The Node.js platform provides Heroku-specific build hooks
Change effective on 11 March 2016
Sometimes, developers need something more production-oriented than the
postinstall hooks in package.json. For instance, some apps need to set up extra authentication before installing dependencies. Some need to build assets, but not in a development environment. Further examples can be found in the discussion on GitHub.
Node.js developers can now use
heroku-postbuild hooks to tailor the build process to their apps.