Call to jquery ajax – .fail vs. :error

The two options are equivalent.

However, the promise-style interface (.fail() and .done()) allow you to separate the code creating the request from the code handling the response.

You can write a function that sends an AJAX request and returns the jqXHR object, and then call that function elsewhere and add a handler.

When combined with the .pipe() function, the promise-style interface can also help reduce nesting when making multiple AJAX calls:

$.ajax(...)
    .pipe(function() { 
        return $.ajax(...);
    })
    .pipe(function() { 
        return $.ajax(...);
    })
    .pipe(function() { 
        return $.ajax(...);
    });