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UML

In UML, you never duplicate properties or methods that are inherited unless a method is being overrid-

den (or overloaded, which is not possible in ECMAScript).

The code for the

Triangle

class is:

function Triangle(iBase, iHeight) {

Polygon.call(this, 3);

this.base = iBase;

this.height = iHeight;

}

Triangle.prototype = new Polygon();

Triangle.prototype.getArea = function () {

return 0.5 * this.base * this.height;

};

Note that the

Triangle

constructor accepts two arguments,

base

and

height

, even though the

Polygon

constructor accepts just one,

sides

. This is because you already know the number of sides in a triangle,

and you don’t want to allow the developer to change that. So, when you use object masquerading, the

number

is passed to the

Polygon

constructor as the number of sides for this object. Then, the values for

base

and

height

are assigned the appropriate properties.

After using prototype chaining to inherit the methods,

Triangle

then overrides the

getArea()

method

to provide the custom calculation required for the calculation of triangle areas.

The last class is

Rectangle

, which also inherits from

Polygon

. Rectangles have four sides and the area

is calculated by multiplying the length by the width, which are two properties needed for the class.

Rectangle

fits into the earlier UML diagram next to

Triangle

because both have

Polygon

as a super-

class (see Figure 4-6).

Figure 4-6

The ECMAScript code for

Rectangle

is as follows:

function Rectangle(iLength, iWidth) {

Polygon.call(this, 4);

this.length = iLength;

this.width = iWidth;

Triangle

base : integer

height: integer

getArea(): integer

Polygon

sides : integer

getArea(): integer

Rectangle

length : integer

width : integer

getArea(): integer

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Inheritance

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