The actual build component (compile) is just changing into a specific directory, and running an npm invocation to compile the SPA into a new location as an artifact rather than a VCS changeset.

There's nothing of note in either detect or release, though the assumption is that there is a buildpack providing node and npm higher up in the chain...

That is, the entire thing is:

cd to-directory
npm run my-cool-command

🤨😮‍😮‍💨

Don't forget to use shellcheck if you make changes to this.