XMLHttpRequest (XHR)

XMLHttpRequest (XHR) is a DOM API that can be used by JavaScript and other web browser scripting languages to transfer XML and other text data between a web server and a browser. This type of AJAX architecture should not be confused with (XDR) XMLDomainRequest which is a lightweight form of XMLHttpRequest design by Microsoft which doesn't utilize XML-RPC.

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.

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 Maps, Windows Live's Virtual Earth, the MapQuest dynamic map interface, Facebook, and many others.


Methods

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.[2]
* 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. content can be a string or reference to a document.
setRequestHeader(label, value)
Adds a label/value pair to the HTTP header to be sent.

Properties

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 [3])
* 4 = loaded – Done.

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:

if(typeof ActiveXObject != "undefined") {
xmlhttp = new ActiveXObject("MSXML2.XMLHTTP");
xmlhttp.open("GET", "#", false);
xmlhttp.send(null);
alert(xmlhttp.responseBody);
} else {
alert("This browser does not support Microsoft ActiveXObjects.")
}

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.[4] If the 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").

History and support

The XMLHttpRequest concept was originally developed by Microsoft as a server side API call for Outlook Web Access 2000. At the time, it was not a standards-based web client feature; however, de facto support for it was implemented by many major web browsers. The Microsoft implementation is called XMLHTTP. It has been available since the introduction of Internet Explorer 5.0[5] and is accessible via JScript, VBScript and other scripting languages supported by IE browsers. In JScript, Internet Explorer versions prior to 7.0 require XMLHTTP to be invoked as an ActiveXObject, so they cannot instantiate the XMLHttpRequest class without help as follows:

// Provide the XMLHttpRequest class for IE 5.x-6.x:
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("Msxml2.XMLHTTP") } catch(e) {}
try { return new ActiveXObject("Microsoft.XMLHTTP") } catch(e) {}
throw new Error( "This browser does not support XMLHttpRequest." )
};

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, iCab since 3.0b352, and Microsoft since Internet Explorer 7.0.

The World Wide Web Consortium published a Working Draft specification for the XMLHttpRequest object's API on 15 April 2008.[6] 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. As of April 2008, the W3C standard was still a work in progress.

On 25 February 2008 published a Working Draft of XMLHttpRequest2 object's API with some extended functionality (such as progress events, cross-site requests, handling of byte streams).

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.

Traditionally, there have been other methods to achieve a similar effect of server dynamic applications using scripting languages and/or plugins:

* Invisible IFrames (or Netscape Navigator's equivalent ilayers)
* Remote Scripting
* Netscape's LiveConnect
* Microsoft's ActiveX
* Microsoft's XML Data Islands
* Adobe Flash Player
* Java applets

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.[citation needed] These include:

* 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.

Example Code

Here is a cross-browser general purpose example of an AJAX/XMLHttpRequest JavaScript function. See the history and support section for compatibility with versions of Internet Explorer prior to 7.0.

function ajax(url, vars, callbackFunction) {
var request = new XMLHttpRequest();
request.open("POST", url, true);
request.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded");

request.onreadystatechange = function() {
var done = 4, ok = 200;
if (request.readyState == done && request.status == ok) {
if (request.responseText) {
callbackFunction(request.responseText);
}
}
};
request.send(vars);
}

[edit] 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 data 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, since a bug exists in Internet Explorer, where the cached response is never refreshed.[9]

It is possible to unify the caching behavior on the client. The following script illustrates an example approach (See the history and support section for compatibility with versions of Internet Explorer prior to 7.0):

var request = new XMLHttpRequest();
request.open("GET", url, false);
request.send(null);
if(!request.getResponseHeader("Date")) {
var cached = request;
request = new XMLHttpRequest();
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.

If script executes operation that has side effects (e.g. adding a comment, marking message as read) which requires that request always reaches the end server, it should use POST method instead.

Workaround

Internet Explorer will also cache dynamic pages, this is a problem 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 an identical 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.[5] To avoid excess conditionals, see the source code in the history and support section.
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