LibJS: Implement Intl.Locale.prototype.toString()

This isn't particularly testable yet without the Intl.Locale constructor
but having this defined will make testing the constructor possible. So
more specific tests for this prototype will come later.

Userland/Libraries/LibJS/Runtime/Intl/LocalePrototype.cpp

@@ -6,10 +6,27 @@
 
 #include <AK/TypeCasts.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Intl/Locale.h>
 #include <LibJS/Runtime/Intl/LocalePrototype.h>
 
 namespace JS::Intl {
 
+static Locale* typed_this(GlobalObject& global_object)
+{
+    auto& vm = global_object.vm();
+
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return nullptr;
+
+    if (!is<Locale>(this_object)) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::NotA, "Intl.Locale");
+        return nullptr;
+    }
+
+    return static_cast<Locale*>(this_object);
+}
+
 // 14.3 Properties of the Intl.Locale Prototype Object, https://tc39.es/ecma402/#sec-properties-of-intl-locale-prototype-object
 LocalePrototype::LocalePrototype(GlobalObject& global_object)
     : Object(*global_object.object_prototype())
@@ -22,8 +39,24 @@ void LocalePrototype::initialize(GlobalObject& global_object)
 
     auto& vm = this->vm();
 
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(vm.names.toString, to_string, 0, attr);
+
     // 14.3.2 Intl.Locale.prototype[ @@toStringTag ], https://tc39.es/ecma402/#sec-Intl.Locale.prototype-@@tostringtag
     define_direct_property(*vm.well_known_symbol_to_string_tag(), js_string(vm, "Intl.Locale"), Attribute::Configurable);
 }
 
+// 14.3.5 Intl.Locale.prototype.toString ( ), https://tc39.es/ecma402/#sec-Intl.Locale.prototype.toString
+JS_DEFINE_NATIVE_FUNCTION(LocalePrototype::to_string)
+{
+    // 1. Let loc be the this value.
+    // 2. Perform ? RequireInternalSlot(loc, [[InitializedLocale]]).
+    auto* locale_object = typed_this(global_object);
+    if (!locale_object)
+        return {};
+
+    // 3. Return loc.[[Locale]].
+    return js_string(vm, locale_object->locale());
+}
+
 }

Userland/Libraries/LibJS/Runtime/Intl/LocalePrototype.h

@@ -17,6 +17,9 @@ public:
     explicit LocalePrototype(GlobalObject&);
     virtual void initialize(GlobalObject&) override;
     virtual ~LocalePrototype() override = default;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(to_string);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Intl/Locale/Locale.prototype.toString.js

@@ -0,0 +1,3 @@
+test("length is 0", () => {
+    expect(Intl.Locale.prototype.toString).toHaveLength(0);
+});