JavaScript: The Definitive Guide, Fourth Edition

Chapter 1. Introduction to JavaScript

JavaScript is a lightweight, interpreted programming language with object-oriented capabilities. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers and embellished for web programming with the addition of objects that represent the web browser window and its contents. This client-side version of JavaScript allows executable content to be included in web pages -- it means that a web page need no longer be static HTML, but can include programs that interact with the user, control the browser, and dynamically create HTML content.

Syntactically, the core JavaScript language resembles C, C++, and Java, with programming constructs such as the if statement, the while loop, and the && operator. The similarity ends with this syntactic resemblance, however. JavaScript is an untyped language, which means that variables do not need to have a type specified. Objects in JavaScript are more like Perl’s associative arrays than they are like structures in C or objects in C++ or Java. The object-oriented inheritance mechanism of JavaScript is like those of the little-known languages Self and NewtonScript; it is quite different from inheritance in C++ and Java. Like Perl, JavaScript is an interpreted language, and it draws inspiration from Perl in a number of places, such as its regular expression and array-handling features.

This chapter provides a quick overview of JavaScript; it explains what JavaScript can and cannot do and exposes some myths about the language. It distinguishes the core JavaScript language from embedded and extended versions of the language, such as the client-side JavaScript that is embedded in web browsers and the server-side JavaScript that is embedded in Netscape’s web servers. (This book documents core and client-side JavaScript.) This chapter also demonstrates real-world web programming with some client-side JavaScript examples.

JavaScript Myths

JavaScript is the subject of a fair bit of misinformation and confusion. Before proceeding any further with our exploration of JavaScript, it is important that we debunk some common and persistent myths about the language.

JavaScript Is Not Java

One of the most common misconceptions about JavaScript is that it is a simplified version of Java, the programming language from Sun Microsystems. Other than an incomplete syntactic resemblance and the fact that both Java and JavaScript can provide executable content in web browsers, the two languages are entirely unrelated. The similarity of names is purely a marketing ploy (the language was originally called LiveScript; its name was changed to JavaScript at the last minute).

JavaScript and Java do, however, make a good team. The two languages have different sets of capabilities. JavaScript can control browser behavior and content but cannot draw graphics or perform networking. Java has no control over the browser as a whole but can do graphics, networking, and multithreading. Client-side JavaScript can interact with and control Java applets embedded in a web page, and, in this sense, JavaScript really can script Java (see Chapter 22 for details).

JavaScript Is Not Simple

JavaScript is touted as a scripting language instead of a programming language, the implication being that scripting languages are simpler, that they are programming languages for non-programmers. Indeed, JavaScript appears at first glance to be a fairly simple language, perhaps of the same complexity as BASIC. JavaScript does have a number of features designed to make it more forgiving and easier to use for new and unsophisticated programmers. Non-programmers can use JavaScript for limited, cookbook-style programming tasks.

Beneath its thin veneer of simplicity, however, JavaScript is a full-featured programming language, as complex as any and more complex than some. Programmers who attempt to use JavaScript for nontrivial tasks often find the process frustrating if they do not have a solid understanding of the language. This book documents JavaScript comprehensively, so you can develop a sophisticated understanding of the language.