LibJS: Support "hello friends".length

The above snippet is a MemberExpression that necessitates the implicit
construction of a StringObject wrapper around a PrimitiveString.

We then do a property lookup (a "get") on the StringObject, where we
find the "length" property. This is pretty neat! :^)

Libraries/LibJS/AST.cpp

@@ -387,7 +387,6 @@ Value VariableDeclaration::execute(Interpreter& interpreter) const
     return js_undefined();
 }
 
-
 void VariableDeclaration::dump(int indent) const
 {
     ASTNode::dump(indent);
@@ -406,4 +405,26 @@ Value ObjectExpression::execute(Interpreter& interpreter) const
     return Value(interpreter.heap().allocate<Object>());
 }
 
+void MemberExpression::dump(int indent) const
+{
+    ASTNode::dump(indent);
+    m_object->dump(indent + 1);
+    m_property->dump(indent + 1);
+}
+
+Value MemberExpression::execute(Interpreter& interpreter) const
+{
+    auto object_result = m_object->execute(interpreter).to_object(interpreter.heap());
+    ASSERT(object_result.is_object());
+
+    String property_name;
+    if (m_property->is_identifier()) {
+        property_name = static_cast<const Identifier&>(*m_property).string();
+    } else {
+        ASSERT_NOT_REACHED();
+    }
+
+    return object_result.as_object()->get(property_name);
+}
+
 }

Libraries/LibJS/AST.h

@@ -363,4 +363,22 @@ private:
     virtual const char* class_name() const override { return "ObjectExpression"; }
 };
 
+class MemberExpression final : public Expression {
+public:
+    MemberExpression(NonnullOwnPtr<Expression> object, NonnullOwnPtr<Expression> property)
+        : m_object(move(object))
+        , m_property(move(property))
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "MemberExpression"; }
+
+    NonnullOwnPtr<Expression> m_object;
+    NonnullOwnPtr<Expression> m_property;
+};
+
 }

Libraries/LibJS/Cell.cpp

@@ -27,14 +27,15 @@
 #include <AK/LogStream.h>
 #include <LibJS/Cell.h>
 #include <LibJS/Object.h>
+#include <LibJS/PrimitiveString.h>
 #include <LibJS/Value.h>
 
 namespace JS {
 
 void Cell::Visitor::visit(Value value)
 {
-    if (value.is_object())
-        visit(value.as_object());
+    if (value.is_cell())
+        visit(value.as_cell());
 }
 
 const LogStream& operator<<(const LogStream& stream, const Cell* cell)

Libraries/LibJS/Value.cpp

@@ -25,7 +25,10 @@
  */
 
 #include <AK/String.h>
+#include <LibJS/Heap.h>
 #include <LibJS/Object.h>
+#include <LibJS/PrimitiveString.h>
+#include <LibJS/StringObject.h>
 #include <LibJS/Value.h>
 
 namespace JS {
@@ -50,6 +53,9 @@ String Value::to_string() const
         return String::format("{%s}", as_object()->class_name());
     }
 
+    if (is_string())
+        return m_value.as_string->string();
+
     ASSERT_NOT_REACHED();
 }
 
@@ -64,7 +70,7 @@ bool Value::to_boolean() const
     case Type::Undefined:
         return false;
     case Type::String:
-        return String(as_string()).is_empty();
+        return !as_string()->string().is_empty();
     case Type::Object:
         return true;
     default:
@@ -72,6 +78,17 @@ bool Value::to_boolean() const
     }
 }
 
+Value Value::to_object(Heap& heap) const
+{
+    if (is_object())
+        return Value(as_object());
+
+    if (is_string())
+        return Value(heap.allocate<StringObject>(m_value.as_string));
+
+    ASSERT_NOT_REACHED();
+}
+
 Value greater_than(Value lhs, Value rhs)
 {
     ASSERT(lhs.is_number());

Libraries/LibJS/Value.h

@@ -50,6 +50,7 @@ public:
     bool is_string() const { return m_type == Type::String; }
     bool is_object() const { return m_type == Type::Object; }
     bool is_boolean() const { return m_type == Type::Boolean; }
+    bool is_cell() const { return is_string() || is_object(); }
 
     explicit Value(bool value)
         : m_type(Type::Boolean)
@@ -75,6 +76,12 @@ public:
         m_value.as_object = object;
     }
 
+    explicit Value(PrimitiveString* string)
+        : m_type(Type::String)
+    {
+        m_value.as_string = string;
+    }
+
     explicit Value(Type type)
         : m_type(type)
     {
@@ -106,23 +113,38 @@ public:
         return m_value.as_object;
     }
 
-    const StringImpl* as_string() const
+    PrimitiveString* as_string()
     {
         ASSERT(is_string());
         return m_value.as_string;
     }
 
+    const PrimitiveString* as_string() const
+    {
+        ASSERT(is_string());
+        return m_value.as_string;
+    }
+
+    Cell* as_cell()
+    {
+        ASSERT(is_cell());
+        return m_value.as_cell;
+    }
+
     String to_string() const;
     bool to_boolean() const;
 
+    Value to_object(Heap&) const;
+
 private:
     Type m_type { Type::Undefined };
 
     union {
         bool as_bool;
         double as_double;
-        StringImpl* as_string;
+        PrimitiveString* as_string;
         Object* as_object;
+        Cell* as_cell;
     } m_value;
 };
 

Userland/js.cpp

@@ -28,21 +28,22 @@
 #include <LibJS/AST.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Object.h>
+#include <LibJS/PrimitiveString.h>
 #include <LibJS/Value.h>
 #include <stdio.h>
 
-#define PROGRAM 2
+#define PROGRAM 4
 
-static void build_program(JS::Program&);
+static void build_program(JS::Program&, JS::Heap&);
 
 int main()
 {
-    auto program = make<JS::Program>();
-    build_program(*program);
+    JS::Interpreter interpreter;
 
+    auto program = make<JS::Program>();
+    build_program(*program, interpreter.heap());
     program->dump(0);
 
-    JS::Interpreter interpreter;
     auto result = interpreter.run(*program);
     dbg() << "Interpreter returned " << result;
 
@@ -125,4 +126,13 @@ void build_program(JS::Program& program)
     program.append<JS::FunctionDeclaration>("foo", move(block));
     program.append<JS::CallExpression>("foo");
 }
+#elif PROGRAM == 4
+void build_program(JS::Program& program, JS::Heap& heap)
+{
+    // "hello friends".length
+
+    program.append<JS::MemberExpression>(
+        make<JS::Literal>(JS::Value(js_string(heap, "hello friends"))),
+        make<JS::Identifier>("length"));
+}
 #endif