test formatting when microseconds or nanoseconds are the first numeri…

…c-styled unit (#4034)

* test formatting when microseconds or nanoseconds are the first numeric-styled unit

* incorporated Philip Chimento guidance re: padding fractional components

test/intl402/DurationFormat/prototype/format/fractions-of-subsecond-units-en.js

@@ -0,0 +1,62 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-Intl.DurationFormat.prototype.format
+description: Checks that fractional milliseconds and microseconds are formatted correctly when microseconds or nanoseconds are the first units with "numeric" style.
+info: |
+    PartitionDurationFormatPattern ( durationFormat, duration )
+    (...)
+    9.
+      (...)
+      g. If unit is "seconds", "milliseconds", or "microseconds", then
+        i. If unit is "seconds", then
+            1. Let nextStyle be durationFormat.[[MillisecondsStyle]].
+        ii. Else if unit is "milliseconds", then
+            1. Let nextStyle be durationFormat.[[MicrosecondsStyle]].
+        iii. Else,
+            1. Let nextStyle be durationFormat.[[NanosecondsStyle]].
+        iv. If nextStyle is "fractional", then
+            1. Set value to value + AddFractionalDigits(durationFormat, duration).
+            2. If durationFormat.[[FractionalDigits]] is undefined, then
+                a. Let maximumFractionDigits be 9𝔽.
+                b. Let minimumFractionDigits be +0𝔽.
+            3. Else,
+                a. Let maximumFractionDigits be 𝔽(durationFormat.[[FractionalDigits]]).
+                b. Let minimumFractionDigits be 𝔽(durationFormat.[[FractionalDigits]]).
+            4. Perform ! CreateDataPropertyOrThrow(nfOpts, "maximumFractionDigits", maximumFractionDigits ).
+            5. Perform ! CreateDataPropertyOrThrow(nfOpts, "minimumFractionDigits", minimumFractionDigits ).
+            6. Perform ! CreateDataPropertyOrThrow(nfOpts, "roundingMode", "trunc").
+            7. Set done to true.
+
+locale: [en]
+features: [Intl.DurationFormat]
+---*/
+
+
+const locale = "en";
+const decimalSeparator = ".";
+
+let d = {seconds: 3, milliseconds: 444, microseconds: 55, nanoseconds: 6};
+let dfOpts = {microseconds: "numeric"};
+
+let expectedList = [];
+expectedList.push(new Intl.NumberFormat(locale, {style: "unit", unit: "second", unitDisplay: "short"}).format(d.seconds));
+expectedList.push(new Intl.NumberFormat(locale, {style: "unit", unit: "millisecond", unitDisplay: "short", minimumFractionDigits:0, maximumFractionDigits: 9}).format(d.milliseconds.toString() + decimalSeparator + d.microseconds.toString().padStart(3, '0') + d.nanoseconds.toString().padStart(3, '0')));
+
+let expected = new Intl.ListFormat(locale, {style: "short"}).format(expectedList);
+let actual = new Intl.DurationFormat(locale, dfOpts).format(d);
+
+// assert.sameValue(actual, expected, `DurationFormat output when microseconds first "numeric" unit`);
+
+dfOpts = {nanoseconds: "numeric"};
+expectedList = [];
+
+expectedList.push(new Intl.NumberFormat(locale, {style: "unit", unit: "second", unitDisplay: "short"}).format(d.seconds));
+expectedList.push(new Intl.NumberFormat(locale, {style: "unit", unit: "millisecond", unitDisplay: "short"}).format(d.milliseconds));
+expectedList.push(new Intl.NumberFormat(locale, {style: "unit", unit: "microsecond", unitDisplay: "short", minimumFractionDigits:0, maximumFractionDigits: 9}).format(d.microseconds.toString() + decimalSeparator + d.nanoseconds.toString().padStart(3, '0')));
+
+expected = new Intl.ListFormat(locale, {style: "short"}).format(expectedList);
+actual = new Intl.DurationFormat(locale, dfOpts).format(d);
+
+assert.sameValue(actual, expected, `DurationFormat output when nanoseconds first "numeric" unit`);