GetFormatterParts failing on latest Firefox Nightly

From https://bugzilla.mozilla.org/show_bug.cgi?id=1742572:

Standalone test case:

function GetFormatterParts(timeZone, epochMilliseconds){
  const formatter = new Intl.DateTimeFormat("en-us", {
    timeZone,
    hour12: false,
    era: "short",
    year: "numeric",
    month: "numeric",
    day: "numeric",
    hour: "numeric",
    minute: "numeric",
    second: "numeric"
  });
  const datetime = formatter.format(new Date(epochMilliseconds));
  const [date, fullYear, time] = datetime.split(/,\s+/);
  const [month, day] = date.split(" ");
  const [year, era] = fullYear.split(" ");
  const [hour, minute, second] = time.split(":");
  return {
    year: era === "BC" ? -year + 1 : +year,
    month: +month,
    day: +day,
    hour: hour === "24" ? 0 : +hour,
    minute: +minute,
    second: +second
  };
}

GetFormatterParts("America/Los_Angeles", Date.now());

GetFormatterParts expects that the formatted date-time string can be split into three parts through the regular expression /,\s+/. This worked before the update, because the formatted string looked like "11 23, 2021 AD, 05:00:00". But after the update to ICU 70, which includes an update to CLDR 40, the string now looks like "11/23/2021 A, 05:00:00", i.e. can no longer be separated into three parts when searching for a comma.

[marco-c]

@justingrant mind releasing a new version with this fix?

[ptomato]

I was thinking of a change for proposal-temporal such as this one:

--- a/polyfill/lib/ecmascript.mjs
+++ b/polyfill/lib/ecmascript.mjs
@@ -2310,14 +2310,14 @@ export const ES = ObjectAssign({}, ES2020, {
   },
   GetFormatterParts: (timeZone, epochMilliseconds) => {
     const formatter = getIntlDateTimeFormatEnUsForTimeZone(timeZone);
-    // FIXME: can this use formatToParts instead?
-    const datetime = formatter.format(new Date(epochMilliseconds));
-    const [date, fullYear, time] = datetime.split(/,\s+/);
-    const [month, day] = date.split(' ');
-    const [year, era] = fullYear.split(' ');
-    const [hour, minute, second] = time.split(':');
+    const values = {};
+    const parts = formatter.formatToParts(new Date(epochMilliseconds));
+    for (const { type, value } of parts) {
+      if (type !== 'literal') values[type] = value;
+    }
+    const { era, year, month, day, hour, minute, second } = values;
     return {
-      year: era === 'BC' ? -year + 1 : +year,
+      year: era.startsWith('B') ? -year + 1 : +year, // era 'B' or 'BC'
       month: +month,
       day: +day,
       hour: hour === '24' ? 0 : +hour, // bugs.chromium.org/p/chromium/issues/detail?id=1045791

We already use formatToParts in the calendar code so maybe it shouldn’t be a problem. On the other hand, it’s only used for non-ISO calendars, and this would require it more generally for ISO calendar usage as well. I’m not sure what formatToParts is transpiled to for old browsers that don’t support it. For the reference code in proposal-temporal I’m not concerned about old browsers (it’s been generally supported since 2018 or so) but for the polyfill it might matter.