All client side code is available to read, and all network requests can be viewed in developer tools of most browsers (including options to go offline so you dont need to disconnect from the internet altogether but still see requests being sent)

What if you open incognito mode, open the page, cut off your network, then close when you're done with the demo?

I overread and I thought you were being sarcastic with "[..], then close your eyes when you're done with the demo".

I overuse sarcasm in reality to the point that people don't believe me when I'm sincere, so no surprises I didn't write this out clearly enough :D

Seriously though I think if it's all JS based you should be good to use incognito and offline to gate off the code from uploading.

Not sure why you'd care. You're just going to look at the screen and move your eyeballs around a bit then close the tab. If they want to waste bandwidth / storage to capture video of me looking around with my eyeballs that's ok by me.

I didn't do this myself, but couldn't you load the page and then disconnect from wifi before running/testing?