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<pre class="metadata">
title: Iterator Helpers
status: proposal
stage: 3
location: https://tc39.es/proposal-iterator-helpers
copyright: false
contributors: Gus Caplan
</pre>
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<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_ &lt; 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_ &lt; 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>