Hello There,
I solved the issue by replacing the code I had used what I found here:
https://gist.github.com/vosters/0be4a6429fbd89ff4e624d45c2b1518a
Also I wanted customers to be able to stay on the current page after adding an item to the cart, not to be redirected to the cart page. I did that using this:
https://github.com/carolineschnapp/ajaxify-cart