What Is JavaScript?

1. 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. JavaScript 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 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 written to describe the way the JavaScript language should work so that the various pieces of software that claimed to support JavaScript were actually talking about the same language. This is called the ECMAScript 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 JavaScript. 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 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 version 4 was abandoned in 2008, leading to a much less ambi­tious version 5. Released in 2009, version 5 made only some uncontrover­sial improvements. Then in 2015 version 6 came out, a major update that included some of the ideas planned for version 4. Since then we’ve had new, small updates every year.

The fact that the language is evolving means that browsers have to con­stantly keep up, and if you’re using an older browser, it may not support every feature. The language designers are careful to not make any changes that could break existing programs, so new browsers can still run old pro­grams. In this book, I’m using the 2017 version ofJavaScript.

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), provide an environment for programming JavaScript outside of the browser.

2. 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 code. I believe reading code and writing code are indis­pensable parts of learning to program. 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’ll 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 experiment with it, is to look it up in the online version of the book at https://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 https://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 defined in this book outside of the book’s website, some care will be required. Many examples stand on their own and should work in any JavaScript environment. But code in later chap­ters is often written for a specific environment (the browser or Node.js) and can run only there. In addition, many chapters define bigger programs, and the pieces of code that appear in them depend on each other or on external files. The sandbox on the website provides links to Zip files containing all the scripts and data files necessary to run the code for a given chapter.

3. Overview of This Book

This book contains three parts. The first 12 chapters discuss the JavaScript language. The next seven chapters are about web browsers and the way JavaScript is used to program them. Finally, two chapters are devoted to Node.js, another environment to program JavaScript in.

Throughout the book, there are five project chapters, which describe larger example programs to give you a taste of actual programming. In order of appearance, we will work through building a delivery robot, a pro­gramming language, a platform game, a pixel paint program, and a dynamic website.

The language part of the book starts with four chapters that introduce the basic structure of the JavaScript language. They introduce control struc­tures (such as the while word you saw in this introduction), functions (writ­ing your own building blocks), and data structures. After these, you will be able to write basic programs. Next, Chapters 5 and 6 introduce techniques to use functions and objects to write more abstract code and keep complexity under control.

After a first project chapter, the language part of the book continues with chapters on error handling and bug fixing, regular expressions (an important tool for working with text), modularity (another defense against complexity), and asynchronous programming (dealing with events that take time). The second project chapter concludes the first part of the book.

The second part, Chapters 13 to 19, describes the tools that browser JavaScript has access to. You’ll learn to display things on the screen (Chapters 14 and 17), respond to user input (Chapter 15), and communicate over the network (Chapter 18). There are again two project chapters in this part.

After that, Chapter 20 describes Node.js, and Chapter 21 builds a small website using that tool.

Finally, Chapter 22 describes some of the considerations that come up when optimizing JavaScript programs for speed.

4. 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 (of which you have already seen a few) are written as follows:

function factorial(n) {

if (n == 0) {

return 1;

} else {

return factorial(n – 1) * n;



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


// → 40320

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,

No Starch Press; 3rd edition.

Leave a Reply

Your email address will not be published. Required fields are marked *