This a JavaScript action but uses TypeScript to generate that JavaScript.
You can bootstrap your environment with a modern version of npm and by running npm i at the root of this repo.
Tests can be found under under __tests__ directory and are runnable with the npm t command.
Source code can be found under the src directory. Running npm run build will generate the JavaScript that will run within GitHub workflows.
A minimal attempt at keeping a consistent code style is can be applied by running npm run fmt.