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Defining Classes and Objects

In this code, the function

showColor()

is defined first (using

this

). Then, two objects (

oCar1

and

oCar2

) are created, one with a

color

property set to

“red”

, and the other with a

color

property set

“blue”

. Both objects are assigned a property called

showColor

that points to the original function

named

showColor()

(note that no naming problem exists because one is a global function and the other

is a property of an object). When calling

showColor()

on each object, the

oCar1

outputs

“red”

whereas

oCar2

outputs

“blue”

. This happens because the this keyword in the function is equal to

car1

when

oCar1.showColor()

is called and equal to

oCar2

when

oCar2.showColor()

is called.

Note that the

this

keyword

must

be used when referring to properties of an object. For instance,

showColor()

wouldn’t work if it were written like this:

function showColor() {

alert(color);

}

Whenever a variable is referenced without an object or

this

before it, ECMAScript thinks that it is a

local or global variable. This function then looks for a local or global variable named

color

, which it

won’t find. The result? The function displays

“null”

in the alert.

Defining Classes and Objects

The capability to use predefined objects is only one part of an object-oriented language. The true power

comes because you can create your own classes and objects for specific uses. As with many things in

ECMAScript, you can accomplish this in a variety of ways.

Factory paradigm

Because properties of an object can be defined dynamically after its creation, a lot of developers wrote

code similar to the following when JavaScript was first introduced:

var oCar = new Object;

oCar.color = “red”;

oCar.doors = 4;

oCar.mpg = 23;

oCar.showColor = function () {

alert(this.color);

};

In this code, an object is created named

car

. The object is then given several properties: Its color is red, it

has four doors, and it gets 23 miles per gallon. The last property is actually a pointer to a function, which

means the property is a method. After this code is executed, you can use an object called

car

. The prob-

lem is that you may need to create more than one instance of a car.

To solve the problem, developers created

factory functions

, which create and return an object of a specific

type. For example, a function called

createCar()

could be used to encapsulate the creation of the

car

object described previously:

Chapter 3

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