Mastering JavaScript Functional Programming: Write clean, robust, and maintainable web and server code using functional JavaScript and TypeScript. 3 Ed

Cover....1

Title Page....2

Copyright and Credits....3

Contributors....5

About the reviewers....6

Table of Contents....8

Preface....14

Chapter 1: Becoming Functional – Several Questions....22

What is functional programming?....22

Theory versus practice....23

A different way of thinking....23

FP and other programming paradigms....24

What FP is not....25

Why use FP?....26

What we need....26

What we get....27

Not all is gold....27

Is JavaScript functional?....28

JavaScript as a tool....28

Going functional with JavaScript....29

Key features of JavaScript....30

How do we work with JavaScript?....36

Using transpilers....38

Working online....40

A step further – TypeScript....40

Testing....43

Summary....43

Questions....43

Chapter 2: Thinking Functionally – A First Example....46

Our problem – doing something only once....46

Solution 1 – hoping for the best!....47

Solution 2 – using a global flag....48

Solution 3 – removing the handler....49

Solution 4 – changing the handler....50

Solution 5 – disabling the button....50

Solution 6 – redefining the handler....51

Solution 7 – using a local flag....51

A functional solution to our problem....52

A higher-order solution....53

Testing the solution manually....56

Testing the solution automatically....56

Producing an even better solution....58

Summary....60

Questions....61

Chapter 3: Starting Out with Functions – A Core Concept....62

All about functions....62

Of lambdas and functions....63

Arrow functions – the modern way....66

Functions as objects....71

Using functions in FP ways....77

Injection – sorting it out....77

Callbacks and promises....81

Continuation-passing style....82

Polyfills....83

Stubbing....87

Immediate invocation (IIFE)....88

Summary....91

Questions....92

Chapter 4: Behaving Properly – Pure Functions....94

Pure functions....94

Referential transparency....95

Side effects....97

Advantages of pure functions....104

Impure functions....109

Avoiding impure functions....110

Is your function pure?....116

Testing – pure versus impure....118

Testing pure functions....118

Testing purified functions....120

Testing impure functions....124

Summary....128

Questions....128

Chapter 5: Programming Declaratively – A Better Style....132

Transformations....133

Reducing an array to a value....133

Applying an operation – map()....141

Dealing with arrays of arrays....148

More general looping....156

Logical HOFs....158

Filtering an array....158

Searching an array....161

Higher-level predicates – every() and some()....164

Checking negatives – none()....165

Working with async functions....166

Some strange behaviors....167

Async-ready looping....169

Working with parallel functions....174

Unresponsive pages....174

A frontend worker....177

A backend worker....180

Workers, FP style....181

Long-living pooled workers....183

Summary....186

Questions....187

Chapter 6: Producing Functions – Higher-Order Functions....190

Wrapping functions – keeping behavior....190

Logging....191

Timing functions....201

Memoizing functions....203

Altering a function’s behavior....214

Doing things once, revisited....214

Logically negating a function....217

Inverting the results....219

Arity changing....221

Throttling and debouncing....223

Changing functions in other ways....225

Turning operations into functions....226

Turning functions into promises....229

Getting a property from an object....231

Demethodizing – turning methods into functions....233

Methodizing – turning functions into methods....236

Finding the optimum....240

Summary....243

Questions....243

Chapter 7: Transforming Functions – Currying and Partial Application....246

A bit of theory....246

Currying....247

Dealing with many parameters....248

Currying by hand....251

Currying with bind()....253

Partial application....257

Partial application with arrow functions....258

Partial application with closures....259

Partial currying....268

Partial currying with bind()....270

Partial currying with closures....273

Final thoughts....274

Variable number of parameters....275

Parameter order....275

Being functional....277

Summary....278

Questions....279

Chapter 8: Connecting Functions – Pipelining, Composition, and More....282

Pipelining....282

Piping in Unix/Linux....283

Revisiting an example....284

Creating pipelines....286

Debugging pipelines....294

Pointfree style....297

Chaining and fluent interfaces....300

An example of fluent APIs....300

Chaining method calls....302

Composing....305

Some examples of composition....306

Composing with higher-order functions....310

Transducing....316

Composing reducers....319

Generalizing for all reducers....321

Testing connected functions....322

Testing pipelined functions....322

Testing composed functions....325

Testing chained functions....326

Testing transduced functions....328

Summary....330

Questions....330

Chapter 9: Designing Functions – Recursion....334

Using recursion....334

Thinking recursively....336

Higher-order functions revisited....346

Searching and backtracking....354

Mutual recursion....362

Odds and evens....362

Doing arithmetic....364

Recursion techniques....369

Tail call optimization....369

Continuation-passing style....372

Trampolines and thunks....377

Recursion elimination....380

Summary....381

Questions....381

Chapter 10: Ensuring Purity – Immutability....386

Going the straightforward JavaScript way....387

Mutator functions....387

Constants....388

Freezing....389

Cloning and mutating....392

Getters and setters....396

Lenses....401

Prisms....413

Creating persistent data structures....417

Working with lists....417

Updating objects....419

A final caveat....425

Summary....425

Questions....426

Chapter 11: Implementing Design Patterns – The Functional Way....430

Understanding design patterns....430

Design pattern categories....431

Do we need design patterns?....433

Object-oriented design patterns....433

Facade and Adapter....434

Decorator or Wrapper....437

Strategy, Template, and Command....441

Dependency Injection....443

Observers and reactive programming....446

Other patterns....458

Functional design patterns....458

Summary....460

Questions....460

Chapter 12: Building Better Containers – Functional Data Types....464

Specifying data types....464

Signatures for functions....465

Other data type options....468

Building containers....470

Extending current data types....471

Containers and functors....473

Monads....488

Functions as data structures....497

Binary trees in Haskell....498

Functions as binary trees....499

Summary....507

Questions....508

Answers to Questions....512

Chapter 1, Becoming Functional – Several Questions....512

Chapter 2, Thinking Functionally – A First Example....518

Chapter 3, Starting Out with Functions – A Core Concept....524

Chapter 4, Behaving Properly – Pure Functions....528

Chapter 5, Programming Declaratively – A Better Style....535

Chapter 6, Producing Functions – Higher-Order Functions....548

Chapter 7, Transforming Functions – Currying and Partial Application....557

Chapter 8, Connecting Functions – Pipelining, Composition, and More....562

Chapter 9, Designing Functions – Recursion....565

Chapter 10, Ensuring Purity – Immutability....575

Chapter 11, Implementing Design Patterns – The Functional Way....583

Chapter 12, Building Better Containers – Functional Data Types....586

Bibliography....598

Index....600

Other Books You May Enjoy....611

Functional programming is a programming paradigm that uses functions for developing software. This book is filled with examples that enable you to leverage the latest JavaScript and TypeScript versions to produce modern and clean code, as well as teach you to how apply functional programming techniques to develop more efficient algorithms, write more concise code, and simplify unit testing.

This book provides comprehensive coverage of the major topics in functional programming to produce shorter, clearer, and testable programs. You'll begin by getting to grips with writing and testing pure functions, reducing side effects, as well as other key features to make your applications functional in nature. The book specifically explores techniques to simplify coding, apply recursion, perform high-level 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 practical programming skills needed to confidently enhance your applications by adding functional programming to wherever it's most suitable.

What You Will Learn:

• Understand when to use functional programming versus classic object-oriented programming
• Use declarative coding instead of imperative coding for clearer, more understandable code
• Know how to avoid side effects and create more reliable code with closures and immutable data
• Use recursion to help design and implement more understandable solutions to complex problems
• Define functional programing data types with or without TypeScript, add type checking, and implement immutability
• Apply advanced containers to get better structures to tackle errors and implement async programming

Who this book is for:

If you are a JavaScript or TypeScript developer looking to enhance your programming skills, then this book is for you. The book applies to both frontend developers working with frameworks such as React, Vue, or Angular as well as backend developers using Node.js or Deno.