Eloquent JavaScript 2e（标注版）：现代编程入门指南

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"《Eloquent JavaScript 2e（标注版）》是一本由Marijn Haverbeke撰写的现代编程入门书籍，专为JavaScript语言设计。本书旨在提供一个优雅且易懂的JavaScript学习路径，适合初学者和有经验的开发者深入理解这门动态脚本语言。版权方面，作品采用Creative Commons Attribution-NonCommercial 3.0许可，允许在非商业环境下分享和修改，同时也部分代码遵循MIT许可协议。书中涵盖了广泛的JavaScript内容，包括但不限于：从基础语法和数据类型讲解，到函数、对象、闭包、原型链等核心概念；还涉及高级主题，如异步编程、模块系统、DOM操作、正则表达式、类和面向对象编程，以及游戏编程中的概念。每一章都配有插图，由多位艺术家创作，如电脑图形出自Max Xiantu，海洋生物与松鼠形象由Margarita Martínez和José Menor绘制，以及Octopuses和开关物体等。第二版的出版得益于454位金融支持者，使得这本教程更加完善。对于纸质版，读者可以在Amazon购买带有额外附录的印刷版本，由No Starch Press发行，具体网址为：http://www.amazon.com/gp/product/1593275846/ref=as_li_qf_sp_asi。这本书不仅适合自学，也是学校教育和专业培训中常用的教材，对于提升JavaScript技能具有很高的实用价值。"

will always be new challenges and new territory to explore. Program-

mers who refuse to keep exploring will stagnate, forget their joy, and get

bored with their craft.

Why language matters

In the beginning, at the birth of computing, there were no programming

languages. Programs looked something like this:

001 100 01 000 000 00 000 000 00

001 100 01 000 000 01 000 000 01

001 100 11 000 000 01 000 000 10

010 100 01 000 010 11 000 000 10

001 000 10 000 000 10 000 010 00

010 000 11 000 000 01 000 000 00

010 000 01 000 000 01 000 000 01

000 100 00 000 000 10 000 000 00

011 000 10 000 000 00 000 000 00

That is a program to add the numbers from 1 to 10 together and print

out the result: 1 + 2 + ... + 10 = 55. It could run on a simple, hypo-

thetical machine. To program early computers, it was necessary to set

large arrays of switches in the right position or punch holes in strips of

cardboard and feed them to the computer. You can probably imagine

how tedious and error-prone this procedure was. Even writing simple

programs required much cleverness and discipline. Complex ones were

nearly inconceivable.

Of course, manually entering these arcane patterns of bits (the ones

and zeros) did give the programmer a profound sense of being a mighty

wizard. And that has to be worth something in terms of job satisfaction.

Each line of the previous program contains a single instruction. It

could be written in English like this:

1. Store the number 0 in m emor y l oca tio n 0.

2. Store the number 1 in m emor y l oca tio n 1.

3. Store the value of memo ry loca tion 1 in memory location 2.

4. Subt rac t the n umber 11 from the v alue in mem ory loc ati on 2.

5. If the v alue in mem ory loc ati on 2 is the num ber 0,

con tin ue with i ns tru ct ion 9.

applications possible—applications with which you can interact directly,

without doing a page reload for every action. But it is also used in

more traditional websites to provide various forms of interactivity and

cleverness.

It is important to note that JavaScript has almost nothing to do

with the programming language named Java. The similar name was in-

spired by marketing considerations, rather than good judgment. When

JavaScript was being introduced, the Java language was being heavily

marketed and was gaining popularity. Someone thought it was a good

idea to try to ride along on this success. Now we are stuck with the

name.

After its adoption outside of Netscape, a standard document was writ-

ten to describe the way the JavaScript language should work to make

sure the various pieces of software that claimed to support JavaScript

were actually talking about the same language. This is called the EC-

MAScript standard, after the Ecma International organization that did

the standardization. In practice, the terms ECMAScript and JavaScript

can be used interchangeably—they are two names for the same language.

There are those who will say terrible things about the JavaScript lan-

guage. Many of these things are true. When I was required to write

something in JavaScript for the rst time, I quickly came to despise it.

It would accept almost anything I typed but interpret it in a way that

was completely dierent from what I meant. This had a lot to do with

the fact that I did not have a clue what I was doing, of course, but there

is a real issue here: JavaScript is ridiculously liberal in what it allows.

The idea behind this design was that it would make programming in

JavaScript easier for beginners. In actuality, it mostly makes nding

problems in your programs harder because the system will not point

them out to you.

This exibility also has its advantages, though. It leaves space for a

lot of techniques that are impossible in more rigid languages, and as you

will see (for example in Chapter 10) it can be used to overcome some

of JavaScript’s shortcomings. After learning the language properly and

working with it for a while, I have learned to actually like JavaScript.

There have been several versions of JavaScript. ECMAScript version

3 was the widely supported version in the time of JavaScript’s ascent

to dominance, roughly between 2000 and 2010. During this time, work

was underway on an ambitious version 4, which planned a number of

radical improvements and extensions to the language. Changing a living,

widely used language in such a radical way turned out to be politically

dicult, and work on the version 4 was abandoned in 2008, leading to

the much less ambitious version 5 coming out in 2009. We’re now at the

point where all major browsers support version 5, which is the language

version that this book will be focusing on. A version 6 is in the process of

being nalized, and some browsers are starting to support new features

from this version.

Web browsers are not the only platforms on which JavaScript is used.

Some databases, such as MongoDB and CouchDB, use JavaScript as

their scripting and query language. Several platforms for desktop and

server programming, most notably the Node.js project (the subject of

Chapter 20) are providing a powerful environment for programming

JavaScript outside of the browser.

Code, and what to do with it

Code is the text that makes up programs. Most chapters in this book

contain quite a lot of it. In my experience, reading code and writing code

are indispensable parts of learning to program, so try to not just glance

over the examples. Read them attentively and understand them. This

may be slow and confusing at rst, but I promise that you will quickly

get the hang of it. The same goes for the exercises. Don’t assume you

understand them until you’ve actually written a working solution.

I recommend you try your solutions to exercises in an actual JavaScript

interpreter. That way, you’ll get immediate feedback on whether what

you are doing is working, and, I hope, you’ll be tempted to experiment

and go beyond the exercises.

The easiest way to run the example code in the book, and to ex-

periment with it, is to look it up in the online version of the book at

eloquentjavascript.net. There, you can click any code example to edit

and run it and to see the output it produces. To work on the exercises,

go to eloquentjavascript.net/code, which provides starting code for each

coding exercise and allows you to look at the solutions.

If you want to run the programs dened in this book outside of the

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