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

Communicating with a computer

We’ve found two effective ways of bridging the communication gap between us, squishy biological organisms with a talent for social and spatial reasoning, and computers, unfeeling manipulators of meaningless data. The first is to appeal to our sense of the physical world and build interfaces that mimic that world and allow us to manipulate shapes on a screen with our fingers. This works very well for casual machine interaction.

But we have not yet found a good way to use the point-and-click approach to communicate things to the computer that the designer of the interface did not anticipate. For open-ended interfaces, such as instructing the computer to perform arbitrary tasks, we’ve had more luck with an approach that makes use of our talent for language: teaching the machine a language.

Human languages allow words and phrases to be combined in many ways, which allows us to say many different things. Computer languages, though typically less grammatically flexible, follow a similar principle.

Casual computing has become much more widespread in the past 20 years, and language-based interfaces, which once were the default way in which people interacted with computers, have largely been replaced with graphical interfaces. But they are still there, if you know where to look. One such language, JavaScript, is built into almost every web browser and is thus available on just about every consumer device.

This book intends to make you familiar enough with this language to be able to make a computer do what you want.

On programming

Besides explaining JavaScript, we’ll also introduce the basic principles of programming. Programming, it turns out, is hard. The fundamental rules are typically simple and clear. But programs built on top of these rules tend to become complex enough to introduce their own rules and complexity. You’re building your own maze, in a way, and you might just get lost in it.

There will be times when reading this book will feel terribly frustrating. If you are new to programming, there will be a lot of new material to digest. Much of this material will then be combined in ways that require you to make additional connections.

It is up to you to make the necessary effort. When you are struggling to follow the book, do not jump to any conclusions about your own capabilities. You are fine—you just need to keep at it. Take a break, reread some material, and always make sure you read and understand the example programs and exercises. Learning is hard work, but everything you learn is yours and will make subsequent learning easier.

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

A computer is a machine built to act as a host for these immaterial machines. Computers themselves can do only incredibly straightforward things. The reason they are so useful is that they do these things at an incredibly high speed. A program can ingeniously combine an enormous number of these simple actions in order to do very complicated things.

To some of us, writing computer programs is a fascinating game. A program is a building of thought. It is costless to build, it is weightless, and it grows easily under our typing hands.

But without care, a program’s size and complexity will grow out of control, confusing even the person who created it. Keeping programs under control is the main problem of programming. When a program works, it is beautiful. The art of programming is the skill of controlling complexity. The great program is subdued—made simple in its complexity.

Many programmers believe that this complexity is best managed by using only a small set of well-understood techniques in their programs. They have composed strict rules (“best practices”) prescribing the form programs should have, and the more zealous among them will consider those who go outside of this safe little zone to be bad programmers.

What hostility to the richness of programming—to try to reduce it to something straightforward and predictable, to place a taboo on all the weird and beautiful programs! The landscape of programming techniques is enormous, fascinating in its diversity, and still largely unexplored. It is certainly dangerous going, luring the inexperienced programmer 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. Programmers who refuse to keep exploring will stagnate, forget their joy, and get bored with their craft.

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 inspired by silly 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.

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.

We 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, we hope, you’ll be tempted to experiment and go beyond the exercises.

When reading this book in your browser, you can edit (and run) all example programs by clicking them.

Typographic conventions

In this book, text written in a monospaced font will represent elements of programs—sometimes they are self-sufficient fragments, and sometimes they just refer to part of a nearby program. Programs are written as follows:

function fac(n) {
  if (n == 0)
    return 1;
    return fac(n - 1) * n;

Sometimes, in order to show the output that a program produces, the expected output is written after it, with two slashes and an arrow in front.

// → 40320

Good luck!