LibJS: Extend Intl.DisplayNames.of to support language tags

Author: https://github.com/trflynn89
Commit: ca77a7c573
Pull-request: https://github.com/SerenityOS/serenity/pull/9622
Reviewed-by: https://github.com/linusg

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

@@ -77,6 +77,7 @@ JS_DEFINE_NATIVE_FUNCTION(DisplayNamesPrototype::of)
 
     switch (display_names->type()) {
     case DisplayNames::Type::Language:
+        result = Unicode::get_locale_language_mapping(display_names->locale(), code.as_string().string());
         break;
     case DisplayNames::Type::Region:
         result = Unicode::get_locale_territory_mapping(display_names->locale(), code.as_string().string());

Userland/Libraries/LibJS/Tests/builtins/Intl/DisplayNames/DisplayNames.prototype.of.js

@@ -29,6 +29,21 @@ describe("correct behavior", () => {
         expect(Intl.DisplayNames.prototype.of).toHaveLength(1);
     });
 
+    test("option type language", () => {
+        const en = new Intl.DisplayNames("en", { type: "language" });
+        expect(en.of("en")).toBe("English");
+
+        const es419 = new Intl.DisplayNames("es-419", { type: "language" });
+        expect(es419.of("en")).toBe("inglés");
+
+        const zhHant = new Intl.DisplayNames(["zh-Hant"], { type: "language" });
+        expect(zhHant.of("en")).toBe("英文");
+
+        expect(en.of("zz")).toBe("zz");
+        expect(es419.of("zz")).toBe("zz");
+        expect(zhHant.of("zz")).toBe("zz");
+    });
+
     test("option type region", () => {
         const en = new Intl.DisplayNames("en", { type: "region" });
         expect(en.of("US")).toBe("United States");