LibJS: Add support for public fields in classes

Userland/Libraries/LibJS/Runtime/FunctionObject.cpp

@@ -66,6 +66,23 @@ BoundFunction* FunctionObject::bind(Value bound_this_value, Vector<Value> argume
     return heap().allocate<BoundFunction>(global_object(), global_object(), target_function, bound_this_object, move(all_bound_arguments), computed_length, constructor_prototype);
 }
 
+void FunctionObject::add_field(StringOrSymbol property_key, FunctionObject* initializer)
+{
+    m_fields.empend(property_key, initializer);
+}
+
+// 7.3.31 DefineField ( receiver, fieldRecord ), https://tc39.es/ecma262/#sec-definefield
+void FunctionObject::InstanceField::define_field(VM& vm, Object& receiver) const
+{
+    Value init_value = js_undefined();
+    if (initializer) {
+        init_value = vm.call(*initializer, receiver.value_of());
+        if (vm.exception())
+            return;
+    }
+    receiver.create_data_property_or_throw(name, init_value);
+}
+
 void FunctionObject::visit_edges(Visitor& visitor)
 {
     Object::visit_edges(visitor);
@@ -75,6 +92,9 @@ void FunctionObject::visit_edges(Visitor& visitor)
 
     for (auto argument : m_bound_arguments)
         visitor.visit(argument);
+
+    for (auto& field : m_fields)
+        visitor.visit(field.initializer);
 }
 
 }

Userland/Libraries/LibJS/Runtime/FunctionObject.h

@@ -63,6 +63,17 @@ public:
     // This is for IsSimpleParameterList (static semantics)
     bool has_simple_parameter_list() const { return m_has_simple_parameter_list; }
 
+    // [[Fields]]
+    struct InstanceField {
+        StringOrSymbol name;
+        FunctionObject* initializer { nullptr };
+
+        void define_field(VM& vm, Object& receiver) const;
+    };
+
+    Vector<InstanceField> const& fields() const { return m_fields; }
+    void add_field(StringOrSymbol property_key, FunctionObject* initializer);
+
 protected:
     virtual void visit_edges(Visitor&) override;
 
@@ -79,6 +90,7 @@ private:
     ConstructorKind m_constructor_kind = ConstructorKind::Base;
     ThisMode m_this_mode { ThisMode::Global };
     bool m_has_simple_parameter_list { false };
+    Vector<InstanceField> m_fields;
 };
 
 }

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -457,6 +457,16 @@ static void append_bound_and_passed_arguments(MarkedValueList& arguments, Vector
     }
 }
 
+// 7.3.32 InitializeInstanceElements ( O, constructor ), https://tc39.es/ecma262/#sec-initializeinstanceelements
+void VM::initialize_instance_elements(Object& object, FunctionObject& constructor)
+{
+    for (auto& field : constructor.fields()) {
+        field.define_field(*this, object);
+        if (exception())
+            return;
+    }
+}
+
 Value VM::construct(FunctionObject& function, FunctionObject& new_target, Optional<MarkedValueList> arguments)
 {
     auto& global_object = function.global_object();
@@ -494,6 +504,14 @@ Value VM::construct(FunctionObject& function, FunctionObject& new_target, Option
 
     // If we are a Derived constructor, |this| has not been constructed before super is called.
     callee_context.this_value = this_argument;
+
+    if (function.constructor_kind() == FunctionObject::ConstructorKind::Base) {
+        VERIFY(this_argument.is_object());
+        initialize_instance_elements(this_argument.as_object(), function);
+        if (exception())
+            return {};
+    }
+
     auto result = function.construct(new_target);
 
     pop_execution_context();

Userland/Libraries/LibJS/Runtime/VM.h

@@ -267,6 +267,8 @@ public:
     Function<void(const Promise&)> on_promise_unhandled_rejection;
     Function<void(const Promise&)> on_promise_rejection_handled;
 
+    void initialize_instance_elements(Object& object, FunctionObject& constructor);
+
 private:
     VM();