LibJS: Implement Temporal.PlainYearMonth.prototype.era

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/b11ea98648e
Pull-request: https://github.com/SerenityOS/serenity/pull/9647
Reviewed-by: https://github.com/IdanHo ✅

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

@@ -36,6 +36,7 @@ void PlainYearMonthPrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.daysInMonth, days_in_month_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.toString, to_string, 0, attr);
@@ -183,6 +184,22 @@ JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::in_leap_year_getter)
     return Value(calendar_in_leap_year(global_object, calendar, *year_month));
 }
 
+// 15.6.9.2 get Temporal.PlainYearMonth.prototype.era, https://tc39.es/proposal-temporal/#sec-get-temporal.plainyearmonth.prototype.era
+JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::era_getter)
+{
+    // 1. Let plainYearMonth be the this value.
+    // 2. Perform ? RequireInternalSlot(plainYearMonth, [[InitializedTemporalYearMonth]]).
+    auto* plain_year_month = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be plainYearMonth.[[Calendar]].
+    auto& calendar = plain_year_month->calendar();
+
+    // 4. Return ? CalendarEra(calendar, plainYearMonth).
+    return calendar_era(global_object, calendar, *plain_year_month);
+}
+
 // 9.3.17 Temporal.PlainYearMonth.prototype.toString ( [ options ] ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype.tostring
 JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::to_string)
 {

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

@@ -27,6 +27,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(days_in_month_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(to_string);
     JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
     JS_DECLARE_NATIVE_FUNCTION(to_json);

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

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