The XMLHttpRequest object is the key to AJAX. It has been available ever since Internet Explorer 5.5 was released in July 2000, but was not fully discovered until AJAX and Web 2.0 in 2005 became popular. The XMLHttpRequest object can be used to exchange data with a server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.
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 such as JSON or even plain text.
Creating an XMLHttpRequest Object
The syntax to create a new request is:
variable = new XMLHttpRequest();
An example will be as follows:
var xhttp = new XMLHttpRequest();
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 the 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.
The following table provides a list of the possible values for the readyState property:
State Description 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() method.
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 has 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 the 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").