what is the meaning of resolved and rejected in deferred object in jQuery?

Question

i have asked a question here : refresh page if internet connection available, then @Fabrizio Calderan gave a very neat solution using the deferred object implementation as follow:

setInterval(function() {
    $.when(
        $.ajax({ 
            url  : "/favicon.ico", /* or other resource */
            type : "HEAD"
        })
    )
    .done(function() {
        location.reload();
    });
}, 120000); /* 120000 ~> 2 minutes */

My question is:

since the ajax call will return jqXHR ; what if the ajax call failed? what is the return type of the ajax then? is it still jqXHR or UNDEFINED or NULL

and since the call of ajax will return jqXHR which is a deferred object; can i conclude the following :

deferred object resolved ==> jqXHR

or rejected ==> UNDEFINED

regards

Answer 1

The jQuery ajax call always returns a jqXHR object whether success or failure. It never returns null or undefined.

The success or failure can be seen via either the optional callback notification functions or via the returned jqXHR object which acts as a promise object and thus will get promise notifications about the ajax call's completion, success or failure.

Because the jqXHR object is also a promise object, your code can be simplified to just this:

setInterval(function() {
    $.ajax({ 
        url  : "/favicon.ico", /* or other resource */
        type : "HEAD"
    }).done(function() {
        location.reload();
    });
}, 120000); /* 120000 ~> 2 minutes */

If you want to be notified of other situations besides just .done(), you can use .fail(), .always() or .then().

If the ajax calls fails immediately, then the promise will be resolved as a failure (e.g rejected). Subsequent .fail() or .always() handlers attached to the promise object will still be called even if it has already failed.

Here's the relevant jQuery documentation:

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.