AJAX XMLHttp

AJAX - The XMLHttpRequest Object

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The keystone of AJAX is the XMLHttpRequest object.

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The XMLHttpRequest Object

All modern browsers support the XMLHttpRequest object.
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.

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Create an XMLHttpRequest Object

All modern browsers (Chrome, Firefox, IE7+, Edge, Safari Opera) have a built-in XMLHttpRequest object.

Syntax for creating an XMLHttpRequest object:

variable = new XMLHttpRequest();

Example

var xhttp = new XMLHttpRequest();

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Access Across Domains

For security reasons, modern browsers do not allow access across domains.
This means that both the web page and the XML file it tries to load, must be located on the same server.
The examples on W3Schools all open XML files located on the W3Schools domain.
If you want to use the example above on one of your own web pages, the XML files you load must be located on your own server.

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Old Versions of Internet Explorer (IE5 and IE6)

Old versions of Internet Explorer (IE5 and IE6) use an ActiveX object instead of the XMLHttpRequest object:

variable = new ActiveXObject("Microsoft.XMLHTTP");

To handle IE5 and IE6, check if the browser supports the XMLHttpRequest object, or else create an ActiveX object:

Example

if (window.XMLHttpRequest) {
    // code for modern browsers    xmlhttp = new XMLHttpRequest();
 } else {
    // code for old IE browsers    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}

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XMLHttpRequest Object Methods

Method	Description
new XMLHttpRequest()	Creates a new XMLHttpRequest object
abort()	Cancels the current request
getAllResponseHeaders()	Returns header information
getResponseHeader()	Returns specific header information
open(method,url,async,user,psw)	Specifies the request method: the request type GET or POST url: the file location async: true (asynchronous) or false (synchronous) user: optional user name psw: optional password
send()	Sends the request to the server Used for GET requests
send(string)	Sends the request to the server. Used for POST requests
setRequestHeader()	Adds a label/value pair to the header to be sent

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XMLHttpRequest Object Properties

Property	Description
onreadystatechange	Defines a function to be called when the readyState property changes
readyState	Holds the status of the XMLHttpRequest. 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready
responseText	Returns the response data as a string
responseXML	Returns the response data as XML data
status	Returns the status-number of a request 200: "OK" 403: "Forbidden" 404: "Not Found" For a complete list go to the Http Messages Reference
statusText	Returns the status-text (e.g. "OK" or "Not Found")

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