LibJS: Implement Temporal.Duration.prototype.months

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/80114094289
Pull-request: https://github.com/SerenityOS/serenity/pull/8794
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -236,6 +236,7 @@ namespace JS {
     P(max)                                   \
     P(message)                               \
     P(min)                                   \
+    P(months)                                \
     P(multiline)                             \
     P(name)                                  \
     P(next)                                  \

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

@@ -26,6 +26,7 @@ void DurationPrototype::initialize(GlobalObject& global_object)
     define_direct_property(*vm.well_known_symbol_to_string_tag(), js_string(vm.heap(), "Temporal.Duration"), Attribute::Configurable);
 
     define_native_accessor(vm.names.years, years_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.months, months_getter, {}, Attribute::Configurable);
 }
 
 static Duration* typed_this(GlobalObject& global_object)
@@ -54,4 +55,17 @@ JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::years_getter)
     return Value(duration->years());
 }
 
+// 7.3.4 get Temporal.Duration.prototype.months, https://tc39.es/proposal-temporal/#sec-get-temporal.duration.prototype.months
+JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::months_getter)
+{
+    // 1. Let duration be the this value.
+    // 2. Perform ? RequireInternalSlot(duration, [[InitializedTemporalDuration]]).
+    auto* duration = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Return duration.[[Months]].
+    return Value(duration->months());
+}
+
 }

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

@@ -20,6 +20,7 @@ public:
 
 private:
     JS_DECLARE_NATIVE_FUNCTION(years_getter);
+    JS_DECLARE_NATIVE_FUNCTION(months_getter);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.prototype.months.js

@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const duration = new Temporal.Duration(0, 123);
+        expect(duration.months).toBe(123);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.Duration object", () => {
+        expect(() => {
+            Reflect.get(Temporal.Duration.prototype, "months", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Duration");
+    });
+});