IMO anything you like. Could be library, jquer/react plugin, theoretical work. Just released my side project: http://libretaxi.org http://libretaxi.org - it is

IMO anything you like. Could be library, jquer/react plugin, theoretical work. Just released my side project: http://libretaxi.org http://libretaxi.org - it is open source, useful for some users, code is kinda clean, plenty of tests. Just show how you can take an idea and implement it. So your employer will know what to expect.