home_: an Object,
_p_: a property key,
_v_: an ECMAScript language value,
): either a normal completion containing ~unused~ or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. If _this_ is not an Object, then
1. Throw a *TypeError* exception.
1. If _this_ is _home_, then
1. NOTE: Throwing here emulates assignment to a non-writable data property on the _home_ object in strict mode code.
1. Throw a *TypeError* exception.
1. Let _desc_ be ? _this_.[[GetOwnProperty]](_p_).
1. If _desc_ is *undefined*, then
1. Perform ? CreateDataPropertyOrThrow(_this_, _p_, _v_).
1. Else,
1. Perform ? Set(_this_, _p_, _v_, *true*).
1. Return ~unused~.
</emu-alg>
</emu-clause>
<emu-clause id="sec-operations-on-iterator-objects">
<h1>Operations on Iterator Objects</h1>
<emu-clause id="sec-getiteratordirect" type="abstract operation">
<h1>
GetIteratorDirect (
_obj_: an Object,
): either a normal completion containing an Iterator Record or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Let _nextMethod_ be ? Get(_obj_, `"next"`).
1. Let _iteratorRecord_ be Record { [[Iterator]]: _obj_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
1. Return _iteratorRecord_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-getiteratorflattenable" type="abstract operation">
<h1>
GetIteratorFlattenable (
_obj_: an ECMAScript language value,
_stringHandling_: ~iterate-strings~ or ~reject-strings~,
): either a normal completion containing an Iterator Record or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. If _obj_ is not an Object, then
1. If _stringHandling_ is ~reject-strings~ or _obj_ is not a String, throw a *TypeError* exception.
1. Let _method_ be ? GetMethod(_obj_, @@iterator).
1. If _method_ is *undefined*, then
1. Let _iterator_ be _obj_.
1. Else,
1. Let _iterator_ be ? Call(_method_, _obj_).
1. If _iterator_ is not an Object, throw a *TypeError* exception.
1. Return ? GetIteratorDirect(_iterator_).
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-control-abstraction-objects">
<h1>Control Abstraction Objects</h1>
<emu-clause id="sec-iteration">
<h1>Iteration</h1>
<emu-clause id="sec-iterator-objects">
<h1>Iterator Objects</h1>
<emu-clause id="sec-iterator-constructor">
<h1>The Iterator Constructor</h1>
<p>The <dfn>Iterator</dfn> constructor:</p>
<ul>
<li>is <dfn>%Iterator%</dfn>.</li>
<li>is the initial value of the *"Iterator"* property of the global object.</li>
<li>is designed to be subclassable. It may be used as the value of an *extends* clause of a class definition.</li>
</ul>
<emu-clause id="sec-iterator">
<h1>Iterator ( )</h1>
<p>When the `Iterator` function is called, the following steps are taken:</p>
<emu-alg>
1. If NewTarget is *undefined* or the active function object, throw a *TypeError* exception.
1. Return ? OrdinaryCreateFromConstructor(NewTarget, *"%Iterator.prototype%"*).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-iterator-constructor">
<h1>Properties of the Iterator Constructor</h1>
<emu-clause id="sec-iterator.prototype">
<h1>Iterator.prototype</h1>
<p>The initial value of Iterator.prototype is %Iterator.prototype%.</p>
<p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>
</emu-clause>
<emu-clause id="sec-iterator.from">
<h1>Iterator.from ( _O_ )</h1>
<emu-alg>
1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_, ~iterate-strings~).
1. Let _hasInstance_ be ? OrdinaryHasInstance(%Iterator%, _iteratorRecord_.[[Iterator]]).
1. If _hasInstance_ is *true*, then
1. Return _iteratorRecord_.[[Iterator]].
1. Let _wrapper_ be OrdinaryObjectCreate(%WrapForValidIteratorPrototype%, « [[Iterated]] »).
1. Set _wrapper_.[[Iterated]] to _iteratorRecord_.
1. Return _wrapper_.
</emu-alg>
<emu-clause id="sec-wrapforvaliditeratorprototype-object">
<h1>The %WrapForValidIteratorPrototype% Object</h1>
<p>The <dfn>%WrapForValidIteratorPrototype%</dfn> object:</p>
<ul>
<li>has a [[Prototype]] internal slot whose value is %Iterator.prototype%.</li>
</ul>
<emu-clause id="sec-wrapforvaliditeratorprototype.next">
<h1>%WrapForValidIteratorPrototype%.next ( )</h1>
<emu-alg>
1. Let _O_ be *this* value.
1. Perform ? RequireInternalSlot(_O_, [[Iterated]]).
1. Let _iteratorRecord_ be _O_.[[Iterated]].
1. Return ? Call(_iteratorRecord_.[[NextMethod]], _iteratorRecord_.[[Iterator]]).
</emu-alg>
</emu-clause>
<emu-clause id="sec-wrapforvaliditeratorprototype.return">
<h1>%WrapForValidIteratorPrototype%.return ( )</h1>
<emu-alg>
1. Let _O_ be *this* value.
1. Perform ? RequireInternalSlot(_O_, [[Iterated]]).
1. Let _iterator_ be _O_.[[Iterated]].[[Iterator]].
1. Assert: _iterator_ is an Object.
1. Let _returnMethod_ be ? GetMethod(_iterator_, *"return"*).
1. If _returnMethod_ is *undefined*, then
1. Return CreateIterResultObject(*undefined*, *true*).
1. Return ? Call(_returnMethod_, _iterator_).
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-iterator-helper-objects">
<h1>Iterator Helper Objects</h1>
<p>An Iterator Helper object is an ordinary object that represents a lazy transformation of some specific source iterator object. There is not a named constructor for Iterator Helper objects. Instead, Iterator Helper objects are created by calling certain methods of Iterator instance objects.</p>
<emu-clause id="sec-%iteratorhelperprototype%-object">
<h1>The %IteratorHelperPrototype% Object</h1>
<p>The <dfn>%IteratorHelperPrototype%</dfn> object:</p>
<ul>
<li>has properties that are inherited by all Iterator Helper Objects.</li>
<li>is an ordinary object.</li>
<li>has a [[Prototype]] internal slot whose value is %Iterator.prototype%.</li>
<li>has the following properties:</li>
</ul>
<emu-clause id="sec-%iteratorhelperprototype%.next">
<h1>%IteratorHelperPrototype%.next ( )</h1>
<emu-alg>
1. Return ? GeneratorResume(*this* value, *undefined*, *"Iterator Helper"*).
</emu-alg>
</emu-clause>
<emu-clause id="sec-%iteratorhelperprototype%.return">
<h1>%IteratorHelperPrototype%.return ( )</h1>
<emu-alg>
1. Let _O_ be *this* value.
1. Perform ? RequireInternalSlot(_O_, [[UnderlyingIterator]]).
1. Assert: _O_ has a [[GeneratorState]] slot.
1. If _O_.[[GeneratorState]] is ~suspended-start~, then
1. Set _O_.[[GeneratorState]] to ~completed~.
1. NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with _O_ can be discarded at this point.
1. Perform ? IteratorClose(_O_.[[UnderlyingIterator]], NormalCompletion(~unused~)).
1. Return CreateIterResultObject(*undefined*, *true*).
1. Let _C_ be Completion { [[Type]]: ~return~, [[Value]]: *undefined*, [[Target]]: ~empty~ }.
1. Return ? GeneratorResumeAbrupt(_O_, _C_, *"Iterator Helper"*).
</emu-alg>
</emu-clause>
<emu-clause id="sec-%iteratorhelperprototype%-@@tostringtag">
<h1>%IteratorHelperPrototype% [ @@toStringTag ]</h1>
<p>The initial value of the @@toStringTag property is the String value *"Iterator Helper"*.</p>
<p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.</p>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-iteratorprototype">
<h1>Iterator.prototype</h1>
<p>The <dfn>Iterator prototype object</dfn>:</p>
<ul>
<li>is <dfn>%Iterator.prototype%</dfn>.</li>
<li>has a [[Prototype]] internal slot whose value is %Object.prototype%.</li>
<li>is an ordinary object.</li>
</ul>
<emu-clause id="sec-iteratorprototype.constructor">
<h1>Iterator.prototype.constructor</h1>
<p>`Iterator.prototype.constructor` is an accessor property with attributes { [[Enumerable]]: *false*, [[Configurable]]: *true* }. The [[Get]] and [[Set]] attributes are defined as follows:</p>
<emu-clause id="sec-get-iteratorprototype-constructor">
<h1>get Iterator.prototype.constructor</h1>
<p>The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:</p>
<emu-alg>
1. Return %Iterator%.
</emu-alg>
</emu-clause>
<emu-clause id="sec-set-iteratorprototype-constructor">
<h1>set Iterator.prototype.constructor</h1>
<p>The value of the [[Set]] attribute is a built-in function that takes an argument _v_. It performs the following steps when called:</p>
<emu-alg>
1. Perform ? SetterThatIgnoresPrototypeProperties(*this* value, %Iterator.prototype%, *"constructor"*, _v_).
1. Return *undefined*.
</emu-alg>
</emu-clause>
<emu-note>
<p>Unlike the *"constructor"* property on most built-in prototypes, for web-compatibility reasons this property must be an accessor.</p>
</emu-note>
</emu-clause>
<emu-clause id="sec-iteratorprototype.map">
<h1>Iterator.prototype.map ( _mapper_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _mapper_ and performs the following steps when called:
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_mapped_, _iterated_).
1. Let _completion_ be Completion(Yield(_mapped_)).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
1. Set _result_.[[UnderlyingIterator]] to _iterated_.
1. Return _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.filter">
<h1>Iterator.prototype.filter ( _predicate_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_predicate_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _predicate_ and performs the following steps when called:
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _selected_ be Completion(Call(_predicate_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_selected_, _iterated_).
1. If ToBoolean(_selected_) is *true*, then
1. Let _completion_ be Completion(Yield(_value_)).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
1. Set _result_.[[UnderlyingIterator]] to _iterated_.
1. Return _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.take">
<h1>Iterator.prototype.take ( _limit_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. Let _numLimit_ be ? ToNumber(_limit_).
1. If _numLimit_ is *NaN*, throw a *RangeError* exception.
1. Let _integerLimit_ be ! ToIntegerOrInfinity(_numLimit_).
1. If _integerLimit_ < 0, throw a *RangeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _integerLimit_ and performs the following steps when called:
1. Let _remaining_ be _integerLimit_.
1. Repeat,
1. If _remaining_ is 0, then
1. Return ? IteratorClose(_iterated_, NormalCompletion(*undefined*)).
1. If _remaining_ is not +∞, then
1. Set _remaining_ to _remaining_ - 1.
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _completion_ be Completion(Yield(_value_)).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
1. Set _result_.[[UnderlyingIterator]] to _iterated_.
1. Return _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.drop">
<h1>Iterator.prototype.drop ( _limit_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. Let _numLimit_ be ? ToNumber(_limit_).
1. If _numLimit_ is *NaN*, throw a *RangeError* exception.
1. Let _integerLimit_ be ! ToIntegerOrInfinity(_numLimit_).
1. If _integerLimit_ < 0, throw a *RangeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _integerLimit_ and performs the following steps when called:
1. Let _remaining_ be _integerLimit_.
1. Repeat, while _remaining_ > 0,
1. If _remaining_ is not +∞, then
1. Set _remaining_ to _remaining_ - 1.
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _completion_ be Completion(Yield(_value_)).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
1. Set _result_.[[UnderlyingIterator]] to _iterated_.
1. Return _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.flatmap">
<h1>Iterator.prototype.flatMap ( _mapper_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _mapper_ and performs the following steps when called:
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_mapped_, _iterated_).
1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~reject-strings~)).
1. IfAbruptCloseIterator(_innerIterator_, _iterated_).
1. Let _innerAlive_ be *true*.
1. Repeat, while _innerAlive_ is *true*,
1. Let _innerValue_ be Completion(IteratorStepValue(_innerIterator_)).
1. IfAbruptCloseIterator(_innerValue_, _iterated_).
1. If _innerValue_ is ~done~, then
1. Set _innerAlive_ to *false*.
1. Else,
1. Let _completion_ be Completion(Yield(_innerValue_)).
1. If _completion_ is an abrupt completion, then
1. Let _backupCompletion_ be Completion(IteratorClose(_innerIterator_, _completion_)).
1. IfAbruptCloseIterator(_backupCompletion_, _iterated_).
1. Return ? IteratorClose(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
1. Set _result_.[[UnderlyingIterator]] to _iterated_.
1. Return _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.reduce">
<h1>Iterator.prototype.reduce ( _reducer_ [ , _initialValue_ ] )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_reducer_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. If _initialValue_ is not present, then
1. Let _accumulator_ be ? IteratorStepValue(_iterated_).
1. If _accumulator_ is ~done~, throw a *TypeError* exception.
1. Let _counter_ be 1.
1. Else,
1. Let _accumulator_ be _initialValue_.
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return _accumulator_.
1. Let _result_ be Completion(Call(_reducer_, *undefined*, « _accumulator_, _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. Set _accumulator_ to _result_.[[Value]].
1. Set _counter_ to _counter_ + 1.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.toarray">
<h1>Iterator.prototype.toArray ( )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _items_ be a new empty List.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return CreateArrayFromList(_items_).
1. Append _value_ to _items_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.foreach">
<h1>Iterator.prototype.forEach ( _fn_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_fn_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _result_ be Completion(Call(_fn_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. Set _counter_ to _counter_ + 1.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.some">
<h1>Iterator.prototype.some ( _predicate_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_predicate_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *false*.
1. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *true*, return ? IteratorClose(_iterated_, NormalCompletion(*true*)).
1. Set _counter_ to _counter_ + 1.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.every">
<h1>Iterator.prototype.every ( _predicate_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_predicate_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *true*.
1. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *false*, return ? IteratorClose(_iterated_, NormalCompletion(*false*)).
1. Set _counter_ to _counter_ + 1.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.find">
<h1>Iterator.prototype.find ( _predicate_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _O_ be the *this* value.
1. If _O_ is not an Object, throw a *TypeError* exception.
1. If IsCallable(_predicate_) is *false*, throw a *TypeError* exception.
1. Let _iterated_ be ? GetIteratorDirect(_O_).
1. Let _counter_ be 0.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *true*, return ? IteratorClose(_iterated_, NormalCompletion(_value_)).
1. Set _counter_ to _counter_ + 1.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype-@@tostringtag">
<h1>Iterator.prototype [ @@toStringTag ]</h1>
<p>`Iterator.prototype[@@toStringTag]` is an accessor property with attributes { [[Enumerable]]: *false*, [[Configurable]]: *true* }. The [[Get]] and [[Set]] attributes are defined as follows:</p>
<emu-clause id="sec-get-iteratorprototype-@@tostringtag">
<h1>get Iterator.prototype [ @@toStringTag ]</h1>
<p>The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:</p>
<emu-alg>
1. Return *"Iterator"*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-set-iteratorprototype-@@tostringtag">
<h1>set Iterator.prototype [ @@toStringTag ]</h1>
<p>The value of the [[Set]] attribute is a built-in function that takes an argument _v_. It performs the following steps when called:</p>
<emu-alg>
1. Perform ? SetterThatIgnoresPrototypeProperties(*this* value, %Iterator.prototype%, %Symbol.toStringTag%, _v_).
1. Return *undefined*.
</emu-alg>
</emu-clause>
<emu-note>
<p>Unlike the @@toStringTag property on most built-in prototypes, for web-compatibility reasons this property must be an accessor.</p>
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="updated-abstract-operations">
<h1>Updated Abstract Operations</h1>
<emu-clause id="sec-createiteratorfromclosure" type="abstract operation">
<h1>
CreateIteratorFromClosure (
_closure_: an Abstract Closure with no parameters,
_generatorBrand_: a String or ~empty~,
_generatorPrototype_: an Object,
<ins>optional _extraSlots_: a List of names of internal slots,</ins>
): a Generator
</h1>
<dl class="header">
</dl>
<emu-alg>
1. NOTE: _closure_ can contain uses of the Yield operation to yield an IteratorResult object.
1. <ins>If _extraSlots_ is not present, set _extraSlots_ to a new empty List.</ins>
1. Let _internalSlotsList_ be <ins>the list-concatenation of _extraSlots_ and</ins> « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
1. Let _generator_ be OrdinaryObjectCreate(_generatorPrototype_, _internalSlotsList_).
1. Set _generator_.[[GeneratorBrand]] to _generatorBrand_.
1. Set _generator_.[[GeneratorState]] to *undefined*.
1. Let _callerContext_ be the running execution context.
1. Let _calleeContext_ be a new execution context.
1. Set the Function of _calleeContext_ to *null*.
1. Set the Realm of _calleeContext_ to the current Realm Record.
1. Set the ScriptOrModule of _calleeContext_ to _callerContext_'s ScriptOrModule.
1. If _callerContext_ is not already suspended, suspend _callerContext_.
1. Push _calleeContext_ onto the execution context stack; _calleeContext_ is now the running execution context.
1. Perform GeneratorStart(_generator_, _closure_).
1. Remove _calleeContext_ from the execution context stack and restore _callerContext_ as the running execution context.
1. Return _generator_.
</emu-alg>
</emu-clause>
</emu-clause>