How to properly unit test jQuery's .ajax() promises using Jasmine and/or Sinon?

Question

It is not that complex actually. It suffices to return a promise and resolve it according to your case.

For example:

spyOn($, 'ajax').andCallFake(function (req) {
    var d = $.Deferred();
    d.resolve(data_you_expect);
    return d.promise();
});

for a success, or

spyOn($, 'ajax').andCallFake(function (req) {
    var d = $.Deferred();
    d.reject(fail_result);
    return d.promise();
});

for a failure.

For Jasmine 2.0 the syntax has changed slightly:

spyOn($, 'ajax').and.callFake(function (req) {});

the method .andCallFake() does not exist in Jasmine 2.0

something along these lines / with sinon and jQuery deferreds

ajaxStub = sinon.stub($, "ajax");

function okResponse() {
  var d = $.Deferred();
  d.resolve( { username: "testuser", userid: "userid", success: true } );
  return d.promise();
};

function errorResponse() {
 var d = $.Deferred();
 d.reject({},{},"could not complete");
 return d.promise();
};

ajaxStub.returns(okResponse());
ajaxStub.returns(errorResponse());