LibJS: Implement the Intl.DisplayNames constructor

There is notably FIXME notations in this commit regarding Unicode locale
extensions. We are not parsing extensions (or private use extensions) at
all yet.

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

@@ -6,8 +6,11 @@
 
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Intl/AbstractOperations.h>
 #include <LibJS/Runtime/Intl/DisplayNames.h>
 #include <LibJS/Runtime/Intl/DisplayNamesConstructor.h>
+#include <LibJS/Runtime/Temporal/AbstractOperations.h>
+#include <LibUnicode/Locale.h>
 
 namespace JS::Intl {
 
@@ -42,11 +45,82 @@ Value DisplayNamesConstructor::construct(FunctionObject& new_target)
     auto& vm = this->vm();
     auto& global_object = this->global_object();
 
+    auto locales = vm.argument(0);
+    auto options = vm.argument(1);
+
     // 2. Let displayNames be ? OrdinaryCreateFromConstructor(NewTarget, "%DisplayNames.prototype%", « [[InitializedDisplayNames]], [[Locale]], [[Style]], [[Type]], [[Fallback]], [[Fields]] »).
     auto* display_names = ordinary_create_from_constructor<DisplayNames>(global_object, new_target, &GlobalObject::intl_display_names_prototype);
     if (vm.exception())
         return {};
 
+    // 3. Let requestedLocales be ? CanonicalizeLocaleList(locales).
+    auto requested_locales = canonicalize_locale_list(global_object, locales);
+    if (vm.exception())
+        return {};
+
+    // 4. If options is undefined, throw a TypeError exception.
+    if (options.is_undefined()) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::IsUndefined, "options"sv);
+        return {};
+    }
+
+    // 5. Set options to ? GetOptionsObject(options).
+    options = Temporal::get_options_object(global_object, options);
+    if (vm.exception())
+        return {};
+
+    // 6. Let opt be a new Record.
+    LocaleOptions opt {};
+
+    // 7. Let localeData be %DisplayNames%.[[LocaleData]].
+
+    // 8. Let matcher be ? GetOption(options, "localeMatcher", "string", « "lookup", "best fit" », "best fit").
+    auto matcher = get_option(global_object, options, vm.names.localeMatcher, Value::Type::String, { "lookup"sv, "best fit"sv }, "best fit"sv);
+    if (vm.exception())
+        return {};
+
+    // 9. Set opt.[[localeMatcher]] to matcher.
+    opt.locale_matcher = matcher;
+
+    // 10. Let r be ResolveLocale(%DisplayNames%.[[AvailableLocales]], requestedLocales, opt, %DisplayNames%.[[RelevantExtensionKeys]]).
+    auto result = resolve_locale(requested_locales, opt, {});
+
+    // 11. Let style be ? GetOption(options, "style", "string", « "narrow", "short", "long" », "long").
+    auto style = get_option(global_object, options, vm.names.style, Value::Type::String, { "narrow"sv, "short"sv, "long"sv }, "long"sv);
+    if (vm.exception())
+        return {};
+
+    // 12. Set displayNames.[[Style]] to style.
+    display_names->set_style(style.as_string().string());
+
+    // 13. Let type be ? GetOption(options, "type", "string", « "language", "region", "script", "currency" », undefined).
+    auto type = get_option(global_object, options, vm.names.type, Value::Type::String, { "language"sv, "region"sv, "script"sv, "currency"sv }, Empty {});
+    if (vm.exception())
+        return {};
+
+    // 14. If type is undefined, throw a TypeError exception.
+    if (type.is_undefined()) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::IsUndefined, "options.type"sv);
+        return {};
+    }
+
+    // 15. Set displayNames.[[Type]] to type.
+    display_names->set_type(type.as_string().string());
+
+    // 16. Let fallback be ? GetOption(options, "fallback", "string", « "code", "none" », "code").
+    auto fallback = get_option(global_object, options, vm.names.fallback, Value::Type::String, { "code"sv, "none"sv }, "code"sv);
+    if (vm.exception())
+        return {};
+
+    // 17. Set displayNames.[[Fallback]] to fallback.
+    display_names->set_fallback(fallback.as_string().string());
+
+    // 18. Set displayNames.[[Locale]] to r.[[locale]].
+    display_names->set_locale(move(result.locale));
+
+    // Note: The remaining steps are skipped in favor of deferring to LibUnicode. We could copy
+    //       the data from LibUnicode to the DisplayNames object, but for now we do not do that.
+
     // 28. Return displayNames.
     return display_names;
 }