Testharness.js API Documentation


testharness.js provides a framework for writing testcases. It is intended to provide a convenient API for making common assertions, and to work both for testing synchronous and asynchronous DOM features in a way that promotes clear, robust, tests.

Basic Usage

The test harness script can be used from HTML or SVG documents and web worker scripts.

From an HTML or SVG document, start by importing both testharness.js and testharnessreport.js scripts into the document:

<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

Refer to the Web Workers section for details and an example on testing within a web worker.

Within each file one may define one or more tests. Each test is atomic in the sense that a single test has a single result (PASS/FAIL/TIMEOUT/NOTRUN). Within each test one may have a number of asserts. The test fails at the first failing assert, and the remainder of the test is (typically) not run.

If the file containing the tests is a HTML file, a table containing the test results will be added to the document after all tests have run. By default this will be added to a div element with id=log if it exists, or a new div element appended to document.body if it does not.

NOTE: By default tests must be created before the load event fires. For ways to create tests after the load event, see "Determining when all tests are complete", below.

Synchronous Tests

To create a synchronous test use the test() function:

test(test_function, name, properties)

test_function is a function that contains the code to test. For example a trivial test for the DOM hasFeature() method (which is defined to always return true) would be:

test(function() {
}, "hasFeature() with no arguments")

The function passed in is run in the test() call.

properties is a javascript object for passing extra options to the test. Currently it is only used to provide test-specific metadata, as described in the metadata section below.

Asynchronous Tests

Testing asynchronous features is somewhat more complex since the result of a test may depend on one or more events or other callbacks. The API provided for testing these features is intended to be rather low-level but hopefully applicable to many situations.

To create a test, one starts by getting a Test object using async_test:

async_test(name, properties)


var t = async_test("DOMContentLoaded")

Assertions can be added to the test by calling the step method of the test object with a function containing the test assertions:

document.addEventListener("DOMContentLoaded", function() {
  t.step(function() {
    assert_true(e.bubbles, "bubbles should be true");

When all the steps are complete, the done() method must be called:


As a convenience, async_test can also takes a function as first argument. This function is called with the test object as both its this object and first argument. The above example can be rewritten as:

async_test(function(t) {
  document.addEventListener("DOMContentLoaded", function() {
    t.step(function() {
      assert_true(e.bubbles, "bubbles should be true");
}, "DOMContentLoaded");

which avoids cluttering the global scope with references to async tests instances.

The properties argument is identical to that for test().

In many cases it is convenient to run a step in response to an event or a callback. A convenient method of doing this is through the step_func method which returns a function that, when called runs a test step. For example:

document.addEventListener("DOMContentLoaded", t.step_func(function() {
  assert_true(e.bubbles, "bubbles should be true");

As a further convenience, the step_func that calls done() can instead use step_func_done, as follows:

document.addEventListener("DOMContentLoaded", t.step_func_done(function() {
  assert_true(e.bubbles, "bubbles should be true");

For asynchronous callbacks that should never execute, unreached_func can be used. For example:

  t.unreached_func("DOMContentLoaded should not be fired on the document element"));

Keep in mind that other tests could start executing before an Asynchronous Test is finished.

Promise Tests

promise_test can be used to test APIs that are based on Promises:

promise_test(test_function, name, properties)

test_function is a function that receives a test as an argument and returns a promise. The test completes when the returned promise resolves. The test fails if the returned promise rejects.


function foo() {
  return Promise.resolve("foo");

promise_test(function() {
  return foo()
    .then(function(result) {
      assert_equals(result, "foo", "foo should return 'foo'");
}, "Simple example");

In the example above, foo() returns a Promise that resolves with the string "foo". The test_function passed into promise_test invokes foo and attaches a resolve reaction that verifies the returned value.

Note that in the promise chain constructed in test_function assertions don't need to wrapped in step or step_func calls.

Unlike Asynchronous Tests, Promise Tests don't start running until after the previous Promise Test finishes.

promise_rejects can be used to test Promises that need to reject:

promise_rejects(test_object, code, promise, description)

The code argument is equivalent to the same argument to the assert_throws function.

Here's an example where the bar() function returns a Promise that rejects with a TypeError:

function bar() {
  return Promise.reject(new TypeError());

promise_test(function(t) {
  return promise_rejects(t, new TypeError(), bar());
}, "Another example");

EventWatcher is a constructor function that allows DOM events to be handled using Promises, which can make it a lot easier to test a very specific series of events, including ensuring that unexpected events are not fired at any point.

Here's an example of how to use EventWatcher:

var t = async_test("Event order on animation start");

var animation = watchedNode.getAnimations()[0];
var eventWatcher = new EventWatcher(t, watchedNode, ['animationstart',

eventWatcher.wait_for('animationstart').then(t.step_func(function() {
  animation.currentTime = END_TIME; // skip to end
  // We expect two animationiteration events then an animationend event on
  // skipping to the end of the animation.
  return eventWatcher.wait_for(['animationiteration',
})).then(t.step_func(function() {

wait_for either takes the name of a single event and returns a Promise that will resolve after that event is fired at the watched node, or else it takes an array of the names of a series of events and returns a Promise that will resolve after that specific series of events has been fired at the watched node.

EventWatcher will assert if an event occurs while there is no wait_for() created Promise waiting to be fulfilled, or if the event is of a different type to the type currently expected. This ensures that only the events that are expected occur, in the correct order, and with the correct timing.

Single Page Tests

Sometimes, particularly when dealing with asynchronous behaviour, having exactly one test per page is desirable, and the overhead of wrapping everything in functions for isolation becomes burdensome. For these cases testharness.js support "single page tests".

In order for a test to be interpreted as a single page test, then it must simply not call test() or async_test() anywhere on the page, and must call the done() function to indicate that the test is complete. All the assert_* functions are avaliable as normal, but are called without the normal step function wrapper. For example:

<!doctype html>
<title>Basic document.body test</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
    assert_equals(document.body, document.getElementsByTagName("body")[0])

The test title for single page tests is always taken from document.title.

Making assertions

Functions for making assertions start assert_. The full list of asserts avaliable is documented in the asserts section below. The general signature is:

assert_something(actual, expected, description)

although not all assertions precisely match this pattern e.g. assert_true only takes actual and description as arguments.

The description parameter is used to present more useful error messages when a test fails.

NOTE: All asserts must be located in a test() or a step of an async_test(), unless the test is a single page test. Asserts outside these places won't be detected correctly by the harness and may cause unexpected exceptions that will lead to an error in the harness.


Occasionally tests may create state that will persist beyond the test itself. In order to ensure that tests are independent, such state should be cleaned up once the test has a result. This can be achieved by adding cleanup callbacks to the test. Such callbacks are registered using the add_cleanup function on the test object. All registered callbacks will be run as soon as the test result is known. For example:

  test(function() {
    var element = document.createElement("div");
    element.setAttribute("id", "null");
    this.add_cleanup(function() { document.body.removeChild(element) });
    assert_equals(document.getElementById(null), element);
  }, "Calling document.getElementById with a null argument.");

Timeouts in Tests

In general the use of timeouts in tests is discouraged because this is an observed source of instability in real tests when run on CI infrastructure. In particular if a test should fail when something doesn't happen, it is good practice to simply let the test run to the full timeout rather than trying to guess an appropriate shorter timeout to use.

In other cases it may be necessary to use a timeout (e.g., for a test that only passes if some event is not fired). In this case it is not permitted to use the standard setTimeout function. Instead one must use the step_timeout function:

async_test(function(t) {
  var gotEvent = false;
  document.addEventListener("DOMContentLoaded", t.step_func(function() {
    assert_false(gotEvent, "Unexpected DOMContentLoaded event");
    gotEvent = true;
    t.step_timeout(function() { t.done(); }, 2000);
}, "Only one DOMContentLoaded");

The difference between setTimeout and step_timeout is that the latter takes account of the timeout multiplier when computing the delay; e.g., in the above case a timeout multiplier of 2 would cause a pause of 4000ms before calling the callback. This makes it less likely to produce unstable results in slow configurations.

Note that timeouts generally need to be a few seconds long in order to produce stable results in all test environments.

For single-page tests, step_timeout is also available as a global function.

Harness Timeout

The overall harness admits two timeout values "normal" (the default) and "long", used for tests which have an unusually long runtime. After the timeout is reached, the harness will stop waiting for further async tests to complete. By default the timeouts are set to 10s and 60s, respectively, but may be changed when the test is run on hardware with different performance characteristics to a common desktop computer. In order to opt-in to the longer test timeout, the test must specify a meta element:

<meta name="timeout" content="long">

Occasionally tests may have a race between the harness timing out and a particular test failing; typically when the test waits for some event that never occurs. In this case it is possible to use test.force_timeout() in place of assert_unreached(), to immediately fail the test but with a status of TIMEOUT. This should only be used as a last resort when it is not possible to make the test reliable in some other way.


Sometimes tests require non-trivial setup that may fail. For this purpose there is a setup() function, that may be called with one or two arguments. The two argument version is:

setup(func, properties)

The one argument versions may omit either argument. func is a function to be run synchronously. setup() becomes a no-op once any tests have returned results. Properties are global properties of the test harness. Currently recognised properties are:

explicit_done - Wait for an explicit call to done() before declaring all tests complete (see below; implicitly true for single page tests)

output_document - The document to which results should be logged. By default this is the current document but could be an ancestor document in some cases e.g. a SVG test loaded in an HTML wrapper

explicit_timeout - disable file timeout; only stop waiting for results when the timeout() function is called (typically for use when integrating with some existing test framework that has its own timeout mechanism).

allow_uncaught_exception - don't treat an uncaught exception as an error; needed when e.g. testing the window.onerror handler.

timeout_multiplier - Multiplier to apply to per-test timeouts.

Determining when all tests are complete

By default the test harness will assume there are no more results to come when:

  1. There are no Test objects that have been created but not completed
  2. The load event on the document has fired

This behaviour can be overridden by setting the explicit_done property to true in a call to setup(). If explicit_done is true, the test harness will not assume it is done until the global done() function is called. Once done() is called, the two conditions above apply like normal.

Dedicated and shared workers don't have an event that corresponds to the load event in a document. Therefore these worker tests always behave as if the explicit_done property is set to true. Service workers depend on the install event which is fired following the completion of running the worker.

Generating tests

There are scenarios in which is is desirable to create a large number of (synchronous) tests that are internally similar but vary in the parameters used. To make this easier, the generate_tests function allows a single function to be called with each set of parameters in a list:

generate_tests(test_function, parameter_lists, properties)

For example:

generate_tests(assert_equals, [
    ["Sum one and one", 1+1, 2],
    ["Sum one and zero", 1+0, 1]

Is equivalent to:

test(function() {assert_equals(1+1, 2)}, "Sum one and one")
test(function() {assert_equals(1+0, 1)}, "Sum one and zero")

Note that the first item in each parameter list corresponds to the name of the test.

The properties argument is identical to that for test(). This may be a single object (used for all generated tests) or an array.

Callback API

The framework provides callbacks corresponding to 4 events:

  • start - triggered when the first Test is created
  • test_state - triggered when a test state changes
  • result - triggered when a test result is received
  • complete - triggered when all results are received

The page defining the tests may add callbacks for these events by calling the following methods:

add_start_callback(callback) - callback called with no arguments

add_test_state_callback(callback) - callback called with a test argument

add_result_callback(callback) - callback called with a test argument

add_completion_callback(callback) - callback called with an array of tests and a status object

Tests have the following properties:

  • status - A status code. This can be compared to the PASS, FAIL, TIMEOUT and NOTRUN properties on the test object

  • message - A message indicating the reason for failure. In the future this will always be a string

The status object gives the overall status of the harness. It has the following properties:

  • status - Can be compared to the OK, ERROR and TIMEOUT properties

  • message - An error message set when the status is ERROR

External API

In order to collect the results of multiple pages containing tests, the test harness will, when loaded in a nested browsing context, attempt to call certain functions in each ancestor and opener browsing context:

  • start - start_callback
  • test_state - test_state_callback
  • result - result_callback
  • complete - completion_callback

These are given the same arguments as the corresponding internal callbacks described above.

External API through cross-document messaging

Where supported, the test harness will also send messages using cross-document messaging to each ancestor and opener browsing context. Since it uses the wildcard keyword (*), cross-origin communication is enabled and script on different origins can collect the results.

This API follows similar conventions as those described above only slightly modified to accommodate message event API. Each message is sent by the harness is passed a single vanilla object, available as the data property of the event object. These objects are structured as follows:

  • start - { type: "start" }
  • test_state - { type: "test_state", test: Test }
  • result - { type: "result", test: Test }
  • complete - { type: "complete", tests: [Test, ...], status: TestsStatus }

Web Workers

The testharness.js script can be used from within dedicated workers, shared workers and service workers.

Testing from a worker script is different from testing from an HTML document in several ways:

  • Workers have no reporting capability since they are runing in the background. Hence they rely on testharness.js running in a companion client HTML document for reporting.

  • Shared and service workers do not have a unique client document since there could be more than one document that communicates with these workers. So a client document needs to explicitly connect to a worker and fetch test results from it using fetch_tests_from_worker. This is true even for a dedicated worker. Once connected, the individual tests running in the worker (or those that have already run to completion) will be automatically reflected in the client document.

  • The client document controls the timeout of the tests. All worker scripts act as if they were started with the explicit_timeout option (see the Harness timeout section).

  • Dedicated and shared workers don't have an equivalent of an onload event. Thus the test harness has no way to know when all tests have completed (see Determining when all tests are complete). So these worker tests behave as if they were started with the explicit_done option. Service workers depend on the oninstall event and don't require an explicit done call.

Here's an example that uses a dedicated worker.



test(function(t) {
  assert_true(true, "true is true");
}, "Simple test");

// done() is needed because the testharness is running as if explicit_done
// was specified.


<!DOCTYPE html>
<title>Simple test</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="log"></div>

fetch_tests_from_worker(new Worker("worker.js"));


The argument to the fetch_tests_from_worker function can be a Worker, a SharedWorker or a ServiceWorker. Once called, the containing document fetches all the tests from the worker and behaves as if those tests were running in the containing document itself.

List of Assertions

assert_true(actual, description)

asserts that actual is strictly true

assert_false(actual, description)

asserts that actual is strictly false

assert_equals(actual, expected, description)

asserts that actual is the same value as expected

assert_not_equals(actual, expected, description)

asserts that actual is a different value to expected. This means that expected is a misnomer.

assert_in_array(actual, expected, description)

asserts that expected is an Array, and actual is equal to one of the members i.e. expected.indexOf(actual) != -1

assert_array_equals(actual, expected, description)

asserts that actual and expected have the same length and the value of each indexed property in actual is the strictly equal to the corresponding property value in expected

assert_approx_equals(actual, expected, epsilon, description)

asserts that actual is a number within ±epsilon of expected

assert_less_than(actual, expected, description)

asserts that actual is a number less than expected

assert_greater_than(actual, expected, description)

asserts that actual is a number greater than expected

assert_between_exclusive(actual, lower, upper, description

asserts that actual is a number between lower and upper but not equal to either of them

assert_less_than_equal(actual, expected, description)

asserts that actual is a number less than or equal to expected

assert_greater_than_equal(actual, expected, description)

asserts that actual is a number greater than or equal to expected

assert_between_inclusive(actual, lower, upper, description

asserts that actual is a number between lower and upper or equal to either of them

assert_regexp_match(actual, expected, description)

asserts that actual matches the regexp expected

assert_class_string(object, class_name, description)

asserts that the class string of object as returned in Object.prototype.toString is equal to class_name.

assert_own_property(object, property_name, description)

assert that object has own property property_name

assert_inherits(object, property_name, description)

assert that object does not have an own property named property_name but that property_name is present in the prototype chain for object

assert_idl_attribute(object, attribute_name, description)

assert that an object that is an instance of some interface has the attribute attribute_name following the conditions specified by WebIDL

assert_readonly(object, property_name, description)

assert that property property_name on object is readonly

assert_throws(code, func, description)

code - the expected exception. This can take several forms:

  • string - the thrown exception must be a DOMException with the given name, e.g., "TimeoutError" (for compatibility with existing tests, a constant is also supported, e.g., "TIMEOUT_ERR")
  • object - the thrown exception must have a property called "name" that matches code.name
  • null - allow any exception (in general, one of the options above should be used)

func - a function that should throw


asserts if called. Used to ensure that some codepath is not taken e.g. an event does not fire.

assert_any(assert_func, actual, expected_array, extra_arg_1, ... extra_arg_N)

asserts that one assert_func(actual, expected_array_N, extra_arg1, ..., extra_arg_N) is true for some expected_array_N in expected_array. This only works for assert_func with signature assert_func(actual, expected, args_1, ..., args_N). Note that tests with multiple allowed pass conditions are bad practice unless the spec specifically allows multiple behaviours. Test authors should not use this method simply to hide UA bugs.

assert_exists(object, property_name, description)

deprecated asserts that object has an own property property_name

assert_not_exists(object, property_name, description)

deprecated assert that object does not have own property property_name


It is possible to add optional metadata to tests; this can be done in one of two ways; either by adding <meta> elements to the head of the document containing the tests, or by adding the metadata to individual [async_]test calls, as properties.