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<!DOCTYPE html>
<meta charset="utf-8">
<pre class="metadata">
title: Iterator Helpers
status: proposal
stage: 2
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>%AsyncIterator%</ins>
</td>
<td>
<ins>`AsyncIterator`</ins>
</td>
<td>
<ins>The `AsyncIterator` constructor (<emu-xref href="#sec-asynciterator-constructor"></emu-xref>)</ins>
</td>
</tr>
<tr>
<td>
%AsyncIteratorPrototype%
</td>
<td>
<ins>`AsyncIterator.prototype`</ins>
</td>
<td>
An object that all standard built-in async iterator objects indirectly inherit from
<p><ins>The initial value of the *"prototype"* data property of %AsyncIterator%; i.e., %AsyncIterator.prototype%</ins></p>
</td>
</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-built-in-function-objects">
<h1>Built-in Function Objects</h1>
<p>
<del>If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:</del>
<ins>Unless otherwise specified, a built-in function object must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:</ins>
</p>
<emu-clause id="sec-createbuiltinfunction" type="abstract operation">
<h1>
CreateBuiltinFunction (
_behaviour_: an Abstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification,
_length_: a non-negative integer or +∞,
_name_: a property key or a Private Name,
_additionalInternalSlotsList_: a List of names of internal slots,
optional _realm_: a Realm Record,
optional _prototype_: an Object or *null*,
optional _prefix_: a String,
): a function object
</h1>
<dl class="header">
<dt>description</dt>
<dd>_additionalInternalSlotsList_ contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object.</dd>
</dl>
<emu-alg>
1. If _realm_ is not present, set _realm_ to the current Realm Record.
1. If _prototype_ is not present, set _prototype_ to _realm_.[[Intrinsics]].[[%Function.prototype%]].
1. Let _internalSlotsList_ be a List containing the names of all the internal slots that <emu-xref href="#sec-built-in-function-objects"></emu-xref> requires for the built-in function object that is about to be created.
1. Append to _internalSlotsList_ the elements of _additionalInternalSlotsList_.
1. <ins>If _behaviour_ is described as async, then</ins>
1. <ins>Let _func_ be a new built-in async function object that, when called, performs the action described by _behaviour_ using the provided arguments as the values of the corresponding parameters specified by _behaviour_. The new function object has internal slots whose names are the elements of _internalSlotsList_, and an [[InitialName]] internal slot.</ins>
1. <ins>Else,</ins>
1. Let _func_ be a new built-in function object that, when called, performs the action described by _behaviour_ using the provided arguments as the values of the corresponding parameters specified by _behaviour_. The new function object has internal slots whose names are the elements of _internalSlotsList_, and an [[InitialName]] internal slot.
1. Set _func_.[[Prototype]] to _prototype_.
1. Set _func_.[[Extensible]] to *true*.
1. Set _func_.[[Realm]] to _realm_.
1. Set _func_.[[InitialName]] to *null*.
1. Perform SetFunctionLength(_func_, _length_).
1. If _prefix_ is not present, then
1. Perform SetFunctionName(_func_, _name_).
1. Else,
1. Perform SetFunctionName(_func_, _name_, _prefix_).
1. Return _func_.
</emu-alg>
<p>Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.</p>
</emu-clause>
</emu-clause>
<emu-clause id="sec-built-in-async-function-objects">
<h1>Built-in Async Function Objects</h1>
<p><dfn variants="built-in async function object,built-in async function objects">Built-in async function objects</dfn> are built-in function objects that provide alternative [[Call]] and [[Construct]] internal methods that conform to the following definitions:</p>
<emu-clause id="sec-built-in-function-objects-call-thisargument-argumentslist" type="internal method">
<h1>
[[Call]] (
_thisArgument_: an ECMAScript language value,
_argumentsList_: a List of ECMAScript language values,
): a normal completion containing an ECMAScript language value
</h1>
<dl class="header">
<dt>for</dt>
<dd>a built-in async function object _F_</dd>
</dl>
<emu-alg>
1. Let _callerContext_ be the running execution context.
1. If _callerContext_ is not already suspended, suspend _callerContext_.
1. Let _calleeContext_ be a new execution context.
1. Set the Function of _calleeContext_ to _F_.
1. Let _calleeRealm_ be _F_.[[Realm]].
1. Set the Realm of _calleeContext_ to _calleeRealm_.
1. Set the ScriptOrModule of _calleeContext_ to *null*.
1. Perform any necessary implementation-defined initialization of _calleeContext_.
1. Push _calleeContext_ onto the execution context stack; _calleeContext_ is now the running execution context.
1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
1. Let _resultsClosure_ be a new Abstract Closure that captures _F_, _thisArgument_, and _argumentsList_ and performs the following steps when called:
1. Return the result of evaluating _F_ in a manner that conforms to the specification of _F_. _thisArgument_ is the *this* value, _argumentsList_ provides the named parameters, and the NewTarget value is *undefined*.
1. Perform AsyncFunctionStart(_promiseCapability_, _resultsClosure_).
1. Remove _calleeContext_ from the execution context stack and restore _callerContext_ as the running execution context.
1. Return _promiseCapability_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-built-in-function-objects-construct-argumentslist-newtarget" type="internal method">
<h1>
[[Construct]] (
_argumentsList_: a List of ECMAScript language values,
_newTarget_: a constructor,
): either a normal completion containing an Object or a throw completion
</h1>
<dl class="header">
<dt>for</dt>
<dd>a built-in async function object _F_</dd>
</dl>
<emu-alg>
1. Throw a *TypeError* exception.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-abstract-operations">
<h1>Abstract Operations</h1>
<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 ECMAScript language value,
): 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, throw a *TypeError* exception.
1. Let _nextMethod_ be ? Get(_obj_, `"next"`).
1. If IsCallable(_nextMethod_) is *false*, throw a *TypeError* exception.
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,
_hint_: ~sync~ or ~async~,
): 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, throw a *TypeError* exception.
1. Let _alreadyAsync_ be *false*.
1. Let _method_ be *undefined*.
1. If _hint_ is ~async~, then
1. Set _method_ to ? Get(_obj_, @@asyncIterator).
1. Set _alreadyAsync_ to *true*.
1. If IsCallable(_method_) is *false*, then
1. Set _method_ to ? Get(_obj_, @@iterator).
1. Set _alreadyAsync_ to *false*.
1. If IsCallable(_method_) is *false*, then
1. Let _iterator_ be _obj_.
1. Set _alreadyAsync_ to *true*.
1. Else,
1. Let _iterator_ be ? Call(_method_, _obj_).
1. Let _nextMethod_ be ? GetV(_iterator_, *"next"*).
1. If IsCallable(_nextMethod_) is *false*, throw a *TypeError* exception.
1. Let _iteratorRecord_ be the Iterator Record { [[Iterator]]: _iterator_, [[NextMethod]]: _nextMethod_, [[Done]]: *false* }.
1. If _hint_ is ~async~ and _alreadyAsync_ is *false*, then
1. Return CreateAsyncFromSyncIterator(_iteratorRecord_).
1. Return _iteratorRecord_.
</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-abstract-operations">
<h1>Iterator Abstract Operations</h1>
<emu-clause id="sec-ifabruptcloseasynciterator" aoid="IfAbruptCloseAsyncIterator">
<h1>IfAbruptCloseAsyncIterator ( _value_, _iteratorRecord_ )</h1>
<p>IfAbruptCloseAsyncIterator is a shorthand for a sequence of algorithm steps that use an Iterator Record. An algorithm step of the form:</p>
<emu-alg>
1. IfAbruptCloseAsyncIterator(_value_, _iteratorRecord_).
</emu-alg>
<p>means the same thing as:</p>
<emu-alg>
1. If _value_ is an abrupt completion, then
1. Perform ? AsyncIteratorClose(_iteratorRecord_, _value_).
1. Return _value_.
1. Else if _value_ is a Completion Record, set _value_ to _value_.[[Value]].
</emu-alg>
</emu-clause>
<emu-clause id="sec-awaitnonprimitive" type="abstract operation">
<h1>
AwaitNonPrimitive (
_value_: an ECMAScript language value,
): either a normal completion containing an ECMAScript language value or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. If _value_ is an Object, return ? Await(_value_).
1. Else, return _value_.
</emu-alg>
</emu-clause>
</emu-clause>
<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_, ~sync~).
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-asynciterator-objects">
<h1>AsyncIterator Objects</h1>
<emu-clause id="sec-asynciterator-constructor">
<h1>The AsyncIterator Constructor</h1>
<p>The <dfn>AsyncIterator</dfn> constructor:</p>
<ul>
<li>is <dfn>%AsyncIterator%</dfn>.</li>
<li>is the initial value of the *AsyncIterator* 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-asynciterator">
<h1>AsyncIterator ( )</h1>
<p>When the `AsyncIterator` 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, *"%AsyncIterator.prototype%"*).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-asynciterator-constructor">
<h1>Properties of the AsyncIterator Constructor</h1>
<emu-clause id="sec-asynciterator.prototype">
<h1>AsyncIterator.prototype</h1>
<p>The initial value of AsyncIterator.prototype is %AsyncIterator.prototype%.</p>
<p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>
</emu-clause>
<emu-clause id="sec-asynciterator.from">
<h1>AsyncIterator.from ( _O_ )</h1>
<emu-alg>
1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_, ~async~).
1. Let _hasInstance_ be ? OrdinaryHasInstance(%AsyncIterator%, _iteratorRecord_.[[Iterator]]).
1. If _hasInstance_ is *true*, then
1. Return _iteratorRecord_.[[Iterator]].
1. Let _wrapper_ be OrdinaryObjectCreate(%WrapForValidAsyncIteratorPrototype%, « [[AsyncIterated]] »).
1. Set _wrapper_.[[AsyncIterated]] to _iteratorRecord_.
1. Return _wrapper_.
</emu-alg>
<emu-clause id="sec-wrapforvalidasynciteratorprototype-object">
<h1>The %WrapForValidAsyncIteratorPrototype% Object</h1>
<p>The <dfn>%WrapForValidAsyncIteratorPrototype%</dfn> object:</p>
<ul>
<li>is an ordinary object.</li>
<li>has a [[Prototype]] internal slot whose value is %AsyncIterator.prototype%.</li>
<li>has the following properties:</li>
</ul>
<emu-clause id="sec-wrapforvalidasynciteratorprototype.next">
<h1>%WrapForValidAsyncIteratorPrototype%.next ( )</h1>
<emu-alg>
1. Let _O_ be *this* value.
1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
1. Let _check_ be Completion(RequireInternalSlot(_O_, [[AsyncIterated]])).
1. IfAbruptRejectPromise(_check_, _promiseCapability_).
1. Let _result_ be Completion(IteratorNext(_O_.[[AsyncIterated]])).
1. IfAbruptRejectPromise(_result_, _promiseCapability_).
1. Return ? PromiseResolve(%Promise%, _result_).
</emu-alg>
</emu-clause>
<emu-clause id="sec-wrapforvalidasynciteratorprototype.return">
<h1>%WrapForValidAsyncIteratorPrototype%.return ( )</h1>
<emu-alg>
1. Let _O_ be *this* value.
1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
1. Let _check_ be Completion(RequireInternalSlot(_O_, [[AsyncIterated]])).
1. IfAbruptRejectPromise(_check_, _promiseCapability_).
1. Let _iterator_ be _O_.[[AsyncIterated]].[[Iterator]].
1. Assert: _iterator_ is an Object.
1. Let _returnMethod_ be Completion(GetMethod(_iterator_, *"return"*)).
1. IfAbruptRejectPromise(_returnMethod_, _promiseCapability_).
1. If _returnMethod_ is *undefined*, then
1. Return ! PromiseResolve(%Promise%, CreateIterResultObject(*undefined*, *true*)).
1. Let _result_ be Completion(Call(_returnMethod_, _iterator_)).
1. IfAbruptRejectPromise(_result_, _promiseCapability_).
1. Return ? PromiseResolve(%Promise%, _result_).
</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 _C_ be Completion { [[Type]]: ~return~, [[Value]]: *undefined*, [[Target]]: ~empty~ }.
1. Return ? GeneratorResumeAbrupt(*this* value, _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-async-iterator-helper-objects">
<h1>Async Iterator Helper Objects</h1>
<p>An Async Iterator Helper object is an ordinary object that represents a lazy transformation of some specific source async iterator object. There is not a named constructor for Async Iterator Helper objects. Instead, Async Iterator Helper objects are created by calling certain methods of AsyncIterator instance objects.</p>
<emu-clause id="sec-%asynciteratorhelperprototype%-object">
<h1>The %AsyncIteratorHelperPrototype% Object</h1>
<p>The <dfn>%AsyncIteratorHelperPrototype%</dfn> object:</p>
<ul>
<li>has properties that are inherited by all Async Iterator Helper Objects.</li>
<li>is an ordinary object.</li>
<li>has a [[Prototype]] internal slot whose value is %AsyncIterator.prototype%.</li>
<li>has the following properties:</li>
</ul>
<emu-clause id="sec-%asynciteratorhelperprototype%.next">
<h1>%AsyncIteratorHelperPrototype%.next ( )</h1>
<emu-alg>
1. Return AsyncGeneratorNext(*this* value, *"Async Iterator Helper"*, *undefined*).
</emu-alg>
</emu-clause>
<emu-clause id="sec-%asynciteratorhelperprototype%.return">
<h1>%AsyncIteratorHelperPrototype%.return ( )</h1>
<emu-alg>
1. Return AsyncGeneratorReturn(*this* value, *"Async Iterator Helper"*, *undefined*).
</emu-alg>
</emu-clause>
<emu-clause id="sec-%asynciteratorhelperprototype%-@@tostringtag">
<h1>%AsyncIteratorHelperPrototype% [ @@toStringTag ]</h1>
<p>The initial value of the @@toStringTag property is the String value *"Async 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>The initial value of %Iterator.prototype%.constructor is %Iterator%.</p>
</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 _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
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 _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
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. Return CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.filter">
<h1>Iterator.prototype.filter ( _filterer_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_filterer_) is *false*, throw a *TypeError* exception.
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _filterer_ and performs the following steps when called:
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _selected_ be Completion(Call(_filterer_, *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. Return CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%).
</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 _iterated_ be ? GetIteratorDirect(*this* value).
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 _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 _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _completion_ be Completion(Yield(? IteratorValue(_next_))).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Return CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%).
</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 _iterated_ be ? GetIteratorDirect(*this* value).
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 _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 _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _completion_ be Completion(Yield(? IteratorValue(_next_))).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Return CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%).
</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 _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
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 _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseIterator(_mapped_, _iterated_).
1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~sync~)).
1. IfAbruptCloseIterator(_innerIterator_, _iterated_).
1. Let _innerAlive_ be *true*.
1. Repeat, while _innerAlive_ is *true*,
1. Let _innerNext_ be Completion(IteratorStep(_innerIterator_)).
1. IfAbruptCloseIterator(_innerNext_, _iterated_).
1. If _innerNext_ is *false*, then
1. Set _innerAlive_ to *false*.
1. Else,
1. Let _innerValue_ be Completion(IteratorValue(_innerNext_)).
1. IfAbruptCloseIterator(_innerValue_, _iterated_).
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. Return CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%).
</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 _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_reducer_) is *false*, throw a *TypeError* exception.
1. If _initialValue_ is not present, then
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, throw a *TypeError* exception.
1. Let _accumulator_ be ? IteratorValue(_next_).
1. Else,
1. Let _accumulator_ be _initialValue_.
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return _accumulator_.
1. Let _value_ be ? IteratorValue(_next_).
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 _iterated_ be ? GetIteratorDirect(*this* value).
1. Let _items_ be a new empty List.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return CreateArrayFromList(_items_).
1. Let _value_ be ? IteratorValue(_next_).
1. Append _value_ to _items_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-iteratorprototype.toasync">
<h1>Iterator.prototype.toAsync ( )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _syncIteratorRecord_ be ? GetIteratorDirect(*this* value).
1. Let _asyncIteratorRecord_ be CreateAsyncFromSyncIterator(_syncIteratorRecord_).
1. Let _wrapper_ be OrdinaryObjectCreate(%WrapForValidAsyncIteratorPrototype%, « [[AsyncIterated]] »).
1. Set _wrapper_.[[AsyncIterated]] to _asyncIteratorRecord_.
1. Return _wrapper_.
</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 _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_fn_) is *false*, throw a *TypeError* exception.
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
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 ( _fn_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_fn_) is *false*, throw a *TypeError* exception.
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *false*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _result_ be Completion(Call(_fn_, *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 ( _fn_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_fn_) is *false*, throw a *TypeError* exception.
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *true*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _result_ be Completion(Call(_fn_, *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 ( _fn_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_fn_) is *false*, throw a *TypeError* exception.
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is *false*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _result_ be Completion(Call(_fn_, *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>The initial value of the @@toStringTag property is the String value "Iterator".</p>
<emu-note>
<p>Unlike the @@toStringTag on most built-in classes, for web-compatibility reasons this property must be writable.</p>
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype">
<h1>AsyncIterator.prototype</h1>
<p>The <dfn>AsyncIterator prototype object</dfn>:</p>
<ul>
<li>is <dfn>%AsyncIterator.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-asynciteratorprototype.constructor">
<h1>AsyncIterator.prototype.constructor</h1>
<p>The initial value of AsyncIterator.prototype.constructor is %AsyncIterator%.</p>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.map">
<h1>AsyncIterator.prototype.map ( _mapper_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
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 _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
1. Set _mapped_ to Completion(AwaitNonPrimitive(_mapped_)).
1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
1. Let _completion_ be Completion(Yield(_mapped_)).
1. IfAbruptCloseAsyncIterator(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Return CreateAsyncIteratorFromClosure(_closure_, *"Async Iterator Helper"*, %AsyncIteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.filter">
<h1>AsyncIterator.prototype.filter ( _filterer_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_filterer_) is *false*, throw a *TypeError* exception.
1. Let _closure_ be a new Abstract Closure with no parameters that captures _iterated_ and _filterer_ and performs the following steps when called:
1. Let _counter_ be 0.
1. Repeat,
1. Let _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _selected_ be Completion(Call(_filterer_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseAsyncIterator(_selected_, _iterated_).
1. Set _selected_ to Completion(AwaitNonPrimitive(_selected_)).
1. IfAbruptCloseAsyncIterator(_selected_, _iterated_).
1. If ToBoolean(_selected_) is *true*, then
1. Let _completion_ be Completion(Yield(_value_)).
1. IfAbruptCloseAsyncIterator(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Return CreateAsyncIteratorFromClosure(_closure_, *"Async Iterator Helper"*, %AsyncIteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.take">
<h1>AsyncIterator.prototype.take ( _limit_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
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 _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 ? AsyncIteratorClose(_iterated_, NormalCompletion(*undefined*)).
1. If _remaining_ is not +∞, then
1. Set _remaining_ to _remaining_ - 1.
1. Let _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Let _completion_ be Completion(Yield(? IteratorValue(_next_))).
1. IfAbruptCloseAsyncIterator(_completion_, _iterated_).
1. Return CreateAsyncIteratorFromClosure(_closure_, *"Async Iterator Helper"*, %AsyncIteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.drop">
<h1>AsyncIterator.prototype.drop ( _limit_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
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 _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 ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Repeat,
1. Let _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Let _completion_ be Completion(Yield(? IteratorValue(_next_))).
1. IfAbruptCloseAsyncIterator(_completion_, _iterated_).
1. Return CreateAsyncIteratorFromClosure(_closure_, *"Async Iterator Helper"*, %AsyncIteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.flatmap">
<h1>AsyncIterator.prototype.flatMap ( _mapper_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_mapper_) is *false*, throw a *TypeError* exception.
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 _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, return *undefined*.
1. Let _value_ be ? IteratorValue(_next_).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
1. Set _mapped_ to Completion(AwaitNonPrimitive(_mapped_)).
1. IfAbruptCloseAsyncIterator(_mapped_, _iterated_).
1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~async~)).
1. IfAbruptCloseAsyncIterator(_innerIterator_, _iterated_).
1. Let _innerAlive_ be *true*.
1. Repeat, while _innerAlive_ is *true*,
1. Let _innerNextPromise_ be Completion(IteratorNext(_innerIterator_)).
1. IfAbruptCloseAsyncIterator(_innerNextPromise_, _iterated_).
1. Let _innerNext_ be Completion(Await(_innerNextPromise_)).
1. IfAbruptCloseAsyncIterator(_innerNext_, _iterated_).
1. Let _innerComplete_ be Completion(IteratorComplete(_innerNext_)).
1. IfAbruptCloseAsyncIterator(_innerComplete_, _iterated_).
1. If _innerComplete_ is *true*, then
1. Set _innerAlive_ to *false*.
1. Else,
1. Let _innerValue_ be Completion(IteratorValue(_innerNext_)).
1. IfAbruptCloseAsyncIterator(_innerValue_, _iterated_).
1. [id="step-async-iterator-flatmap-yield"] Let _completion_ be Completion(Yield(_innerValue_)).
1. If _completion_ is a return completion, then
1. Let _backupCompletion_ be Completion(IteratorClose(_innerIterator_, _completion_)).
1. IfAbruptCloseIterator(_backupCompletion_, _iterated_).
1. Return ? IteratorClose(_completion_, _iterated_).
1. Else if _completion_ is a throw completion, then
1. Assert: Awaiting _innerValue_ during the Yield on step <emu-xref href="#step-async-iterator-flatmap-yield"></emu-xref> threw.
1. Return ? IteratorClose(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Return CreateAsyncIteratorFromClosure(_closure_, *"Async Iterator Helper"*, %AsyncIteratorHelperPrototype%).
</emu-alg>
</emu-clause>
<emu-clause id="sec-asynciteratorprototype.reduce">
<h1>AsyncIterator.prototype.reduce ( _reducer_ [ , _initialValue_ ] )</h1>
<p>This async method performs the following steps when called:</p>
<emu-alg>
1. Let _iterated_ be ? GetIteratorDirect(*this* value).
1. If IsCallable(_reducer_) is *false*, throw a *TypeError* exception.
1. If _initialValue_ is not present, then
1. Let _next_ be ? Await(? IteratorNext(_iterated_)).
1. If ? IteratorComplete(_next_) is *true*, throw a *TypeError* exception.
1. Let _accumulator_ be ? IteratorValue(_next_).