This looks like the perfect case for using Promises...

    loadjs('foobar').then(() => {
      // foobar.js loaded
    });

And then with the newer spec:

    await loadjs('foobar');
    // foobar.js loaded

Such a pity that instead it uses a proprietary format.

Edit: you could still add all other nice things without compromising the main thing, for example to add a "before":

    loadjs.before(fnA).before(fnB).load('foobar').then(() => {});