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Encyclopedia > XMLHttpRequest

XMLHttpRequest (XHR) is an API that can be used by JavaScript, and other web browser scripting languages to transfer XML and other text data to and from a web server using HTTP, by establishing an independent and asynchronous communication channel between a web page's Client-Side and Server-Side. API and Api redirect here. ... JavaScript is a scripting language most often used for client-side web development. ... An example of a Web browser (Mozilla Firefox) A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network. ... Scripting languages (commonly called script languages) are computer programming languages that are typically interpreted. ... The Extensible Markup Language (XML) is a general-purpose markup language. ... The inside/front of a Dell PowerEdge web server The term Web server can mean one of two things: A computer program that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... Client/Server is a network application architecture which separates the client (usually the graphical user interface) from the server. ... In computer networking, the term server-side refers to operations that are performed by the server in a client-server relationship. ...


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 HTML, JSON or plain text. HTML, an initialism of Hypertext Markup Language, is the predominant markup language for web pages. ... JSON (JavaScript Object Notation) (Pronounced like Jason, IPA ) is a lightweight computer data interchange format. ... Computer files can be divided into two broad categories: binary and text. ...


XMLHttpRequest is an important part of the Ajax web development technique, and it is used by many websites to implement responsive and dynamic web applications. Examples of web applications that make use of XMLHttpRequest include Google's Gmail service, Meebo, Google Maps, Windows Live's Virtual Earth, the MapQuest dynamic map interface, Facebook and many others. AJAX redirects here. ... In software engineering, a web application is an application delivered to users from a web server over a network such as the World Wide Web or an intranet. ... This article is about the corporation. ... For other uses, see Gmail (disambiguation). ... Meebo is an in-browser instant messaging program which supports multiple IM services, including Yahoo! Messenger, Windows Live Messenger, Google Talk, AIM, ICQ [1], and Jabber and is based on the free and open source library libpurple created by the software developers of Pidgin[2]. // Although still in its early... Google Maps (for a time named Google Local) is a free web mapping service application and technology provided by Google that powers many map-based services including the Google Maps website, Google Ride Finder and embedded maps on third-party websites via the Google Maps API. It offers street maps... MSN Virtual Earth is a free online virtual globe map service by Microsoft. ... Screenshot from MapQuest MapQuest is a map publisher and free online Web Map Service, owned by AOL. The company was founded in 1967 as Cartographic Services , a division of R.R. Donnelley & Sons in Chicago, Illinois. ... Facebook headquarters in Palo Alto, CA Facebook is a social networking website that allows people to communicate with their friends and exchange information. ...

Contents

Methods

Method Description
abort() Cancels the current request.
getAllResponseHeaders() 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", "HEAD", "PUT", "DELETE", or a variety of other HTTP methods listed in the W3C specification.[1] HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ...


The URL parameter may be either a relative or complete URL.


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.

Properties

Property Description
onreadystatechange Specifies a reference to an event handler for an event that fires at every state change.
readyState Returns the state of the object as follows:
  • 0 = uninitialized - open() has not yet been called.
  • 1 = open - send() has not yet been called.
  • 2 = sent - send() has been called, headers and status are available.
  • 3 = receiving - Downloading, responseText holds partial data (although this functionality is not available in IE [2])
  • 4 = loaded - Finished.
responseText Returns the response as a string.
responseXML 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.
responseBody Returns the response as a binary encoded string. This property is not part of the native XMLHttpRequest wrapper. For this property to be available, the XHR object must be created with an ActiveX component. A JScript example:
 xmlhttp=new ActiveXObject("MSXML2.XMLHTTP") xmlhttp.open("GET","#",false); xmlhttp.send(null); alert(xmlhttp.responseBody); 
status Returns the HTTP status code as a number (e.g. 404 for "Not Found" and 200 for "OK"). Some network-related status codes (e.g. 408 for "Request Timeout") cause errors to be thrown in Firefox if the status fields are accessed [3]. If server does not respond (properly), IE returns a WinInet Error Code (e.g 12029 for "cannot connect").
statusText Returns the status as a string (e.g. "Not Found" or "OK").

Hierarchy of objects in an example HTML DOM - Document Object Model The Document Object Model (DOM) is a platform- and language-independent standard object model for representing HTML or XML and related formats. ... ActiveX is Microsoft technology used for developing reusable object oriented software components. ... JScript is the Microsoft implementation of the ECMAScript scripting programming language specification. ... The following is a list of HTTP response status codes and standard associated phrases, intended to give a short textual description of the status. ...

History and support

The XMLHttpRequest concept was originally developed by Microsoft as part of Outlook Web Access 2000, as a server side API call, so not still a standards-based web client feature, but de facto support for it has been implemented by many major web browsers. The Microsoft implementation is called XMLHTTP. It has been available since Internet Explorer 5.0[4] and is accessible via JScript, VBScript and other scripting languages supported by IE browsers. Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ... Outlook Web Access (OWA) is a webmail service of Microsoft Exchange Server 5. ... Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE), commonly abbreviated to IE, is a series of proprietary graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. ...


The Mozilla project incorporated the first compatible native implementation of XMLHttpRequest in Mozilla 1.0 in 2002. This implementation was later followed by Apple since Safari 1.2, Konqueror, Opera Software since Opera 8.0 and iCab since 3.0b352. The Mozilla Application Suite (originally known as Mozilla, marketed as the Mozilla Suite, and code named Seamonkey) is a free, cross-platform internet suite, whose components include a web browser, an e-mail and news client, an HTML editor, and an IRC client. ... Apple Inc. ... Safari is a web browser developed by Apple Inc. ... Konqueror is a file manager, web browser and file viewer, which was developed as part of the K Desktop Environment (KDE) by volunteers and runs on most Unix-like operating systems. ... Logo of Opera Software. ... Opera is an Internet suite which handles common internet-related tasks, including visiting web sites, sending and receiving e-mail messages, managing contacts, and online chat. ... iCab is a highly-configurable web browser for the Macintosh by iCab Company. ...


The World Wide Web Consortium published a Working Draft specification for the XMLHttpRequest object's API on 5 April 2006.[5] While this is still a work in progress, its goal is "to document a minimum set of interoperable features based on existing implementations, allowing Web developers to use these features without platform-specific code". The draft specification is based upon existing popular implementations, to help improve and ensure interoperability of code across web platforms. It has been suggested that W3C Markup Validation Service be merged into this article or section. ...


Web pages that use XMLHttpRequest or XMLHTTP can mitigate the current minor differences in the implementations either by encapsulating the XMLHttpRequest object in a JavaScript wrapper, or by using an existing framework that does so. In either case, the wrapper should detect the abilities of current implementation and work within its requirements. JavaScript is a scripting language most often used for client-side web development. ...


Traditionally, there have been other methods to achieve a similar effect of server dynamic applications using scripting languages and/or plugin technology: Scripting languages (commonly called script languages) are computer programming languages that are typically interpreted. ... For other uses, see Plug in. ...

In addition, the World Wide Web Consortium has several Recommendations that also allow for dynamic communication between a server and user agent, though few of them are well supported. These include: IFRAME is a tag used in web page designing. ... Remote Scripting [1] is a technology developed by Microsoft. ... LiveConnect is a programming interface. ... ActiveX is Microsoft technology used for developing reusable object oriented software components. ... Macromedia Flash or Flash is a graphics animation program, written and marketed by Macromedia, that uses vector graphics. ... An applet is a small program that runs in the context of a larger program on a client computer. ...

  • The object element defined in HTML 4 for embedding arbitrary content types into documents, (replaces inline frames under XHTML 1.1)
  • The Document Object Model (DOM) Level 3 Load and Save Specification.[6]

Here is a cross-browser general purpose example of an AJAX/XMLHttpRequest JavaScript function

 function ajax(url, vars, callbackFunction){ var request = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("MSXML2.XMLHTTP.3.0"); request.open("POST", url, true); request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); request.onreadystatechange = function(){ if (request.readyState == 4 && request.status == 200) { if (request.responseText){ callbackFunction(request.responseText); } } }; request.send(vars); } 

Here is a patch allowing direct invocation of XMLHttpRequest in browsers that don't supply one directly:

 if( !window.XMLHttpRequest ) XMLHttpRequest = function(){ try{ return new ActiveXObject("MSXML3.XMLHTTP") }catch(e){} try{ return new ActiveXObject("MSXML2.XMLHTTP.3.0") }catch(e){} try{ return new ActiveXObject("Msxml2.XMLHTTP") }catch(e){} try{ return new ActiveXObject("Microsoft.XMLHTTP") }catch(e){} throw new Error("Could not find an XMLHttpRequest alternative.") }; 

Known problems

Caching

Most of the implementations also realize HTTP caching. Internet Explorer and Firefox do, but there is a difference in how and when the cached content is revalidated.


Firefox revalidates the cached response every time the page is refreshed, issuing an "If-Modified-Since" header with value set to the value of the "Last-Modified" header of the cached response.


Internet Explorer does so only if the cached response is expired (i.e., after the date of received "Expires" header).


This raises some issues, and it is widely believed that a bug exists in Internet Explorer, and the cached response is never refreshed.


It is possible to unify the caching behavior on the client. The following script illustrates an example approach:

 var request = (typeof(XMLHttpRequest) != "undefined") ? new XMLHttpRequest() : new ActiveXObject("Msxml2.XMLHTTP"); request.open("GET", url, false); request.send(null); if(!request.getResponseHeader("Date")) { var cached = request; request = (typeof(XMLHttpRequest) != "undefined") ? new XMLHttpRequest() : new ActiveXObject("Msxml2.XMLHTTP"); var ifModifiedSince = cached.getResponseHeader("Last-Modified"); ifModifiedSince = (ifModifiedSince) ? ifModifiedSince : new Date(0); // January 1, 1970 request.open("GET", url, false); request.setRequestHeader("If-Modified-Since", ifModifiedSince); request.send(""); if(request.status == 304) { request = cached; } } 

In Internet Explorer, if the response is returned from the cache without revalidation, the "Date" header is an empty string. The workaround is achieved by checking the "Date" response header and issuing another request if needed. In case a second request is needed, the actual HTTP request is not made twice, as the first call would not produce an actual HTTP request.


The reference to the cached request is preserved, because if the response code/status of the second call is "304 Not Modified", the response body becomes an empty string ("") and then it is needed to go back to the cached object. A way to save memory and expenses of second object creation is to preserve just the needed response data and reuse the XMLHttpRequest object.


The above script relies on the assumption that the "Date" header is always issued by the server, which should be true for most server configurations. Also, it illustrates a synchronous communication between the server and the client. In case of asynchronous communication, the check should be made during the callback.


This problem is often overcome by employing techniques preventing the caching at all. Using these techniques indiscriminately can result in poor performance and waste of network bandwidth.


Workaround

Internet Explorer will also cache dynamic pages, this is because the URL of the page may not change but the content will (For example a news feed). A work around for this situation can be achieved by adding a unique time stamp or random number, or possibly both, typically using the Date object and/or Math.random().


For simple document request the query string delimiter '?' can be used, or for existing queries a final sub-query can be added after a final '&' – to append the unique query term to the existing query. The downside is that each such request will fill up the cache with useless (never reused) content that could otherwise be used for other cached content (more useful data will be purged from cache to make room for these one-time responses).


Reusing XMLHttpRequest Object in IE

In IE, if the open method is called after setting the onreadystatechange callback, there will be a problem when trying to reuse the XHR object. To be able to reuse the XHR object properly, use the open method first and set onreadystatechange later. This happens because IE resets the object implicitly in the open method if the status is 'completed'. For more explanation of reuse: Reusing XMLHttpRequest Object in IE. The downside to calling the open method after setting the callback is a loss of cross-browser support for readystates. See the quirksmode article.


Cross-browser support

Microsoft developers were the first to include the XMLHttp object in their MSXML ActiveX control. Developers at the open source Mozilla project saw this invention and ported their own XMLHttp, not as an ActiveX control but as a native browser object called XMLHttpRequest. Konqueror, Opera and Safari have since implemented similar functionality but more along the lines of Mozilla's XMLHttpRequest. Some Ajax developer and run-time frameworks only support one implementation of XMLHttp while others support both. Developers building Ajax functionality from scratch can provide if/else logic within their client-side JavaScript to use the appropriate XMLHttp object as well. Internet Explorer 7 added native support for the XMLHttpRequest object, but retains backward compatibility with the ActiveX implementation.[4] Microsoft XML Core Services (MSXML) is a set of services that allow applications written in JScript, VBScript, and Microsoft development tools to build Windows-native XML-based applications. ... ActiveX is Microsoft technology used for developing reusable object oriented software components. ... ActiveX control is a term used to denote reusable software components that are based on Microsoft Component Object Model (COM). ... Konqueror is a file manager, web browser and file viewer, which was developed as part of the K Desktop Environment (KDE) by volunteers and runs on most Unix-like operating systems. ... Opera is a cross-platform internet software suite consisting of a web browser, e-mail/news client, address book, newsfeed reader, IRC chat client, and download manager. ... Safari is a web browser developed by Apple Computer, Inc. ... An Ajax framework is a framework that helps to develop web applications that use Ajax, a collection of technologies used to build dynamic web pages on the client side. ... This article or section does not cite its references or sources. ...


Frameworks

Because of the complexity of handling cross-browser distinctions between XMLHttpRequest implementations, a number of frameworks have emerged to abstract these differences into a set of reusable programming constructs. An Ajax framework is a framework that helps to develop web applications that use Ajax, a collection of technologies used to build dynamic web pages on the client side. ...


References

  1. ^ - W3C Working Draft 27 February 2007 section 2.2
  2. ^ [1]
  3. ^ The XMLHttpRequest Object, W3C Working Draft
  4. ^ a b Dutta, Sunava (2006-01-23). Native XMLHTTPRequest object. IEBlog. Microsoft. Retrieved on 2006-11-30.
  5. ^ The XMLHttpRequest Object 18 October 2007
  6. ^ Document Object Model (DOM) Level 3 Load and Save Specification, V 1.0, W3C Recommendation 07 April 2004

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 334th day of the year (335th in leap years) in the Gregorian calendar. ...

See also

AJAX redirects here. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... “REST” redirects here. ...

External links

Documentation/Browser implementations

It has been suggested that W3C Markup Validation Service be merged into this article or section. ...

Cross-Browser implementations

Tutorials

Security


  Results from FactBites:
 
XMLHttpRequest: Information from Answers.com (1274 words)
XMLHttpRequest (XHR) is an API that can be used by JavaScript, JScript, VBScript and other web browser scripting languages to transfer and manipulate XML data to and from a web server using HTTP, establishing an independent connection channel between a web page's Client-Side and Server-Side.
XMLHttpRequest is an important part of the Ajax web development technique, and it is used by many websites to implement responsive and dynamic web applications.
Reusing XMLHttpRequest Object in IE In IE, if the open method is called after setting the onreadystatechange callback, there will be a problem when trying to reuse the XHR object.
  More results at FactBites »

 
 

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