LibJS: Add [[InitialName]] and use it in Function.prototype.toString()

Userland/Libraries/LibJS/Runtime/FunctionObject.cpp

@@ -61,7 +61,7 @@ void FunctionObject::set_function_name(Variant<PropertyKey, PrivateName> const&
     // 4. If F has an [[InitialName]] internal slot, then
     if (is<NativeFunction>(this)) {
         // a. Set F.[[InitialName]] to name.
-        // TODO: Remove FunctionObject::name(), implement NativeFunction::initial_name(), and then do this.
+        static_cast<NativeFunction&>(*this).set_initial_name({}, name);
     }
 
     // 5. If prefix is present, then
@@ -72,7 +72,7 @@ void FunctionObject::set_function_name(Variant<PropertyKey, PrivateName> const&
         // b. If F has an [[InitialName]] internal slot, then
         if (is<NativeFunction>(this)) {
             // i. Optionally, set F.[[InitialName]] to name.
-            // TODO: See above.
+            static_cast<NativeFunction&>(*this).set_initial_name({}, name);
         }
     }
 

Userland/Libraries/LibJS/Runtime/FunctionPrototype.cpp

@@ -155,8 +155,12 @@ JS_DEFINE_NATIVE_FUNCTION(FunctionPrototype::to_string)
     }
 
     // 3. If func is a built-in function object, return an implementation-defined String source code representation of func. The representation must have the syntax of a NativeFunction. Additionally, if func has an [[InitialName]] internal slot and func.[[InitialName]] is a String, the portion of the returned String that would be matched by NativeFunctionAccessor[opt] PropertyName must be the value of func.[[InitialName]].
-    if (is<NativeFunction>(function))
-        return js_string(vm, String::formatted("function {}() {{ [native code] }}", static_cast<NativeFunction&>(function).name()));
+    if (is<NativeFunction>(function)) {
+        // NOTE: once we remove name(), the fallback here can simply be an empty string.
+        auto const& native_function = static_cast<NativeFunction&>(function);
+        auto const name = native_function.initial_name().value_or(native_function.name());
+        return js_string(vm, String::formatted("function {}() {{ [native code] }}", name));
+    }
 
     // 4. If Type(func) is Object and IsCallable(func) is true, return an implementation-defined String source code representation of func. The representation must have the syntax of a NativeFunction.
     // NOTE: ProxyObject, BoundFunction, WrappedFunction

Userland/Libraries/LibJS/Runtime/NativeFunction.h

@@ -1,12 +1,15 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2022, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <AK/Function.h>
+#include <AK/Optional.h>
 #include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/FunctionObject.h>
 
@@ -35,6 +38,9 @@ public:
     virtual bool has_constructor() const override { return false; }
     virtual Realm* realm() const override { return m_realm; }
 
+    Optional<FlyString> const& initial_name() const { return m_initial_name; }
+    void set_initial_name(Badge<FunctionObject>, FlyString initial_name) { m_initial_name = move(initial_name); }
+
 protected:
     NativeFunction(FlyString name, Object& prototype);
     explicit NativeFunction(Object& prototype);
@@ -43,6 +49,7 @@ private:
     virtual bool is_native_function() const final { return true; }
 
     FlyString m_name;
+    Optional<FlyString> m_initial_name; // [[InitialName]]
     Function<ThrowCompletionOr<Value>(VM&, GlobalObject&)> m_native_function;
     Realm* m_realm { nullptr };
 };

Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.toString.js

@@ -127,6 +127,9 @@ describe("correct behavior", () => {
         // Built-in functions
         expect(console.debug.toString()).toBe("function debug() { [native code] }");
         expect(Function.toString()).toBe("function Function() { [native code] }");
+        expect(
+            Object.getOwnPropertyDescriptor(Temporal.TimeZone.prototype, "id").get.toString()
+        ).toBe("function get id() { [native code] }");
 
         const values = [
             // Callable Proxy