Why do I get a syntax error in my ajax based shopping cart?

adding ajax features to cart 1

I am trying to build an online store, and I have opted to include ajax in the shopping cart. I am stuck as I don't understand why ajax is not showing. I am only seeing this error in console which I have failed to understand.

VM1510:2 Uncaught SyntaxError: Private field '#hiding' must be declared in an enclosing class
    at processResponse (rails-ujs.js:281:1)
    at rails-ujs.js:194:1
    at xhr.onreadystatechange (rails-ujs.js:262:1)