#### Overview of this book

Functional programming is a programming paradigm for developing software using functions. Learning to use functional programming is a good way to write more concise code, with greater concurrency and performance. The JavaScript language is particularly suited to functional programming. This book provides comprehensive coverage of the major topics in functional programming with JavaScript to produce shorter, clearer, and testable programs. You’ll delve into functional programming; including writing and testing pure functions, reducing side-effects, and other features to make your applications functional in nature. Specifically, we’ll explore techniques to simplify coding, apply recursion for loopless coding, learn ways to achieve immutability, implement design patterns, and work with data types. By the end of this book, you’ll have developed the JavaScript skills you need to program functional applications with confidence.
Dedication
Title Page
Credits
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Preface
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

## Chapter 5. Programming Declaratively - A Better Style

Up to now, we haven't really been able to appreciate the possibilities of FP, as pertains to working at a higher level, declarative fashion. In this chapter, we will correct that, and start getting shorter, more concise, and easier to understand code, by using some higher-order functions (HOF: functions that take functions as parameters) such as:

• .reduce() and .reduceRight() to apply an operation to a whole array reducing it to a single result
• .map(), to transform an array into another, by applying a function to each of its elements
• .forEach(), to simplify writing loops, by abstracting the necessary looping code

We'll also be able to do searches and selections with:

• .filter(), to pick some elements from an array
• .find() and .findIndex(), to search for elements satisfying a condition
• And a pair of predicates, .every() and .some(), to check an array for some Boolean test

Using these functions lets you work more declaratively, and you'll see that...