Objects - JavaScript by Example - page 203

EXAMPLE 8.14

4 Book.prototype.addTax=function addTax(){

this.price *= 1.18;
return(this.price.toFixed(2));

}

5 var famousBook=new Book("War and Peace","Leo Tolstoy", 30.00);

var myBook=new Book("JavaScript by Example", "Ellie
Quigley", 25);

6 document.write("<br /><b>" + famousBook.show()+"<br />");

document.write("<br /><b>" + myBook.show()+"<br />");

7 document.write("With tax \""+ famousBook.title +"\" costs $"+

famousBook.addTax() +".<br />");
document.write("With tax \""+ myBook.title + "\" costs $" +

8 myBook.addTax() +".<br />");

</script>
</body>
</html>

EXPLANATION
1

The constructor function called Book defines the properties and methods for a
Book class. ( Book is a JavaScript representation of a class.) The constructor func-
tion has a prototype object containing those properties that are inherited by all
Book objects.

2 A function called showProps is defined on line 3. Its name is assigned to the prop-

erty called show , making it a method for the Book class.

3 The function showProps uses the special for loop to iterate through all the proper-

ties of an object.

4 The Book's prototype property is used to add a function called addTax() . It calcu-

lates and returns the price of the book with an 18 percent sales tax added. This
function will be added as a method to the Book class and available to all objects
in that class. JavaScript creates only one copy of the function and all book objects
will point to that copy.

5 A new Book object, called myBook , is created. It has inherited all of the original

properties of the Book class, including the new method defined by the prototype
property, called addTax .

6 The show() method is called for the object, famousBook . The entire object, prop-

erties and methods are displayed. You can see that addTax has been included.

7 The new method, addTax() is called for the object famousBook .

8 Again, addTax() is called for the object myBook . The result is shown in Figure

8.17.