#### 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
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# Chapter 5, Programming Declaratively – a Better Style

5.1. Filtering... but what? Boolean(x) is the same as !!x, and it turns an expression from being truthy or falsy into true or false, respectively. Thus, the .filter() operation removes all falsy elements from the array.

5.2. Generating HTML code, with restrictions: In real life, you wouldn't limit yourself to using only filter(), map(), and reduce(), but the objective of this question was to make you think about how to manage with only those. Using join() or other extra string functions would make the problem easier. For instance, finding out a way to add the enclosing <div><ul> ... </ul></div> tags is tricky, so we had to make the first reduce() operation produce an array so that we could keep on working on it:

`var characters = [  { name: "Fred", plays: "bowling...`