LibJS: Add Reference class to represent a base.property reference

Expression nodes can now be asked to produce a Reference. We then use
this to implement the "delete" operator without downcasting the child
node to a MemberExpression manually.

Libraries/LibJS/AST.cpp

@@ -37,6 +37,7 @@
 #include <LibJS/Runtime/MarkedValueList.h>
 #include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/PrimitiveString.h>
+#include <LibJS/Runtime/Reference.h>
 #include <LibJS/Runtime/ScriptFunction.h>
 #include <LibJS/Runtime/Value.h>
 #include <stdio.h>
@@ -376,21 +377,37 @@ Value LogicalExpression::execute(Interpreter& interpreter) const
     ASSERT_NOT_REACHED();
 }
 
+Reference Expression::to_reference(Interpreter&) const
+{
+    return {};
+}
+
+Reference MemberExpression::to_reference(Interpreter& interpreter) const
+{
+    auto object_value = m_object->execute(interpreter);
+    if (object_value.is_empty())
+        return {};
+    auto* object = object_value.to_object(interpreter.heap());
+    if (!object)
+        return {};
+    auto property_name = computed_property_name(interpreter);
+    if (!property_name.is_valid())
+        return {};
+    return { object, property_name };
+}
+
 Value UnaryExpression::execute(Interpreter& interpreter) const
 {
     if (m_op == UnaryOp::Delete) {
-        if (!m_lhs->is_member_expression())
+        auto reference = m_lhs->to_reference(interpreter);
-            return Value(true);
-        auto object_value = static_cast<const MemberExpression&>(*m_lhs).object().execute(interpreter);
         if (interpreter.exception())
             return {};
-        auto* object = object_value.to_object(interpreter.heap());
+        if (reference.is_unresolvable())
-        if (!object)
+            return Value(true);
+        auto* base_object = reference.base().to_object(interpreter.heap());
+        if (!base_object)
             return {};
-        auto property_name = static_cast<const MemberExpression&>(*m_lhs).computed_property_name(interpreter);
+        return base_object->delete_property(reference.name());
-        if (!property_name.is_valid())
-            return {};
-        return object->delete_property(property_name);
     }
 
     auto lhs_result = m_lhs->execute(interpreter);

Libraries/LibJS/AST.h

@@ -140,6 +140,8 @@ private:
 };
 
 class Expression : public ASTNode {
+public:
+    virtual Reference to_reference(Interpreter&) const;
 };
 
 class Declaration : public Statement {
@@ -755,6 +757,7 @@ public:
 
     virtual Value execute(Interpreter&) const override;
     virtual void dump(int indent) const override;
+    virtual Reference to_reference(Interpreter&) const override;
 
     bool is_computed() const { return m_computed; }
     const Expression& object() const { return *m_object; }

Libraries/LibJS/Forward.h

@@ -66,6 +66,7 @@ class Interpreter;
 class LexicalEnvironment;
 class MarkedValueList;
 class PrimitiveString;
+class Reference;
 class ScopeNode;
 class Shape;
 class Statement;

Libraries/LibJS/Runtime/PropertyName.h

@@ -60,6 +60,13 @@ public:
     i32 as_number() const { return m_number; }
     const FlyString& as_string() const { return m_string; }
 
+    String to_string() const
+    {
+        if (is_string())
+            return as_string();
+        return String::number(as_number());
+    }
+
 private:
     Type m_type { Type::Invalid };
     FlyString m_string;

Libraries/LibJS/Runtime/Reference.h

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <LibJS/Runtime/PropertyName.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+class Reference {
+public:
+    Reference() {}
+    Reference(Value base, const PropertyName& name, bool strict = false)
+        : m_base(base)
+        , m_name(name)
+        , m_strict(strict)
+    {
+    }
+
+    Value base() const { return m_base; }
+    const PropertyName& name() const { return m_name; }
+    bool is_strict() const { return m_strict; }
+
+    bool is_unresolvable() const { return m_base.is_undefined(); }
+    bool is_property() const
+    {
+        return m_base.is_object() || has_primitive_base();
+    }
+
+    bool has_primitive_base() const
+    {
+        return m_base.is_boolean() || m_base.is_string() || m_base.is_number();
+    }
+
+private:
+    Value m_base { js_undefined() };
+    PropertyName m_name;
+    bool m_strict { false };
+};
+
+const LogStream& operator<<(const LogStream&, const Value&);
+
+}