hash_formatter.rb

module ActionDispatch
  module Routing
    class HashFormatter
      attr_reader :result

      def initialize
        @result = {}
      end

      def section_title(title); end

      def section(routes)
        routes.each { |route| incorporate_route(route) }
      end

      def header(routes); end

      def no_routes(routes, filter); end

      private

      def incorporate_route(route)
        name = route[:name]
        return if route.empty? || name.empty?

        prepared_route = route.without(:reqs, :name).merge(
          path: route[:path].remove('(.:format)')
        )

        result.merge!(name => prepared_route)
      end
    end
  end
end

rails_routes_json.rb

require 'action_dispatch/routing/inspector'

module ApplicationHelper
  def client_routes_json
    inspector = ActionDispatch::Routing::RoutesInspector.new(
      Rails.application.routes.routes
    )

    inspector.format(ActionDispatch::Routing::HashFormatter.new).to_json
  end
end