LibJS: Implement Temporal.PlainDate.prototype.toPlainYearMonth()

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -392,6 +392,7 @@ namespace JS {
     P(toPlainDate)                           \
     P(toPlainDateTime)                       \
     P(toPlainTime)                           \
+    P(toPlainYearMonth)                      \
     P(toString)                              \
     P(toTemporalInstant)                     \
     P(toTimeString)                          \

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

@@ -417,6 +417,36 @@ PlainDate* date_from_fields(GlobalObject& global_object, Object& calendar, Objec
     return static_cast<PlainDate*>(date_object);
 }
 
+// 12.1.25 YearMonthFromFields ( calendar, fields [ , options ] ),
+PlainYearMonth* year_month_from_fields(GlobalObject& global_object, Object& calendar, Object& fields, Object* options)
+{
+    auto& vm = global_object.vm();
+
+    // 1. Assert: Type(calendar) is Object.
+    // 2. Assert: Type(fields) is Object.
+    // 3. If options is not present, then
+    //     a. Set options to undefined.
+    // 4. Else,
+    //     a. Assert: Type(options) is Object.
+
+    // 5. Let yearMonth be ? Invoke(calendar, "yearMonthFromFields", « fields, options »).
+    auto year_month = Value(&calendar).invoke(global_object, vm.names.yearMonthFromFields, &fields, options ?: js_undefined());
+    if (vm.exception())
+        return {};
+
+    // 6. Perform ? RequireInternalSlot(yearMonth, [[InitializedTemporalYearMonth]]).
+    auto* year_month_object = year_month.to_object(global_object);
+    if (!year_month_object)
+        return {};
+    if (!is<PlainYearMonth>(year_month_object)) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::NotA, "Temporal.PlainYearMonth");
+        return {};
+    }
+
+    // 7. Return yearMonth.
+    return static_cast<PlainYearMonth*>(year_month_object);
+}
+
 // 12.1.28 CalendarEquals ( one, two ), https://tc39.es/proposal-temporal/#sec-temporal-calendarequals
 bool calendar_equals(GlobalObject& global_object, Object& one, Object& two)
 {

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

@@ -49,6 +49,7 @@ Object* to_temporal_calendar(GlobalObject&, Value);
 Object* to_temporal_calendar_with_iso_default(GlobalObject&, Value);
 Object* get_temporal_calendar_with_iso_default(GlobalObject&, Object&);
 PlainDate* date_from_fields(GlobalObject&, Object& calendar, Object& fields, Object& options);
+PlainYearMonth* year_month_from_fields(GlobalObject&, Object& calendar, Object& fields, Object* options = nullptr);
 bool calendar_equals(GlobalObject&, Object& one, Object& two);
 Object* consolidate_calendars(GlobalObject&, Object& one, Object& two);
 bool is_iso_leap_year(i32 year);

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

@@ -7,9 +7,11 @@
 
 #include <AK/TypeCasts.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/AbstractOperations.h>
 #include <LibJS/Runtime/Temporal/Calendar.h>
 #include <LibJS/Runtime/Temporal/PlainDate.h>
 #include <LibJS/Runtime/Temporal/PlainDatePrototype.h>
+#include <LibJS/Runtime/Temporal/PlainYearMonth.h>
 
 namespace JS::Temporal {
 
@@ -43,6 +45,7 @@ void PlainDatePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.inLeapYear, in_leap_year_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(vm.names.toPlainYearMonth, to_plain_year_month, 0, attr);
     define_native_function(vm.names.getISOFields, get_iso_fields, 0, attr);
     define_native_function(vm.names.withCalendar, with_calendar, 1, attr);
     define_native_function(vm.names.equals, equals, 1, attr);
@@ -267,6 +270,32 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::in_leap_year_getter)
     return Value(calendar_in_leap_year(global_object, calendar, *temporal_date));
 }
 
+// 3.3.16 Temporal.PlainDate.prototype.toPlainYearMonth ( ), https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.toplainyearmonth
+JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::to_plain_year_month)
+{
+    // 1. Let temporalDate be the this value.
+    // 2. Perform ? RequireInternalSlot(temporalDate, [[InitializedTemporalDate]]).
+    auto* temporal_date = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be temporalDate.[[Calendar]].
+    auto& calendar = temporal_date->calendar();
+
+    // 4. Let fieldNames be ? CalendarFields(calendar, « "monthCode", "year" »).
+    auto field_names = calendar_fields(global_object, calendar, { "monthCode"sv, "year"sv });
+    if (vm.exception())
+        return {};
+
+    // 5. Let fields be ? PrepareTemporalFields(temporalDate, fieldNames, «»).
+    auto* fields = prepare_temporal_fields(global_object, *temporal_date, field_names, {});
+    if (vm.exception())
+        return {};
+
+    // 6. Return ? YearMonthFromFields(calendar, fields).
+    return year_month_from_fields(global_object, calendar, *fields);
+}
+
 // 3.3.18 Temporal.PlainDate.prototype.getISOFields ( ), https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.getisofields
 JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::get_iso_fields)
 {

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

@@ -32,7 +32,7 @@ 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(to_plain_year_month);
     JS_DECLARE_NATIVE_FUNCTION(get_iso_fields);
     JS_DECLARE_NATIVE_FUNCTION(with_calendar);
     JS_DECLARE_NATIVE_FUNCTION(equals);

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.toPlainYearMonth.js

@@ -0,0 +1,13 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.PlainDate.prototype.toPlainYearMonth).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const plainDate = new Temporal.PlainDate(2021, 7, 6);
+        const plainYearMonth = plainDate.toPlainYearMonth();
+        expect(plainYearMonth.calendar).toBe(plainDate.calendar);
+        expect(plainYearMonth.year).toBe(2021);
+        expect(plainYearMonth.month).toBe(7);
+    });
+});