Please unify the order of parameters between $.ajax(...).fail() and $.ajax(...).then() so that using $.ajax(...).always() is not unnecessarily painful. Even better, please unify the order of returned parameters for all ajax handlers (including success / error / complete options) so that we can end the guess-work. :P

The offending code can be found in src/ajax.js (line 569 in git master):

if ( isSuccess ) {
  deferred.resolveWith( callbackContext, [ success, statusText, jqXHR ] );
} else {
  deferred.rejectWith( callbackContext, [ jqXHR, statusText, error ] );
}

This means that for AJAX calls, separate .fail() and .then() callbacks need to be created, instead of just being able to do it all in .always(). Combined with the fact that certain result are deemed by jQuery to be failures (e.g. status === 0 for local files), this means we need to have some of our success code duplicated from .then() into .fail().

Example necessary with jQuery 1.7.2:

$.ajax('/my/xhr/request.php')
.fail(function(jqxhr, status, response) {
  if (jqxhr.status === 0) { // for HTML5 Application Cache or local resource
    // TODO: parse response as per data-type
    // TODO: do post-success stuff
  } else {
    // TODO: do post-error stuff
  }
)
.then(function(response, status, jqxhr) {
  // TODO: do post-success stuff
);

Example with consistent parameter order (fingers crossed):

$.ajax('/my/xhr/request.php')
.always(function(jqxjr, status, response) {
  // for HTML5 Application Cache or local resource, status === 0
  if ($.inArray(jqxhr.status, [0, 200, 304] !== -1) { 
    // TODO: parse response as per data-type
    // TODO: do post-success stuff
  } else {
    // TODO: do post-error stuff
  }  
});

It was bad enough that the order of parameters is different to the old success / error / complete handlers specified as options to $.ajax(...), but to make them different between resolve and reject makes this extra frustrating.

I understand completely that this breaks compatibility with older versions of jQuery, but I think this is really important for the long-term and worth it. I'm happy to wait for jQuery 1.8 or 1.9 for this breakage to occur. I think it is important to at least discuss this issue on its merits (and I would not have captured them all here in this initial somewhat-emotional post :P).

Please comment.