LibJS: Implement Temporal.Now.plainDateISO()

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/f2a2e8e13cd
Pull-request: https://github.com/SerenityOS/serenity/pull/9031
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -283,6 +283,7 @@ namespace JS {
     P(parseFloat)                            \
     P(parseInt)                              \
     P(plainDate)                             \
+    P(plainDateISO)                          \
     P(pop)                                   \
     P(pow)                                   \
     P(preventExtensions)                     \

Userland/Libraries/LibJS/Runtime/Temporal/Now.cpp

@@ -35,6 +35,7 @@ void Now::initialize(GlobalObject& global_object)
     define_native_function(vm.names.timeZone, time_zone, 0, attr);
     define_native_function(vm.names.instant, instant, 0, attr);
     define_native_function(vm.names.plainDate, plain_date, 1, attr);
+    define_native_function(vm.names.plainDateISO, plain_date_iso, 0, attr);
 }
 
 // 2.1.1 Temporal.Now.timeZone ( ), https://tc39.es/proposal-temporal/#sec-temporal.now.timezone
@@ -66,6 +67,25 @@ JS_DEFINE_NATIVE_FUNCTION(Now::plain_date)
     return create_temporal_date(global_object, date_time->iso_year(), date_time->iso_month(), date_time->iso_day(), date_time->calendar());
 }
 
+// 2.1.8 Temporal.Now.plainDateISO ( [ temporalTimeZoneLike ] ), https://tc39.es/proposal-temporal/#sec-temporal.now.plaindateiso
+JS_DEFINE_NATIVE_FUNCTION(Now::plain_date_iso)
+{
+    auto temporal_time_zone_like = vm.argument(0);
+
+    // 1. Let calendar be ? GetISO8601Calendar().
+    auto* calendar = get_iso8601_calendar(global_object);
+    if (vm.exception())
+        return {};
+
+    // 2. Let dateTime be ? SystemDateTime(temporalTimeZoneLike, calendar).
+    auto* date_time = system_date_time(global_object, temporal_time_zone_like, calendar);
+    if (vm.exception())
+        return {};
+
+    // 3. Return ? CreateTemporalDate(dateTime.[[ISOYear]], dateTime.[[ISOMonth]], dateTime.[[ISODay]], dateTime.[[Calendar]]).
+    return create_temporal_date(global_object, date_time->iso_year(), date_time->iso_month(), date_time->iso_day(), date_time->calendar());
+}
+
 // 2.2.1 SystemTimeZone ( ), https://tc39.es/proposal-temporal/#sec-temporal-systemtimezone
 TimeZone* system_time_zone(GlobalObject& global_object)
 {

Userland/Libraries/LibJS/Runtime/Temporal/Now.h

@@ -22,6 +22,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(time_zone);
     JS_DECLARE_NATIVE_FUNCTION(instant);
     JS_DECLARE_NATIVE_FUNCTION(plain_date);
+    JS_DECLARE_NATIVE_FUNCTION(plain_date_iso);
 };
 
 TimeZone* system_time_zone(GlobalObject&);

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDateISO.js

@@ -0,0 +1,25 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.Now.plainDateISO).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const plainDate = Temporal.Now.plainDateISO();
+        expect(plainDate).toBeInstanceOf(Temporal.PlainDate);
+        expect(plainDate.calendar.id).toBe("iso8601");
+    });
+
+    test("custom time zone", () => {
+        const timeZone = {
+            getOffsetNanosecondsFor() {
+                return 86400000000000;
+            },
+        };
+        const plainDate = Temporal.Now.plainDateISO();
+        const plainDateWithOffset = Temporal.Now.plainDateISO(timeZone);
+        // Yes, this will fail if a day, month, or year change happens between the above two lines :^)
+        expect(plainDateWithOffset.year).toBe(plainDate.year);
+        expect(plainDateWithOffset.month).toBe(plainDate.month);
+        expect(plainDateWithOffset.day).toBe(plainDate.day + 1);
+    });
+});