this isn't defined in ES6 if you don't use a constructor or don't have an object already; you'll need to use either the traditional syntax (function ClassName) or the new syntax (class ClassName) to create a constructor, and then call that constructor.

In your case, it doesn't seem like a full-blown class is necessary, so try using a normal object:

export let LifecycleFactoryObj = {
    myCollection: new WeakSet(),
    //.... More here
    add: function(opts) {
        let tempObject = new MyCollection(opts);
        this.myCollection.add(tempObject);
        if (this.myCollection.has(tempObject)) {
            return tempObject;
        }
        return null;
    }
};