Web applications have come a long way from only a few years ago. Gone are the days when JavaScript code was embedded, sort of as an afterthought, inside a webpage. In today's web, we build JavaScript applications, and if you're reading this book, applications that scale. This means our architecture needs to be designed with portability in mind; the idea that the backend that serves our application and feeds it data, is replaceable.

Along with portability comes the idea of testability. We can't make assumptions about the backend when we're developing large scale JavaScript code, and that means having the ability to run with no backend at all. This chapter looks at these two closely related topics and what they mean for us in the face of changing scaling influences.