jQuery jQuery.ajax()

The $.ajax() function underlies all Ajax requests sent by jQuery. It is often unnecessary to directly call this function, as several higher-level alternatives like $.get() and .load() are available and are easier to use. If less common options are required, though, $.ajax() can be used more flexibly.

At its simplest, the $.ajax() function can be called with no arguments:

$.ajax();

Note: Default settings can be set globally by using the $.ajaxSetup() function.

This example, using no options, loads the contents of the current page, but does nothing with the result. To use the result, you can implement one of the callback functions.

The jqXHR Object

The jQuery XMLHttpRequest (jqXHR) object returned by $.ajax() as of jQuery 1.5 is a superset of the browser’s native XMLHttpRequest object. For example, it contains responseText and responseXML properties, as well as a getResponseHeader() method. When the transport mechanism is something other than XMLHttpRequest (for example, a script tag for a JSONP request) the jqXHR object simulates native XHR functionality where possible.

As of jQuery 1.5.1, the jqXHR object also contains the overrideMimeType() method (it was available in jQuery 1.4.x, as well, but was temporarily removed in jQuery 1.5). The .overrideMimeType() method may be used in the beforeSend() callback function, for example, to modify the response content-type header:

$.ajax({
 url: "http://fiddle.jshell.net/favicon.png",
 beforeSend: function( xhr ) {
 xhr.overrideMimeType( "text/plain; charset=x-user-defined" );
 }
})
 .done(function( data ) {
 if ( console && console.log ) {
 console.log( "Sample of data:", data.slice( 0, 100 ) );
 }
 });

The jqXHR objects returned by $.ajax() as of jQuery 1.5 implement the Promise interface, giving them all the properties, methods, and behavior of a Promise (see Deferred object for more information). These methods take one or more function arguments that are called when the $.ajax() request terminates. This allows you to assign multiple callbacks on a single request, and even to assign callbacks after the request may have completed. (If the request is already complete, the callback is fired immediately.) Available Promise methods of the jqXHR object include:

• jqXHR.done(function( data, textStatus, jqXHR ) {});

  An alternative construct to the success callback option, the .done() method replaces the deprecated jqXHR.success() method. Refer to deferred.done() for implementation details.

• jqXHR.fail(function( jqXHR, textStatus, errorThrown ) {});

  An alternative construct to the error callback option, the .fail() method replaces the deprecated .error() method. Refer to deferred.fail() for implementation details.

• jqXHR.always(function( data|jqXHR, textStatus, jqXHR|errorThrown ) { });

  An alternative construct to the complete callback option, the .always() method replaces the deprecated .complete() method.

  In response to a successful request, the function’s arguments are the same as those of .done(): data, textStatus, and the jqXHR object. For failed requests the arguments are the same as those of .fail(): the jqXHR object, textStatus, and errorThrown. Refer to deferred.always() for implementation details.

• jqXHR.then(function( data, textStatus, jqXHR ) {}, function( jqXHR, textStatus, errorThrown ) {});

  Incorporates the functionality of the .done() and .fail() methods, allowing (as of jQuery 1.8) the underlying Promise to be manipulated. Refer to deferred.then() for implementation details.

// Assign handlers immediately after making the request,
// and remember the jqXHR object for this request
var jqxhr = $.ajax( "example.php" )
 .done(function() {
 alert( "success" );
 })
 .fail(function() {
 alert( "error" );
 })
 .always(function() {
 alert( "complete" );
 });
 
// Perform other work here ...
 
// Set another completion function for the request above
jqxhr.always(function() {
 alert( "second complete" );
});

The this reference within all callbacks is the object in the context option passed to $.ajax in the settings; if context is not specified, this is a reference to the Ajax settings themselves.

For backward compatibility with XMLHttpRequest, a jqXHR object will expose the following properties and methods:

• readyState
• status
• statusText
• responseXML and/or responseText when the underlying request responded with xml and/or text, respectively
• setRequestHeader(name, value) which departs from the standard by replacing the old value with the new one rather than concatenating the new value to the old one
• getAllResponseHeaders()
• getResponseHeader()
• statusCode()
• abort()

No onreadystatechange mechanism is provided, however, since done, fail, always, and statusCode cover all conceivable requirements.

Callback Function Queues

The beforeSend, error, dataFilter, success and complete options all accept callback functions that are invoked at the appropriate times.

As of jQuery 1.5, the fail and done, and, as of jQuery 1.6, always callback hooks are first-in, first-out managed queues, allowing for more than one callback for each hook. See Deferred object methods, which are implemented internally for these $.ajax() callback hooks.

The callback hooks provided by $.ajax() are as follows:

1. beforeSend callback option is invoked; it receives the jqXHR object and the settings object as parameters.
2. error callback option is invoked, if the request fails. It receives the jqXHR, a string indicating the error type, and an exception object if applicable. Some built-in errors will provide a string as the exception object: "abort", "timeout", "No Transport".
3. dataFilter callback option is invoked immediately upon successful receipt of response data. It receives the returned data and the value of dataType, and must return the (possibly altered) data to pass on to success.
4. success callback option is invoked, if the request succeeds. It receives the returned data, a string containing the success code, and the jqXHR object.
5. Promise callbacks — .done(), .fail(), .always(), and .then() — are invoked, in the order they are registered.
6. complete callback option fires, when the request finishes, whether in failure or success. It receives the jqXHR object, as well as a string containing the success or error code.

Data Types

Different types of response to $.ajax() call are subjected to different kinds of pre-processing before being passed to the success handler. The type of pre-processing depends by default upon the Content-Type of the response, but can be set explicitly using the dataType option. If the dataType option is provided, the Content-Type header of the response will be disregarded.

The available data types are text, html, xml, json, jsonp, and script.

If text or html is specified, no pre-processing occurs. The data is simply passed on to the success handler, and made available through the responseText property of the jqXHR object.

If xml is specified, the response is parsed using jQuery.parseXML before being passed, as an XMLDocument, to the success handler. The XML document is made available through the responseXML property of the jqXHR object.

If json is specified, the response is parsed using jQuery.parseJSON before being passed, as an object, to the success handler. The parsed JSON object is made available through the responseJSON property of the jqXHR object.

If script is specified, $.ajax() will execute the JavaScript that is received from the server before passing it on to the success handler as a string.

If jsonp is specified, $.ajax() will automatically append a query string parameter of (by default) callback=? to the URL. The jsonp and jsonpCallback properties of the settings passed to $.ajax() can be used to specify, respectively, the name of the query string parameter and the name of the JSONP callback function. The server should return valid JavaScript that passes the JSON response into the callback function. $.ajax() will execute the returned JavaScript, calling the JSONP callback function, before passing the JSON object contained in the response to the $.ajax() success handler.

For more information on JSONP, see the original post detailing its use.

Sending Data to the Server

By default, Ajax requests are sent using the GET HTTP method. If the POST method is required, the method can be specified by setting a value for the type option. This option affects how the contents of the data option are sent to the server. POST data will always be transmitted to the server using UTF-8 charset, per the W3C XMLHTTPRequest standard.

The data option can contain either a query string of the form key1=value1&key2=value2, or an object of the form {key1: 'value1', key2: 'value2'}. If the latter form is used, the data is converted into a query string using jQuery.param() before it is sent. This processing can be circumvented by setting processData to false. The processing might be undesirable if you wish to send an XML object to the server; in this case, change the contentType option from application/x-www-form-urlencoded to a more appropriate MIME type.

Advanced Options

The global option prevents handlers registered using .ajaxSend(), .ajaxError(), and similar methods from firing when this request would trigger them. This can be useful to, for example, suppress a loading indicator that was implemented with .ajaxSend() if the requests are frequent and brief. With cross-domain script and JSONP requests, the global option is automatically set to false. See the descriptions of these methods below for more details.

If the server performs HTTP authentication before providing a response, the user name and password pair can be sent via the username and password options.

Ajax requests are time-limited, so errors can be caught and handled to provide a better user experience. Request timeouts are usually either left at their default or set as a global default using $.ajaxSetup() rather than being overridden for specific requests with the timeout option.

By default, requests are always issued, but the browser may serve results out of its cache. To disallow use of the cached results, set cache to false. To cause the request to report failure if the asset has not been modified since the last request, set ifModified to true.

The scriptCharset allows the character set to be explicitly specified for requests that use a <script> tag (that is, a type of script or jsonp). This is useful if the script and host page have differing character sets.

The first letter in Ajax stands for "asynchronous," meaning that the operation occurs in parallel and the order of completion is not guaranteed. The async option to $.ajax() defaults to true, indicating that code execution can continue after the request is made. Setting this option to false (and thus making the call no longer asynchronous) is strongly discouraged, as it can cause the browser to become unresponsive.

The $.ajax() function returns the XMLHttpRequest object that it creates. Normally jQuery handles the creation of this object internally, but a custom function for manufacturing one can be specified using the xhr option. The returned object can generally be discarded, but does provide a lower-level interface for observing and manipulating the request. In particular, calling .abort() on the object will halt the request before it completes.

Extending Ajax

As of jQuery 1.5, jQuery’s Ajax implementation includes prefilters, transports, and converters that allow you to extend Ajax with a great deal of flexibility.

Using Converters

$.ajax() converters support mapping data types to other data types. If, however, you want to map a custom data type to a known type (e.g json), you must add a correspondence between the response Content-Type and the actual data type using the contents option:

$.ajaxSetup({
 contents: {
 mycustomtype: /mycustomtype/
 },
 converters: {
 "mycustomtype json": function( result ) {
 // Do stuff
 return newresult;
 }
 }
});

This extra object is necessary because the response Content-Types and data types never have a strict one-to-one correspondence (hence the regular expression).

To convert from a supported type (e.g text, json) to a custom data type and back again, use another pass-through converter:

$.ajaxSetup({
 contents: {
 mycustomtype: /mycustomtype/
 },
 converters: {
 "text mycustomtype": true,
 "mycustomtype json": function( result ) {
 // Do stuff
 return newresult;
 }
 }
});

The above now allows passing from text to mycustomtype and then mycustomtype to json.