Functional Programming in JavaScript (Node.js)

This book is a comprehensive guide to implementing the principles of functional programming (FP) in everyday JavaScript and Node.js development. Unlike abstract academic works, the material is built around real-world server-side development tasks: data stream processing, file system operations, building APIs, and asynchronous chains.

What the book covers

The book systematically reveals key FP concepts:

• Pure functions and immutability — how to avoid side effects and make code predictable.
• Closures and currying — creating flexible functions with partial application of arguments.
• Composition and pipelines — building complex logic from simple functions without nested calls.
• Recursion and tail recursion — replacing loops with functional approaches.
• Monads (Maybe, Either, IO) — safe error handling and working with side effects.
• Functors and applicative functors — abstractions for working with contexts.
• Asynchronous FP in Node.js — promise composition, Observable, and functional reactive streams.

Who this book is for

• JavaScript developers wanting to write more reliable and testable code.
• Node.js programmers tired of "callback hell" and side effects.
• Those who want to understand how functional programming applies to real projects, not just academic examples.
• Developers transitioning from OOP to a functional style.

What you will learn

• How to rewrite typical imperative code (loops, mutations, side effects) in a functional style.
• How to use built-in array methods (map, filter, reduce) to build chains.
• How to implement middleware composition in Express/Koa using functional patterns.
• How to handle errors without try/catch using monads.
• How to test functional code with minimal mocks and stubs.
• How to use libraries like Ramda, Sanctuary, and Folktale to speed up development.

Each chapter contains practical examples ready to run in Node.js and exercises to reinforce the material. By the end of the book, you will be able to design application architectures where functions are first-class citizens and complexity is managed through composition, not inheritance.

The book is suitable for both beginners and experienced developers — a basic knowledge of JavaScript (ES6+) is sufficient.