In HedgeDoc we use an event system based on [EventEmitter2][eventemitter2]. It's used to reduce circular dependencies between different services and inform these services about changes.

HedgeDoc's system is basically [the system NestJS offers][nestjs/eventemitter].  
The config for the `EventEmitterModule` is stored in `events.ts` and exported as `eventModuleConfig`.  
In the same file enums for the event keys are defined. Each of these events is expected to be sent with an additional value. In the enum defintion a comment should tell you what exactly this value should be.

[eventemitter2]: https://github.com/EventEmitter2/EventEmitter2
[nestjs/eventemitter]: https://docs.nestjs.com/techniques/events