LibJS: Implement Temporal.PlainDateTime.prototype.nanosecond

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

@@ -37,6 +37,7 @@ void PlainDateTimePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.second, second_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.millisecond, millisecond_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.microsecond, microsecond_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.nanosecond, nanosecond_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.valueOf, value_of, 0, attr);
@@ -199,6 +200,19 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::microsecond_getter)
     return Value(date_time->iso_microsecond());
 }
 
+// 5.3.13 get Temporal.PlainDateTime.prototype.nanosecond, https://tc39.es/proposal-temporal/#sec-get-temporal.plaindatetime.prototype.nanosecond
+JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::nanosecond_getter)
+{
+    // 1. Let dateTime be the this value.
+    // 2. Perform ? RequireInternalSlot(dateTime, [[InitializedTemporalDateTime]]).
+    auto* date_time = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Return 𝔽(dateTime.[[ISONanosecond]]).
+    return Value(date_time->iso_nanosecond());
+}
+
 // 5.3.35 Temporal.PlainDateTime.prototype.valueOf ( ), https://tc39.es/proposal-temporal/#sec-temporal.plaindatetime.prototype.valueof
 JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::value_of)
 {

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

@@ -29,6 +29,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(second_getter);
     JS_DECLARE_NATIVE_FUNCTION(millisecond_getter);
     JS_DECLARE_NATIVE_FUNCTION(microsecond_getter);
+    JS_DECLARE_NATIVE_FUNCTION(nanosecond_getter);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
     JS_DECLARE_NATIVE_FUNCTION(to_plain_date);
     JS_DECLARE_NATIVE_FUNCTION(get_iso_fields);

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

@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 30, 1, 4, 32, 111, 420, 963);
+        expect(plainDateTime.nanosecond).toBe(963);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.PlainDateTime object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDateTime.prototype, "nanosecond", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDateTime");
+    });
+});