Hello,
While trying to run the code examples from the book I always encounter the same issues that seem to be caused, maybe among other potential suspects, by webpacker.
I mostly get Unexpected token errors when webpacker tries to compile. It seems that Typescript isn’t well parsed.
By following some of the instructions described in webpacker’s README I managed sometimes before the use of Stimulus and React in the book to make it works. However once both frameworks present, I did not find my way through all those errors, to be honest.
So at this point, I can’t move forward in the book it became quite a struggle to fix here and there potential versions conflicts.
Would not it be interesting to freeze the versions of some gems/library in the Gemfile and the package.json?
Also before trying to run anything in a project directory, yarn always asks to proceed to do an integrity check, a bundle exec rails whatever can’t be run until it’s done. I don’t know if it can also play in upgrading automatically the versions that aren’t locked and thus maybe cause some issues later on.
error Couldn't find an integrity file
error Found 1 errors.
========================================
Your Yarn packages are out of date!
Please run `yarn install --check-files` to update.
========================================
To disable this check, please change `check_yarn_integrity`
to `false` in your webpacker config file (config/webpacker.yml).
yarn check v1.22.4
info Visit https://yarnpkg.com/en/docs/cli/check for documentation about this command.
Thanks for the help, I really enjoy the book and would love to move forward through the practical aspect. 
Simon