The XMLHttpRequest object is the key to AJAX. It has been available ever since Internet Explorer 5.5 was released in July 2000, but not fully discovered before people started to talk about AJAX and Web 2.0 in 2005.
The data returned from XMLHttpRequest calls will often be provided by back-end databases. Besides XML, XMLHttpRequest can be used to fetch data in other formats, e.g. JSON or even plain text.
You already have seen couple of examples on how to create a XMLHttpRequest object.
Below is listed some of the methods and properties you have to become familiar with.
Cancels the current request.
Returns the complete set of HTTP headers as a string.
getResponseHeader( headerName )
Returns the value of the specified HTTP header.
open( method, URL )
open( method, URL, async )
open( method, URL, async, userName )
open( method, URL, async, userName, password )
Specifies the method, URL, and other optional attributes of a request.
The method parameter can have a value of "GET", "POST", or "HEAD". Other HTTP methods, such as "PUT" and "DELETE" (primarily used in REST applications), may be possible
The "async" parameter specifies whether the request should be handled asynchronously or not . "true" means that script processing carries on after the send() method, without waiting for a response, and "false" means that the script waits for a response before continuing script processing.
send( content )
Sends the request.
setRequestHeader( label, value )
Adds a label/value pair to the HTTP header to be sent.
An event handler for an event that fires at every state change.
The readyState property defines the current state of the XMLHttpRequest object.
Here are the possible values for the readyState propery:
|0||The request is not initialized|
|1||The request has been set up|
|2||The request has been sent|
|3||The request is in process|
|4||The request is completed|
readyState=0 after you have created the XMLHttpRequest object, but before you have called the open()
readyState=1 after you have called the
open() method, but before you have called send().
readyState=2 after you have called send().
readyState=3 after the browser has established a communication with the
server, but before the server has completed the response.
readyState=4 after the request has been
completed, and the response data have been completely received
from the server.
Returns the response as a string.
Returns the response as XML. This property returns an XML document object, which can be examined and parsed using W3C DOM node tree methods and properties.
Returns the status as a number (e.g. 404 for "Not Found" and 200 for "OK").
Returns the status as a string (e.g. "Not Found" or "OK").