In complex web applications, low-level details of JavaScript code can make the program difficult to analyze and affect the performance of the system as a whole. Functional programming (FP), a style of writing code, promotes loosely coupled relationships between individual application components and provides an overview of the project, simplifying its development, communication with customers, and maintenance. This book explains techniques for improving web applications, including their extensibility, modularity, reusability and testability, as well as performance. In an easy-to-read form, with specific examples and clear explanations, it is demonstrated how to use FP techniques in practice. Beginners in mastering FP will appreciate many successful examples of comparing FP with imperative and object-oriented programming, which allows them to better understand the features of functional design. After reading this book, the reader will learn to think about their projects functionally, and perhaps will grow to understand monads! The main topics of the book. Application of valuable FP techniques in practice and where it is most appropriate. Separating the logic of the system from the details of its implementation. Error handling, testing and debugging of application code in the FP style. Demonstration and discussion of all JavaScript code examples written according to the ES6 standard (ES 2015). The book is addressed to developers who have firmly grasped the basics of JavaScript programming and have sufficient experience in designing web applications.