Ajax (also AJAX; /ˈæks/; short for "Asynchronous JavaScript And XML")[1] is a set of Web development techniques using many Web technologies on the client side to create asynchronous Web applications. With Ajax, Web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows for Web pages, and by extension Web applications, to change content dynamically without the need to reload the entire page.[2] In practice, modern implementations commonly substitute JSON for XML due to the advantages of JSON being native to JavaScript.[3]

Ajax is not a single technology, but rather a group of technologies. HTML and CSS can be used in combination to mark up and style information. The webpage can then be modified by JavaScript to dynamically display – and allow the user to interact with — the new information. The built-in XMLHttpRequest object within JavaScript is commonly used to execute Ajax on webpages allowing for websites to load content onto the screen without refreshing the page. Ajax is also not a new technology, or another different language, just existing technologies used in new ways.

History

In the early-to-mid 1990s, most Web sites were based on complete HTML pages. Each user action required that a complete new page be loaded from the server. This process was inefficient, as reflected by the user experience: all page content disappeared, then the new page appeared. Each time the browser reloaded a page because of a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and made bandwidth a limiting factor on performance.

In 1996, the iframe tag was introduced by Internet Explorer; like the object element it can load or fetch content asynchronously. In 1998, the Microsoft Outlook Web App team developed the concept behind the XMLHttpRequest scripting object.[4] It appeared as XMLHTTP in the second version of the MSXML library,[4][5] which shipped with Internet Explorer 5.0 in March 1999.[6]

The functionality of the XMLHTTP ActiveX control in IE 5 was later implemented by Mozilla, Safari, Opera and other browsers as the XMLHttpRequest JavaScript object.[7] Microsoft adopted the native XMLHttpRequest model as of Internet Explorer 7. The ActiveX version is still supported in Internet Explorer, but not in Microsoft Edge. The utility of these background HTTP requests and asynchronous Web technologies remained fairly obscure until it started appearing in large scale online applications such as Outlook Web App (2000)[8] and Oddpost (2002).

Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail (2004) and Google Maps (2005).[9] In October 2004 Kayak.com's public beta release was among the first large-scale e-commerce uses of what their developers at that time called "the xml http thing".[10] This increased interest in AJAX among web program developers.

The term Ajax was publicly used on 18 February 2005 by Jesse James Garrett in an article titled Ajax: A New Approach to Web Applications, based on techniques used on Google pages.[1]

On 5 April 2006, the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official Web standard.[11][12] The latest draft of the XMLHttpRequest object was published on 30 January 2014.[13]

Technologies

The conventional model for a Web Application versus an application using Ajax

The term Ajax has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax,[1][2] Jesse James Garrett explained that the following technologies are incorporated:

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange,[14] although other formats such as preformatted HTML or plain text can also be used.[15] A variety of popular JavaScript libraries, including JQuery, include abstractions to assist in executing Ajax requests.

Asynchronous HTML and HTTP (AHAH) involves using XMLHTTPRequest to retrieve (X)HTML fragments, which are then inserted directly into the Web page.

Drawbacks

Examples

JavaScript example

An example of a simple Ajax request using the GET method, written in JavaScript.

get-ajax-data.js:

// This is the client-side script.

// Initialize the HTTP request.
var xhr = new XMLHttpRequest();
xhr.open('GET', 'send-ajax-data.php');

// Track the state changes of the request.
xhr.onreadystatechange = function () {
    var DONE = 4; // readyState 4 means the request is done.
    var OK = 200; // status 200 is a successful return.
    if (xhr.readyState === DONE) {
        if (xhr.status === OK) {
            console.log(xhr.responseText); // 'This is the output.'
        } else {
            console.log('Error: ' + xhr.status); // An error occurred during the request.
        }
    }
};

// Send the request to send-ajax-data.php
xhr.send(null);

send-ajax-data.php:

<?php
// This is the server-side script.

// Set the content type.
header('Content-Type: text/plain');

// Send the data back.
echo "This is the output.";
?>

Many developers dislike the syntax used in the XMLHttpRequest object, so some of the following workarounds have been created.

Fetch example

Fetch is a new native JavaScript API. Although not yet supported by all browsers, it is gaining momentum as a more popular way to execute Ajax. According to Google Developers Documentation, "Fetch makes it easier to make web requests and handle responses than with the older XMLHttpRequest."

fetch('send-ajax-data.php').then(function(response) {
	return response.text();
}).then(function(data) {
	console.log(data);
}).catch(function(error) {
	console.log('Error: ' + error);
});

As seen above, fetch relies on JavaScript promises.

jQuery example

The popular JavaScript library jQuery has implemented abstractions which enable developers to use Ajax more efficiently. Although it still uses XMLHttpRequest behind the scenes, the following is the same example as above using the 'ajax' method.

$.ajax({
	type: 'GET',
	url: 'send-ajax-data.php',
	dataType: "JSON", // data type expected from server
	success: function (data) {
		console.log(data);
	},
	error: function() {
	    console.log('Error: ' + data);
	}
});

jQuery also implements a 'get' method which allows the same code to be written more concisely.

$.get('send-ajax-data.php').done(function(data) {
	console.log(data);
}).fail(function(data) {
	console.log('Error: ' + data);
});

See also

References

  1. ^ a b c Jesse James Garrett (18 February 2005). "Ajax: A New Approach to Web Applications". AdaptivePath.com. Retrieved 19 June 2008.
  2. ^ a b Ullman, Chris (March 2007). Beginning Ajax. wrox. ISBN 978-0-470-10675-4. Archived from the original on 5 July 2008. Retrieved 24 June 2008. ((cite book)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  3. ^ "JSON: The Fat-Free Alternative to XML". www.json.org. Retrieved 17 February 2017.
  4. ^ a b "Article on the history of XMLHTTP by an original developer". Alexhopmann.com. 31 January 2007. Archived from the original on 23 June 2007. Retrieved 14 July 2009. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  5. ^ "Specification of the IXMLHTTPRequest interface from the Microsoft Developer Network". Msdn.microsoft.com. Retrieved 14 July 2009.
  6. ^ Dutta, Sunava (23 January 2006). "Native XMLHTTPRequest object". IEBlog. Microsoft. Retrieved 30 November 2006.
  7. ^ "Dynamic HTML and XML: The XMLHttpRequest Object". Apple Inc. Retrieved 25 June 2008.
  8. ^ Hopmann, Alex. "Story of XMLHTTP". Alex Hopmann’s Blog. Archived from the original on 30 March 2010. Retrieved 17 May 2010. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  9. ^ "A Brief History of Ajax". Aaron Swartz. 22 December 2005. Retrieved 4 August 2009.
  10. ^ English, Paul. "Kayak User Interface". OFFICIAL KAYAK.COM TECHNOBLOG. Retrieved 22 May 2014.
  11. ^ a b "The XMLHttpRequest Object". World Wide Web Consortium. 5 April 2006. Archived from the original on 16 May 2008. Retrieved 25 June 2008. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  12. ^ van Kesteren, Anne; Jackson, Dean. "The XMLHttpRequest Object". W3.org. W3C. Retrieved 14 November 2015.
  13. ^ Kesteren, Anne; Aubourg, Julian; Song, Jungkee; Steen, Hallvord R. M. "XMLHttpRequest Level 1". W3.org. W3C. Retrieved 14 November 2015.
  14. ^ "JavaScript Object Notation". Apache.org. Archived from the original on 16 June 2008. Retrieved 4 July 2008. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  15. ^ "Speed Up Your Ajax-based Apps with JSON". DevX.com. Archived from the original on 4 July 2008. Retrieved 4 July 2008. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  16. ^ Quinsey, Peter. "User-proofing Ajax".
  17. ^ "WAI-ARIA Overview". W3C. Archived from the original on 26 October 2010. Retrieved 21 October 2010. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  18. ^ Edwards, James (5 May 2006). "Ajax and Screenreaders: When Can it Work?". sitepoint.com. Retrieved 27 June 2008.
  19. ^ "Access Control for Cross-Site Requests". World Wide Web Consortium. Archived from the original on 14 May 2008. Retrieved 27 June 2008. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  20. ^ "Secure Cross-Domain Communication in the Browser". The Architecture Journal (MSDN). Archived from the original on 29 March 2010. Retrieved 27 April 2010. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  21. ^ Cuthbertson, Tim. "What is asynchronous programming, and why is it so damn awkward?". GFX Monk. Retrieved 19 October 2010.
  22. ^ "Selenium documentation: Fetching a Page". Selenium. Retrieved 6 October 2011.
    It is worth noting that if your page uses a lot of Ajax on load then WebDriver may not know when it has completely loaded. If you need to ensure such pages are fully loaded, then you can use Explicit and Implicit Waits.
  23. ^ Pimentel, Victoria; Nickerson, Bradford G. (8 May 2012). "Communicating and Displaying Real-Time Data with WebSocket". Internet Computing, IEEE. 16 (4): 45–53. doi:10.1109/MIC.2012.64.
  24. ^ a b "Why use Ajax?". InterAKT. 10 November 2005. Archived from the original on 29 May 2008. Retrieved 26 June 2008. ((cite web)): Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  25. ^ a b "Deep Linking for AJAX".
  26. ^ a b "HTML5 specification". Retrieved 21 October 2011.
  27. ^ Hendriks, Erik (23 May 2014). "Official news on crawling and indexing sites for the Google index". Google. Retrieved 24 May 2015.
  28. ^ Prokoph, Andreas (8 May 2007). "Help Web crawlers efficiently crawl your portal sites and Web sites". IBM. Retrieved 22 April 2009.
  29. ^ http://googlewebmastercentral.blogspot.co.uk/2015/10/deprecating-our-ajax-crawling-scheme.html