Delay External JS until Object rendered

I’ve searched around a bit and wasn’t sure if there was an easy way to do this…

Basically I am looking to delay running this script until a Server Connect Function is complete (image is loaded and fully rendered).

<script src="../../assets/js/cropper.js" id="cropperScript"></script>

It seems so simple but I am not sure how to properly use the async / defer function as they did not seem to do anything but simply checking the box.

Any thoughts?