The XMLHTTPrequest object

“Anatomy of an Ajax Application,” introduced the building blocks of an Ajax application and discussed how these pieces fit together.This lesson examines the object at the heart of every Ajax application—the XMLHTTPRequest object.

An object can be thought of as a single package containing a set of prop rties, which contain and classify data, and a set of methods with which the object can perform actions on that data.

Suppose, for example, that we had an object of type wheelbarrow. Such an object might have a property contents, which describes how many items the wheelbarrow holds at any given moment. Methods might include fill() tip(), forward(), and stop().When using JavaScript you can design such objects as you see fit.

However, in addition to user-defined objects, JavaScript has a range of ready-made objects for use in scripts.These are referred to as native objects. Examples of JavaScript’s native objects include Math(), String(), and Date().

Creating an Instance of an Object
Many objects, such as the document object, already exist and therefore do not need you to create an instance of them. Others, however, require you to create an instance of the object in question before you can use it.

You can create an instance of an object by calling a method known as the object’s constructor, using the new keyword:

var myBarrow = new Wheelbarrow();

Having created an instance myBarrow of the object wheelbarrow, properties and methods for the object may be manipulated using a simple syntax:

myBarrow.contents  = 20;
myBarrow.forward();
myBarrow.stop();
myBarrow.tip();

Of course, you are at liberty to create other instances of the same object and have them exist concurrently:

var myBarrow = new Wheelbarrow();
var yourBarrow = new Wheelbarrow();
myBarrow.contents = 20;
yourBarrow.contents = 50;

The Document Object Model or DOM
We mentioned briefly in Lesson "client side coding using javascript" the hierarchy of objects “built in” to a web page and known as the Document Object Model.You access these objects and their properties and methods in the same way as native objects and objects that you devise and create yourself.