Testing WebUI with Mocha

As Chromium focuses on building modular WebUI using web components and well-defined API calls, we aim to phase out and ultimately replace WebUIBrowserTest with a mocha-based alternative. Mocha is a flexible, lightweight test framework with strong async support. With mocha, we can write small unit tests for individual features of components instead of grouping everything into a monolithic browser test.

Advantages of using mocha over vanilla browser tests include:
  • Allowing multiple test definitions within one browser test, amortizing the browser spin-up cost over several tests
  • Pinpointing failure locations, even when several tests fail within one browser test or when assertions fail inside in helper functions
  • Associating uncaught exceptions with the correct test
  • Reporting test durations and flagging slow tests
  • Fast test iteration -- changes to tests may not require recompiling
This framework is a work in progress; its first consumers are Polymer projects in Chromium.

Adding the BrowserTest

Polymer tests should use PolymerTest from polymer_browser_test_base.js as their prototype. The following test will set the command-line args --my-switch=my-value and navigate to chrome://my-webui/.

/** @const {string} Path to source root. */
var ROOT_PATH = '../../../../../';

// Polymer BrowserTest fixture.
    [ROOT_PATH + 'chrome/test/data/webui/polymer_browser_test_base.js']);

 * Test fixture for FooBar element.
 * @constructor
 * @extends {PolymerTest}
function FooBarBrowserTest() {}

FooBarBrowserTest.prototype = {
  __proto__: PolymerTest.prototype,

   * Override browsePreload like this if you need to navigate to a particular WebUI.
   * @override
  browsePreload: 'chrome://my-webui',

   * Set any command line switches needed by your features.
  commandLineSwitches: [{
    switchName: 'my-switch',
    switchValue: 'my-value'  // omit switchValue for boolean switches

   * Set extra libraries relative to source root. PolymerTest.getLibraries(ROOT_PATH) is required even if you aren't importing other files.
   * @override
  extraLibraries: PolymerTest.getLibraries(ROOT_PATH).concat([
    'some_file.js',  // Loads ./some_file.js (relative to source root).
    ROOT_PATH + 'ui/webui/resources/js/some_file.js',  // Path must be included in browser_tests.isolate.

Then, create one or more TEST_F functions. Each should register some number of tests with mocha:

TEST_F('FooBarBrowserTest', 'FooBarTest', function() {
  suite('FooBar', function() {
    var fooBar;

    // Import foo_bar.html before running suite.
    suiteSetup(function() {
      // Returns a promise, ensuring mocha waits until the resource loads.
      return PolymerTest.importHtml('chrome://resources/foo_bar.html');

    // Create and initialize a FooBar before each test.
    setup(function() {
      fooBar = document.createElement('foo-bar');  // Immediately calls created() and ready().
      document.body.appendChild(fooBar);  // Immediately calls attached().

    test('is a FooBar', function() {
      assertEquals('FOO-BAR', fooBar.tagName);
      // ... more work and assertions

    // ... more synchronous and asynchronous tests

  // ... more test suites

  // Run all the above tests.

Should you write a large number of tests (woohoo!), it may help to break tests into separate files. For example, if you have foo_bar_tests.js:

cr.define('foo_bar', function() {
  function registerTests() {
    suite('FooBar', function() { /* ... */ });

  return {registerTests: registerTests};

You can then include these tests in your main foo_bar_browsertest.js by adding the file to FooBarBrowserTest.prototype.extraLibraries. Then, in your TEST_F, simply call foo_bar.registerTests() before the mocha.run() call.

Including JS files in tests

Files imported via extraLibraries are read when the test runs. In isolated testing, only certain files are copied to the bots. Files in chrome/test/data are always copied over, but if your test requires files in other directories, be sure the file or some parent directory is included in chrome/browser_tests.isolate.

Explanation: js2gtest allows #include-style imports of JavaScript using GEN_INCLUDE and the extraLibraries property of the test fixture prototype. These should be relative to the file defining your test fixture.

Use GEN_INCLUDE when the file needs to be included before the rest of the script is run. This will also result in the code being eval'd during the js2gtest step.

Use the extraLibraries property of your test fixture's prototype when the file is only needed within your tests. The content of these files, along with any GEN_INCLUDEd files, are added to each GTest body before each test is run.

Because these files are read at runtime, they must be present when running the browser_tests binary. This is only a concern in isolated testing, and placing included files in chrome/test/data should always work.

Running the tests

Just build and run like any other browser_tests. You may filter tests by class or test name (the arguments to TEST_F) via gtest_filter.

ninja -C out/Release browser_tests
./out/Release/browser_tests --gtest_filter=FooBarBrowserTest.Foo*

Tests and suites

With mocha, we can define any number of suites; each suite can consist of any number of tests. Top-level tests are in the global suite, but it is recommended to always create an outer suite for your tests. Each suite and test call takes a name and a function.

We can also define a suiteSetup to run at the beginning of a suite, a setup to run before each test in a suite, a teardown to run after each test in a suite, and a suiteTeardown to run once a suite is completed.

Suites can be nested; each setup and teardown also applies to all tests in nested suites.

By default, hooks and tests are synchronous. Any of the functions used in these calls can take a done callback as a parameter, causing mocha to wait until the done parameter is called before moving to the next test or hook. The done callback must be called with an Error on failure, or no arguments on success.

suite('Outer suite', function() {
  suiteSetup(function() {
    // Function to run at the beginning of this suite.

  setup(function() {
    // Function to run before each test within this suite.

  test(function() {
    // A test that runs after setup is called.

  test(function(done) {
    // An asynchronous test that runs after setup is called again.
    setTimeout(done, 1000);

  teardown(function() {
    // Function to run after each test in this suite.

  suiteTeardown(function() {
    // Function to run at the very end of this suite.

  suite('Inner suite', function() {
    // Another suite. We can define separate suiteSetup and setup functions here that only run within this suite.

Instead of taking a done parameter, an asynchronous hook or test can return a Promise. The test succeeds or fails once the promise is resolved or rejected.

// Notice no done callback is taken.
suiteSetup(function() {
  return new Promise(function(resolve, reject) {
    window.addEventListener('thingCreated', resolve);

test('Something doesn\'t happen', function() {
  return new Promise(function(resolve, reject) {
    // Ensure no event fires.
    window.addEventListener('change', reject);
    setTimeout(resolve, 1000);

More features

For more mocha features, see the documentation. The API is well documented within the source.