Frontend testing standards and style guidelines

There are two types of test suites you’ll encounter while developing frontend code at GitLab. We use Karma and Jasmine for JavaScript unit and integration testing, and RSpec feature tests with Capybara for e2e (end-to-end) integration testing.

Unit and feature tests need to be written for all new features. Most of the time, you should use RSpec for your feature tests.

Regression tests should be written for bug fixes to prevent them from recurring in the future.

See the Testing Standards and Style Guidelines page for more information on general testing practices at GitLab.

Jest

We have started to migrate frontend tests to the Jest testing framework (see also the corresponding epic).

Jest tests can be found in /spec/frontend and /ee/spec/frontend in EE.

It is not yet a requirement to use Jest. You can view the epic of issues we need to solve before being able to use Jest for all our needs.

Differences to Karma

  • Jest runs in a Node.js environment, not in a browser. Support for running Jest tests in a browser is planned.
  • Because Jest runs in a Node.js environment, it uses jsdom by default. See also its limitations below.
  • Jest does not have access to Webpack loaders or aliases. The aliases used by Jest are defined in its own config.
  • All calls to setTimeout and setInterval are mocked away. See also Jest Timer Mocks.
  • rewire is not required because Jest supports mocking modules. See also Manual Mocks.
  • No context object is passed to tests in Jest. This means sharing this.something between beforeEach() and it() for example does not work. Instead you should declare shared variables in the context that they are needed (via const / let).
  • The following will cause tests to fail in Jest:
    • Unmocked requests.
    • Unhandled Promise rejections.
    • Calls to console.warn, including warnings from libraries like Vue.

Limitations of jsdom

As mentioned above, Jest uses jsdom instead of a browser for running tests. This comes with a number of limitations, namely:

See also the issue for support running Jest tests in browsers.

Debugging Jest tests

Running yarn jest-debug will run Jest in debug mode, allowing you to debug/inspect as described in the Jest docs.

Timeout error

The default timeout for Jest is set in /spec/frontend/test_setup.js.

If your test exceeds that time, it will fail.

If you cannot improve the performance of the tests, you can increase the timeout for a specific test using setTestTimeout.

import { setTestTimeout } from 'helpers/timeout';

describe('Component', () => {
  it('does something amazing', () => {
    setTestTimeout(500);
    // ...
  });
});

Remember that the performance of each test depends on the environment.

Karma test suite

GitLab uses the Karma test runner with Jasmine as its test framework for our JavaScript unit and integration tests.

JavaScript tests live in spec/javascripts/, matching the folder structure of app/assets/javascripts/: app/assets/javascripts/behaviors/autosize.js has a corresponding spec/javascripts/behaviors/autosize_spec.js file.

Keep in mind that in a CI environment, these tests are run in a headless browser and you will not have access to certain APIs, such as Notification, which will have to be stubbed.

Best practices

Naming unit tests

When writing describe test blocks to test specific functions/methods, please use the method name as the describe block name.

// Good
describe('methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

// Bad
describe('#methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

// Bad
describe('.methodName', () => {
  it('passes', () => {
    expect(true).toEqual(true);
  });
});

Testing promises

When testing Promises you should always make sure that the test is asynchronous and rejections are handled. Your Promise chain should therefore end with a call of the done callback and done.fail in case an error occurred.

// Good
it('tests a promise', done => {
  promise
    .then(data => {
      expect(data).toBe(asExpected);
    })
    .then(done)
    .catch(done.fail);
});

// Good
it('tests a promise rejection', done => {
  promise
    .then(done.fail)
    .catch(error => {
      expect(error).toBe(expectedError);
    })
    .then(done)
    .catch(done.fail);
});

// Bad (missing done callback)
it('tests a promise', () => {
  promise.then(data => {
    expect(data).toBe(asExpected);
  });
});

// Bad (missing catch)
it('tests a promise', done => {
  promise
    .then(data => {
      expect(data).toBe(asExpected);
    })
    .then(done);
});

// Bad (use done.fail in asynchronous tests)
it('tests a promise', done => {
  promise
    .then(data => {
      expect(data).toBe(asExpected);
    })
    .then(done)
    .catch(fail);
});

// Bad (missing catch)
it('tests a promise rejection', done => {
  promise
    .catch(error => {
      expect(error).toBe(expectedError);
    })
    .then(done);
});

Stubbing and Mocking

Jasmine provides useful helpers spyOn, spyOnProperty, jasmine.createSpy, and jasmine.createSpyObject to facilitate replacing methods with dummy placeholders, and recalling when they are called and the arguments that are passed to them. These tools should be used liberally, to test for expected behavior, to mock responses, and to block unwanted side effects (such as a method that would generate a network request or alter window.location). The documentation for these methods can be found in the Jasmine introduction page.

Sometimes you may need to spy on a method that is directly imported by another module. GitLab has a custom spyOnDependency method which utilizes babel-plugin-rewire to achieve this. It can be used like so:

// my_module.js
import { visitUrl } from '~/lib/utils/url_utility';

export default function doSomething() {
  visitUrl('/foo/bar');
}
// my_module_spec.js
import doSomething from '~/my_module';

describe('my_module', () => {
  it('does something', () => {
    const visitUrl = spyOnDependency(doSomething, 'visitUrl');

    doSomething();
    expect(visitUrl).toHaveBeenCalledWith('/foo/bar');
  });
});

Unlike spyOn, spyOnDependency expects its first parameter to be the default export of a module who’s import you want to stub, rather than an object which contains a method you wish to stub (if the module does not have a default export, one is be generated by the babel plugin). The second parameter is the name of the import you wish to change. The result of the function is a Spy object which can be treated like any other Jasmine spy object.

Further documentation on the babel rewire pluign API can be found on its repository Readme doc.

Waiting in tests

Sometimes a test needs to wait for something to happen in the application before it continues. Avoid using setTimeout because it makes the reason for waiting unclear and if passed a time larger than zero it will slow down our test suite. Instead use one of the following approaches.

Promises and Ajax calls

Register handler functions to wait for the Promise to be resolved.

const askTheServer = () => {
  return axios
    .get('/endpoint')
    .then(response => {
      // do something
    })
    .catch(error => {
      // do something else
    });
};

in Jest:

it('waits for an Ajax call', () => {
  return askTheServer().then(() => {
    expect(something).toBe('done');
  });
});

in Karma:

it('waits for an Ajax call', done => {
  askTheServer()
    .then(() => {
      expect(something).toBe('done');
    })
    .then(done)
    .catch(done.fail);
});

If you are not able to register handlers to the Promise—for example because it is executed in a synchronous Vue life cycle hook—you can flush all pending Promises:

in Jest:

it('waits for an Ajax call', () => {
  synchronousFunction();
  jest.runAllTicks();

  expect(something).toBe('done');
});

in Karma:

You are out of luck. The following only works sometimes and may lead to flaky failures:

it('waits for an Ajax call', done => {
  synchronousFunction();

  // create a new Promise and hope that it resolves after the rest
  Promise.resolve()
    .then(() => {
      expect(something).toBe('done');
    })
    .then(done)
    .catch(done.fail);
});
Vue rendering

To wait until a Vue component is re-rendered, use either of the equivalent Vue.nextTick() or vm.$nextTick().

in Jest:

it('renders something', () => {
  wrapper.setProps({ value: 'new value' });

  return wrapper.vm.$nextTick().then(() => {
    expect(wrapper.text()).toBe('new value');
  });
});

in Karma:

it('renders something', done => {
  wrapper.setProps({ value: 'new value' });

  wrapper.vm
    .$nextTick()
    .then(() => {
      expect(wrapper.text()).toBe('new value');
    })
    .then(done)
    .catch(done.fail);
});
setTimeout() / setInterval() in application

If the application itself is waiting for some time, mock await the waiting. In Jest this is already done by default (see also Jest Timer Mocks). In Karma you can use the Jasmine mock clock.

const doSomethingLater = () => {
  setTimeout(() => {
    // do something
  }, 4000);
};

in Jest:

it('does something', () => {
  doSomethingLater();
  jest.runAllTimers();

  expect(something).toBe('done');
});

in Karma:

it('does something', () => {
  jasmine.clock().install();

  doSomethingLater();
  jasmine.clock().tick(4000);

  expect(something).toBe('done');
  jasmine.clock().uninstall();
});
Events

If the application triggers an event that you need to wait for in your test, register an event handler which contains the assertions:

it('waits for an event', done => {
  eventHub.$once('someEvent', eventHandler);

  someFunction();

  function eventHandler() {
    expect(something).toBe('done');
    done();
  }
});

In Jest you can also use a Promise for this:

it('waits for an event', () => {
  const eventTriggered = new Promise(resolve => eventHub.$once('someEvent', resolve));

  someFunction();

  return eventTriggered.then(() => {
    expect(something).toBe('done');
  });
});

Migrating flaky Karma tests to Jest

Some of our Karma tests are flaky because they access the properties of a shared scope. This also means that they are not easily parallelized.

Migrating flaky Karma tests to Jest will help significantly as each test is executed in an isolated scope, improving performance and predictability.

Vue.js unit tests

See this section.

Running frontend tests

For running the frontend tests, you need the following commands:

  • rake karma:fixtures (re-)generates fixtures.
  • yarn test executes the tests.

As long as the fixtures don’t change, yarn test is sufficient (and saves you some time).

Live testing and focused testing

While developing locally, it may be helpful to keep Karma running so that you can get instant feedback on as you write tests and modify code. To do this you can start Karma with yarn run karma-start. It will compile the javascript assets and run a server at http://localhost:9876/ where it will automatically run the tests on any browser which connects to it. You can enter that url on multiple browsers at once to have it run the tests on each in parallel.

While Karma is running, any changes you make will instantly trigger a recompile and retest of the entire test suite, so you can see instantly if you’ve broken a test with your changes. You can use Jasmine focused or excluded tests (with fdescribe or xdescribe) to get Karma to run only the tests you want while you’re working on a specific feature, but make sure to remove these directives when you commit your code.

It is also possible to only run Karma on specific folders or files by filtering the run tests via the argument --filter-spec or short -f:

# Run all files
yarn karma-start
# Run specific spec files
yarn karma-start --filter-spec profile/account/components/update_username_spec.js
# Run specific spec folder
yarn karma-start --filter-spec profile/account/components/
# Run all specs which path contain vue_shared or vie
yarn karma-start -f vue_shared -f vue_mr_widget

You can also use glob syntax to match files. Remember to put quotes around the glob otherwise your shell may split it into multiple arguments:

# Run all specs named `file_spec` within the IDE subdirectory
yarn karma -f 'spec/javascripts/ide/**/file_spec.js'

RSpec feature integration tests

Information on setting up and running RSpec integration tests with Capybara can be found in the Testing Best Practices.

Frontend test fixtures

Code that is added to HAML templates (in app/views/) or makes Ajax requests to the backend has tests that require HTML or JSON from the backend. Fixtures for these tests are located at:

  • spec/javascripts/fixtures/, for running tests in CE.
  • ee/spec/javascripts/fixtures/, for running tests in EE.

Fixture files in:

  • The Karma test suite are served by jasmine-jquery.
  • Jest use spec/frontend/helpers/fixtures.js.

The following are examples of tests that work for both Karma and Jest:

it('makes a request', () => {
  const responseBody = getJSONFixture('some/fixture.json'); // loads spec/javascripts/fixtures/some/fixture.json
  axiosMock.onGet(endpoint).reply(200, responseBody);
  
  myButton.click();
  
  // ...
});

it('uses some HTML element', () => {
  loadFixtures('some/page.html'); // loads spec/javascripts/fixtures/some/page.html and adds it to the DOM
  
  const element = document.getElementById('#my-id');
  
  // ...
});

HTML and JSON fixtures are generated from backend views and controllers using RSpec (see spec/javascripts/fixtures/*.rb).

For each fixture, the content of the response variable is stored in the output file. This variable gets automagically set if the test is marked as type: :request or type: :controller. Fixtures are regenerated using the bin/rake karma:fixtures command but you can also generate them individually, for example bin/rspec spec/javascripts/fixtures/merge_requests.rb. When creating a new fixture, it often makes sense to take a look at the corresponding tests for the endpoint in (ee/)spec/controllers/ or (ee/)spec/requests/.

Gotchas

RSpec errors due to JavaScript

By default RSpec unit tests will not run JavaScript in the headless browser and will simply rely on inspecting the HTML generated by rails.

If an integration test depends on JavaScript to run correctly, you need to make sure the spec is configured to enable JavaScript when the tests are run. If you don’t do this you’ll see vague error messages from the spec runner.

To enable a JavaScript driver in an rspec test, add :js to the individual spec or the context block containing multiple specs that need JavaScript enabled:

# For one spec
it 'presents information about abuse report', :js do
  # assertions...
end

describe "Admin::AbuseReports", :js do
  it 'presents information about abuse report' do
    # assertions...
  end
  it 'shows buttons for adding to abuse report' do
    # assertions...
  end
end

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