Faking XMLHttpRequest with SinonJS

 // uses sinon and jQuery

var server = sinon.fakeServer.create();
server.respondWith([200, { "Content-Type": "application/json" }, '{"myData":3}']);
$.getJSON('some/url');
console.log(server.response);
server.restore();

JSFiddle:  http://jsfiddle.net/stevenhollidge/crDCT/4/ 

// uses sinon and jquery

var xhr = sinon.useFakeXMLHttpRequest();
var requests = [];

xhr.onCreate = function (request) {
    requests.push(request);
};

var responseData = '{"myData":3}';
$.getJSON('some/url', function(data) { console.log(data); });
requests[0].respond(200, { "Content-Type": "application/json" }, responseData);
console.log(requests[0].url);
xhr.restore();

JSFiddle:  http://jsfiddle.net/stevenhollidge/crDCT/2/

An example combined with Jasmine to mock out the http call:

// uses jasmine, sinon and jQuery

describe("useFakeXMLHttpRequest", function() {
    var xhr, requests;
     
    beforeEach(function () {
        xhr = sinon.useFakeXMLHttpRequest();
        requests = [];

        xhr.onCreate = function (request) {
            requests.push(request);
        };
    },

    afterEach: function () {
        xhr.restore();
    },

    it('should be able to handle responses', function () {
        var responseData = '{"myData":3}';
        $.getJSON('some/url', function(data) { console.log(data); });
        requests[0].respond(200, { "Content-Type": "application/json" }, responseData);
        expect(requests[0].url).toBe('some/url');
    }
}

JSFiddle:  http://jsfiddle.net/stevenhollidge/crDCT/