#### Overview of this book

Functional programming is a paradigm for developing software with better performance. It helps you write concise and testable code. To help you take your programming skills to the next level, this comprehensive book will assist you in harnessing the capabilities of functional programming with JavaScript and writing highly maintainable and testable web and server apps using functional JavaScript. This second edition is updated and improved to cover features such as transducers, lenses, prisms and various other concepts to help you write efficient programs. By focusing on functional programming, you’ll not only start to write but also to test pure functions, and reduce side effects. The book also specifically allows you to discover techniques for simplifying code and applying recursion for loopless coding. Gradually, you’ll understand how to achieve immutability, implement design patterns, and work with data types for your application, before going on to learn functional reactive programming to handle complex events in your app. Finally, the book will take you through the design patterns that are relevant to functional programming. By the end of this book, you’ll have developed your JavaScript skills and have gained knowledge of the essential functional programming techniques to program effectively.
Preface
Technical Requirements
Free Chapter
Becoming Functional - Several Questions
Thinking Functionally - A First Example
Starting Out with Functions - A Core Concept
Behaving Properly - Pure Functions
Programming Declaratively - A Better Style
Producing Functions - Higher-Order Functions
Transforming Functions - Currying and Partial Application
Connecting Functions - Pipelining and Composition
Designing Functions - Recursion
Ensuring Purity - Immutability
Implementing Design Patterns - The Functional Way
Building Better Containers - Functional Data Types
Bibliography
Other Books You May Enjoy

# Chapter 9, Designing Functions – Recursion

9.1. Into reverse: An empty string is reversed by simply doing nothing. To reverse a non-empty string, remove its first character, reverse the rest, and append the removed character at the end. For example, reverse("MONTEVIDEO") can be found by using reverse("ONTEVIDEO")+"M". In the same way, reverse("ONTEVIDEO") would be equal to reverse("NTEVIDEO")+"O", and so on:

`const reverse = str =>  str.length === 0 ? "" : reverse(str.slice(1)) + str[0];`

9.2. Climbing steps: Suppose we want to climb a ladder with n steps. We can do this in two ways:

• Climbing one single step and then climbing an (n-1) steps ladder
• Climbing two steps at once and then climbing an (n-2) steps ladder

So, if we call ladder(n) the number of ways to climb an steps ladder...