HTML and CSS Reference
In-Depth Information
EXAMPLE
8.15 (
CONTINUED
)
8
if(
tale.hasOwnProperty("author")
){
document.write("The object has an author property.<br />");
}
9
if(
Book.prototype.isPrototypeOf(tale)
){
document.write("Book is a prototype of object tale.<br />");
}
10
if(
tale instanceof Book
){
document.write("tale is an instance of Book.<br />");
}
11
if(
tale instanceof Object
){
document.write("tale is an instance of Object.<br />");
}
</script>
</big>
</body>
</html>
EXPLANATION
1 The constructor function called
Book
defines the properties and methods for a
Book
class.
2 The Book class starts with an empty prototype object. Now we assign it a property.
The
publisher
property and its value are available to all Book objects.
3 By assigning a property to the
Object
object's prototype, all instances of the Book
class (or any class) will have access to the
category
property.
4 A ew i t e f t e k l s is t , a f e to n j t ll d
tale
.
5
The value of the
Object
object's
prototype
property,
category
, is displayed.
6
The
constructor
property is inherited from the
Object
object and can be used with
any of its descendants. This line tests if this object has a constructor. It does, so
the block is entered.
7
The value assigned to the
constructor
property is the constructor function and the
names of all of its arguments and properties.
8
The
hasOwnProperty()
method returns true if the property (its argument) belongs
to the object being tested; in this case the object is
tale
and the method is checking
to see if the
author
property belongs to it and is not inherited from the prototype
chain.
9
Next we test to see if the
tale
object is inheriting from the Book class; that is, Book
is a prototype for the
tale
class.
10
The
instanceof
operator returns
true
if the specified object,
tale,
is of the specified
object type; that is,
tale
is a Book type.
11
The
instanceof
operator returns
true
if the specified object,
tale
, is of the specified
object type; that is,
tale
is also an Object type. The output is shown in Figure 8.18.
