Reliable JavaScript How to Code Safely in the World's Most Dangerous Language

Reliable JavaScript, How to Code Safely in the World's Most Dangerous Language demonstrates how to create test-driven development for large-scale JavaScript applications that will stand the test of time and stay accurate through long-term use and maintenance. Taking a test-first approach to software architecture, this book walks you through several patterns and practices and explains what they are supposed to do by having you write unit tests. Write the code to pass the unit tests, so you not only develop your technique for structuring large-scale applications, but you also learn how to test your work. You'll come away with hands-on practice that results in code that is correct from the start, and has the test coverage to ensure that it stays correct during subsequent maintenance. All code is provided both in the text and on the web, so you can immediately get started designing more complete, robust applications.

JavaScript has graduated from field-validation scripts to full-scale applications, but many developers still approach their work as if they were writing simple scripts. If you're one of those developers, this book is the solution you need to whip your code into shape and create JavaScript applications that work.

• Write more concise and elegant code by thinking in JavaScript
• Test the implementation and use of common design patterns
• Master the use of advanced JavaScript features
• Ensure your code's conformance to your organization's standards

If you're ready to step up your code and develop more complete software solutions, Reliable JavaScript is your essential resource.

About the authors

Larry Spencer leads an international team of developers at ScerIS, a software and services company near Boston. He has over 35 years' experience as an executive, developer, consultant, teacher, and frequent presenter at programming conferences.

Seth Richards has been crafting software professionally since 2002. His past work focused on web-based, enterprise-class geographic information system applications.

Visit us at wrox.com where you have access to free code samples, Programmer to Programmer forums, and discussions on the latest happenings in the industry from around the world.

PART I: LAYING A SOLID FOUNDATION

CHAPTER 1: PRACTICING SKILLFUL SOFTWARE 3

Writing Code That Starts Correct 4

Mastering the Features of JavaScript 4

Case Study: D3.js 5

JavaScript Is Single-Threaded 15

Avoiding JavaScript’s Pitfalls in Larger Systems 16

Scripts Are Not Modules 16

Nested Functions Control Scope 16

Coding by Contract 17

Applying the Principles of Software Engineering 18

The SOLID Principles 18

The DRY Principle 21

Writing Code That Stays Correct 22

Investing for the Future with Unit Tests 22

Practicing Test-Driven Development 23

Engineering Your Code to Be Easy to Test 24

Summary 27

CHAPTER 2: TOOLING UP 29

Using a Testing Framework 29

Identifying Incorrect Code 33

Designing for Testability 35

Writing the Minimum Required Code 36

Safe Maintenance and Refactoring 36

Runnable Specification 37

Current Open-Source and Commercial Frameworks 37

QUnit 37

D.O.H. 38

Introducing Jasmine 39

Suites and Specs 39

Expectations and Matchers 41

Spies 41

Using a Dependency-Injection Framework 45

What Is Dependency Injection? 45

Making Your Code More Reliable with Dependency Injection 46

Mastering Dependency Injection 47

Case Study: Writing a Lightweight Dependency-Injection Framework 47

Using a Dependency-Injection Framework 55

Current Dependency-Injection Frameworks 56

RequireJS 56

AngularJS 57

Using an Aspect Toolkit 58

Case Study: Caching with and without AOP 58

Implementing Caching without AOP 58

Making Your Code More Reliable with AOP 60

Case Study: Building the Aop.js Module 60

Other AOP Libraries 72

AspectJS 72

AopJS jQuery Plugin 73

YUI’s Do Class 73

Conclusion 73

Using a Code-Checking Tool 73

Making Your Code More Reliable with Linting Tools 74

Introducing JSHint 76

Using JSHint 76

If You Don’t Run It, Bugs Will Come 79

Alternatives to JSHint 79

JSLint 79

ESLint 79

Strict Mode 80

Summary 80

CHAPTER 3: CONSTRUCTING RELIABLE OBJECTS 81

Using Primitives 81

Using Object Literals 83

Using the Module Pattern 84

Creating Modules]at]Will 84

Creating Immediate]Execution Modules 85

Creating Reliable Modules 86

Using Object Prototypes and Prototypal Inheritance 87

The Default Object Prototype 87

Prototypal Inheritance 88

Prototype Chains 88

Creating Objects with New 89

The new Object Creation Pattern 89

Potential for Bad Things to Happen 90

Enforcing the Use of new 90

Using Classical Inheritance 95

Emulating Classical Inheritance 95

Repetition Killed the Kangaroo 96

Using Functional Inheritance 98

Monkey]Patching 100

Summary 102

PART II: TESTING PATTERN-BASED CODE

CHAPTER 4: REVIEWING THE BENEFITS OF PATTERNS 107

Case Study 107

Producing More Elegant Code by Using a Broader Vocabulary 108

Producing Reliable Code with Well-Engineered, Well-Tested Building Blocks 109

Summary 110

CHAPTER 5: ENSURING CORRECT USE OF THE CALLBACK PATTERN 111

Understanding the Pattern Through Unit Tests 112

Writing and Testing Code That Uses Callback Functions 112

Writing and Testing Callback Functions 117

Avoiding Problems 121

Flattening the Callback Arrow 121

Minding this 123

Summary 128

CHAPTER 6: ENSURING CORRECT USE OF THE PROMISE PATTERN 129

Understanding Promises Through Unit Tests 130

Using a Promise 130

Constructing and Returning a Promise 135

Testing an XMLHttpRequest 138

Chaining Promises 141

Using a Promise Wrapper 142

Understanding States and Fates 143

Distinguishing Standard Promises from jQuery Promises 143

Summary 143

CHAPTER 7: ENSURING CORRECT USE OF PARTIAL FUNCTION APPLICATION 145

Unit-Testing a Partial Function Application 145

Creating an Aspect for Partial Function Application 147

Distinguishing Between Partial Function Application and Currying 149

Currying 149

Partial Function Application 149

Summary 150

CHAPTER 8: ENSURING CORRECT USE OF THE MEMOIZATION PATTERN 151

Understanding the Pattern Through Unit Tests 152

Adding Memoization with AOP 155

Creating the Memoization Aspect 155

Applying the returnValueCache Aspect to restaurantApi 159

Summary 160

CHAPTER 9: ENSURING CORRECT IMPLEMENTATION OF THE SINGLETON PATTERN 161

Understanding the Pattern Through Unit Tests 162

Implementing a Singleton Shared Cache with an Object Literal 162

Implementing a Singleton Shared Cache with a Module 166

Summary 170

CHAPTER 10: ENSURING CORRECT IMPLEMENTATION OF THE FACTORY PATTERN 173

Writing Unit Tests for a Factory 173

Implementing the Factory Pattern 179

Considering Other Factory Types 181

Summary 181

CHAPTER 11: ENSURING CORRECT IMPLEMENTATION AND USE OF THE SANDBOX PATTERN 183

Understanding the Pattern Through Unit Tests 184

Creating a Widget Sandbox 185

Instantiating a Widget Sandbox 185

Providing Tools to the Widget via the Sandbox 187

Creating and Testing Sandbox Tools 197

Creating Functions for Use with a Sandbox 201

Summary 203

CHAPTER 12: ENSURING CORRECT IMPLEMENTATION OF THE DECORATOR PATTERN 205

Developing a Decorator the Test]Driven Way 207

Writing a Fake for the Decorated Object 207

Writing Tests for Pass]Through of Errors 208

Writing a Do]Nothing Decorator 209

Adding Pass]Through Functionality to the Decorator 210

Verifying Pass]Through of Successes 213

Adding the Decorator’s Features 215

Generalizing the Decorator 222

Summary 222

CHAPTER 13: ENSURING CORRECT IMPLEMENTATION OF THE STRATEGY PATTERN 223

Understanding the Pattern Through Unit Tests 223

Implementing the transportScheduler Without the Strategy Pattern 224

Implementing the transportScheduler Using the Strategy Pattern 226

Creating transportScheduler Using Test]Driven Development 227

Creating a Strategy for Use with transportScheduler 235

Summary 237

CHAPTER 14: ENSURING CORRECT IMPLEMENTATION OF THE PROXY PATTERN 239

Developing a Proxy the Test]Driven Way 240

Summary 256

CHAPTER 15: ENSURING CORRECT IMPLEMENTATION OF CHAINABLE METHODS 257

Understanding the Pattern Through Unit Tests 259

Chaining then 266

Summary 267

PART III: TESTING AND WRITING WITH ADVANCED JAVASCRIPT FEATURES

CHAPTER 16: CONFORMING TO INTERFACES IN AN INTERFACE-FREE LANGUAGE 271

Understanding the Benefits of Interfaces 272

Understanding the Interface Segregation Principle 273

Using Test-Driven Development to Create a Contract Registry 275

Defining a Contract 275

Determining Whether a Contract Is Fulfilled 278

Asserting That a Contract Is Fulfi lled 282

Bypassing Contract Enforcement 283

Creating an Aspect to Enforce a Contract on a Returned (Created) Object 283

Summary 288

CHAPTER 17: ENSURING CORRECT ARGUMENT TYPES 289

Introduction 289

Understanding the Opportunities and Risks Posed by JavaScript’s Type-Free Parameters 290

Extending the ContractRegistry to Check Arguments 290

Scoping Out the Task 291

Determining Whether Every Variable in a Set Fulfi lls Its Contract 291

Asserting That Every Variable in a Set Fulfi lls Its Contract 300

Packaging Argument-Checking in an Aspect 301

Supporting Contract Libraries 303

Putting It All Together 303

Creating the Contracts Modules 304

Creating the Application’s ContractRegistry 307

Bypassing Contracts for Production 307

Comparing the Aspect-Oriented Solution to a Static Solution 307

Considering the Advantages of TypeScript 308

Considering the Advantages of Aspects 308

Summary 308

CHAPTER 18: ENSURING CORRECT USE OF CALL, APPLY, AND BIND 311

Exploring How this Is Bound 312

Default Binding 312

Default Binding and strict Mode 313

Implicit Binding 314

new Binding 316

Explicit Binding 317

Creating and Testing Code That Uses call, apply, and bind 317

Using call and apply 318

Creating an Array.prototype.forEach Polyfill Using Test-Driven Development 320

Using bind 329

Summary 334

CHAPTER 19: ENSURING THE CORRECT USE OF METHODBORROWING 335

Ensuring the Borrowing Object Is Suitable 336

Making the Borrowed Function Qualify the Borrower 336

Attaching an Aspect to the Borrowed Object 338

Using a borrow() Method 342

Adding an Object]Validator to the ContractRegistry 342

Anticipating Side Effects on the Borrower 343

Considering Side Effects from an Isolated Function 343

Considering Side Effects from a Function That Calls Other Functions 345

Anticipating Side Effects on the Donor Object 351

Summary 351

CHAPTER 20: ENSURING CORRECT USE OF MIXINS 353

Creating and Using Mixins 355

Creating and Using a Traditional Mixin 356

Creating the extend Function Using Test]driven Development 356

Creating a Traditional Mixin Using Test]driven Development 367

Creating and Using a Functional Mixin 373

Summary 380

CHAPTER 21: TESTING ADVANCED PROGRAM ARCHITECTURES 383

Ensuring Reliable Use of the Observer Pattern 384

Examining the Observer Pattern 384

Enhancing the Reliability of the Observer Pattern 391

Ensuring Reliable Use of the Mediator Pattern 395

Examining the Mediator Pattern 396

Enhancing the Reliability of Mediator-Based Code 397

Developing a Colleague 398

Testing a Colleague 399

Segregating the Mediator’s Interfaces 402

Deciding Where to Put the Contracts 403

Ensuring the Colleague Gets a Mediator with the Expected Interface 404

Developing a Mediator 406

Testing the Mediator 408

Summary 410

PART IV: SPECIAL SUBJECTS IN TESTING

CHAPTER 22: TESTING ADVANCED PROGRAM ARCHITECTURES 383

Ensuring Reliable Use of the Observer Pattern 384

Examining the Observer Pattern 384

Enhancing the Reliability of the Observer Pattern 391

Ensuring Reliable Use of the Mediator Pattern 395

Examining the Mediator Pattern 396

Enhancing the Reliability of Mediator-Based Code 397

Developing a Colleague 398

Testing a Colleague 399

Segregating the Mediator’s Interfaces 402

Deciding Where to Put the Contracts 403

Ensuring the Colleague Gets a Mediator with the Expected Interface 404

Developing a Mediator 406

Testing the Mediator 408

Summary 410

CHAPTER 23: ENSURING CONFORMANCE TO STANDARDS 435

Using ESLint 436

Installing ESLint 436

Installing Node and npm 436

Installing ESLint Using npm 439

Running ESLint 439

Executing ESLint on a Single File 442

Executing ESLint on All the JavaScript Files in a Directory 443

Enforcing Coding Standards with ESLint 444

Creating a Custom ESLint Rule 445

Running ESLint with Custom Rules 448

Enforcing Architectural Divisions 449

The Family-Secret Technique 450

The Imprinting Technique 452

The Mission Impossible Technique 454

The Magic Wand Technique 459

Do Not Use the Call Stack Technique 460

Other Techniques 460

Other Architectures 460

Summary 460

PART V: SUMMARY

CHAPTER 24: SUMMARY OF THE PRINCIPLES OF TEST-DRIVEN DEVELOPMENT 465

Recalling Why Test-Driven Development Is Worthwhile 465

Practicing Test-Driven Development 466

Writing Unit-Testable Code 466

Mastering the Mechanics of Test-Driven Development 466

Writing the Test Before the Code 467

Keeping Your Tests DRY 467

Testing Error Conditions First 467

Testing the Simple before the Complex 467

Being Specifi c 467

Testing Just One Thing 468

Your Test Data Are Just As Important As the Test 468

Using Jasmine Effectively 468

Testing the Patterns in This Book 468

Testing Aspect-Oriented Programming 468

Testing Object Construction 469

Testing Callbacks 469

Testing Promise-Based Code 469

Testing a Partial Function Application 470

Testing Memoization 470

Testing a Singleton 470

Testing a Factory Method 470

Testing a Sandbox 470

Testing the Decorator Pattern 471

Testing the Strategy Pattern 471

Testing the Proxy Pattern 471

Testing Chainable Methods 471

Testing Conformance to an Interface 472

Testing the Use of call and apply 472

Testing the Method-Borrowing Pattern 472

Testing Mixins 472

Testing Mediators and Observers 473

Testing DOM Access 473

Tests to Enforce Architectural Divisions 473

Summary 473

CHAPTER 25: SUMMARY OF JAVASCRIPT IDIOMS IN THIS BOOK 475

Reviewing Objects 475

Object Properties May Be Added and Removed 476

Objects May Be Used as a Dictionary 476

Reviewing Variables 477

Variable Declarations Are Hoisted 477

Variables Have Function Scope 478

Reviewing Functions 481

Functions Are Objects 481

Functions Declarations Are Hoisted 481

Functions Don’t Have Return Types 482

Functions May Be Anonymous 482

Functions May Be Nested 483

Functions May Be Invoked with Any Number of Arguments 484

Functions May Be Invoked Immediately 485

Reviewing Boolean Operations 486

Types May Be Coerced When Testing Equality 486

Values May Be Truthy or Falsy 487

Summary 487

INDEX 489

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