jQuery.ajax()

Arguments

A map of fieldName-fieldValue pairs to set on the native XHR object. For example, you can use it to set withCredentials to true for cross-domain requests if needed.

$.ajax({
   url: a_cross_domain_url,
   xhrFields: {
      withCredentials: true
   }
});

In jQuery 1.5, the withCredentials property was not propagated to the native XHR and thus CORS requests requiring it would ignore this flag. For this reason, we recommend using jQuery 1.5.1+ should you require the use of it.

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, we 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' );
  },
  success: function( data ) {
    if (console && console.log){
      console.log( 'Sample of data:', data.slice(0,100) );
    }
  }
});

The jqXHR objects returned by $.ajax() implement the Promise interface, giving them all the properties, methods, and behavior of a Promise (see Deferred object for more information). For convenience and consistency with the callback names used by $.ajax(), jqXHR also provides .error(), .success(), and .complete() methods. These methods take a function argument that is called when the $.ajax() request terminates, and the function receives the same arguments as the correspondingly-named $.ajax() callback. In jQuery 1.5 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.)

// Assign handlers immediately after making the request,
// and remember the jqxhr object for this request
var jqxhr = $.ajax({ url: "example.php" })
    .success(function() { alert("success"); })
    .error(function() { alert("error"); })
    .complete(function() { alert("complete"); });

// perform other work here ...

// Set another completion function for the request above
jqxhr.complete(function(){ alert("second complete"); });

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()
• abort()

No onreadystatechange mechanism is provided, however, since success, error, complete and statusCode cover all conceivable requirements.

Note: jqXHR.success() and jqXHR.error() will be deprecated in jQuery 1.8 in favour of using jqXHR.done() and jqXHR.fail() instead. This is to avoid any confusion regarding the set of callback names that should ideally be used.

Callback Functions

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

1. beforeSend is called before the request is sent, and is passed the XMLHttpRequest object as a parameter.
2. error is called if the request fails. It is passed the XMLHttpRequest object, a string indicating the error type, and an exception object if applicable.
3. dataFilter is called on success. It is passed the returned data and the value of dataType, and must return the (possibly altered) data to pass on to success.
4. success is called if the request succeeds. It is passed the returned data, a string containing the success code, and the XMLHttpRequest object.
5. complete is called when the request finishes, whether in failure or success. It is passed the XMLHttpRequest object, as well as a string containing the success or error code.

Data Types

The $.ajax() function relies on the server to provide information about the retrieved data. If the server reports the return data as XML, the result can be traversed using normal XML methods or jQuery's selectors. If another type is detected, such as HTML in the example above, the data is treated as text.

Different data handling can be achieved by using the dataType option. Besides plain xml, the dataType can be html, json, jsonp, script, or text.

The text and xml types return the data with no processing. The data is simply passed on to the success handler, either through the responseText or responseHTML property of the XMLHttpRequest object, respectively.

Note: We must ensure that the MIME type reported by the web server matches our choice of dataType. In particular, XML must be declared by the server as text/xml or application/xml for consistent results.

If html is specified, any embedded JavaScript inside the retrieved data is executed before the HTML is returned as a string. Similarly, script will execute the JavaScript that is pulled back from the server, then return the script itself as textual data.

The json type parses the fetched data file as a JavaScript object and returns the constructed object as the result data. To do so, it uses JSON.parse() when the browser supports it; otherwise it uses a Function constructor. JSON data is convenient for communicating structured data in a way that is concise and easy for JavaScript to parse. If the fetched data file exists on a remote server, the jsonp type can be used instead. This type will cause a query string parameter of callback=? to be appended to the URL; the server should prepend the JSON data with the callback name to form a valid JSONP response. If a specific parameter name is desired instead of callback, it can be specified with the jsonp option to $.ajax().

Note: JSONP is an extension of the JSON format, requiring some server-side code to detect and handle the query string parameter. More information about it can be found in the original post detailing its use.

When data is retrieved from remote servers (which is only possible using the script or jsonp data types), the operation is performed using a <script> tag rather than an XMLHttpRequest object. In this case, no XMLHttpRequest object is returned from $.ajax(), nor is one passed to the handler functions such as beforeSend.

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.

The data option can contain either a query string of the form key1=value1&key2=value2, or a map of the form {key1: 'value1', key2: 'value2'}. If the latter form is used, the data is converted into a query string before it is sent. This processing can be circumvented by setting processData to false. The processing might be undesirable if we wish to send an XML object to the server; in this case, we would also want to 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. 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.

Examples

Example 1

$.ajax({
   type: "GET",
   url: "test.js",
   dataType: "script"
 });

Example 2

$.ajax({
   type: "POST",
   url: "some.php",
   data: "name=John&location=Boston",
   success: function(msg){
     alert( "Data Saved: " + msg );
   }
 });

Example 3

$.ajax({
  url: "test.html",
  cache: false,
  success: function(html){
    $("#results").append(html);
  }
});

Example 4

var html = $.ajax({
  url: "some.php",
  async: false
 }).responseText;

Example 5

var xmlDocument = [create xml document];
 $.ajax({
   url: "page.php",
   processData: false,
   data: xmlDocument,
   success: handleResponse
 });

Example 6

bodyContent = $.ajax({
      url: "script.php",
      global: false,
      type: "POST",
      data: ({id : this.getAttribute('id')}),
      dataType: "html",
      success: function(msg){
         alert(msg);
      }
   }
).responseText;