LibJS: Add a mechanism for callback-based object properties

This patch adds NativeProperty, which can be used to implement object
properties that have C++ getters and/or setters.

Use this to move String.prototype.length to its correct place. :^)

Libraries/LibJS/Makefile

@@ -8,6 +8,7 @@ OBJS = \
     Interpreter.o \
     Lexer.o \
     NativeFunction.o \
+    NativeProperty.o \
     Object.o \
     ObjectPrototype.o \
     Parser.o \

Libraries/LibJS/NativeProperty.cpp

@@ -0,0 +1,56 @@
+/*
+ * 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.
+ */
+
+#include <LibJS/NativeProperty.h>
+#include <LibJS/Value.h>
+
+namespace JS {
+
+NativeProperty::NativeProperty(AK::Function<Value(Object*)> getter, AK::Function<void(Object*, Value)> setter)
+    : m_getter(move(getter))
+    , m_setter(move(setter))
+{
+}
+
+NativeProperty::~NativeProperty()
+{
+}
+
+Value NativeProperty::get(Object* object) const
+{
+    if (!m_getter)
+        return js_undefined();
+    return m_getter(object);
+}
+
+void NativeProperty::set(Object* object, Value value)
+{
+    if (!m_setter)
+        return;
+    m_setter(object, move(value));
+}
+
+}

Libraries/LibJS/NativeProperty.h

@@ -0,0 +1,50 @@
+/*
+ * 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/Function.h>
+#include <LibJS/Object.h>
+
+namespace JS {
+
+class NativeProperty final : public Object {
+public:
+    NativeProperty(AK::Function<Value(Object*)> getter, AK::Function<void(Object*, Value)> setter);
+    virtual ~NativeProperty() override;
+
+    Value get(Object*) const;
+    void set(Object*, Value);
+
+private:
+    virtual bool is_native_property() const override { return true; }
+    virtual const char* class_name() const override { return "NativeProperty"; }
+
+    AK::Function<Value(Object*)> m_getter;
+    AK::Function<void(Object*, Value)> m_setter;
+};
+
+}

Libraries/LibJS/Object.cpp

@@ -28,6 +28,7 @@
 #include <LibJS/Heap.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/NativeFunction.h>
+#include <LibJS/NativeProperty.h>
 #include <LibJS/Object.h>
 #include <LibJS/Value.h>
 
@@ -47,8 +48,11 @@ Value Object::get(String property_name) const
     const Object* object = this;
     while (object) {
         auto value = object->m_properties.get(property_name);
-        if (value.has_value())
+        if (value.has_value()) {
+            if (value.value().is_object() && value.value().as_object()->is_native_property())
+                return static_cast<const NativeProperty*>(value.value().as_object())->get(const_cast<Object*>(this));
             return value.value();
+        }
         object = object->prototype();
     }
     return js_undefined();
@@ -64,6 +68,11 @@ void Object::put_native_function(String property_name, AK::Function<Value(Interp
     put(property_name, heap().allocate<NativeFunction>(move(native_function)));
 }
 
+void Object::put_native_property(String property_name, AK::Function<Value(Object*)> getter, AK::Function<void(Object*, Value)> setter)
+{
+    put(property_name, heap().allocate<NativeProperty>(move(getter), move(setter)));
+}
+
 void Object::visit_children(Cell::Visitor& visitor)
 {
     Cell::visit_children(visitor);

Libraries/LibJS/Object.h

@@ -42,10 +42,12 @@ public:
     void put(String property_name, Value);
 
     void put_native_function(String property_name, AK::Function<Value(Interpreter&, Vector<Value>)>);
+    void put_native_property(String property_name, AK::Function<Value(Object*)> getter, AK::Function<void(Object*, Value)> setter);
 
     virtual bool is_function() const { return false; }
     virtual bool is_native_function() const { return false; }
     virtual bool is_string_object() const { return false; }
+    virtual bool is_native_property() const { return false; }
 
     virtual const char* class_name() const override { return "Object"; }
     virtual void visit_children(Cell::Visitor&) override;

Libraries/LibJS/StringObject.cpp

@@ -37,7 +37,6 @@ StringObject::StringObject(PrimitiveString* string)
     : m_string(string)
 {
     set_prototype(interpreter().string_prototype());
-    put("length", Value(static_cast<i32>(m_string->string().length())));
 }
 
 StringObject::~StringObject()

Libraries/LibJS/StringPrototype.cpp

@@ -36,6 +36,12 @@ namespace JS {
 
 StringPrototype::StringPrototype()
 {
+    put_native_property(
+        "length", [](Object* object) {
+            ASSERT(object->is_string_object());
+            return Value((i32) static_cast<const StringObject*>(object)->primitive_string()->string().length());
+        },
+        nullptr);
     put_native_function("charAt", [](Interpreter& interpreter, Vector<Value> arguments) -> Value {
         i32 index = 0;
         if (!arguments.is_empty())