Rails HMR with rack-proxy and webpack-dev-server, not using Webpacker

webpack_dev_server.rb

# config/initializers/webpack_dev_server.rb

if Rails.development?
  Rails.application.config.middleware.use WebpackDevServerProxy, dev_server_host: "localhost:9000"
end

webpack_dev_server_proxy.rb

# lib/webpack_dev_server_proxy.rb

# A proxy to forward requests to GET assets to webpack-dev-server behind Rails server.
class WebpackDevServerProxy < Rack::Proxy
  def initialize(app = nil, opts = {})
    super
    @dev_server_host = opts[:dev_server_host]
  end

  def perform_request(env)
    if env['PATH_INFO'].start_with?('/assets/pack/') # Specify asset paths to proxy
      env['HTTP_HOST'] = @dev_server_host
      super
    else
      @app.call(env)
    end
  end
end

webpack_manifest.rb

# in config/initializers/webpack_manifest.rb

WebpackManifest::Rails.configuration do |c|
  c.cache = !Rails.env.development?

  c.manifest = if Rails.env.development?
                 'http://localhost:9000/assets/pack/manifest.json'
               else
                 Rails.root.join('public', 'assets', 'manifest.json')
               end
end