Eloquent JavaScript(2nd版)：JavaScript编程入门经典

JavaScript

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grammer into all kinds of confusion, but that only means you should

proceed with caution and keep your wits about you. As you learn, there

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, hypothet-

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

arrays of switches in the right position or punch holes in strips of card-

board and feed them to the computer. You can probably imagine how

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.

What is JavaScript?

JavaScript was introduced in 1995 as a way to add programs to web pages

in the Netscape Navigator browser. The language has since been adopted

by all other major graphical web browsers. It has made modern web

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 in popularity. Someone thought it 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 organization that did the standard-

ization. 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 ﬁrst time, I quickly came to despise it.

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

was completely diﬀerent 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 ﬁnding

problems in your programs harder because the system will not point

them out to you.

This ﬂexibility 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 dominant, 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 diﬃcult, and work on the fourth edition was abandoned in

2008, leading to the much less ambitious ﬁfth edition coming out in

2009. We’re now at the point where all major browsers support this ﬁfth

edition, which is the language version that this book will be focusing on.

A sixth edition is in the process of being ﬁnalized, and some browsers

are starting to support new features from this edition.

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 ﬁrst, 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

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