Project Server Get project field value using Javascript

I've been using javascript to access to libraries in a site, but now I want to get the value of a custom field in a project.

I've been looking to do this but I've found nothing, clientcontext does not seem to have any way to access a project's data and I've found ProjectContext, it seems the way to go, but haven't found any example.

Anyone can point me out to the right direction?