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Creating the base class

ClassA.call(this, sColor);

this.name = sName;

}

ClassB.prototype = new ClassA();

ClassB.prototype.sayName = function () {

alert(this.name);

};

In this example, inheritance is accomplished with the two highlighted lines. First, in the

ClassB

con-

structor, object masquerading is used to inherit the

color

property from

ClassA

. In the second high-

lighted line, prototype chaining is used to inherit the methods of

ClassA

. Because this hybrid method

uses prototype chaining, the

instanceof

operator still works correctly.

The following example tests this code:

var objA = new ClassA(“red”);

var objB = new ClassB(“blue”, “Nicholas”);

objA.sayColor(); //outputs “red”

objB.sayColor(); //outputs “blue”

objB.sayName(); //outputs “Nicholas”

A more practical example

In real Web sites and applications, chances are you won’t be creating classes named

ClassA

and

ClassB

It’s far more likely that you will create classes that represent specific things, such as shapes. If you con-

sider the shapes example from the beginning of the chapter, the

Polygon

Triangle

, and

Rectangle

classes form a nice set of data to explore.

Creating the base class

Think of the

Polygon

class first. What sort of properties and methods are necessary? First, it’s important

to know the number of sides the polygon has, so an integer property named

sides

should be included.

What else might be necessary for a polygon? You may want to determine the area of polygon, so add a

method named

getArea()

to calculate it. Figure 4-3 shows the UML representation of this class.

Figure 4-3

In UML, properties are represented by the property name and type in the section immediately under the

class name. Methods are located under the properties, indicating the method name and the type of the

return value.

Polygon

sides : integer

getArea(): integer

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Inheritance

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