LibJS: Remove extra property check from Instant#toZonedDateTimeISO

This is a normative change in the Temporal spec.

See: 7dfbd80

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

@@ -388,34 +388,20 @@ JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::to_zoned_date_time)
     return TRY(create_temporal_zoned_date_time(vm, instant->nanoseconds(), *time_zone, *calendar));
 }
 
-// 8.3.18 Temporal.Instant.prototype.toZonedDateTimeISO ( item ), https://tc39.es/proposal-temporal/#sec-temporal.instant.prototype.tozoneddatetimeiso
+// 8.3.18 Temporal.Instant.prototype.toZonedDateTimeISO ( timeZone ), https://tc39.es/proposal-temporal/#sec-temporal.instant.prototype.tozoneddatetimeiso
 JS_DEFINE_NATIVE_FUNCTION(InstantPrototype::to_zoned_date_time_iso)
 {
-    auto item = vm.argument(0);
-
     // 1. Let instant be the this value.
     // 2. Perform ? RequireInternalSlot(instant, [[InitializedTemporalInstant]]).
     auto* instant = TRY(typed_this_object(vm));
 
-    // 3. If Type(item) is Object, then
+    // 3. Set timeZone to ? ToTemporalTimeZone(timeZone).
-    if (item.is_object()) {
+    auto* time_zone = TRY(to_temporal_time_zone(vm, vm.argument(0)));
-        // a. Let timeZoneProperty be ? Get(item, "timeZone").
-        auto time_zone_property = TRY(item.as_object().get(vm.names.timeZone));
 
-        // b. If timeZoneProperty is not undefined, then
+    // 4. Let calendar be ! GetISO8601Calendar().
-        if (!time_zone_property.is_undefined()) {
-            // i. Set item to timeZoneProperty.
-            item = time_zone_property;
-        }
-    }
-
-    // 4. Let timeZone be ? ToTemporalTimeZone(item).
-    auto* time_zone = TRY(to_temporal_time_zone(vm, item));
-
-    // 5. Let calendar be ! GetISO8601Calendar().
     auto* calendar = get_iso8601_calendar(vm);
 
-    // 6. Return ? CreateTemporalZonedDateTime(instant.[[Nanoseconds]], timeZone, calendar).
+    // 5. Return ? CreateTemporalZonedDateTime(instant.[[Nanoseconds]], timeZone, calendar).
     return TRY(create_temporal_zoned_date_time(vm, instant->nanoseconds(), *time_zone, *calendar));
 }
 

Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.toZonedDateTimeISO.js

@@ -25,6 +25,25 @@ describe("correct behavior", () => {
         const zonedDateTime = instant.toZonedDateTimeISO({ timeZone });
         expect(zonedDateTime.timeZone).toBe(timeZone);
     });
+
+    test("avoids extra timeZone property lookup", () => {
+        const instant = new Temporal.Instant(1625614921123456789n);
+
+        let timesGetterCalled = 0;
+        const timeZoneObject = {
+            get timeZone() {
+                timesGetterCalled++;
+                return "UTC";
+            },
+
+            toString() {
+                return "UTC";
+            },
+        };
+
+        instant.toZonedDateTimeISO({ timeZone: timeZoneObject });
+        expect(timesGetterCalled).toBe(0);
+    });
 });
 
 describe("errors", () => {