[v8-users] 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?

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