Fake XMLHttpRequest

Provides a fake implementation of XMLHttpRequest and provides several interfaces for manipulating objects created by it.

Also fakes native XMLHttpRequest and ActiveXObject (when available, and only for XMLHTTP progids). Helps with testing requests made with XHR.

The fake server and XHR can be used completely stand-alone by downloading sinon-server.js.

    setUp: function () {
        this.xhr = sinon.useFakeXMLHttpRequest();
        var requests = this.requests = [];

        this.xhr.onCreate = function (xhr) {

    tearDown: function () {

    "test should fetch comments from server" : function () {
        var callback = sinon.spy();
        myLib.getCommentsFor("/some/article", callback);
        assertEquals(1, this.requests.length);

        this.requests[0].respond(200, { "Content-Type": "application/json" },
                                 '[{ "id": 12, "comment": "Hey there" }]');
        assert(callback.calledWith([{ id: 12, comment: "Hey there" }]));


var xhr = sinon.useFakeXMLHttpRequest();

Causes Sinon to replace the native XMLHttpRequest object in browsers that support it with a custom implementation which does not send actual requests.

In browsers that support ActiveXObject, this constructor is replaced, and fake objects are returned for XMLHTTP progIds. Other progIds, such as XMLDOM are left untouched.

The native XMLHttpRequest object will be available at sinon.xhr.XMLHttpRequest

xhr.onCreate = function (xhr) {};

By assigning a function to the onCreate property of the returned object from useFakeXMLHttpRequest() you can subscribe to newly created FakeXMLHttpRequest objects. See below for the fake xhr object API.

Using this observer means you can still reach objects created by e.g. jQuery.ajax (or other abstractions/frameworks).


Restore original function(s).


String request.url

The URL set on the request object.

String request.method

The request method as a string.

Object request.requestHeaders

An object of all request headers, i.e.:

    "Accept": "text/html, */*",
    "Connection": "keep-alive"

String request.requestBody

The request body

int request.status

The request’s status code.

undefined if the request has not been handled (see respond below)

String request.statusText

Only populated if the respond method is called (see below).

boolean request.async

Whether or not the request is asynchronous.

String request.username

Username, if any.

String request.password

Password, if any.

Document request.responseXML

When using respond, this property is populated with a parsed document if response headers indicate as much (see the spec)

String request.getResponseHeader(header);

The value of the given response header, if the request has been responded to (see respond).

Object request.getAllResponseHeaders();

All response headers as an object.

Filtered requests

When using Sinon.JS for mockups or partial integration/functional testing, you might want to fake some requests, while allowing others to go through to the backend server. With filtered FakeXMLHttpRequests (new in v1.3.0), you can.


Default false.

When set to true, Sinon will check added filters if certain requests should be “unfaked”


Add a filter that will decide whether or not to fake a request.

The filter will be called when xhr.open is called, with the exact same arguments (method, url, async, username, password). If the filter returns true, the request will not be faked.

Simulating server responses


Sets response status (status and statusText properties).

Status should be a number, the status text is looked up from sinon.FakeXMLHttpRequest.statusCodes.


Sets response headers (e.g. { "Content-Type": "text/html", /* ... */ }, updates the readyState property and fires onreadystatechange.


Sets the respond body, updates the readyState property and fires onreadystatechange.

Additionally, populates responseXML with a parsed document if response headers indicate as much.

request.respond(status, headers, body);

Calls the above three methods.


Simulates a network error on the request. The onerror handler will be called and the status will be 0.

Boolean request.autoRespond

When set to true, causes the server to automatically respond to incoming requests after a timeout.

The default timeout is 10ms but you can control it through the autoRespondAfter property.

Note that this feature is intended to help during mockup development, and is not suitable for use in tests.

Number request.autoRespondAfter

When autoRespond is true, respond to requests after this number of milliseconds. Default is 10.

Fake server

High-level API to manipulate FakeXMLHttpRequest instances.

// example.test.js in Mocha TDD
var sinon = require("sinon");

suite("sinon.fakeServer", function () {
  setup(function () {
    this.server = sinon.fakeServer.create();

  teardown(function () {

  test("test should fetch comments from server", function () {
    this.server.respondWith("GET", "/some/article/comments.json", [
      { "Content-Type": "application/json" },
      '[{ "id": 12, "comment": "Hey there" }]',

    var callback = sinon.spy();
    myLib.getCommentsFor("/some/article", callback);

    sinon.assert.calledWith(callback, [{ id: 12, comment: "Hey there" }]);

    assert(server.requests.length > 0);


var server = sinon.fakeServer.create([config]);

Creates a new server.

This function also calls sinon.useFakeXMLHttpRequest().

create() accepts an optional properties object to configure the fake server. See Configuration Properties below for available options.

var server = sinon.fakeServerWithClock.create();

Creates a server that also manages fake timers.

This is useful when testing XHR objects created with e.g. jQuery 1.3.x, which uses a timer to poll the object for completion, rather than the usual onreadystatechange.

Just as a create() above, an optional properties object can be provided to set configuration parameters during instantiation.


Changes the configuration the fake server after it has been created.

This can be useful for changing the delay of an automatic response for a specific test requirement. For more options, see Configuration Properties below.


Causes the server to respond to any request not matched by another response with the provided data. The default catch-all response is [404, {}, ""].

response can be one of three things:

  1. A String representing the response body
  2. An Array with status, headers and response body, e.g. [200, { "Content-Type": "text/html", "Content-Length": 2 }, "OK"]
  3. A Function.

Default status is 200 and default headers are none.

When the response is a Function, it will be passed the request object. You must manually call respond on it to complete the request.

server.respondWith(url, response);

Responds to all requests to given URL, e.g. /posts/1.

server.respondWith(method, url, response);

Responds to all method requests to the given URL with the given response.

method is an HTTP verb.

server.respondWith(urlRegExp, response);

URL may be a regular expression, e.g. /\\/post\\//\\d+

If the response is a Function, it will be passed any capture groups from the regular expression along with the XMLHttpRequest object:

server.respondWith(/\/todo-items\/(\d+)/, function (xhr, id) {
    { "Content-Type": "application/json" },
    '[{ "id": ' + id + " }]"

server.respondWith(method, urlRegExp, response);

Responds to all method requests to URLs matching the regular expression.


Causes all queued asynchronous requests to receive a response.

If none of the responses added through respondWith() match, the default response is [404, {}, ""].

Synchronous requests are responded to immediately so make sure to call respondWith() to configure the server response before calling respond(). If not, you will receive the default 404 NOT FOUND response.

If called with arguments, respondWith() will be called with those arguments before responding to requests.


Used internally to determine the HTTP method used with the provided request.

By default this method returns request.method. When server.fakeHTTPMethods is true, the method will return the value of the _method parameter if the method is “POST”.

This method can be overridden to provide custom behavior.


Restores the native XHR constructor.


Array[] server.requests

server.requests is an array of all the FakeXMLHttpRequest objects that have been created.

This property allows you to inspect the received requests to verify request ordering, find unmatched requests or check that no requests has been done.

Configuration Properties

The Fake Server exposes configurable properties to modify the behavior of the server as desired. These properties can be set directly or with an object literal passed into create(options) and/or server.configure(options).

// Defaults
server.autoRespond = false;
server.autoRespondAfter = 10; // in milliseconds
server.respondImmediately = false;
server.fakeHTTPMethods = false;

// configure fakeServer to autoRespond
server.autoRespond = true;

// Change server now to respondImmediately
server.configure({ respondImmediately: true });

Boolean autoRespond

If set, will automatically respond to every request after a timeout. Default: false.

The default timeout is 10ms but you can control it through the autoRespondAfter property.

Note that this feature is intended to help during mockup development, and is not suitable for use in tests. For synchronous immediate responses, use respondImmediately instead.

Number autoRespondAfter (ms)

Causes the server to automatically respond to incoming requests after a timeout. Requires server.autoRespond to be set to true to have an effect. If server.respondImmediately is set to true, this setting is ignored.

Boolean respondImmediately

If set, the server will respond to every request immediately and synchronously. Default: false

This is ideal for faking the server from within a test without having to call server.respond() after each request made in that test.

As this is synchronous and immediate, this is not suitable for simulating actual network latency in tests or mockups. To simulate network latency with automatic responses, see autoRespond and autoRespondAfter properties. If server.respondImmediately == true, it will override all autoRespond behavior.

Boolean fakeHTTPMethods

If set to true, server will find _method parameter in POST body and recognize that as the actual method.

Supports a pattern common to Ruby on Rails applications. For custom HTTP method faking, override server.getHTTPMethod(request).