Config is done in JSON at .minecraft/config/jscore/

// master.config.json
{
    "replLanguagePriorities": ["js"],
    "clientEntryPoint": "client/index.js",
    "serverEntryPoint": "server/index.js"
}

• replLanguagePriorities

  List of language IDs to use a the default REPL language, JSCore goes along the list until it finds a language it supports.

• clientEntryPoint / serverEntryPoint

  The script file at the entry point is start/stopped on the following events.

  Entry point	Start	Stop
  Client	Client started	Client stoping
  Server	Server started	Server stopping