LibJS: Implement Iterator.prototype [ @@toStringTag ] according to spec

The spec allows setting the prototype on non built-in Iterator objects.

This is a normative change in the Iterator Helpers proposal. See:
30b3501

Userland/Libraries/LibJS/Runtime/Iterator.cpp

@@ -353,6 +353,42 @@ ThrowCompletionOr<MarkedVector<Value>> iterator_to_list(VM& vm, IteratorRecord&
     }
 }
 
+// 2.1 SetterThatIgnoresPrototypeProperties ( this, home, p, v ), https://tc39.es/proposal-iterator-helpers/#sec-SetterThatIgnoresPrototypeProperties
+ThrowCompletionOr<void> setter_that_ignores_prototype_properties(VM& vm, Value this_, Object const& home, PropertyKey const& property, Value value)
+{
+    // 1. If this is not an Object, then
+    if (!this_.is_object()) {
+        // a. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(ErrorType::NotAnObject, this_);
+    }
+
+    auto& this_object = this_.as_object();
+
+    // 2. If this is home, then
+    if (&this_object == &home) {
+        // a. NOTE: Throwing here emulates assignment to a non-writable data property on the home object in strict mode code.
+        // b. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(ErrorType::DescWriteNonWritable, this_);
+    }
+
+    // 3. Let desc be ? this.[[GetOwnProperty]](p).
+    auto desc = TRY(this_object.internal_get_own_property(property));
+
+    // 4. If desc is undefined, then
+    if (!desc.has_value()) {
+        // a. Perform ? CreateDataPropertyOrThrow(this, p, v).
+        TRY(this_object.create_data_property_or_throw(property, value));
+    }
+    // 5. Else,
+    else {
+        // a. Perform ? Set(this, p, v, true).
+        TRY(this_object.set(property, value, Object::ShouldThrowExceptions::Yes));
+    }
+
+    // 6. Return unused.
+    return {};
+}
+
 // Non-standard
 Completion get_iterator_values(VM& vm, Value iterable, IteratorValueCallback callback)
 {

Userland/Libraries/LibJS/Runtime/Iterator.h

@@ -83,6 +83,7 @@ Completion iterator_close(VM&, IteratorRecord const&, Completion);
 Completion async_iterator_close(VM&, IteratorRecord const&, Completion);
 NonnullGCPtr<Object> create_iterator_result_object(VM&, Value, bool done);
 ThrowCompletionOr<MarkedVector<Value>> iterator_to_list(VM&, IteratorRecord&);
+ThrowCompletionOr<void> setter_that_ignores_prototype_properties(VM&, Value this_, Object const& home, PropertyKey const& property, Value value);
 
 using IteratorValueCallback = Function<Optional<Completion>(Value)>;
 Completion get_iterator_values(VM&, Value iterable, IteratorValueCallback callback);

Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp

@@ -29,9 +29,6 @@ void IteratorPrototype::initialize(Realm& realm)
     auto& vm = this->vm();
     Base::initialize(realm);
 
-    // 3.1.3.13 Iterator.prototype [ @@toStringTag ], https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype-@@tostringtag
-    define_direct_property(vm.well_known_symbol_to_string_tag(), PrimitiveString::create(vm, "Iterator"_string), Attribute::Configurable | Attribute::Writable);
-
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(realm, vm.well_known_symbol_iterator(), symbol_iterator, 0, attr);
     define_native_function(realm, vm.names.map, map, 1, attr);
@@ -45,6 +42,9 @@ void IteratorPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.some, some, 1, attr);
     define_native_function(realm, vm.names.every, every, 1, attr);
     define_native_function(realm, vm.names.find, find, 1, attr);
+
+    // 3.1.3.13 Iterator.prototype [ @@toStringTag ], https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype-@@tostringtag
+    define_native_accessor(realm, vm.well_known_symbol_to_string_tag(), to_string_tag_getter, to_string_tag_setter, Attribute::Configurable);
 }
 
 // 27.1.2.1 %IteratorPrototype% [ @@iterator ] ( ), https://tc39.es/ecma262/#sec-%iteratorprototype%-@@iterator
@@ -749,4 +749,23 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::find)
     }
 }
 
+// 3.1.3.13.1 get Iterator.prototype [ @@toStringTag ], https://tc39.es/proposal-iterator-helpers/#sec-get-iteratorprototype-@@tostringtag
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::to_string_tag_getter)
+{
+    // 1. Return "Iterator".
+    return PrimitiveString::create(vm, vm.names.Iterator.as_string());
+}
+
+// 3.1.3.13.2 set Iterator.prototype [ @@toStringTag ], https://tc39.es/proposal-iterator-helpers/#sec-set-iteratorprototype-@@tostringtag
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::to_string_tag_setter)
+{
+    auto& realm = *vm.current_realm();
+
+    // 1. Perform ? SetterThatIgnoresPrototypeProperties(this value, %Iterator.prototype%, %Symbol.toStringTag%, v).
+    TRY(setter_that_ignores_prototype_properties(vm, vm.this_value(), realm.intrinsics().iterator_prototype(), vm.well_known_symbol_to_string_tag(), vm.argument(0)));
+
+    // 2. Return undefined.
+    return js_undefined();
+}
+
 }

Userland/Libraries/LibJS/Runtime/IteratorPrototype.h

@@ -34,6 +34,9 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(some);
     JS_DECLARE_NATIVE_FUNCTION(every);
     JS_DECLARE_NATIVE_FUNCTION(find);
+
+    JS_DECLARE_NATIVE_FUNCTION(to_string_tag_getter);
+    JS_DECLARE_NATIVE_FUNCTION(to_string_tag_setter);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.@@toStringTag.js

@@ -1,3 +1,33 @@
-test("basic functionality", () => {
-    expect(Iterator.prototype[Symbol.toStringTag]).toBe("Iterator");
+const sentinel = "whf :^)";
+
+describe("errors", () => {
+    test("setter called on non-object", () => {
+        let { get, set } = Object.getOwnPropertyDescriptor(Iterator.prototype, Symbol.toStringTag);
+
+        expect(() => {
+            set.call(undefined, sentinel);
+        }).toThrowWithMessage(TypeError, "undefined is not an object");
+    });
+
+    test("cannot set the built-in Iterator's toStringTag", () => {
+        expect(() => {
+            Iterator.prototype[Symbol.toStringTag] = sentinel;
+        }).toThrowWithMessage(
+            TypeError,
+            "Cannot write to non-writable property '[object IteratorPrototype]'"
+        );
+    });
+});
+
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        expect(Iterator.prototype[Symbol.toStringTag]).toBe("Iterator");
+    });
+
+    test("toStringTag setter", () => {
+        let Proto = Object.create(Iterator.prototype);
+        Proto[Symbol.toStringTag] = sentinel;
+
+        expect(Proto[Symbol.toStringTag]).toBe(sentinel);
+    });
 });