LibJS: Implement Temporal.PlainDateTime.prototype.era

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

@@ -49,6 +49,7 @@ void PlainDateTimePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.daysInYear, days_in_year_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.monthsInYear, months_in_year_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.inLeapYear, in_leap_year_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.era, era_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.withPlainTime, with_plain_time, 1, attr);
@@ -359,6 +360,22 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::in_leap_year_getter)
     return calendar_in_leap_year(global_object, calendar, *date_time);
 }
 
+// 15.6.6.2 get Temporal.PlainDateTime.prototype.era, https://tc39.es/proposal-temporal/#sec-get-temporal.plaindatetime.prototype.era
+JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::era_getter)
+{
+    // 1. Let plainDateTime be the this value.
+    // 2. Perform ? RequireInternalSlot(plainDateTime, [[InitializedTemporalDateTime]]).
+    auto* plain_date_time = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be plainDateTime.[[Calendar]].
+    auto& calendar = plain_date_time->calendar();
+
+    // 4. Return ? CalendarEra(calendar, plainDateTime).
+    return calendar_era(global_object, calendar, *plain_date_time);
+}
+
 // 5.3.23 Temporal.PlainDateTime.prototype.withPlainTime ( [ plainTimeLike ] ), https://tc39.es/proposal-temporal/#sec-temporal.plaindatetime.prototype.withplaintime
 JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::with_plain_time)
 {

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

@@ -38,6 +38,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(days_in_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(months_in_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(in_leap_year_getter);
+    JS_DECLARE_NATIVE_FUNCTION(era_getter);
     JS_DECLARE_NATIVE_FUNCTION(with_plain_time);
     JS_DECLARE_NATIVE_FUNCTION(with_plain_date);
     JS_DECLARE_NATIVE_FUNCTION(with_calendar);

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDateTime/PlainDateTime.prototype.era.js

@@ -0,0 +1,24 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 6, 18, 14, 47);
+        expect(plainDateTime.era).toBeUndefined();
+    });
+
+    test("calendar with custom era function", () => {
+        const calendar = {
+            era() {
+                return "foo";
+            },
+        };
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 6, 18, 14, 47, 0, 0, 0, calendar);
+        expect(plainDateTime.era).toBe("foo");
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainDateTime object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDateTime.prototype, "era", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDateTime");
+    });
+});