Reacting to reactive variables outside a component


I would like to use reactive vars to handle my global local state in my app. One thing I can’t seem to figure out is how to react to a variable change when I’m outside of a component. Basically I want to do simple stuff like: When this value is updated, run that function to save the new value to a localStorage. I wan tthis reaction to be handled outside of the component because I don’t want it to be linked to a specific component. Whenever this var gets updated from anywhere in the app, I want to react to the change and do some stuff.

I tried putting the stuff I want to run in

typePolicies: {
    Query: {
      fields: {
        something: {
          merge: (_, incoming) => {
            // Run some custom function
            return incoming

and then updateing the reactive var using somethingVar(newValue), but it doesn’t feel like the correct thing to do and the ‘merged’ is not getting called anyway.