Primer on JavaScript Objects
JavaScript is an
object-oriented language. However, it is not a class-based based language
like Java or C++, but a prototype-based OO language. This means that
JavaScript does not make a distinction between a class and its instance.
In JavaScript, for example you can dynamically add a method to a prototype
and all objects will have it, no matter when they were created.
There are two ways
to construct an object in JavaScript:
- using object
initializers (JavaScript 1.2 and later only - not explained here)
- using constructor
function
You can use any
method to create an object. You just have to call it with new
operator. Here is an example:
function createCar(
make, model )
{
this.make
= make;
this.model = model;
this.model = printValues; // printValues()
is a global function
}
myCar = new
CreateCar("Audi", "100 CS");
alert(myCar.make + " " + myCar.model);
The above example
also illustrates that in order to create a new property of a function
all you have to do is assign it some value. We can also do that after
the object was already created:
myCar.color
= "Blue";
In the same fashion
we can assign a pointer to a function:
myCar.toString
= printProperties;
In JavaScript 1.0
you cannot delete the objects - they exist until you leave the page.
In JavaScript 1.1 you can assign the object a null value and
it will get deleted from memory. In later versions you can use delete
myObject;
Inheritance (Object Prototypes)
Inheritance is possible
in JavaScript. All you have to do is assign the prototype property
of the object to the new instance of the object you would like to extend.
In the following example Manager object extends from Employee object:
function Manager
() {...}
Manager.prototype = new Employee;
JavaScript does
not allow multiple inheritance, but the constructor can call other constructors
because for JavaScript their simply methods.
Dynamic Object Modification
In JavaScript, you
can add properties to any object at run time. You are not constrained
to use only the properties provided by the constructor function. To
add a property that is specific to a single object, you assign a value
to the object, as follows:
mark = new
Manager("Mark");
mark.bonus = 3000;
Now, the mark object
has a bonus property, but no other Manager has this property.
If you add a new
property to an object that is being used as the prototype for a constructor
function, you add that property to all objects that inherit properties
from the prototype. For example, you can add a specialty property to
all employees with the following statement:
Employee.prototype.specialty
= "none";
As soon as JavaScript
executes this statement, the mark object also has the specialty property
with the value of "none".
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