Every multiplayer project needs to run the game code on the backend side. Game Service perfectly does that task. But sometimes you don't need that kind of complicity and just to want a simple script that players can connect to and do things here and there. In Javascript!
The service uses Git to host your code on, so you can setup a repository, put your javascript there, configure it on the service, and you're all set.
Please refer to the Documentation for more information.