LibJS: Add the CreateMappedArgumentsObject abstract operation

This patch adds a new ArgumentsObject class to represent what the spec
calls "Arguments Exotic Objects"

These are constructed by the new CreateMappedArgumentsObject when the
`arguments` identifier is resolved in a callee context.

The implementation is incomplete and doesn't yet support mapping of
the parameter variables to the indexed properties of `arguments`.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -26,6 +26,7 @@ set(SOURCES
     Runtime/AggregateError.cpp
     Runtime/AggregateErrorConstructor.cpp
     Runtime/AggregateErrorPrototype.cpp
+    Runtime/ArgumentsObject.cpp
     Runtime/Array.cpp
     Runtime/ArrayBuffer.cpp
     Runtime/ArrayBufferConstructor.cpp

Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp

@@ -11,6 +11,7 @@
 #include <LibJS/Interpreter.h>
 #include <LibJS/Parser.h>
 #include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/ArgumentsObject.h>
 #include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/ArrayPrototype.h>
 #include <LibJS/Runtime/BoundFunction.h>
@@ -214,6 +215,9 @@ Object* create_unmapped_arguments_object(GlobalObject& global_object, Vector<Val
     if (vm.exception())
         return nullptr;
 
+    for (auto& argument : arguments)
+        object->indexed_properties().append(argument);
+
     // 4. Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
     auto length = arguments.size();
     object->define_property(vm.names.length, Value(length), Attribute::Writable | Attribute::Configurable);
@@ -222,9 +226,6 @@ Object* create_unmapped_arguments_object(GlobalObject& global_object, Vector<Val
 
     object->define_property(*vm.well_known_symbol_iterator(), global_object.array_prototype()->get(vm.names.values), Attribute::Writable | Attribute::Configurable);
 
-    for (auto& argument : arguments)
-        object->indexed_properties().append(argument);
-
     // 8. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false }).
     object->define_accessor(vm.names.callee, global_object.throw_type_error_function(), global_object.throw_type_error_function(), 0);
     if (vm.exception())
@@ -233,4 +234,40 @@ Object* create_unmapped_arguments_object(GlobalObject& global_object, Vector<Val
     return object;
 }
 
+// 10.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env ), https://tc39.es/ecma262/#sec-createmappedargumentsobject
+Object* create_mapped_arguments_object(GlobalObject& global_object, FunctionObject& function, Vector<FunctionNode::Parameter> const& formals, Vector<Value> const& arguments, EnvironmentRecord&)
+{
+    // FIXME: This implementation is incomplete and doesn't support the actual identifier mappings yet.
+    (void)formals;
+
+    auto& vm = global_object.vm();
+    auto* object = vm.heap().allocate<ArgumentsObject>(global_object, global_object);
+    if (vm.exception())
+        return nullptr;
+
+    // 14. Let index be 0.
+    // 15. Repeat, while index < len,
+    //     a. Let val be argumentsList[index].
+    //     b . Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
+    //     c. Set index to index + 1.
+    for (auto& argument : arguments)
+        object->indexed_properties().append(argument);
+
+    // 16. Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor { [[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
+    auto length = arguments.size();
+    object->define_property(vm.names.length, Value(length), Attribute::Writable | Attribute::Configurable);
+    if (vm.exception())
+        return nullptr;
+
+    // 20. Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
+    object->define_property(*vm.well_known_symbol_iterator(), global_object.array_prototype()->get(vm.names.values), Attribute::Writable | Attribute::Configurable);
+
+    // 21. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor { [[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }).
+    object->define_property(vm.names.callee, Value(&function), Attribute::Writable | Attribute::Configurable);
+    if (vm.exception())
+        return nullptr;
+
+    return object;
+}
+
 }

Userland/Libraries/LibJS/Runtime/AbstractOperations.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Forward.h>
+#include <LibJS/AST.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Value.h>
@@ -24,6 +25,7 @@ FunctionObject* species_constructor(GlobalObject&, Object const&, FunctionObject
 GlobalObject* get_function_realm(GlobalObject&, FunctionObject const&);
 Object* get_prototype_from_constructor(GlobalObject&, FunctionObject const& constructor, Object* (GlobalObject::*intrinsic_default_prototype)());
 Object* create_unmapped_arguments_object(GlobalObject&, Vector<Value> const& arguments);
+Object* create_mapped_arguments_object(GlobalObject&, FunctionObject&, Vector<FunctionNode::Parameter> const&, Vector<Value> const& arguments, EnvironmentRecord&);
 
 enum class CallerMode {
     Strict,

Userland/Libraries/LibJS/Runtime/ArgumentsObject.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/ArgumentsObject.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS {
+
+ArgumentsObject::ArgumentsObject(GlobalObject& global_object)
+    : Object(*global_object.object_prototype())
+{
+}
+
+void ArgumentsObject::initialize(GlobalObject& global_object)
+{
+    Base::initialize(global_object);
+}
+
+ArgumentsObject::~ArgumentsObject()
+{
+}
+
+}

Userland/Libraries/LibJS/Runtime/ArgumentsObject.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class ArgumentsObject final : public Object {
+    JS_OBJECT(ArgumentsObject, Object);
+
+public:
+    explicit ArgumentsObject(GlobalObject&);
+
+    virtual void initialize(GlobalObject&) override;
+    virtual ~ArgumentsObject() override;
+};
+
+}

Userland/Libraries/LibJS/Runtime/Object.h

@@ -108,6 +108,7 @@ public:
     virtual bool is_global_object() const { return false; }
     virtual bool is_proxy_object() const { return false; }
     virtual bool is_native_function() const { return false; }
+    virtual bool is_ordinary_function_object() const { return false; }
 
     // B.3.7 The [[IsHTMLDDA]] Internal Slot, https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot
     virtual bool is_htmldda() const { return false; }

Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.h

@@ -41,6 +41,7 @@ protected:
     virtual bool is_strict_mode() const final { return m_is_strict; }
 
 private:
+    virtual bool is_ordinary_function_object() const { return true; }
     virtual FunctionEnvironmentRecord* create_environment_record(FunctionObject&) override;
     virtual void visit_edges(Visitor&) override;
 

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -372,13 +372,7 @@ Value VM::get_variable(const FlyString& name, GlobalObject& global_object)
                 if (context.function->is_strict_mode() || !context.function->has_simple_parameter_list()) {
                     context.arguments_object = create_unmapped_arguments_object(global_object, context.arguments);
                 } else {
-                    // FIXME: This code path is completely ad-hoc.
+                    context.arguments_object = create_mapped_arguments_object(global_object, *context.function, verify_cast<OrdinaryFunctionObject>(context.function)->parameters(), context.arguments, *lexical_environment());
-                    context.arguments_object = Array::create(global_object);
-                    context.arguments_object->put(names.callee, context.function);
-
-                    for (auto argument : context.arguments) {
-                        context.arguments_object->indexed_properties().append(argument);
-                    }
                 }
             }
             return context.arguments_object;