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Editorial: Introduce an operation for backing To{Int,Uint}{8,16,32} and To{BigInt,BigUint}64 #3832

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Editorial: Introduce an operation for backing To{Int,Uint}{8,16,32} and To{BigInt,BigUint}64 #3832

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bf286aa
Editorial: Introduce ToFixedSizeInteger for backing To{Int,Uint}{8,16…
gibson042 Apr 12, 2026
0b5401f
Editorial: Refactor ToFixedSizeInteger to be infallible
gibson042 Apr 12, 2026
24b3ce1
fixup! Editorial: Refactor ToFixedSizeInteger to be infallible
gibson042 Apr 13, 2026
85ca958
Editorial: Reword the ToFixedSizeInteger note
gibson042 Apr 23, 2026
8d9d33f
fixup! Editorial: Reword the ToFixedSizeInteger note
gibson042 Apr 28, 2026
06ccf1e
fixup! Editorial: Refactor ToFixedSizeInteger to be infallible
gibson042 May 1, 2026
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110 changes: 37 additions & 73 deletions spec.html
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Original file line number Diff line number Diff line change
Expand Up @@ -5316,6 +5316,30 @@ <h1>
</emu-note>
</emu-clause>

<emu-clause id="sec-tofixedsizeinteger" type="abstract operation">
<h1>
ToFixedSizeInteger (
_int_: an integer, +∞, or -∞,
_signed_: ~unsigned~ or ~signed~,
_bitWidth_: a positive integer,
): an integer
</h1>
<dl class="header">
<dt>description</dt>
<dd>It maps _int_ to one of 2<sup>_bitWidth_</sup> integers in the inclusive interval from 0 to 2<sup>_bitWidth_</sup> - 1 (if _signed_ is ~unsigned~) or -2<sup>_bitWidth_ - 1</sup> to 2<sup>_bitWidth_ - 1</sup> - 1 (if _signed_ is ~signed~).</dd>
</dl>
<emu-alg>
1. If _int_ = +∞ or _int_ = -∞, return 0.
1. Let _fixedInt_ be _int_ modulo 2<sup>_bitWidth_</sup>.
1. NOTE: The following step does not change the two's complement representation of _fixedInt_.
1. If _signed_ is ~signed~ and _fixedInt_ ≥ 2<sup>_bitWidth_ - 1</sup>, set _fixedInt_ to _fixedInt_ - 2<sup>_bitWidth_</sup>.
1. Return _fixedInt_.
</emu-alg>
<emu-note>
<p>ToFixedSizeInteger is idempotent: for any ECMAScript language value _x_, ToFixedSizeInteger(ToFixedSizeInteger(_x_, _signed_, _bitWidth_), _signed_, _bitWidth_) is the same as ToFixedSizeInteger(_x_, _signed_, _bitWidth_). In fact, because +∞ and -∞ are mapped to 0, this invariant even survives inversion of _signed_ (e.g., ToFixedSizeInteger(ToFixedSizeInteger(_x_, ~signed~, 32), ~unsigned~, 32) is the same as ToFixedSizeInteger(_x_, ~unsigned~, 32)).</p>
</emu-note>
</emu-clause>

<emu-clause id="sec-toint32" type="abstract operation">
<h1>
ToInt32 (
Expand All @@ -5324,29 +5348,12 @@ <h1>
</h1>
<dl class="header">
<dt>description</dt>
<dd>It converts _argument_ to one of 2<sup>32</sup> integral Number values in the inclusive interval from 𝔽(-2<sup>31</sup>) to 𝔽(2<sup>31</sup> - 1).</dd>
<dd>It converts _argument_ to one of 2<sup>32</sup> integral Number values in the inclusive interval from 𝔽(-2<sup>31</sup>) to 𝔽(2<sup>31</sup> - 1), excluding *-0*<sub>𝔽</sub>.</dd>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. Let _int32bit_ be _int_ modulo 2<sup>32</sup>.
1. If _int32bit_ ≥ 2<sup>31</sup>, return 𝔽(_int32bit_ - 2<sup>32</sup>).
1. Return 𝔽(_int32bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~signed~, 32)).
</emu-alg>
<emu-note>
<p>Given the above definition:</p>
<ul>
<li>
It is idempotent: for any ECMAScript language value _x_, ToInt32(ToInt32(_x_)) is the same as ToInt32(_x_).
</li>
<li>
For any ECMAScript language value _x_, ToInt32(ToUint32(_x_)) is the same as ToInt32(_x_). (It is to preserve this property that *+∞*<sub>𝔽</sub> and *-∞*<sub>𝔽</sub> are mapped to *+0*<sub>𝔽</sub>.)
</li>
<li>
ToInt32 maps *-0*<sub>𝔽</sub> to *+0*<sub>𝔽</sub>.
</li>
</ul>
</emu-note>
</emu-clause>

<emu-clause id="sec-touint32" type="abstract operation">
Expand All @@ -5361,27 +5368,8 @@ <h1>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. Let _int32bit_ be _int_ modulo 2<sup>32</sup>.
1. [id="step-touint32-return"] Return 𝔽(_int32bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~unsigned~, 32)).
</emu-alg>
<emu-note>
<p>Given the above definition:</p>
<ul>
<li>
Step <emu-xref href="#step-touint32-return"></emu-xref> is the only difference between ToUint32 and ToInt32.
</li>
<li>
It is idempotent: for any ECMAScript language value _x_, ToUint32(ToUint32(_x_)) is the same as ToUint32(_x_).
</li>
<li>
For any ECMAScript language value _x_, ToUint32(ToInt32(_x_)) is the same as ToUint32(_x_). (It is to preserve this property that *+∞*<sub>𝔽</sub> and *-∞*<sub>𝔽</sub> are mapped to *+0*<sub>𝔽</sub>.)
</li>
<li>
ToUint32 maps *-0*<sub>𝔽</sub> to *+0*<sub>𝔽</sub>.
</li>
</ul>
</emu-note>
</emu-clause>

<emu-clause id="sec-toint16" type="abstract operation">
Expand All @@ -5392,14 +5380,11 @@ <h1>
</h1>
<dl class="header">
<dt>description</dt>
<dd>It converts _argument_ to one of 2<sup>16</sup> integral Number values in the inclusive interval from 𝔽(-2<sup>15</sup>) to 𝔽(2<sup>15</sup> - 1).</dd>
<dd>It converts _argument_ to one of 2<sup>16</sup> integral Number values in the inclusive interval from 𝔽(-2<sup>15</sup>) to 𝔽(2<sup>15</sup> - 1), excluding *-0*<sub>𝔽</sub>.</dd>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. Let _int16bit_ be _int_ modulo 2<sup>16</sup>.
1. If _int16bit_ ≥ 2<sup>15</sup>, return 𝔽(_int16bit_ - 2<sup>16</sup>).
1. Return 𝔽(_int16bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~signed~, 16)).
</emu-alg>
</emu-clause>

Expand All @@ -5415,21 +5400,8 @@ <h1>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. [id="step-touint16-mod"] Let _int16bit_ be _int_ modulo 2<sup>16</sup>.
1. Return 𝔽(_int16bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~unsigned~, 16)).
</emu-alg>
<emu-note>
<p>Given the above definition:</p>
<ul>
<li>
The substitution of 2<sup>16</sup> for 2<sup>32</sup> in step <emu-xref href="#step-touint16-mod"></emu-xref> is the only difference between ToUint32 and ToUint16.
</li>
<li>
ToUint16 maps *-0*<sub>𝔽</sub> to *+0*<sub>𝔽</sub>.
</li>
</ul>
</emu-note>
</emu-clause>

<emu-clause id="sec-toint8" type="abstract operation">
Expand All @@ -5440,14 +5412,11 @@ <h1>
</h1>
<dl class="header">
<dt>description</dt>
<dd>It converts _argument_ to one of 2<sup>8</sup> integral Number values in the inclusive interval from *-128*<sub>𝔽</sub> to *127*<sub>𝔽</sub>.</dd>
<dd>It converts _argument_ to one of 2<sup>8</sup> integral Number values in the inclusive interval from *-128*<sub>𝔽</sub> to *127*<sub>𝔽</sub>, excluding *-0*<sub>𝔽</sub>.</dd>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. Let _int8bit_ be _int_ modulo 2<sup>8</sup>.
1. If _int8bit_ ≥ 2<sup>7</sup>, return 𝔽(_int8bit_ - 2<sup>8</sup>).
1. Return 𝔽(_int8bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~signed~, 8)).
</emu-alg>
</emu-clause>

Expand All @@ -5463,9 +5432,7 @@ <h1>
</dl>
<emu-alg>
1. Let _int_ be ? ToIntegerOrInfinity(_argument_).
1. If _int_ is not finite, return *+0*<sub>𝔽</sub>.
1. Let _int8bit_ be _int_ modulo 2<sup>8</sup>.
1. Return 𝔽(_int8bit_).
1. Return 𝔽(ToFixedSizeInteger(_int_, ~unsigned~, 8)).
</emu-alg>
</emu-clause>

Expand Down Expand Up @@ -5640,10 +5607,8 @@ <h1>
<dd>It converts _argument_ to one of 2<sup>64</sup> BigInt values in the inclusive interval from ℤ(-2<sup>63</sup>) to ℤ(2<sup>63</sup> - 1).</dd>
</dl>
<emu-alg>
1. Let _n_ be ? ToBigInt(_argument_).
1. Let _int64bit_ be ℝ(_n_) modulo 2<sup>64</sup>.
1. If _int64bit_ ≥ 2<sup>63</sup>, return ℤ(_int64bit_ - 2<sup>64</sup>).
1. Return ℤ(_int64bit_).
1. Let _int_ be ℝ(? ToBigInt(_argument_)).
1. Return ℤ(ToFixedSizeInteger(_int_, ~signed~, 64)).
</emu-alg>
</emu-clause>

Expand All @@ -5658,9 +5623,8 @@ <h1>
<dd>It converts _argument_ to one of 2<sup>64</sup> BigInt values in the inclusive interval from *0*<sub>ℤ</sub> to ℤ(2<sup>64</sup> - 1).</dd>
</dl>
<emu-alg>
1. Let _n_ be ? ToBigInt(_argument_).
1. Let _int64bit_ be ℝ(_n_) modulo 2<sup>64</sup>.
1. Return ℤ(_int64bit_).
1. Let _int_ be ℝ(? ToBigInt(_argument_)).
1. Return ℤ(ToFixedSizeInteger(_int_, ~unsigned~, 64)).
</emu-alg>
</emu-clause>

Expand Down
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