Theory versus practice

In this book, we won't be going about FP in a theoretical way: our point is, rather, to show how some of its techniques and tenets can be successfully applied for common, everyday JavaScript programming. But, and this is important, we won't be going about this in a dogmatic fashion, but rather in a very practical way. We won't dismiss useful JS constructs, only because they don't happen to fulfill the academic expectations of FP. We won't avoid practical JS features just to fit the FP paradigm. In fact, we could almost say we'll be doing SFP—Sorta Functional Programming because our code will be a mixture of FP features and more classical imperative and Object Oriented Programming (OOP).

(This doesn't mean that we'll be leaving all the theory by the side. We'll be picky, and just touch the main theoretical points, give some vocabulary and definitions, and explain core FP concepts... but we'll always be keeping in sight the idea of helping to produce actual, useful, JS code, and not to try to achieve some mystical, dogmatic FP criteria.)

OOP has been a way to solve the inherent complexity of writing large programs and systems, and developing clean, extensible, scalable application architectures. However, because of the scale of today's web applications, the complexity of all codebases is continuously growing. Also, the newer features of JS make it possible to develop applications that wouldn't even have been possible just a few years ago; think of mobile (hybrid) apps done with Ionic, Apache Cordova, or React Native, or desktop apps done with Electron or NW.js, for example. JS has also migrated to the backend with Node.js, so today the scope of usage for the language has grown in a serious way, and dealing with all the added complexity taxes all designs.