How to force jquery $.when(...) to fail

Question

$.when(a1, ...).fail(function() { alert('fail'); });

Is there a value for a1 that would force the deferred object to fail, while not doing any malformed ajax calls within the object ?

Answer 1

You could pass a rejected Deferred object:

$.when($.Deferred().reject()).fail(function() {
    alert("fail");
});

You can test it in this fiddle.

Answer 2

To elaborate on @Frédéric Hamidi, if you have a "async" function which is not already "promisized" then it will often have a success and failure callback.

Then you do something like:

// @returns {promise} a promise
function callAsync(args){
  var deferred = $.Deferred();

  ApiNotReturningPromise.ajax(args, function success(data) {
    deferred.resolve(data);
    // if you wish to always fail: o.O
    // deferred.reject(data);
  }, function fail(error) {
    deferred.reject(error);
  });

  return deferred.promise();
}

Again, you can just fail in both cases here if you wish to, although that seems to make little sense to me, by

But, if it already is a Promise, (and AFAIK promise-like is enough for jQuery) then you can just reuse that promise.