I recently finished reworking my site's CSS using Tailwind CSS. Everything went smoothly locally - especially cool since I have no prior experience using Webpacker, or even Yarn. Tests were passing with no issues. After pushing my finished changes, TravisCI immediately broke - complaining about something wrong with Webpacker:

$ bundle exec rake webpacker:compile
Compilation failed:
/home/travis/build/taylorthurlow/thurlow.io/vendor/bundle/ruby/2.5.0/gems/webpacker-3.5.5/lib/webpacker/webpack_runner.rb:11:in `exec': No such file or directory - /home/travis/build/taylorthurlow/thurlow.io/node_modules/.bin/webpack (Errno::ENOENT)
    from /home/travis/build/taylorthurlow/thurlow.io/vendor/bundle/ruby/2.5.0/gems/webpacker-3.5.5/lib/webpacker/webpack_runner.rb:11:in `block in run'
    from /home/travis/build/taylorthurlow/thurlow.io/vendor/bundle/ruby/2.5.0/gems/webpacker-3.5.5/lib/webpacker/webpack_runner.rb:10:in `chdir'
    from /home/travis/build/taylorthurlow/thurlow.io/vendor/bundle/ruby/2.5.0/gems/webpacker-3.5.5/lib/webpacker/webpack_runner.rb:10:in `run'
    from /home/travis/build/taylorthurlow/thurlow.io/vendor/bundle/ruby/2.5.0/gems/webpacker-3.5.5/lib/webpacker/runner.rb:6:in `run'
    from ./bin/webpack:15:in `<main>'

To me, it wasn't exactly clear what was going on. It looked to me, at first, like the Webpacker gem just wasn't being installed, but the gem was in the gemfile, and after all, this worked locally. Here's the travis.yml that eventually solved my problem, with some of my stuff removed for brevity:

language: ruby
  - 2.5.1

    - NODE_ENV=test

  - bundle exec rake db:schema:load RAILS_ENV=test

  bundler: true
    - node_modules
  yarn: true

  - bundle install
  - nvm install node
  - node -v
  - npm i -g yarn
  - yarn

  - bundle exec rails webpacker:compile
  - bundle exec rake

I'm not sure which of these changes fixed my problem, but it's likely the entire install section which wasn't there before. I hope this is helpful in getting Webpacker working properly on TravisCI.

Happy hacking!