How extend a C++ created class in JavaScript

[George Marques]

I'm trying to embed V8 5.4 into a game engine. I want to expose the engine types to JavaScript as ES6 classes.

I am actually able to do that, but when I extend the class in JavaScript the methods are not being overridden by the JS code.

Here's a sample of how I am doing it:

class Point {
public:
  int x;
  int y;
  int sum() { return x + y; }
};


void PointConstructor(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  if (!args.IsConstructCall()) {
    isolate->ThrowException(String::NewFromUtf8(isolate, "Can only be called as constructor."));
  return;
  }

  Point* p = new Point();
  p->x = args[0]->IntegerValue();
  p->y = args[1]->IntegerValue();


  args.This()->SetInternalField(0, External::New(isolate, p));
}


void PointSum(const FunctionCallbackInfo<Value>& args) {
  Local<External> field = Local<External>::Cast(args.This()->GetInternalField(0));
  Point* p = static_cast<Point*>(field->Value());

  args.GetReturnValue().Set(p->sum());
}


int main(int argc, char* argv[]) {
  // ... initialize V8
 
  // Create the global object with print function for test
  Local<ObjectTemplate> global = ObjectTemplate::New(isolate);
  global->Set(String::NewFromUtf8(isolate, "print"), FunctionTemplate::New(isolate, PrintCallback));

  // Create the point constructor template
  Local<FunctionTemplate> point_constructor = FunctionTemplate::New(isolate, PointConstructor);
  Local<ObjectTemplate> point_template = point_constructor->InstanceTemplate();
  point_template->SetInternalFieldCount(1);
  point_template->Set(String::NewFromUtf8(isolate, "sum"), FunctionTemplate::New(isolate, PointSum));
  point_template->SetHandler(NamedPropertyHandlerConfiguration(NamedGet));

  // Add the constructor to the global object as the type
  global->Set(String::NewFromUtf8(isolate, "Point"), point_constructor);
 
  // ... compile and run code.
}

And here is the Java Script code:

class MyPoint extends Point {
  sum() {
    return 25;
  }
}

var p = new MyPoint(2, 3);
print(p.sum()) // Prints 5, using the native function

My problem is that the sum() method of the child type is not called if the parent method is native. Is there a way to enforce the overriding?

[George Marques]

Solved my problem. I had to use point_constructor->InstanceTemplate() only to set the internal field count. To set the methods I need to use point_constructor->PrototypeTemplate().

Still struggling a bit to understand why it has to be that way. V8 internals is kind of a mystery.

[Ben Noordhuis]

On Tue, Nov 8, 2016 at 5:49 AM, George Marques <george...@gmail.com> wrote:
> Solved my problem. I had to use point_constructor->InstanceTemplate() only
> to set the internal field count. To set the methods I need to use
> point_constructor->PrototypeTemplate().
>
> Still struggling a bit to understand why it has to be that way. V8 internals
> is kind of a mystery.

It sounds like you figured it out but just in case: in your example
the C++ code is overriding the JS method. The prototype template is
the base class (insofar that concept applies to JS) whereas the
instance template is the derived class.