Get data from javascript
There does not seem to be a single answer for this question, but I will try to give some directions in possible solutions. First of all I presume that "the scene graph technique" means storing the object data in some kind of data structure (I know what a scenegraph is, but there are many different possibilities for indoor/outdoor scenes etc). There seem to be some Javascript libraries, but they do much more then simply providing the datastructure.

For defining the scenegraph structure you could look at glTF(https://github.com/KhronosGroup/glTF) defined by Khronos themselves. This really depends on what kind of features your scene graph requires.

If the C++ library is required for the processing you can look into providing Javascript bindings for it. You can either try to compile the library to javascript using emscripten (https://github.com/kripken/emscripten). Otherwise provide a C-interface and call it directly using ccall(). This way you can directly share this datastructure via parameters.