Add node modules to Rails assets paths

node_modules.rb

# Allow Rails to use assets from Node packages
package_json = Rails.root.join("package.json")
Rails.configuration.watchable_files << package_json
Rails.configuration.to_prepare do
  node_config = JSON.parse(package_json.read)
  paths = Rails.configuration.assets.paths
  node_modules = Rails.root.join("node_modules")
  paths.reject! { |p| p.to_s.start_with?(node_modules.to_s) }
  node_config["dependencies"].keys.each do |node_module|
    module_dir = node_modules.join(node_module)
    fail "Please run `npm install`" unless module_dir.exist?
    module_config = JSON.parse(module_dir.join("package.json").read)
    if module_config["main"]
      main_dir = module_dir.join(module_config["main"]).expand_path.dirname
      paths << main_dir.to_s unless paths.include?(main_dir.to_s)
    end
    if module_config["directories"] && module_config["directories"]["lib"]
      lib_dir = module_dir.join(module_config["directories"]["lib"]).expand_path
      paths << lib_dir.to_s if lib_dir != main_dir && !paths.include?(lib_dir.to_s)
    end
  end
end