LibJS: Implement Temporal.PlainDate.prototype.era

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

@@ -288,6 +288,30 @@ Value calendar_in_leap_year(GlobalObject& global_object, Object& calendar, Objec
     return Value(&calendar).invoke(global_object, vm.names.inLeapYear, &date_like);
 }
 
+// 15.6.1.2 CalendarEra ( calendar, dateLike ), https://tc39.es/proposal-temporal/#sec-temporal-calendarera
+Value calendar_era(GlobalObject& global_object, Object& calendar, Object& date_like)
+{
+    auto& vm = global_object.vm();
+
+    // 1. Assert: Type(calendar) is Object.
+
+    // 2. Let result be ? Invoke(calendar, "era", « dateLike »).
+    auto result = Value(&calendar).invoke(global_object, vm.names.era, &date_like);
+    if (vm.exception())
+        return {};
+
+    // 3. If result is not undefined, set result to ? ToString(result).
+    if (!result.is_undefined()) {
+        auto result_string = result.to_string(global_object);
+        if (vm.exception())
+            return {};
+        result = js_string(vm, move(result_string));
+    }
+
+    // 4. Return result.
+    return result;
+}
+
 // 12.1.21 ToTemporalCalendar ( temporalCalendarLike ), https://tc39.es/proposal-temporal/#sec-temporal-totemporalcalendar
 Object* to_temporal_calendar(GlobalObject& global_object, Value temporal_calendar_like)
 {