Core of the codechecks is an Codechecks' API.
It allows for storing, retrieving and browsing artifacts. An artifact can be anything: piece of json, file or whole directory. Also, it can be a servable content like HTML, this way you can present any complicated data to users in form of HTML report.
Another important task of API is communication with Github. Codechecks require installing GitHub app on repos that you're interested in. This allows us to provide feedback information using GitHub Checks API. Support for other services like BitBucket and GitLab is coming soon.
Client (currently written in TypeScript) allows for easy communication with API. It provides execution context for plugins by parsing environment variables, getting more information about the project from API etc. In future there maybe another implementation, written in other language than TypeScript but it's out of scope for now and we threat JS client as the only official implementation.
JS client enables to write reusable "plugins" (we call them simply codechecks) in JS and distribute them directly as NPM packages.