Posted 12 months ago. Visible to the public.

How to make Webpacker compile once for parallel tests, and only if necessary

Webpack is the future. We're using it in our latest Rails applications.

For tests, we want to compile assets like for production.
For parallel tests, we want to avoid 8 workers compiling the same files at the same time.
When assets did not change, we do not want to spend time compiling them.

Here is our solution for all that.

Its concept should work for all test suites. For Cucumber integration tests, place the following into features/support/webpacker.rb.

module WebpackerTestSupport module_function def compile_once digest_file = Rails.root.join("tmp/webpacker_#{Rails.env}_digest") app_code = Dir[Webpacker.config.source_path.join('**/*')] npm_code = Dir[Rails.root.join('yarn.lock')] packable_contents = (app_code + npm_code) .sort .map { |filename| if File.file?(filename) } .join digest = Digest::SHA256.hexdigest(packable_contents) return if digest_file.exist? && == digest if ENV['TEST_ENV_NUMBER'].to_i < 1 public_output_path = Webpacker.config.public_output_path FileUtils.rm_r(public_output_path) if File.exist?(public_output_path) puts "Removed Webpack output directory #{public_output_path}" Webpacker.compile digest_file.write(digest) else loop do break if digest_file.exist? && == digest sleep 0.1 end end end end WebpackerTestSupport.compile_once # The configured asset host will become a `<base>` tag in the HTML's `<head>` and allow resolving test assets via the # development Rails server. This makes HTML-only screenshots for non-`@javascript` Scenarios prettier. # Note that you won't be using development assets, but the compiled files from inside `RAILS_ROOT/public/packs-test/`. Capybara.asset_host = 'http://localhost:3000/'

That's it. Enjoy! 🎉

By refactoring problematic code and creating automated tests, makandra can vastly improve the maintainability of your Rails application.

Author of this card:

Arne Hartherz
Last edit:
3 months ago
by Emanuel De
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Arne Hartherz to makandra dev
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