Issues with importing complex modules

Scriptable
4

I seem to be suffering from the same issue, or lack of JavaScript knowledge in my case…

I have a file that defines a class RTM (an implementation of the Remember The Milk API) that I want to export, so that I can import it in other scripts:

class RTM {
    constructor() {
        this.token = ...
        ...
    }

    async call(method, params, options) {
        ...
    }

    addTask(task) {
        return this.call(...)
    }
}

(How) Can this be done? What modules.exports syntax should I use?

Whatever I try I end up with TypeError: undefined is not an object…