Well you sure as shit aren't going to see "hoodie.account.signUp(username, password);" in Gmail's client-side code.
Gmail's code base is not a "offline first, mobile first" platform. The API that is exposed to the client-side is fairly light (watch the traffic).
I highly suggest use research "business logic flaws" in web apps. Anything Jeremiah Grossman has said on this topic is good stuff.