If you are testing web platform code, prefer using use a wpt test (preferably upstreamable ones).


Mochitest is an automated testing framework built on top of the MochiKit JavaScript libraries.

Only things that can be tested using JavaScript (with chrome privileges!) can be tested with this framework. Given some creativity, that’s actually much more than you might first think, but it’s not possible to write Mochitest tests to directly test a non-scripted C++ component, for example. (Use a compiled-code test like GTest to do that.)

Running tests

To run a single test (perhaps a new test you just added) or a subset of the entire Mochitest suite, pass a path parameter to the mach command.

For example, to run only the test test_CrossSiteXHR.html in the Mozilla source tree, you would run this command:

./mach test dom/security/test/cors/test_CrossSiteXHR.html

To run all the tests in dom/svg/, this command would work:

./mach test dom/svg/

You can also pass a manifest path to run all tests on that manifest:

./mach test dom/base/test/mochitest.ini

Running flavors and subsuites

Flavors are variations of the default configuration used to run Mochitest. For example, a flavor might have a slightly different set of prefs set for it, a custom extension installed or even run in a completely different scope.

The Mochitest flavors are:

  • plain - The most basic and common Mochitest. They run in content scope, but can access certain privileged APIs with SpecialPowers.

  • browser - These often test the browser UI itself and run in browser window scope.

  • chrome - These run in chrome scope and are typically used for testing privileged JavaScript APIs. More information can be found here.

  • a11y - These test the accessibility interfaces. They can be found under the top accessible directory and run in chrome scope. Note that these run without e10s / fission.

A subsuite is similar to a flavor, except that it has an identical configuration. It is just logically separated from the “default” subsuite for display purposes. For example, devtools is a subsuite of the browser flavor. There is no difference in how these two jobs are run. It exists so that the devtools team can easily see and run their tests.

Note: There are also tags, which are similar to subsuites. Although they both are used to logically group related sets of tests, they behave differently. For example, applying a subsuite to a test removes that test from the default set, whereas, a tag does not remove it.

By default, mach finds and runs every test in the given subdirectory no matter which flavor or subsuite it belongs to. But sometimes, you might only want to run a specific flavor or subsuite. This can be accomplished using the --flavor (or -f) and --subsuite options respectively. For example:

./mach mochitest -f plain                        # runs all plain tests
./mach mochitest -f browser --subsuite devtools  # runs all browser tests in the devtools subsuite
./mach mochitest -f chrome dom/indexedDB         # runs all chrome tests in the dom/indexedDB subdirectory

In many cases, it won’t be necessary to filter by flavor or subsuite as running specific directories will do it implicitly. For example running:

./mach mochitest devtools/

Is a rough equivalent to running the devtools subsuite. There might be situations where you might want to run tests that don’t belong to any subsuite. To do this, use:

./mach mochitest --subsuite default

Debugging individual tests

If you need to debug an individual test, you could reload the page containing the test with the debugger attached. If attaching a debugger before the problem shows up is hard (for example, if the browser crashes as the test is loading), you can specify a debugger when you run mochitest:

./mach mochitest --debugger=gdb ...

See also the --debugger-args and --debugger-interactive arguments. You can also use the --jsdebugger argument to debug JavaScript.

Finding errors

Search for the string TEST-UNEXPECTED-FAIL to find unexpected failures. You can also search for SimpleTest FINISHED to see the final test summary.

Logging results

The output from a test run can be sent to the console and/or a file (by default the results are only displayed in the browser). There are several levels of detail to choose from. The levels are DEBUG, INFO, WARNING, ERROR and CRITICAL, where DEBUG produces the highest detail (everything), and CRITICAL produces the least.

Mochitest uses structured logging. This means that you can use a set of command line arguments to configure the log output. To log to stdout using the mach formatter and log to a file in JSON format, you can use --log-mach=- --log-raw=mochitest.log. By default the file logging level for all your formatters is INFO but you can change this using --log-mach-level=<level>.

To turn on logging to the console use --console-level=<level>.

For example, to log test run output with the default (tbpl) formatter to the file ~/mochitest.log at DEBUG level detail you would use:

./mach mochitest --log-tbpl=~/mochitest.log --log-tbpl-level=DEBUG

Headless mode

The tests must run in a focused window, which effectively prevents any other user activity on the engaged computer. You can avoid this by using the --headless argument or MOZ_HEADLESS=1 environment variable.

./mach mochitest --headless ...

Writing tests

A Mochitest plain test is simply an HTML or XHTML file that contains some JavaScript to test for some condition.

Asynchronous Tests

Sometimes tests involve asynchronous patterns, such as waiting for events or observers. In these cases, you need to use add_task:

add_task(async function my_test() {
  let keypress = new Promise(...);
  // .. simulate keypress
  await keypress;
  // .. run test

Use add_setup() when asynchronous test task is meant to prepare test for run. All setup tasks are executed once in order they appear prior to any test tasks.

add_setup(async () => {
  await clearStorage();

Or alternatively, manually call waitForExplicitFinish and finish:

addEventListener("keypress", function() {
  // ... run test ...
}, false);
// ... simulate key press ...

If you need more time, requestLongerTimeout(number) can be quite useful. requestLongerTimeout() takes an integer factor that is a multiplier for the default 45 seconds timeout. So a factor of 2 means: “Wait for at last 90s (2*45s)”. This is really useful if you want to pause execution to do a little debugging.

Test functions

Each test must contain some JavaScript that will run and tell Mochitest whether the test has passed or failed. SimpleTest.js provides a number of functions for the test to use, to communicate the results back to Mochitest. These include:

  • ok(expressionThatShouldBeTrue, "Description of the check") – tests a value for its truthfulness

  • is(actualValue, expectedValue, "Description of the check") – compares two values (using

  • isnot(actualValue, unexpectedValue, "Description of the check") – opposite of is()

If you want to include a test for something that currently fails, don’t just comment it out! Instead, use one of the “todo” equivalents so we notice if it suddenly starts passing (at which point the test can be re-enabled):

  • todo(falseButShouldBeTrue, "Description of the check")

  • todo_is(actualValue, expectedValue, "Description of the check")

  • todo_isnot(actualValue, unexpectedValue, "Description of the check")

Tests can call a function info("Message string") to write a message to the test log.

In addition to mochitest assertions, mochitest supports the CommonJS standard assertions, like nodejs’ assert module but implemented in Assert.sys.mjs. These are auto-imported in the browser flavor, but need to be imported manually in other flavors.

Helper functions

Right now, useful helpers derived from MochiKit are available in testing/mochitest/tests/SimpleTest/SimpleTest.js.

Although all of Mochikit is available at testing/mochitest/MochiKit, only include files that you require to minimize test load times. Bug 367569 added sendChar, sendKey, and sendString helpers. These are available in testing/mochitest/tests/SimpleTest/EventUtils.js.

If you need to access some data files from your Mochitest, you can get an URI for them by using SimpleTest.getTestFileURL("relative/path/to/data.file"). Then you can eventually fetch their content by using XMLHttpRequest or so.

Adding tests to the tree

mach addtest is the preferred way to add a test to the tree:

./mach addtest --suite mochitest-{plain,chrome,browser-chrome} path/to/new/test

That will add the manifest entry to the relevant manifest (mochitest.ini, chrome.ini, etc. depending on the flavor) to tell the build system about your new test, as well as creating the file based on a template.


Optionally, you can specify metadata for your test, like whether to skip the test on certain platforms:

skip-if = os == 'win'

The mochitest.ini format, which is recognized by the parser, defines a long list of metadata.

Adding a new mochitest.ini or chrome.ini file

If a mochitest.ini or chrome.ini file does not exist in the test directory where you want to add a test, add them and update the file in the directory for your test. For example, in gfx/layers/, we add these two manifest files:

MOCHITEST_MANIFESTS += ['apz/test/mochitest.ini']
MOCHITEST_CHROME_MANIFESTS += ['apz/test/chrome.ini']


See the Mochitest FAQ page for other features and such that you may want to use, such as SSL-enabled tests, custom http headers, async tests, leak debugging, prefs…