Chapter 1

Building Abstractions with Functions

We are about to study the idea of a computational process. Computational processes are abstract beings that inhabit computers. As they evolve, processes manipulate other abstract things called data. The evolution of a process is directed by a pattern of rules called a program. People create programs to direct processes. In effect, we conjure the spirits of the computer with our spells.

A computational process is indeed much like a sorcerer’s idea of a spirit. It cannot be seen or touched. It is not composed of matter at all. However, it is very real. It can perform intellectual work. It can answer questions. It can affect the world by disbursing money at a bank or by controlling a robot arm in a factory. The programs we use to conjure processes are like a sorcerer’s spells. They are carefully composed from symbolic expressions in arcane and esoteric programming languages that prescribe the tasks we want our processes to perform.

A computational process, in a correctly working computer, executes programs precisely and accurately. Thus, like the sorcerer’s apprentice, novice programmers must learn to understand and to anticipate the consequences of their conjuring. Even small errors (usually called bugs or glitches) in programs can have complex and unanticipated consequences.

Fortunately, learning to program is considerably less dangerous than learning sorcery, because the spirits we deal with are conveniently contained in a secure way. Real-world programming, however, requires care, expertise, and wisdom. A small bug in a computer-aided design program, for example, can lead to the catastrophic collapse of an airplane or a dam or the self-destruction of an industrial robot.

Master software engineers have the ability to organize programs so that they can be reasonably sure that the resulting processes will perform the tasks intended. They can visualize the behavior of their systems in advance. They know how to structure programs so that unanticipated problems do not lead to catastrophic consequences, and when problems do arise, they can debug their programs. Well-designed computational systems, like well-designed automobiles or nuclear reactors, are designed in a modular manner, so that the parts can be constructed, replaced, and debugged separately.

We need an appropriate language for describing processes, and we will use for this purpose the programming language JavaScript. Just as our everyday thoughts are usually expressed in our natural language (such as English, French, or Japanese), and descriptions of quantitative phenomena are expressed with mathematical notations, our process descriptions will be expressed in JavaScript. JavaScript was developed in the early 1990s as a programming language for controlling the behavior of World Wide Web browsers through scripts that appear in web pages. The language was conceived by Brendan Eich, originally under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. The name “JavaScript” is a trademark of Sun Microsystems.

Despite its inception as a language for controlling browsers, JavaScript is a general-purpose programming language. A JavaScript interpreter is a machine that carries out processes described in the JavaScript language. The first JavaScript interpreter was implemented by Eich at Netscape Communications Corporation, for the Netscape Navigator web browser. The main features of JavaScript are inherited from the Scheme and Self programming languages. Scheme is a dialect of Lisp, and was used as programming language for the original version of this book. From Scheme, JavaScript inherited its most fundamental design principles such as statically-scoped first-class functions and dynamic typing, and as a result, it was fairly straightforward to translate the programs in this book from Scheme to JavaScript.

JavaScript bears only superficial resemblance to the language Java, after which it was (eventually) named; both Java and JavaScript use the block structure of the language C. In contrast with Java and C, which employ compilation to lower-level languages, JavaScript programs are usually interpreted by web browsers. After Netscape Navigator, other web browsers provided interpreters for the language, including Microsoft’s Internet Explorer, whose JavaScript version is called JScript. The popularity of JavaScript for controlling web browsers gave rise to a standardization effort, culminating in a standardized version of the language, called ECMAScript. The first edition of the ECMAScript standard was led by Guy Lewis Steele Jr. and completed in June 1997 (Ecma 1997). The third edition, which is used in this book, was led by Mike Cowlishaw and completed in December 1999.

The practice of embedding JavaScript programs in web pages encouraged the developers of web browsers to implement JavaScript interpreters. As these programs became more complex, the interpreters became more efficient in executing them, eventually using sophisticated implementation techniques such as Just-In-Time (JIT) compilation. The majority of JavaScript programs (as of 2013) is embedded in web pages and interpreted by browsers, but JavaScript is also used for scripting dashboard widgets in Apple computers running the OS X operating system, for controlling software systems such as Adobe Reader and Adobe Flash and devices such as universal remote panels.

However, it is the ability of browsers to execute JavaScript programs that makes it an ideal language for an online version of a programming textbook. Executing programs by clicking on things on a web page comes naturally in JavaScript—after all that is what JavaScript was designed for! More fundamentally, JavaScript possesses features that make it an excellent medium for studying important programming constructs and data structures and for relating them to the linguistic features that support them. JavaScript’s statically-scoped first-class functions provide direct and concise access to abstraction mechanisms, and dynamic typing removes the need for declaring the types of the data being manipulated by the program. Above and beyond these considerations, programming in JavaScript is great fun.