Return fake jqXHR from custom Ajax method

Question

I have several methods that wrap the $.ajax() method and return its jqXHR, but I need to force fail before even calling the original $.ajax() and still allow to use success/error methods of the result.

//just a very simple example, actuall code is more complicated
$.rpc = function(method, args) {
 if (!method || 'string' === typeof method ||) {
 return false; //PROBLEM - cannot use success/error handlers!
 }
 return $.ajax({
 type: 'POST',
 url: '/rpc',
 data: { method: method, params: args },
 }); //AJAX()
};
//usage:
$.rpc(loginMethod, credentials).success(gotoProfile).fail(gotoLogin);
//if loginMethod is for any reason undefined it crash instead of calling fail handler

So I use this helper method, since the jqXHR is created directly inside the Ajax method:

$.ajax.getFailed = function(message) {
 var
 jqDef = new $.Deferred(),
 jqXHR = jqDef.promise();
 jqXHR.success = jqXHR.done; //deprecated $.ajax methods
 jqXHR.error = jqXHR.fail;
 jqDef.reject.defer(1, jqXHR, [jqXHR, message]);
 //defer is from extJS, alternative to setTimeout
 return jqXHR;
};
$.rpc = function(method, args) {
 if (!method || 'string' === typeof method ||) {
 return $.ajax.getFailed('Missing method');
 }
 return $.ajax({
 type: 'POST',
 url: '/rpc',
 data: { method: method, params: args },
 }); //AJAX()
};

Is this code OK or is there anything else I should add?

• Simon Forsberg: Does your approach work? If it works, I guess it is OK :)
• Radek Pech: I've made a jQuery plugin that supports this approach: github.com/nothrem/jQueryProcess (see the last part of the README).

Answer 1

Looking at the documentation for $.ajax reveals a beforeSend option that would allow you to cancel the request before it's sent by returning false.

$.rpc = function(method, args) {
 var beforeSend = function () {
 return method && 'string' !== typeof method; 
 }
 return $.ajax({
 type: 'POST',
 url: '/rpc',
 data: { method: method, params: args },
 beforeSend: beforeSend
 }); 
};

• Radek Pech: This seems to work for fail case correctly and answers the original question. But in the meanwhile I've created similar method getDone() which I use in case the data are already cached on client - and this method cannot cancel the AJAX while calling the success handlers.