XMLHttpRequest

XMLHttpRequest (XHR) is an API available in web browser scripting languages such as JavaScript. It is used to send HTTP or HTTPS requests directly to a web server and load the server response data directly back into the script. Development versions of all major browsers support URI schemes beyond http: and https:, in particular, blob: URLs are supported. The data might be received from the server as XML text or as plain text. Data from the response can be used directly to alter the DOM of the currently active document in the browser window without loading a new web page document. The response data can also be evaluated by client-side scripting. For example, if it was formatted as JSON by the web server, it can easily be converted into a client-side data object for further use.

XMLHttpRequest has an important role in the Ajax web development technique. It is currently used by many websites to implement responsive and dynamic web applications. Examples of these web applications include Gmail, Google Maps, Facebook and many others.

XMLHttpRequest is subject to the browser's same origin policy in that, for security reasons, requests will only succeed if they are made to the same server that served the original web page. There are alternative ways to circumvent this policy if required.

History and support

The concept behind the XMLHttpRequest object was originally created by the developers of Outlook Web Access (by Microsoft) for Microsoft Exchange Server 2000. An interface called IXMLHTTPRequest was developed and implemented into the second version of the MSXML library using this concept. The second version of the MSXML library was shipped with Internet Explorer 5.0 in March 1999, allowing access, via ActiveX, to the IXMLHTTPRequest interface using the XMLHTTP wrapper of the MSXML library.

The Mozilla project developed and implemented an interface called nsIXMLHttpRequest into the Gecko layout engine. This interface was modelled to work as closely to Microsoft's IXMLHTTPRequest interface as possible. Mozilla created a wrapper to use this interface through a JavaScript object which they called XMLHttpRequest. The XMLHttpRequest object was accessible as early as Gecko version 0.6 released on December 6 of 2000, but it was not completely functional until as late as version 1.0 of Gecko released on June 5, 2002. The XMLHttpRequest object became a de facto standard in other major web clients, implemented in Safari 1.2 released in February 2004, Konqueror, Opera 8.0 released in April 2005, and iCab 3.0b352 released in September 2005.

The World Wide Web Consortium published a Working Draft specification for the XMLHttpRequest object on April 5, 2006, edited by Anne van Kesteren of Opera Software and Dean Jackson of W3C. 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 last revision to the XMLHttpRequest object specification was on November 19 of 2009, being a last call working draft.

Microsoft added the XMLHttpRequest object identifier to its scripting languages in Internet Explorer 7.0 released in October 2006.

With the advent of cross-browser JavaScript libraries such as jQuery and the Prototype JavaScript Framework, developers can invoke XMLHttpRequest functionality without coding directly to the API. Prototype provides an asynchronous requester object called Ajax.Request that wraps the browser's underlying implementation and provides access to it. jQuery objects represent or wrap elements from the current client-side DOM. They all have a .load() method that takes a URI parameter and makes an XMLHttpRequest to that URI, then by default places any returned HTML into the HTML element represented by the jQuery object.

The W3C has since published another Working Draft specification for the XMLHttpRequest object, "XMLHttpRequest Level 2", on February 25 of 2008. Level 2 consists of extended functionality to the XMLHttpRequest object, including, but not currently limited to, progress events, support for cross-site requests, and the handling of byte streams. The latest revision of the XMLHttpRequest Level 2 specification is that of 16 August 2011, which is still a working draft.

As of 5 December 2011, XMLHttpRequest version 2 has been merged into the main XMLHttpRequest specification, and there is no longer a version 1 and a version 2.

Support in Internet Explorer versions 5, 5.5 and 6

Internet Explorer versions 5 and 6 did not define the XMLHttpRequest object identifier in their scripting languages as the XMLHttpRequest identifier itself was not standard at the time of their releases. Backward compatibility can be achieved through object detection if the XMLHttpRequest identifier does not exist.

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.

Since Microsoft still supports Windows XP with Internet Explorer 6, a JavaScript encapsulation example is provided below.

/*
   Provide the XMLHttpRequest constructor for Internet Explorer 5.x-6.x:
   Other browsers (including Internet Explorer 7.x-9.x) do not redefine
   XMLHttpRequest if it already exists.

   This example is based on findings at:
   http://blogs.msdn.com/xmlteam/archive/2006/10/23/using-the-right-version-of-msxml-in-internet-explorer.aspx
*/
if (typeof XMLHttpRequest == "undefined")
  XMLHttpRequest = function () {
    try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); }
      catch (e) {}
    try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); }
      catch (e) {}
    try { return new ActiveXObject("Microsoft.XMLHTTP"); }
      catch (e) {}
    //Microsoft.XMLHTTP points to Msxml2.XMLHTTP and is redundant
    throw new Error("This browser does not support XMLHttpRequest.");
  };

HTTP request

The following sections demonstrate how a request using the XMLHttpRequest object functions within a conforming user agent based on the W3C Working Draft. As the W3C standard for the XMLHttpRequest object is still a draft, user agents may not abide by all the functionings of the W3C definition and any of the following is subject to change. Extreme care should be taken into consideration when scripting with the XMLHttpRequest object across multiple user agents. This article will try to list the inconsistencies between the major user agents.

The open method

The HTTP and HTTPS requests of the XMLHttpRequest object must be initialized through the open method. This method must be invoked prior to the actual sending of a request to validate and resolve the request method, URL, and URI user information to be used for the request. This method does not assure that the URL exists or the user information is correct. This method can accept up to five parameters, but requires only two, to initialize a request.

open( Method, URL, Asynchronous, UserName, Password )

The first parameter of the method is a text string indicating the HTTP request method to use. The request methods that must be supported by a conforming user agent, defined by the W3C draft for the XMLHttpRequest object, are currently listed as the following.

• GET (Supported by Internet Explorer 7 (and later), Mozilla 1+)
• POST (Supported by Internet Explorer 7 (and later), Mozilla 1 (and later))
• HEAD (Supported by Internet Explorer 7 (and later))
• PUT
• DELETE
• OPTIONS (Supported by Internet Explorer 7 (and later))

However, request methods are not limited to the ones listed above. The W3C draft states that a browser may support additional request methods at their own discretion.

The second parameter of the method is another text string, this one indicating the URL of the HTTP request. The W3C recommends that browsers should raise an error and not allow the request of a URL with either a different port or ihost URI component from the current document.

The third parameter, a boolean value indicating whether or not the request will be asynchronous, is not a required parameter by the W3C draft. The default value of this parameter should be assumed to be true by a W3C conforming user agent if it is not provided. An asynchronous request ("true") will not wait on a server response before continuing on with the execution of the current script. It will instead invoke the onreadystatechange event listener of the XMLHttpRequest object throughout the various stages of the request. A synchronous request ("false") however will block execution of the current script until the request has been completed, thus not invoking the onreadystatechange event listener.

The fourth and fifth parameters are the username and password, respectively. These parameters, or just the username, may be provided for authentication and authorization if required by the server for this request.

The setRequestHeader method

Upon successful initialization of a request, the setRequestHeader method of the XMLHttpRequest object can be invoked to send HTTP headers with the request.

setRequestHeader( Name, Value )

The first parameter of this method is the text string name of the header. The second parameter is the text string value. This method must be invoked for each header that needs to be sent with the request. Any headers attached here will be removed the next time the open method is invoked in a W3C conforming user agent.

The send method

To send an HTTP request, the send method of the XMLHttpRequest must be invoked. This method accepts a single parameter containing the content to be sent with the request.

send( Data )

This parameter may be omitted if no content needs to be sent. The W3C draft states that this parameter may be any type available to the scripting language as long as it can be turned into a text string, with the exception of the DOM document object. If a user agent cannot serialise the parameter, then the parameter should be ignored.

If the parameter is a DOM document object, a user agent should assure the document is turned into well-formed XML using the encoding indicated by the inputEncoding property of the document object. If the Content-Type request header was not added through setRequestHeader yet, it should automatically be added by a conforming user agent as "application/xml;charset=charset," where charset is the encoding used to encode the document.

If the user agent is configured to use a proxy server, then the XMLHttpRequest object will modify the request appropriately so as to connect to the proxy instead of the origin server, and send Proxy-Authorization headers as configured.

The onreadystatechange event listener

If the open method of the XMLHttpRequest object was invoked with the third parameter set to true for an asynchronous request, the onreadystatechange event listener will be automatically invoked for each of the following actions that change the readyState property of the XMLHttpRequest object.

State changes work like this:

• After the open method has been invoked successfully, the readyState property of the XMLHttpRequest object should be assigned a value of 1.
• After the send method has been invoked and the HTTP response headers have been received, the readyState property of the XMLHttpRequest object should be assigned a value of 2.
• Once the HTTP response content begins to load, the readyState property of the XMLHttpRequest object should be assigned a value of 3.
• Once the HTTP response content has finished loading, the readyState property of the XMLHttpRequest object should be assigned a value of 4.

The listener will only respond to state changes which occur after the listener is defined. To detect states 1 and 2, the listener must be defined before the open method is invoked. The open method must be invoked before the send method is invoked.

xmlhttp.onreadystatechange = function() {
	alert(xmlhttp.readyState);
};
xmlhttp.open("GET","somepage.xml",true);
xmlhttp.send(null);

The HTTP response

After a successful and completed call to the send method of the XMLHttpRequest, if the server response was valid XML and the Content-Type header sent by the server is understood by the user agent as an Internet media type for XML, the responseXML property of the XMLHttpRequest object will contain a DOM document object. Another property, responseText will contain the response of the server in plain text by a conforming user agent, regardless of whether or not it was understood as XML.

Cross-domain requests

In the early development of the World Wide web, it was found possible to breach users' security by the use of JavaScript to exchange information from one web site with that from another less reputable one. All modern browsers therefore implement a same origin policy that prevents many such attacks, such as cross-site scripting. XMLHttpRequest data is subject to this security policy, but sometimes web developers want intentionally to circumvent its restrictions. This is sometimes due to the legitimate use of subdomains as, for example, making an XMLHttpRequest from a page created by foo.example.com for information from bar.example.com will normally fail.

Various alternatives exist to circumvent this security feature, including using JSONP, Cross-Origin Resource Sharing or alternatives with plugins such as Flash or Silverlight. XMLHttpRequest Level 2 also includes a feature to communicate with other domains. This is implemented in Firefox 3.5, Google Chrome, and Safari 4. Internet Explorer 8 has the non-standard XDomainRequest, which can do a similar thing.

Headers added to a server's HTTP response headers can allow cross-domain requests to succeed. For example, Access-Control-Allow-Origin: *, can allow all domains to access a server. Access-Control-Allow-Origin can be used in all browsers that support cross-domain requests, which includes Internet Explorer 8. The W3C's specification is defined in Cross-Origin Resource Sharing.

See also

• WebSockets
• Hypertext Transfer Protocol
• Representational State Transfer
• Ajax

External links

Specifications

• XMLHttpRequest (level 1) specification from W3C (abandoned Candidate Recommendation)
• XMLHttpRequest level 2 specification from W3C (Working Draft)
  • Cross-Origin Resource Sharing
  • Progress Events 1.0

Browsers

• MSDN Library
  • Specification of the XMLHttpRequest object for Microsoft developers
  • XDomainRequest
• Specification of the XMLHttpRequest object for Mozilla developers
• Specification of the XMLHttpRequest object for Opera developers

Other

• "Attacking AJAX Applications", a presentation given at the Black Hat security conference. Discusses several issues involving XHR and the future of cross-domain Ajax.