It is this. I should not need an internet connection in order to make my calculator add 4 and 5.
A web app which does not require its client-side scripting, which works on the server and then is progressively enhanced, does not work in an offline environment. It doesn’t work when you’re in and out of train tunnels, or at a conference with bad wifi, or on a metered data connection. The offline first concept, which should be informing how we build apps, is right about this.
window.Array to be something else on the client. I bet there isn’t one.)