Editorial: refactor FormatSecondsStringPart

This editorial commit refactors FormatSecondsStringPart to include
similar editorial changes made to GetOffsetStringFor in tc39#2607.

Author: justingrant

polyfill/lib/ecmascript.mjs

@@ -2358,18 +2358,18 @@ export function ISODateTimePartString(part) {
 export function FormatSecondsStringPart(second, millisecond, microsecond, nanosecond, precision) {
   if (precision === 'minute') return '';
 
-  const secs = `:${ISODateTimePartString(second)}`;
-  let fraction = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
+  const secondsString = `:${ISODateTimePartString(second)}`;
+  let subSecondNanoseconds = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
 
+  let fractionString;
   if (precision === 'auto') {
-    if (fraction === 0) return secs;
-    fraction = `${fraction}`.padStart(9, '0');
-    while (fraction[fraction.length - 1] === '0') fraction = fraction.slice(0, -1);
+    if (subSecondNanoseconds === 0) return secondsString;
+    fractionString = `${subSecondNanoseconds}`.padStart(9, '0').replace(/0+$/, '');
   } else {
-    if (precision === 0) return secs;
-    fraction = `${fraction}`.padStart(9, '0').slice(0, precision);
+    if (precision === 0) return secondsString;
+    fractionString = `${subSecondNanoseconds}`.padStart(9, '0').slice(0, precision);
   }
-  return `${secs}.${fraction}`;
+  return `${secondsString}.${fractionString}`;
 }
 
 export function TemporalInstantToString(instant, timeZone, precision) {

spec/abstractops.html

@@ -686,16 +686,16 @@ <h1>FormatSecondsStringPart ( _second_, _millisecond_, _microsecond_, _nanosecon
       1. Assert: _second_, _millisecond_, _microsecond_, and _nanosecond_ are integers.
       1. If _precision_ is *"minute"*, return *""*.
       1. Let _secondsString_ be the string-concatenation of the code unit 0x003A (COLON) and ToZeroPaddedDecimalString(_second_, 2).
-      1. Let _fraction_ be _millisecond_ &times; 10<sup>6</sup> + _microsecond_ &times; 10<sup>3</sup> + _nanosecond_.
+      1. Let _subSecondNanoseconds_ be _millisecond_ &times; 10<sup>6</sup> + _microsecond_ &times; 10<sup>3</sup> + _nanosecond_.
       1. If _precision_ is *"auto"*, then
-        1. If _fraction_ is 0, return _secondsString_.
-        1. Set _fraction_ to ToZeroPaddedDecimalString(_fraction_, 9).
-        1. Set _fraction_ to the longest possible substring of _fraction_ starting at position 0 and not ending with the code unit 0x0030 (DIGIT ZERO).
+        1. If _subSecondNanoseconds_ is 0, return _secondsString_.
+        1. Let _fractionString_ be ToZeroPaddedDecimalString(_subSecondNanoseconds_, 9).
+        1. Set _fractionString_ to the longest prefix of _fractionString_ ending with a code unit other than 0x0030 (DIGIT ZERO).
       1. Else,
         1. If _precision_ is 0, return _secondsString_.
-        1. Set _fraction_ to ToZeroPaddedDecimalString(_fraction_, 9).
-        1. Set _fraction_ to the substring of _fraction_ from 0 to _precision_.
-      1. Return the string-concatenation of _secondsString_, the code unit 0x002E (FULL STOP), and _fraction_.
+        1. Let _fractionString_ be ToZeroPaddedDecimalString(_subSecondNanoseconds_, 9).
+        1. Set _fractionString_ to the substring of _fractionString_ from 0 to _precision_.
+      1. Return the string-concatenation of _secondsString_, the code unit 0x002E (FULL STOP), and _fractionString_.
     </emu-alg>
   </emu-clause>