Treat Web services the same as any other data access:
To approach client side MVC architecture, without frameworks, we treated Web services in the same manner as we did localStorage and the local IndexedDB database. All code that involves requests to remote servers or that query a database or that read/write to localStorage happen in the model layer as DAO objects.
These were written using either the prototype pattern, the module pattern, or in some cases a combination of the two, using a modified factory pattern to treat objects as singletons in production but allowing us to "reset" them when unit testing so we keep tests self-contained.
They're generalized so that no application logic exists in this layer, and most of the code at this level can essentially be reused across future applications.
The services layer encapsulates the DAO layer, keeping the specific storage technology separate from the controller logic. Thus, pushing data to a remote server is seen as the same as inserting data into IndexedDB.
Used raw XMLHttpRequest to interface with Web services:
We chose not to use jQuery AJAX. Instead, we wrote a wrapper around XMLHttpRequest. There's no right or wrong answer here, but choosing not to use jQuery here allows us to stay focused on the rule of having no DOM manipulations in this layer. By wrapping the AJAX logic inside a prototype class, we set certain default headers, since most of what Web services deals with in our case is application/json data.
However, we did make an exception for jQuery Deferred. It didn't seem to make sense to write our own Observer implementation simply to be purists, and our logic behind this decision is that future professional programmers who work on this project with us have a better chance of understanding $.Deferred than some hand-rolled implementation. Many developers, on the other hand, we argued would have been exposed to XMLHttpRequest at some point in time, so being a purist here seemed less risky from the perspective of communicating to other developers what a piece of code does.
Earlier, I mentioned rules. We wrote up a series of "rules" designed to keep the code maintainable. For instance, rule #1 is that jQuery DOM manipulations only happen in a "view" layer. So the only jQuery we use in other layers is the $.Deferred object and nothing else.
We use $.Deferred throughout the application to keep certain logic in the layers where that belongs. For instance, we keep a persistent $.Deferred observer in the DOM click handlers in order to "notify" the controller that a click event has occurred so the controller can then delegate to other layers of the application to get/set, fetch/store, or perform other actions.
Using TDD as a development methodology, we've created small functions, where the largest is never more than a page, and most layers have a great deal of test coverage.
After a few months, the jury is still out on whether we should have used a framework, as much of what we've done could be generalized further into a reusable framework. We've learned quite a bit trying this, and I'm hoping this experience helps others who wish to develop client-side applications that involve a lot of asynchronous operations.
To give credit where it's due, I took inspiration from the following people:
For further reading on going framework-less, see Joe Gregorio's Zero Framework Manifesto