How to make a unit test for ajax calls

I am a newbie in creating unit tests for JavaScripts. In my unit test I keep on getting "AssertError: expected "ok" but was [undefined] " as a result. Can any teach me how to do a unit test for ajax calls? Thanks. By the way, I'm using JsTestDriver for testing my unit tests.

Here's my source file:

function getReport(m, y)
{
$.ajax({
    url: "url/to",
    data: {
        m:m,
        y:y
    },
    dataType: 'json',
    success:function (res) {
            return res;

    }       
});

}

Here's my test file:

TestCase("Report", {

  "test get report": function () {
 assertEquals("ok", getReport(1, 2015));
}   
});