A rake-based approach to integrate jsbundling-rails into RSpec runs that only rebuilds when JS has changed.

assets.rake

# lib/tasks/asset.rake
#
# Hook up to jsbundling-rails by making spec:prepare dependent
# on app/assets/builds/application.js, a file-based equivalent to
# javascript:build. Will only rebuild when JS files have changed
if Rails.env.test?
  ASSET_SOURCE_DIR = 'app/javascript'.freeze
  ASSET_BUILDS_DIR = 'app/assets/builds'.freeze
  JS_SOURCE = FileList["#{ASSET_SOURCE_DIR}/**/*"]

  #
  # application.js is symbolic for "all the app JS and sourcemaps, timestamped".
  # It will only rebuild if any file found in the JS_SOURCE FileList is newer.
  file "#{ASSET_BUILDS_DIR}/application.js" => JS_SOURCE do
    unless system 'yarn install && yarn build'
      raise 'assets.rake: Command build failed, ensure yarn is installed and `yarn build` runs without errors'
    end
  end

  Rake::Task['spec:prepare'].enhance(["#{ASSET_BUILDS_DIR}/application.js"])
end

rails_helper.rb

# /spec/rails_helper.rb

require 'spec_helper'
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../config/environment', __dir__)
# Prevent database truncation if the environment is production
abort('The Rails environment is running in production mode!') if Rails.env.production?
require 'rspec/rails'
# Add additional requires below this line. Rails is not loaded until this point!

Rails.application.load_tasks
Rake::Task['spec:prepare'].invoke

Dir[Rails.root.join('spec', 'support', 'config', '*.rb')].each { |f| require f }

# Checks for pending migrations and applies them before tests are run.
# If you are not using ActiveRecord, you can remove these lines.
begin
  ActiveRecord::Migration.maintain_test_schema!
rescue ActiveRecord::PendingMigrationError => e
  puts e.to_s.strip
  exit 1
end

require 'byebug/core'
Byebug.mode = :off if ENV['DISABLE_BYEBUG']

RSpec.configure do |config|
  # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
  config.fixture_path = "#{::Rails.root}/spec/fixtures"

  # If you're not using ActiveRecord, or you'd prefer not to run each of your
  # examples within a transaction, remove the following line or assign false
  # instead of true.
  config.use_transactional_fixtures = true

  # You can uncomment this line to turn off ActiveRecord support entirely.
  # config.use_active_record = false

  # RSpec Rails can automatically mix in different behaviours to your tests
  # based on their file location, for example enabling you to call `get` and
  # `post` in specs under `spec/controllers`.
  #
  # You can disable this behaviour by removing the line below, and instead
  # explicitly tag your specs with their type, e.g.:
  #
  #     RSpec.describe UsersController, type: :controller do
  #       # ...
  #     end
  #
  # The different available types are documented in the features, such as in
  # https://relishapp.com/rspec/rspec-rails/docs
  config.infer_spec_type_from_file_location!

  # Filter lines from Rails gems in backtraces.
  config.filter_rails_from_backtrace!
  # arbitrary gems may also be filtered via:
  # config.filter_gems_from_backtrace("gem name")

  config.include ActiveJob::TestHelper, type: :feature
end