#### 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

# Questions

7.1. Sum as you will: The following exercise will help you understand some of the concepts we dealt with in this chapter, even if you solve it without using any of the functions we looked at. Write a sumMany() function that lets you sum an indeterminate quantity of numbers, in the following fashion. Note that when the function is called with no arguments, the sum is returned:

`let result = sumMany((9)(2)(3)(1)(4)(3)()); // 22`

7.2. Working stylishly: Write an applyStyle() function that will let you apply basic styling to strings, in the following way. Use either currying or partial application:

`const makeBold = applyStyle("b");document.getElementById("myCity").innerHTML =    makeBold("Montevideo");// <b>Montevideo</b>, to produce Montevideoconst makeUnderline = applyStyle("u");document.getElementById("myCountry&quot...`