> Since the library is meant to be used in a web app, how would the app be accessed without internet access

If a website still works when wifi is disabled, it's because the entirety of the application has been downloaded and cached by the browser; no further Network requests are needed—or invoked by something the user might want to do, like scanning another barcode.

If scanning a barcode requires a GET, POST, PUT etc. somewhere, it won't work offline.

If scanning a barcode only requires reusing a cached script/function/asset, it should work offline.