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Object Basics

ECMAScript objects are one of the unique (and useful) features of JavaScript. Chapter 2,

“ECMAScript Basics,” introduced the concept that everything is an object, including functions.

This chapter focuses on how to manipulate and use those objects, as well as how to create your

own objects to add functionality specific to your needs.

Object-Oriented Terminology

ECMA-262 defines an

object

as an “unordered collection of properties each of which contains a

primitive value, object, or function.” Strictly speaking, this means that an object is an array of val-

ues in no particular order. Although this is ECMAScript’s interpretation, an

object

is more generi-

cally defined to be a code-based representation of a noun (person, place, or thing).

Each object is defined by a

class

, which can be thought of as a recipe for an object. The class defines

both the

interface

of an object (the properties and methods that can be accessed by developers) as

well as the inner workings of the object (the code that makes the properties and methods work).

The compiler or interpreter uses the class to build objects according to its specifications.

When a program uses a class to create an object, the resulting object is said to be an

instance

of the

class. The only limit to the number of instances that can be created from a single class is the physi-

cal memory limitations of the machine on which the code is running. Each instance behaves the

same way, but each can handle separate sets of data. The process of creating an object instance

from a class is called

instantiation

As I discussed briefly in Chapter 1, ECMAScript has no formal classes. Instead, ECMA-262 describes

object definitions

as the recipes for an object. This is a logical compromise for ECMAScript, because

object definitions actually are objects in and of themselves (which I explain shortly). Even though

classes don’t actually exist, this book refers to object definitions as classes because the term is more

familiar to most developers and, functionally, the two are equivalent.

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