# 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 with Jasmine and Jest 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](index.md) page for more
information on general testing practices at GitLab.

## Vue.js testing

If you are looking for a guide on Vue component testing, you can jump right away to this [section][vue-test].

## Jest

We have started to migrate frontend tests to the [Jest](https://jestjs.io) testing framework (see also the corresponding
[epic](https://gitlab.com/groups/gitlab-org/-/epics/895)).

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

> **Note:**
>
> Most examples have a Jest and Karma example. See the Karma examples only as explanation to what's going on in the code, should you stumble over some usescases during your discovery. The Jest examples are the one you should follow.

## Karma test suite

While GitLab is switching over to [Jest][jest] you'll still find Karma tests in our application. [Karma][karma] is a test runner which uses [Jasmine] as its test
framework. Jest also uses Jasmine as foundation, that's why it's looking quite similar.

Karma tests live in `spec/javascripts/` and `/ee/spec/javascripts` in EE.

`app/assets/javascripts/behaviors/autosize.js`
might have 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`](https://developer.mozilla.org/en-US/docs/Web/API/notification),
which have to be stubbed.

### When should I use Jest over Karma?

If you need to update an existing Karma test file (found in `spec/javascripts`), you do not
need to migrate the whole spec to Jest. Simply updating the Karma spec to test your change
is fine. It is probably more appropriate to migrate to Jest in a separate merge request.

If you create a new test file, it needs to be created in Jest. This will

help support our migration and we think you'll love using Jest.

As always, please use discretion. Jest solves a lot of issues we experienced in Karma and
provides a better developer experience, however there are potentially unexpected issues
which could arise (especially with testing against browser specific features).

### Differences to Karma

- Jest runs in a Node.js environment, not in a browser. Support for running Jest tests in a browser [is planned](https://gitlab.com/gitlab-org/gitlab/-/issues/26982).

- Because Jest runs in a Node.js environment, it uses [jsdom](https://github.com/jsdom/jsdom) by default. See also its [limitations](#limitations-of-jsdom) below.

- Jest does not have access to Webpack loaders or aliases.

  The aliases used by Jest are defined in its [own config](https://gitlab.com/gitlab-org/gitlab/blob/master/jest.config.js).

- All calls to `setTimeout` and `setInterval` are mocked away. See also [Jest Timer Mocks](https://jestjs.io/docs/en/timer-mocks).
- `rewire` is not required because Jest supports mocking modules. See also [Manual Mocks](https://jestjs.io/docs/en/manual-mocks).

- No [context object](https://jasmine.github.io/tutorials/your_first_suite#section-The_%3Ccode%3Ethis%3C/code%3E_keyword) 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](#differences-to-karma), Jest uses jsdom instead of a browser for running tests.
This comes with a number of limitations, namely:

- [No scrolling support](https://github.com/jsdom/jsdom/blob/15.1.1/lib/jsdom/browser/Window.js#L623-L625)
- [No element sizes or positions](https://github.com/jsdom/jsdom/blob/15.1.1/lib/jsdom/living/nodes/Element-impl.js#L334-L371)
- [No layout engine](https://github.com/jsdom/jsdom/issues/1322) in general

See also the issue for [support running Jest tests in browsers](https://gitlab.com/gitlab-org/gitlab/-/issues/26982).

### Debugging Jest tests

Running `yarn jest-debug` will run Jest in debug mode, allowing you to debug/inspect as described in the [Jest docs](https://jestjs.io/docs/en/troubleshooting#tests-are-failing-and-you-don-t-know-why).

### Timeout error

The default timeout for Jest is set in

[`/spec/frontend/test_setup.js`](https://gitlab.com/gitlab-org/gitlab/blob/master/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`](https://gitlab.com/gitlab-org/gitlab/blob/master/spec/frontend/helpers/timeout.js).

```javascript

import { setTestTimeout } from 'helpers/timeout';

describe('Component', () => {

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

});
```

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

## What and how to test

Before jumping into more gritty details about Jest-specific workflows like mocks and spies, we should briefly cover what to test with Jest.

### Don't test the library

Libraries are an integral part of any JavaScript developer's life. The general advice would be to not test library internals, but expect that the library knows what it's supposed to do and has test coverage on its own.
A general example could be something like this

```javascript
import { convertToFahrenheit } from 'temperatureLibrary'

function getFahrenheit(celsius) {
  return convertToFahrenheit(celsius)
}
```

It does not make sense to test our `getFahrenheit` function because underneath it does nothing else but invoking the library function, and we can expect that one is working as intended. (Simplified, I know)

Let's take a short look into Vue land. Vue is a critical part of the GitLab JavaScript codebase. When writing specs for Vue components, a common gotcha is to actually end up testing Vue provided functionality, because it appears to be the easiest thing to test. Here's an example taken from our codebase.

```javascript
// Component
{
  computed: {
    hasMetricTypes() {
      return this.metricTypes.length;
    },
}
```

and here's the corresponding spec

```javascript
 describe('computed', () => {
    describe('hasMetricTypes', () => {
      it('returns true if metricTypes exist', () => {
        factory({ metricTypes });
        expect(wrapper.vm.hasMetricTypes).toBe(2);
      });

      it('returns true if no metricTypes exist', () => {
        factory();
        expect(wrapper.vm.hasMetricTypes).toBe(0);
      });
    });
});
```

Testing the `hasMetricTypes` computed prop would seem like a given, but to test if the computed property is returning the length of `metricTypes`, is testing the Vue library itself. There is no value in this, besides it adding to the test suite. Better is to test it in the way the user interacts with it. Probably through the template.

Keep an eye out for these kinds of tests, as they just make updating logic more fragile and tedious than it needs to be. This is also true for other libraries.

Some more examples can be found in the [Frontend unit tests section](testing_levels.md#frontend-unit-tests)

### Don't test your mock

Another common gotcha is that the specs end up verifying the mock is working. If you are using mocks, the mock should support the test, but not be the target of the test.

**Bad:**

```javascript

const spy = jest.spyOn(idGenerator, 'create')
spy.mockImplementation = () = '1234'

expect(idGenerator.create()).toBe('1234')
```

**Good:**

```javascript
const spy = jest.spyOn(idGenerator, 'create')
spy.mockImplementation = () = '1234'

// Actually focusing on the logic of your component and just leverage the controllable mocks output
expect(wrapper.find('div').html()).toBe('<div id="1234">...</div>')

```

### Follow the user

The line between unit and integration tests can be quite blurry in a component heavy world. The most important guideline to give is the following:

- Write clean unit tests if there is actual value in testing a complex piece of logic in isolation to prevent it from breaking in the future
- Otherwise, try to write your specs as close to the user's flow as possible

For example, it's better to use the generated markup to trigger a button click and validate the markup changed accordingly than to call a method manually and verify data structures or computed properties. There's always the chance of accidentally breaking the user flow, while the tests pass and provide a false sense of security.

## Common practices

Following you'll find some general common practices you will find as part of our testsuite. Should you stumble over something not following this guide, ideally fix it right away. 🎉

### How to query DOM elements

When it comes to querying DOM elements in your tests, it is best to uniquely target the element, without adding additional attributes specifically for testing purposes. Sometimes this cannot be done feasibly. In these cases, adding test attributes to simplify the selectors might be the best option.

Preferentially, in component testing with `@vue/test-utils`, you should query for child components using the component itself. Otherwise, try to use an existing attribute like `name` or a Vue `ref` (if using `@vue/test-utils`):

```javascript
it('exists', () => {
    wrapper.find(FooComponent);
    wrapper.find('input[name=foo]');
    wrapper.find({ ref: 'foo'});
    wrapper.find('.js-foo');
});
```

It is not recommended that you add `.js-*` classes just for testing purposes. Only do this if there are no other feasible options available.

Do not use a `.qa-*` class or `data-qa-selector` attribute for any tests other than QA end-to-end testing.

### Naming unit tests

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

```javascript
// 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. It's now possible to use the `async/await` syntax in the test suite:

```javascript
it('tests a promise', async () => {
  const users = await fetchUsers()
  expect(users.length).toBe(42)
});

it('tests a promise rejection', async () => {
  expect.assertions(1);
  try {
    await user.getUserName(1);
  } catch (e) {
    expect(e).toEqual({
      error: 'User with 1 not found.',
    });
  }
});
```

You can also work with Promise chains. In this case, you can make use of the `done` callback and `done.fail` in case an error occurred. Following are some examples:

```javascript
// 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);

});
```

### Manipulating Time

Sometimes we have to test time-sensitive code. For example, recurring events that run every X amount of seconds or similar. Here you'll find some strategies to deal with that:

#### `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](https://gitlab.com/gitlab-org/gitlab/blob/a2128edfee799e49a8732bfa235e2c5e14949c68/jest.config.js#L47)
(see also [Jest Timer Mocks](https://jestjs.io/docs/en/timer-mocks)). In Karma you can use the
[Jasmine mock clock](https://jasmine.github.io/api/2.9/Clock.html).

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

```

**in Jest:**

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

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

});
```

**in Karma:**

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

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

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

### Waiting in tests

Sometimes a test needs to wait for something to happen in the application before it continues.
Avoid using [`setTimeout`](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout)

because it makes the reason for waiting unclear and if used within Karma with 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.

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

**in Jest:**

```javascript

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

});
```

**in Karma:**

```javascript
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, please take a look at the [waitFor](#wait-until-axios-requests-finish) helpers or you can flush all pending `Promise`s:

**in Jest:**

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

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

#### Vue rendering

To wait until a Vue component is re-rendered, use either of the equivalent
[`Vue.nextTick()`](https://vuejs.org/v2/api/#Vue-nextTick) or `vm.$nextTick()`.

**in Jest:**

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

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

**in Karma:**

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

  wrapper.vm
    .$nextTick()
    .then(() => {
      expect(wrapper.text()).toBe('new value');
    })
    .then(done)
    .catch(done.fail);
});
```

#### Events

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

```javascript
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:

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

  someFunction();

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

### Ensuring that tests are isolated

Tests are normally architected in a pattern which requires a recurring setup and breakdown of the component under test. This is done by making use of the `beforeEach` and `afterEach` hooks.

Example

```javascript
  let wrapper;

  beforeEach(() => {
    wrapper = mount(Component);
  });

  afterEach(() => {
    wrapper.destroy();
  });
```

When looking at this initially you'd suspect that the component is setup before each test and then broken down afterwards, providing isolation between tests.

This is however not entirely true as the `destroy` method does not remove everything which has been mutated on the `wrapper` object. For functional components, destroy only removes the rendered DOM elements from the document.

In order to ensure that a clean wrapper object and DOM are being used in each test, the breakdown of the component should rather be performed as follows:

```javascript
  afterEach(() => {
    wrapper.destroy();
    wrapper = null;
  });
```

See also the [Vue Test Utils documentation on `destroy`](https://vue-test-utils.vuejs.org/api/wrapper/#destroy).

## Factories

TBU

## Mocking Strategies with Jest

### Stubbing and Mocking

Jasmine provides stubbing and mocking capabilities. There are some subtle differences in how to use it within Karma and Jest.

Stubs or spies are often used synonymously. In Jest it's quite easy thanks to the `.spyOn` method. [Official docs][jestspy]
The more challenging part are mocks, which can be used for functions or even dependencies.

### Manual module mocks

Jest supports [manual module mocks](https://jestjs.io/docs/en/manual-mocks) by placing a mock in a `__mocks__/` directory next to the source module. **Don't do this.** We want to keep all of our test-related code in one place (the `spec/` folder), and the logic that Jest uses to apply mocks from `__mocks__/` is rather inconsistent.

Instead, our test runner detects manual mocks from `spec/frontend/mocks/`. Any mock placed here is automatically picked up and injected whenever you import its source module.

- Files in `spec/frontend/mocks/ce` will mock the corresponding CE module from `app/assets/javascripts`, mirroring the source module's path.
  - Example: `spec/frontend/mocks/ce/lib/utils/axios_utils` will mock the module `~/lib/utils/axios_utils`.
- Files in `spec/frontend/mocks/node` will mock NPM packages of the same name or path.
- We don't support mocking EE modules yet.

If a mock is found for which a source module doesn't exist, the test suite will fail. 'Virtual' mocks, or mocks that don't have a 1-to-1 association with a source module, are not supported yet.

### Writing a mock

Create a JS module in the appropriate place in `spec/frontend/mocks/`. That's it. It will automatically mock its source package in all tests.

Make sure that your mock's export has the same format as the mocked module. So, if you're mocking a CommonJS module, you'll need to use `module.exports` instead of the ES6 `export`.

It might be useful for a mock to expose a property that indicates if the mock was loaded. This way, tests can assert the presence of a mock without calling any logic and causing side-effects. The `~/lib/utils/axios_utils` module mock has such a property, `isMock`, that is `true` in the mock and undefined in the original class. Jest's mock functions also have a `mock` property that you can test.

### Bypassing mocks

If you ever need to import the original module in your tests, use [`jest.requireActual()`](https://jestjs.io/docs/en/jest-object#jestrequireactualmodulename) (or `jest.requireActual().default` for the default export). The `jest.mock()` and `jest.unmock()` won't have an effect on modules that have a manual mock, because mocks are imported and cached before any tests are run.

### Keep mocks light

Global mocks introduce magic and can affect how modules are imported in your tests. Try to keep them as light as possible and dependency-free. A global mock should be useful for any unit test. For example, the `axios_utils` and `jquery` module mocks throw an error when an HTTP request is attempted, since this is useful behaviour in >99% of tests.

When in doubt, construct mocks in your test file using [`jest.mock()`](https://jestjs.io/docs/en/jest-object#jestmockmodulename-factory-options), [`jest.spyOn()`](https://jestjs.io/docs/en/jest-object#jestspyonobject-methodname), etc.

## Running Frontend Tests

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

- `rake frontend:fixtures` (re-)generates [fixtures](#frontend-test-fixtures).

- `yarn test` executes the tests.

- `yarn jest` executes only the Jest tests.

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

### Live testing and focused testing -- Jest

While you work on a testsuite, you may want to run these specs in watch mode, so they rerun automatically on every save.

```shell

# Watch and rerun all specs matching the name icon
yarn jest --watch icon

# Watch and rerun one specifc file
yarn jest --watch path/to/spec/file.spec.js
```

You can also run some focused tests without the `--watch` flag

```shell

# Run specific jest file
yarn jest ./path/to/local_spec.js
# Run specific jest folder
yarn jest ./path/to/folder/
# Run all jest files which path contain term
yarn jest term
```

### Live testing and focused testing -- Karma

Karma allows something similar, but it's way more costly.

Running Karma with `yarn run karma-start` 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][jasmine-focus] 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`:

```shell

# 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:

```shell

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

## 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/frontend/fixtures/`, for running tests in CE.
- `ee/spec/frontend/fixtures/`, for running tests in EE.

Fixture files in:

- The Karma test suite are served by [jasmine-jquery](https://github.com/velesin/jasmine-jquery).
- Jest use `spec/frontend/helpers/fixtures.js`.

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

```javascript
it('makes a request', () => {

  const responseBody = getJSONFixture('some/fixture.json'); // loads spec/frontend/fixtures/some/fixture.json

  axiosMock.onGet(endpoint).reply(200, responseBody);

  myButton.click();

  // ...
});

it('uses some HTML element', () => {

  loadFixtures('some/page.html'); // loads spec/frontend/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/frontend/fixtures/*.rb`).

For each fixture, the content of the `response` variable is stored in the output file.

This variable gets automatically set if the test is marked as `type: :request` or `type: :controller`.

Fixtures are regenerated using the `bin/rake frontend:fixtures` command but you can also generate them individually,

for example `bin/rspec spec/frontend/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/`.

## Data-driven tests

Similar to [RSpec's parameterized tests](best_practices.md#table-based--parameterized-tests),
Jest supports data-driven tests for:

- Individual tests using [`test.each`](https://jestjs.io/docs/en/api#testeachtable-name-fn-timeout) (aliased to `it.each`).
- Groups of tests using [`describe.each`](https://jestjs.io/docs/en/api#describeeachtable-name-fn-timeout).

These can be useful for reducing repetition within tests. Each option can take an array of
data values or a tagged template literal.

For example:

```javascript
// function to test
const icon = status => status ? 'pipeline-passed' : 'pipeline-failed'
const message = status => status ? 'pipeline-passed' : 'pipeline-failed'

// test with array block
it.each([
    [false, 'pipeline-failed'],
    [true, 'pipeline-passed']
])('icon with %s will return %s',
 (status, icon) => {
    expect(renderPipeline(status)).toEqual(icon)
 }
);
```

```javascript
// test suite with tagged template literal block
describe.each`
    status   | icon                 | message
    ${false} | ${'pipeline-failed'} | ${'Pipeline failed - boo-urns'}
    ${true}  | ${'pipeline-passed'} | ${'Pipeline succeeded - win!'}
`('pipeline component', ({ status, icon, message }) => {
    it(`returns icon ${icon} with status ${status}`, () => {
        expect(icon(status)).toEqual(message)
    })

    it(`returns message ${message} with status ${status}`, () => {
        expect(message(status)).toEqual(message)
    })
});
```