Chapter 3. Data Types and Values

Computer programs work by manipulating values , such as the number 3.14 or the text "Hello World". The types of values that can be represented and manipulated in a programming language are known as data types, and one of the most fundamental characteristics of a programming language is the set of data types it supports. JavaScript allows you to work with three primitive data types: numbers, strings of text (known as "strings"), and boolean truth values (known as "booleans"). JavaScript also defines two trivial data types, null and undefined, each of which defines only a single value.

In addition to these primitive data types, JavaScript supports a composite data type known as object. An object (that is, a member of the data type object) represents a collection of values (either primitive values, like numbers and strings, or composite values, like other objects). Objects in JavaScript have a dual nature: an object can represent an unordered collection of named values or an ordered collection of numbered values. In the latter case, the object is called an array . Although objects and arrays are fundamentally the same data type in JavaScript, they behave quite differently and will usually be considered distinct types throughout this book.

JavaScript defines another special kind of object, known as a function . A function is an object that has executable code associated with it. A function may be invoked to perform some kind of operation. Like arrays, functions behave differently from other kinds of objects, and JavaScript defines special language syntax for working with them. Thus, we'll treat the function data type independently of the object and array types.

In addition to functions and arrays, core JavaScript defines a few other specialized kinds of objects. These objects do not represent new data types, just new classes of objects. The Date class defines objects that represent dates, the RegExp class defines objects that represent regular expressions (a powerful pattern-matching tool described in Chapter 10), and the Error class defines objects that represent syntax and runtime errors that can occur in a JavaScript program.

The remainder of this chapter documents each of the primitive data types in detail. It also introduces the object, array, and function data types, which are fully documented in Chapter 7, Chapter 8, and Chapter 9. Finally, it provides an overview of the Date, RegExp, and Error classes, which are documented in full detail in the core reference section of this book.