Hey Alix, it should be fairly easy. RenJS creates a global object called RenJS, which contains everything about the game. When you set a variable in game, it is stored into this global object, specifically here: RenJS.logicManager.vars
This is a map that contains all of the variables, and you can access it from wherever you want, be it in game or from any javascript you have. What's more, if you add variables to this map externally (let's say, you have an input and save its value into RenJS.logicManager.vars.myInput, then in game you can call this variable as you would normally do with {}.
I'm not sure I'm explaining myself too well, but let me know if you need to know more about this. The last part of this doc (https://lunafromthemoon.github.io/RenJS/actions/call/) has a list of things you can use to interact with the game from outside (or inside).