How to interop with javascript code that changes an argument in place?

I want to interop with some javascript code. If I did things in javascript, then what I’d want to do would look like this:

var textToSVG
TextToSVG.load("", (err, t2s) => {
  textToSVG = t2s; // This is the function I care about getting

What I want to get out of this code is the textToSVG function (t2s), but I don’t know how to get it a nice way in clojurescript because the javascript code is not in functional style. One way is like this:

(defonce text-to-svg (atom nil))
    (fn [err t2s]
      (reset! text-to-svg t2s)))

Is that what you’d do, or is there a better way?

(edited for clarity)

if you want to guarantee that the fn will exist when you try to interact with it, you might put it in a promise that clients then chain off. or have it as a component in a clip system (clip can have async components)

Thanks. The whole async way of thinking is new to me. I might try out the promesa library so that I can write code as if it was synchronous, and halt the execution of everything else until the font has been loaded.