it-ebooks - Eloquent JavaScript 2e

Written by Marijn Haverbeke.

Licensed under a Creative Commons attribution-noncommercial license. All code in this book may also be considered licensed under an MIT license.

Illustrations by various artists: Cover by Wasif Hyder. Computer (introduction) and unicycle people (Chapter 21) by Max Xiantu. Sea of bits (Chapter 1) and weresquirrel (Chapter 4) by Margarita Martnez and Jos Menor. Octopuses (Chapter 2 and 4) by Jim Tierney. Object with on/off switch (Chapter 6) by Dyle MacGregor. Regular expression diagrams in Chapter 9 generated with regexper.com by Jeff Avallone. Game concept for Chapter 15 by Thomas Palef. Pixel art in Chapter 16 by Antonio Perdomo Pastor.

The second edition of Eloquent JavaScript was made possible by 454 financial backers.

A paper version of Eloquent JavaScript, including a bonus chapter, is being brought out by No Starch Press. They also sell a more polished EPUB version that includes the bonus chapter.

This is a book about getting computers to do what you want them todo. Computers are about as common as screwdrivers today, but they contain alot more hidden complexity and thus are harder to operate andunderstand. To many, they remain alien, slightly threatening things.

Weve found two effective ways ofbridging the communication gap between us, squishy biologicalorganisms with a talent for social and spatial reasoning, andcomputers, unfeeling manipulators of meaningless data. The first is toappeal to our sense of the physical world and build interfaces thatmimic that world and allow us to manipulate shapes on a screen withour fingers. This works very well for casual machine interaction.

But we have not yet found a good way to usethe point-and-click approach to communicate things to the computerthat the designer of the interface did not anticipate. For open-endedinterfaces, such as instructing the computer to perform arbitrarytasks, weve had more luck with an approach that makes use of ourtalent for language: teaching the machine a language.

Human languages allow words andphrases to be combined in many ways, which allows us to saymany different things. Computer languages, though typically lessgrammatically flexible, follow a similar principle.

Casual computinghas become much more widespread in the past 20 years, andlanguage-based interfaces, which once were the default way in whichpeople interacted with computers, have largely been replaced withgraphical interfaces. But they are still there, if you know where tolook. One such language, JavaScript, is built into almost everyweb browser and is thus available on just about every consumerdevice.

This book intends to make you familiarenough with this language to be able to make a computer do what youwant.

I do not enlighten those who are not eager to learn,nor arouse those who are not anxious to give an explanationthemselves. If I have presented one corner of the square and theycannot come back to me with the other three, I should not go over thepoints again.

Confucius

Besides explaining JavaScript, I alsowill introduce the basic principles of programming. Programming, itturns out, is hard. The fundamental rules are typically simple andclear. But programs built on top of these rules tend to become complexenough to introduce their own rules and complexity. Youre buildingyour own maze, in a way, and you might just get lost in it.

There will be times when reading this book feels terriblyfrustrating. If you are new to programming, there will be a lot of newmaterial to digest. Much of this material will then be combined inways that require you to make additional connections.

It is up to you to make the necessary effort. When you are strugglingto follow the book, do not jump to any conclusions about your owncapabilities. You are fineyou just need to keep at it. Take a break,reread some material, and always make sure you read and understandthe example programs and exercises. Learning is hard work, buteverything you learn is yours and will make subsequent learningeasier.

The computer programmer is a creator ofuniverses for which he [sic] alone is responsible. Universes of virtuallyunlimited complexity can be created in the form of computer programs.

Joseph Weizenbaum, Computer Power and Human Reason

A program is many things. It is apiece of text typed by a programmer, it is the directing force thatmakes the computer do what it does, it is data in the computersmemory, yet it controls the actions performed on this same memory.Analogies that try to compare programs to objects we are familiar withtend to fall short. A superficially fitting one is that of amachinelots of separate parts tend to be involved, and to make thewhole thing tick, we have to consider the ways in which these partsinterconnect and contribute to the operation of the whole.

A computer is a machine built to act as a host for theseimmaterial machines. Computers themselves can do only stupidlystraightforward things. The reason they are so useful is that they dothese things at an incredibly high speed. A program can ingeniouslycombine an enormous number of these simple actions in order to do verycomplicated things.

To some of us, writing computer programs is afascinating game. A program is a building of thought. It is costlessto build, it is weightless, and it grows easily under our typinghands.

But without care, a programs size and complexity will grow out ofcontrol, confusing even the person who created it. Keeping programsunder control is the main problem of programming. When a programworks, it is beautiful. The art of programming is the skill ofcontrolling complexity. The great program is subduedmade simple inits complexity.

Many programmers believethat this complexity is best managed by using only a small set ofwell-understood techniques in their programs. They have composedstrict rules (best practices) prescribing the form programs shouldhave, and the more zealous among them will consider those who gooutside of this safe little zone to be bad programmers.

What hostility to the richness ofprogrammingto try to reduce it to something straightforward andpredictable, to place a taboo on all the weird and beautiful programs!The landscape of programming techniques is enormous, fascinating inits diversity, and still largely unexplored. It is certainly dangerousgoing, luring the inexperienced programmer into all kinds ofconfusion, but that only means you should proceed with caution andkeep your wits about you. As you learn there will always be newchallenges and new territory to explore. Programmers who refuse tokeep exploring will stagnate, forget their joy, and get bored withtheir craft.

In thebeginning, at the birth of computing, there were no programminglanguages. Programs looked something like this:

00110001 00000000 0000000000110001 00000001 0000000100110011 00000001 0000001001010001 00001011 0000001000100010 00000010 0000100001000011 00000001 0000000001000001 00000001 0000000100010000 00000010 0000000001100010 00000000 00000000

That is aprogram to add the numbers from 1 to 10 together and print out theresult: 1 + 2 + ... + 10 = 55. It could run on a simple,hypothetical machine. To program early computers, it was necessary toset large arrays of switches in the right position or punch holes instrips of cardboard and feed them to the computer. You can probably imaginehow tedious and error-prone this procedure was. Even writing simpleprograms required much cleverness and discipline. Complex ones werenearly inconceivable.