LibJS: Implement basic support for the "delete" operator

It turns out "delete" is actually a unary op :)
This patch implements deletion of object properties, it doesn't yet
work for casually deleting properties from the global object.

When deleting a property from an object, we switch that object to
having a unique shape, no longer sharing shapes with others.
Once an object has a unique shape, it no longer needs to care about
shape transitions.

1 files changed, 8 insertions, 0 deletions

diff --git a/Libraries/LibJS/Runtime/Shape.h b/Libraries/LibJS/Runtime/Shape.h
index 70c75fef78..26c8eb574b 100644
--- a/Libraries/LibJS/Runtime/Shape.h
+++ b/Libraries/LibJS/Runtime/Shape.h
@@ -78,6 +78,9 @@ public:
     Shape* create_configure_transition(const FlyString& name, u8 attributes);
     Shape* create_prototype_transition(Object* new_prototype);
 
+    bool is_unique() const { return m_unique; }
+    Shape* create_unique_clone() const;
+
     Object* prototype() { return m_prototype; }
     const Object* prototype() const { return m_prototype; }
 
@@ -87,6 +90,10 @@ public:
 
     void set_prototype_without_transition(Object* new_prototype) { m_prototype = new_prototype; }
 
+    void remove_property_from_unique_shape(const FlyString&, size_t offset);
+    void add_property_to_unique_shape(const FlyString&, u8 attributes);
+    void reconfigure_property_in_unique_shape(const FlyString& property_name, u8 attributes);
+
 private:
     virtual const char* class_name() const override { return "Shape"; }
     virtual void visit_children(Visitor&) override;
@@ -99,6 +106,7 @@ private:
     Shape* m_previous { nullptr };
     FlyString m_property_name;
     u8 m_attributes { 0 };
+    bool m_unique { false };
     Object* m_prototype { nullptr };
     TransitionType m_transition_type { TransitionType::Invalid };
 };