深入理解ECMAScript 6：JavaScript开发者的权威指南

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"Understanding ECMAScript 6: The Definitive Guide for JavaScript Developers" by Nicholas C. Zakas ECMAScript 6（ES6），也称为ECMAScript 2015，是JavaScript语言的一个重大更新，引入了大量新特性以增强其功能和表达能力。在《Understanding ECMAScript 6》一书中，专家开发者Nicholas C. Zakas全面介绍了这个版本带来的对象类型、语法和其他激动人心的变化。 1. **类（Class）语法**：ES6引入了类的概念，尽管它们在语法上类似于传统面向对象语言中的类，但JavaScript的类实际上仍然是基于原型的。类语法提供了一种更简洁的构造函数和继承的表示方式，使得代码更易读和维护。 2. **迭代器（Iterators）与生成器（Generators）**：迭代器允许遍历可迭代对象，如数组或Set。生成器则是一种特殊的函数，可以暂停执行并保持状态，使异步编程更加高效。它们可以用于创建自定义迭代行为，比如在数据流控制和处理大量数据时非常有用。 3. **箭头函数（Arrow Functions）**：箭头函数简化了函数声明，同时也改变了`this`的绑定规则。它们不拥有自己的`this`，而是从父作用域继承，这解决了ES5中回调函数中`this`指向问题。 4. **数据结构扩展**：ES6新增了Set和Map数据结构。Set存储唯一值，而Map提供了键值对的存储，这些新数据结构提供了更丰富的操作集合的方法，如查找、添加和删除元素。 5. **继承改进**：ES6通过类语法和`extends`关键字增强了原型链继承。这使得多层继承和接口实现更加清晰，同时引入了`super`关键字来调用父类的方法。 6. **Promise对象**：Promise是处理异步操作的新方法，它将异步代码结构化为一系列有顺序的步骤，而不是嵌套的回调函数，提高了代码的可读性和可维护性。 7. **模块系统**：ES6引入了模块系统，通过`import`和`export`关键字，可以更好地组织和重用代码，实现了模块间的封装和依赖管理。这本书不仅适合Web开发者，也适合Node.js开发者，无论你处于从ES5向ES6过渡的哪个阶段，都能从中受益。每个章节都包含了大量的示例代码，帮助读者理解新特性的实际应用。通过深入学习《Understanding ECMAScript 6》，开发者能够充分利用这些新特性，提升JavaScript编程的效率和代码质量，从而在现代前端和后端开发中发挥更大的潜力。

xiv Contents in Detail

Global Promise Rejection Handling .................................... 224

Node.js Rejection Handling ................................... 225

Browser Rejection Handling ................................... 227

Chaining Promises ................................................ 228

Catching Errors............................................ 229

Returning Values in Promise Chains.............................. 230

Returning Promises in Promise Chains ............................ 231

Responding to Multiple Promises ...................................... 233

The Promise.all() Method ..................................... 234

The Promise.race() Method.................................... 235

Inheriting from Promises ............................................ 236

Promise-Based Asynchronous Task Running ............................... 237

Summary ...................................................... 241

PROXIES AND THE REFLECTION API 243

The Array Problem ................................................ 244

Introducing Proxies and Reflection ..................................... 244

Creating a Simple Proxy ............................................ 245

Validating Properties Using the set Trap ................................. 246

Object Shape Validation Using the get Trap .............................. 247

Hiding Property Existence Using the has Trap ............................. 249

Preventing Property Deletion with the deleteProperty Trap ..................... 250

Prototype Proxy Traps .............................................. 252

How Prototype Proxy Traps Work ............................... 252

Why Two Sets of Methods? ................................... 254

Object Extensibility Traps ........................................... 255

Two Basic Examples ........................................ 255

Duplicate Extensibility Methods................................. 256

Property Descriptor Traps ........................................... 257

Blocking Object.defineProperty()................................ 258

Descriptor Object Restrictions .................................. 259

Duplicate Descriptor Methods .................................. 260

The ownKeys Trap ................................................ 261

Function Proxies with the apply and construct Traps ......................... 262

Validating Function Parameters................................. 264

Calling Constructors Without new ............................... 265

Overriding Abstract Base Class Constructors ....................... 266

Callable Class Constructors ................................... 267

Revocable Proxies ................................................ 268

Solving the Array Problem........................................... 269

Detecting Array Indexes...................................... 270

Increasing length When Adding New Elements...................... 270

Deleting Elements When Reducing length.......................... 272

Implementing the MyArray Class................................ 273

Using a Proxy as a Prototype......................................... 275

Using the get Trap on a Prototype ............................... 276

Using the set Trap on a Prototype ............................... 277

Using the has Trap on a Prototype............................... 278

Proxies as Prototypes on Classes................................ 279

Summary ...................................................... 282

Contents in Detail xv

ENCAPSULATING CODE WITH MODULES 283

What Are Modules? .............................................. 283

Basic Exporting .................................................. 284

Basic Importing .................................................. 285

Importing a Single Binding.................................... 286

Importing Multiple Bindings ................................... 286

Importing an Entire Module ................................... 286

A Subtle Quirk of Imported Bindings ............................. 288

Renaming Exports and Imports........................................ 288

Default Values in Modules........................................... 289

Exporting Default Values ..................................... 289

Importing Default Values ..................................... 290

Re-exporting a Binding ............................................. 291

Importing Without Bindings .......................................... 292

Loading Modules ................................................. 293

Using Modules in Web Browsers ............................... 293

Browser Module Specifier Resolution ............................. 297

Summary ...................................................... 298

MINOR CHANGES IN ECMASCRIPT 6 299

Working with Integers ............................................. 299

Identifying Integers ......................................... 300

Safe Integers ............................................. 300

New Math Methods ............................................... 301

Unicode Identifiers ................................................ 302

Formalizing the __proto__ Property .................................... 303

UNDERSTANDING ECMASCRIPT 7 (2016) 305

The Exponentiation Operator......................................... 306

Order of Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

Operand Restriction ........................................ 306

The Array.prototype.includes() Method .................................. 307

How to Use Array.prototype.includes()............................ 307

Value Comparison ......................................... 308

A Change to Function-Scoped Strict Mode ............................... 308

INDEX 311

xviii Foreword

upcoming ECMAScript 6 (ES6) speciﬁcation, codenamed Harmony, was far

from ﬁnished and seemed like a distant future. “Maybe in 10 years I’ll get to

write some ES6 code,” I thought.

There were some experimental “transpilers” like Google Traceur that

translated code from ES6 into ES5. Most of them were very limited or hard

to plug into an existing JavaScript build pipeline. But then a new transpiler

called 6to5 came along and changed everything. It was easy to install, inte-

grated well with the existing tools, and produced readable code. It spread

like wildﬁre. Now called Babel, 6to5 brought ES6 features to a mainstream

audience even before the speciﬁcation was ﬁnalized. In a matter of months,

ES6 was everywhere.

ES6 has divided the community for a number of reasons. As this book

goes to press, it is still not fully implemented in many major browsers.

Having a build step can be intimidating when you’re just learning the

language. Some libraries have documentation and examples in ES6, and

you might wonder if it is possible to use those libraries in ES5 at all. This

contributes to the confusion. Many people didn’t expect any new features

in the language because it had almost never changed before. Others anx-

iously awaited the new features’ arrival and used all of them together—in

some cases beyond what was necessary.

Just as I was becoming proﬁcient with JavaScript, I felt that somebody

pulled the rug from under my feet, and now I had to learn a new language.

I felt bad about this for a few months. Finally, on Christmas Eve, I started

reading a draft of this book. I couldn’t put it down. Next thing I knew, it

was 3 am, everybody at the party was asleep, and I understood ES6!

Nicholas is an incredibly gifted teacher. He conveys deep details in a

straightforward way so they don’t go over your head. Apart from this book,

he is also known for creating ESLint, a JavaScript code analyzer that has

been downloaded millions of times.

Nicholas knows JavaScript like very few people do. Don’t miss the

chance to soak up some of his knowledge. Read this book, and you, too,

will become conﬁdent in your understanding of ES6.

Dan Abramov

React core team member and creator of Redux

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