Functional Programming in TypeScript

The book "Functional Programming in TypeScript" is a practical guide for developers who want to go beyond imperative code and master the functional programming (FP) paradigm in modern TypeScript. Unlike abstract academic textbooks, this book focuses on real-world tasks: data processing, building reliable APIs, state management, and asynchronous streams.

What is the book about?

The author systematically reveals key FP concepts:

• Pure functions and immutability — how to write predictable code without side effects.
• Composition and currying — building complex logic from simple building blocks.
• Functors, applicatives, and monads — working with Option, Either, Task, and IO in TypeScript.
• Lazy evaluation and infinite structures — performance optimization.
• Type safety — using TypeScript's strict type system to verify FP patterns at compile time.

Who is this book for?

• JavaScript/TypeScript developers familiar with the basics of the language (variables, functions, classes).
• Backend engineers (Node.js, Deno) looking to write more reliable and testable services.
• Frontend developers (React, Angular) striving for predictable state management.
• Those who want to understand how libraries like fp-ts, Effect, or Ramda work "under the hood."

What will you learn and be able to do after reading?

• Write code that is easy to refactor and test — thanks to the absence of hidden dependencies.
• Handle errors without try/catch using the Either monad and the Railway Oriented Programming pattern.
• Work with asynchrony (Promise, Observable) in a functional style via the Task monad.
• Build data transformation chains through function composition (pipe, flow).
• Use advanced TypeScript features: custom types, discriminated unions, conditional types for FP.
• Develop your own functional libraries and utilities.

The book contains dozens of code examples that can be run immediately in a Node.js environment or browser. All examples are accompanied by explanations and comparisons with the imperative approach. At the end of each chapter, there are exercises to reinforce the material.

"Functional Programming in TypeScript" is not just a translation of Haskell or Scala concepts into TypeScript, but an adaptation of the best FP practices to the realities of web development. The book will help you write cleaner, safer, and more expressive code that is easy to maintain and evolve.