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Normative: use Numbers instead of reals as counters in Iterator builtins #3776

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Normative: use Numbers instead of reals as counters in Iterator builtins
michaelficarra Mar 26, 2026
20e6fb7
fix syntax error
michaelficarra Apr 30, 2026
635a2b1
oops
michaelficarra May 1, 2026
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70 changes: 37 additions & 33 deletions spec.html
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Expand Up @@ -48683,16 +48683,18 @@ <h1>Iterator.prototype.drop ( _limit_ )</h1>
1. If _numLimit_ is *NaN*, then
1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. If _numLimit_ is finite and _numLimit_ > 𝔽(2<sup>53</sup> - 1), then
1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Let _integerLimit_ be ! ToIntegerOrInfinity(_numLimit_).
1. If _integerLimit_ &lt; 0, then
1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
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_ ≠ +∞, then
1. Set _remaining_ to _remaining_ - 1.
1. Let _remaining_ be 𝔽(_integerLimit_).
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@nicolo-ribaudo nicolo-ribaudo May 5, 2026

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Given the check above that _numLimit_ is either Infinity or a safe integer, what is the observable difference between doing all of these steps in the floats space vs mathematical values space?

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@michaelficarra michaelficarra May 5, 2026

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I don't think there is one. Just makes it very obvious to implementers that it's okay to do all of this with floats.

1. Repeat, while _remaining_ > *+0*<sub>𝔽</sub>,
1. Set _remaining_ to _remaining_ - *1*<sub>𝔽</sub>.
1. Let _next_ be ? IteratorStep(_iterated_).
1. If _next_ is ~done~, return ReturnCompletion(*undefined*).
1. Repeat,
Expand All @@ -48717,14 +48719,14 @@ <h1>Iterator.prototype.every ( _predicate_ )</h1>
1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
1. Let _counter_ be 0.
1. Let _counter_ be *+0*<sub>𝔽</sub>.
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. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *false*, return ? IteratorClose(_iterated_, NormalCompletion(*false*)).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
</emu-alg>
</emu-clause>

Expand All @@ -48740,16 +48742,16 @@ <h1>Iterator.prototype.filter ( _predicate_ )</h1>
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
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. Let _counter_ be *+0*<sub>𝔽</sub>.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return ReturnCompletion(*undefined*).
1. Let _selected_ be Completion(Call(_predicate_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. Let _selected_ be Completion(Call(_predicate_, *undefined*, « _value_, _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. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterators]] »).
1. Set _result_.[[UnderlyingIterators]] to « _iterated_ ».
1. Return _result_.
Expand All @@ -48767,14 +48769,14 @@ <h1>Iterator.prototype.find ( _predicate_ )</h1>
1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
1. Let _counter_ be 0.
1. Let _counter_ be *+0*<sub>𝔽</sub>.
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. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *true*, return ? IteratorClose(_iterated_, NormalCompletion(_value_)).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
</emu-alg>
</emu-clause>

Expand All @@ -48790,11 +48792,11 @@ <h1>Iterator.prototype.flatMap ( _mapper_ )</h1>
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
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. Let _counter_ be *+0*<sub>𝔽</sub>.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return ReturnCompletion(*undefined*).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_mapped_, _iterated_).
1. Let _innerIterator_ be Completion(GetIteratorFlattenable(_mapped_, ~reject-primitives~)).
1. IfAbruptCloseIterator(_innerIterator_, _iterated_).
Expand All @@ -48810,7 +48812,7 @@ <h1>Iterator.prototype.flatMap ( _mapper_ )</h1>
1. Let _backupCompletion_ be Completion(IteratorClose(_innerIterator_, _completion_)).
1. IfAbruptCloseIterator(_backupCompletion_, _iterated_).
1. Return ? IteratorClose(_iterated_, _completion_).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterators]] »).
1. Set _result_.[[UnderlyingIterators]] to « _iterated_ ».
1. Return _result_.
Expand All @@ -48828,13 +48830,13 @@ <h1>Iterator.prototype.forEach ( _procedure_ )</h1>
1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
1. Let _counter_ be 0.
1. Let _counter_ be *+0*<sub>𝔽</sub>.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return *undefined*.
1. Let _result_ be Completion(Call(_procedure_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. Let _result_ be Completion(Call(_procedure_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
</emu-alg>
</emu-clause>

Expand All @@ -48850,15 +48852,15 @@ <h1>Iterator.prototype.map ( _mapper_ )</h1>
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
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. Let _counter_ be *+0*<sub>𝔽</sub>.
1. Repeat,
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return ReturnCompletion(*undefined*).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, 𝔽(_counter_) »)).
1. Let _mapped_ be Completion(Call(_mapper_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_mapped_, _iterated_).
1. Let _completion_ be Completion(Yield(_mapped_)).
1. IfAbruptCloseIterator(_completion_, _iterated_).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
1. Let _result_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterators]] »).
1. Set _result_.[[UnderlyingIterators]] to « _iterated_ ».
1. Return _result_.
Expand All @@ -48879,17 +48881,17 @@ <h1>Iterator.prototype.reduce ( _reducer_ [ , _initialValue_ ] )</h1>
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. Let _counter_ be *1*<sub>𝔽</sub>.
1. Else,
1. Let _accumulator_ be _initialValue_.
1. Let _counter_ be 0.
1. Let _counter_ be *+0*<sub>𝔽</sub>.
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. Let _result_ be Completion(Call(_reducer_, *undefined*, « _accumulator_, _value_, _counter_ »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. Set _accumulator_ to _result_.
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
</emu-alg>
</emu-clause>

Expand All @@ -48904,14 +48906,14 @@ <h1>Iterator.prototype.some ( _predicate_ )</h1>
1. Let _error_ be ThrowCompletion(a newly created *TypeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
1. Let _counter_ be 0.
1. Let _counter_ be *+0*<sub>𝔽</sub>.
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. Let _result_ be Completion(Call(_predicate_, *undefined*, « _value_, _counter_ »)).
1. IfAbruptCloseIterator(_result_, _iterated_).
1. If ToBoolean(_result_) is *true*, return ? IteratorClose(_iterated_, NormalCompletion(*true*)).
1. Set _counter_ to _counter_ + 1.
1. Set _counter_ to _counter_ + *1*<sub>𝔽</sub>.
</emu-alg>
</emu-clause>

Expand All @@ -48927,18 +48929,20 @@ <h1>Iterator.prototype.take ( _limit_ )</h1>
1. If _numLimit_ is *NaN*, then
1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. If _numLimit_ is finite and _numLimit_ > 𝔽(2<sub>53</sub> - 1), then

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1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Let _integerLimit_ be ! ToIntegerOrInfinity(_numLimit_).
1. If _integerLimit_ &lt; 0, then
1. Let _error_ be ThrowCompletion(a newly created *RangeError* object).
1. Return ? IteratorClose(_iterated_, _error_).
1. Set _iterated_ to ? GetIteratorDirect(_obj_).
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. Let _remaining_ be 𝔽(_integerLimit_).
1. Repeat,
1. If _remaining_ = 0, then
1. If _remaining_ is *+0*<sub>𝔽</sub>, then
1. Return ? IteratorClose(_iterated_, ReturnCompletion(*undefined*)).
1. If _remaining_ ≠ +∞, then
1. Set _remaining_ to _remaining_ - 1.
1. Set _remaining_ to _remaining_ - *1*<sub>𝔽</sub>.
1. Let _value_ be ? IteratorStepValue(_iterated_).
1. If _value_ is ~done~, return ReturnCompletion(*undefined*).
1. Let _completion_ be Completion(Yield(_value_)).
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