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Requirements of object-oriented languages

The object definition is contained within a single function called a

constructor

. The constructor isn’t a

special kind of function; it’s just a regular function that is used to create an object. Later in this chapter,

you learn how to create your own constructors.

Requirements of object-oriented languages

Before a language can be called object-oriented, it must provide four basic capabilities to developers:

Encapsulation

— the capability to store related information, whether data or methods, together

in an object

Aggregation

— the capability to store one object inside of another object

Inheritance

— the capability of a class to rely upon another class (or number of classes) for some

of its properties and methods

Polymorphism

— the capability to write one function or method that works in a variety of differ-

ent ways

ECMAScript supports all four of these requirements and so is considered to be object-oriented.

Composition of an object

In ECMAScript, objects are composed of

attributes

, which are either primitive or reference values. If an

attribute contains a function, it is considered to be a

method

of the object; otherwise, the attribute is con-

sidered a

property

Working with Objects

The previous chapter touched briefly on how to work with objects, but now it’s time to go into more

detail. Objects are created and destroyed throughout the execution of JavaScript code, and understand-

ing the implications of this paradigm is vital to your understanding of the language as a whole.

Declaration and instantiation

Objects are created by using the

new

keyword followed by the name of the class you wish to instantiate:

var oObject = new Object();

var oStringObject = new String();

The first line creates a new instance of

Object

and stores it in the variable

oObject

; the second line cre-

ates a new instance of

String

and stores it in the variable

oStringObject

. The parentheses aren’t

required when the constructor doesn’t require arguments, so these two lines could be rewritten as

follows:

var oObject = new Object;

var oStringObject = new String;

Chapter 3

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