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Inheritance

function Polygon(iSides) {

this.sides = iSides;

}

Polygon.prototype.getArea = function () {

return 0;

};

function Triangle(iBase, iHeight) {

Polygon.call(this, 3);

this.base = iBase;

this.height = iHeight;

}

Triangle.prototype.inheritFrom(Polygon);

Triangle.prototype.getArea = function () {

return 0.5 * this.base * this.height;

};

function Rectangle(iLength, iWidth) {

Polygon.call(this, 4);

this.length = iLength;

this.width = iWidth;

}

Rectangle.prototype.inheritFrom(Polygon);

Rectangle.prototype.getArea = function () {

return this.length * this.width;

};

To test this code, you can use the same example as before and add in a couple extra lines to test out the

instanceOf()

method:

var triangle = new Triangle(12, 4);

var rectangle = new Rectangle(22, 10);

alert(triangle.sides);

alert(triangle.getArea());

alert(rectangle.sides);

alert(rectangle.getArea());

alert(triangle.instanceOf(Triangle)); //outputs “true”

alert(triangle.instanceOf(Polygon)); //outputs “true”

alert(rectangle.instanceOf(Rectangle)); //outputs “true”

alert(rectangle.instanceOf(Polygon)); //outputs “true”

The last four lines test

instanceOf()

and should all return

true

117

Inheritance

07_579088 ch04.qxd 3/28/05 11:36 AM Page 117