Functional Programming in JavaScript (Node.js)

This book is a practical guide to applying the functional programming paradigm in the JavaScript and Node.js environment. Unlike academic works, the material is focused on real backend development tasks: data stream processing, asynchronous computations, building reliable and testable microservices.

The book starts with the basics: data immutability, pure functions, absence of side effects. Gradually, you will move on to advanced concepts: currying, partial application, function composition, functors and monads (Maybe, Either, IO). Special attention is paid to integrating the functional approach with existing code and popular libraries (Ramda, Lodash/fp, Sanctuary).

A separate chapter is devoted to reactive programming with RxJS — you will learn how to build complex asynchronous event processing chains, manage data streams, and avoid "callback hell". You will discover how a functional style helps write code that is easy to debug, test, and maintain.

Who this book is for:

• JavaScript developers who want to go beyond OOP and the imperative approach;
• Node.js programmers striving to write reliable, predictable, and concurrently executable code;
• Beginners in functional programming who want to apply theory in practice without learning Haskell;
• Teams that want to introduce functional practices into existing projects.

What you will learn:

• How to write pure functions and avoid state mutations;
• Function composition instead of deeply nested calls;
• Error handling without try/catch using monads;
• Building reactive streams with RxJS for Node.js;
• Testing functional code (property-based testing);
• Performance optimization with a functional approach.

The book contains many working examples in Node.js (reading files, handling HTTP requests, working with databases) and exercises to reinforce the material. After reading it, you will be able to apply functional techniques in everyday development, making your code more concise, declarative, and resilient to errors.

Perfect for those who are already familiar with the basics of JavaScript and want to take the quality of their code to the next level.