1248 lines
45 KiB
Markdown
1248 lines
45 KiB
Markdown
---
|
||
stage: none
|
||
group: unassigned
|
||
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/product/ux/technical-writing/#assignments
|
||
---
|
||
|
||
# Frontend testing standards and style guidelines
|
||
|
||
There are two types of test suites encountered while developing frontend code
|
||
at GitLab. We use 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](https://github.com/rspec/rspec-rails#feature-specs) 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](../fe_guide/vue.md#testing-vue-components).
|
||
|
||
## Jest
|
||
|
||
We use Jest to write frontend unit and integration tests.
|
||
Jest tests can be found in `/spec/frontend` and `/ee/spec/frontend` in EE.
|
||
|
||
### Limitations of jsdom
|
||
|
||
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` runs Jest in debug mode, allowing you to debug/inspect as described in the [Jest docs](https://jestjs.io/docs/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 fails.
|
||
|
||
If you cannot improve the performance of the tests, you can increase the timeout
|
||
for a specific test using [`jest.setTimeout`](https://jestjs.io/docs/27.x/jest-object#jestsettimeouttimeout)
|
||
|
||
```javascript
|
||
describe('Component', () => {
|
||
it('does something amazing', () => {
|
||
jest.setTimeout(500);
|
||
// ...
|
||
});
|
||
});
|
||
```
|
||
|
||
Remember that the performance of each test depends on the environment.
|
||
|
||
### Test-specific stylesheets
|
||
|
||
To help facilitate RSpec integration tests we have two test-specific stylesheets. These can be used to do things like disable animations to improve test speed, or to make elements visible when they need to be targeted by Capybara click events:
|
||
|
||
- `app/assets/stylesheets/disable_animations.scss`
|
||
- `app/assets/stylesheets/test_environment.scss`
|
||
|
||
Because the test environment should match the production environment as much as possible, use these minimally and only add to them when necessary.
|
||
|
||
## 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 script
|
||
{
|
||
computed: {
|
||
hasMetricTypes() {
|
||
return this.metricTypes.length;
|
||
},
|
||
}
|
||
```
|
||
|
||
```html
|
||
<!-- Component template -->
|
||
<template>
|
||
<gl-dropdown v-if="hasMetricTypes">
|
||
<!-- Dropdown content -->
|
||
</gl-dropdown>
|
||
</template>
|
||
```
|
||
|
||
Testing the `hasMetricTypes` computed prop would seem like a given here. 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. It's better to test a component in the way the user interacts with it: checking the rendered template.
|
||
|
||
```javascript
|
||
// Bad
|
||
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);
|
||
});
|
||
});
|
||
});
|
||
|
||
// Good
|
||
it('displays a dropdown if metricTypes exist', () => {
|
||
factory({ metricTypes });
|
||
expect(wrapper.findComponent(GlDropdown).exists()).toBe(true);
|
||
});
|
||
|
||
it('does not display a dropdown if no metricTypes exist', () => {
|
||
factory();
|
||
expect(wrapper.findComponent(GlDropdown).exists()).toBe(false);
|
||
});
|
||
```
|
||
|
||
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. A suggestion here is: if you are checking a `wrapper.vm` property, you should probably stop and rethink the test to check the rendered template instead.
|
||
|
||
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.
|
||
|
||
```javascript
|
||
const spy = jest.spyOn(idGenerator, 'create')
|
||
spy.mockImplementation = () = '1234'
|
||
|
||
// Bad
|
||
expect(idGenerator.create()).toBe('1234')
|
||
|
||
// Good: 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
|
||
|
||
These some general common practices included as part of our test suite. 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 and semantically target
|
||
the element.
|
||
|
||
Preferentially, this is done by targeting what the user actually sees using [DOM Testing Library](https://testing-library.com/docs/dom-testing-library/intro/).
|
||
When selecting by text it is best to use the [`byRole`](https://testing-library.com/docs/queries/byrole/) query
|
||
as it helps enforce accessibility best practices. `findByRole` and the other [DOM Testing Library queries](https://testing-library.com/docs/queries/about/) are available when using [`shallowMountExtended` or `mountExtended`](#shallowmountextended-and-mountextended).
|
||
|
||
When writing Vue component unit tests, it can be wise to query children by component, so that the unit test can focus on comprehensive value coverage
|
||
rather than dealing with the complexity of a child component's behavior.
|
||
|
||
Sometimes, neither of the above are feasible. In these cases, adding test attributes to simplify the selectors might be the best option. A list of
|
||
possible selectors include:
|
||
|
||
- A semantic attribute like `name` (also verifies that `name` was setup properly)
|
||
- A `data-testid` attribute ([recommended by maintainers of `@vue/test-utils`](https://github.com/vuejs/vue-test-utils/issues/1498#issuecomment-610133465))
|
||
optionally combined with [`shallowMountExtended` or `mountExtended`](#shallowmountextended-and-mountextended)
|
||
- a Vue `ref` (if using `@vue/test-utils`)
|
||
|
||
```javascript
|
||
import { shallowMountExtended } from 'helpers/vue_test_utils_helper'
|
||
|
||
const wrapper = shallowMountExtended(ExampleComponent);
|
||
|
||
it('exists', () => {
|
||
// Best (especially for integration tests)
|
||
wrapper.findByRole('link', { name: /Click Me/i })
|
||
wrapper.findByRole('link', { name: 'Click Me' })
|
||
wrapper.findByText('Click Me')
|
||
wrapper.findByText(/Click Me/i)
|
||
|
||
// Good (especially for unit tests)
|
||
wrapper.findComponent(FooComponent);
|
||
wrapper.find('input[name=foo]');
|
||
wrapper.find('[data-testid="my-foo-id"]');
|
||
wrapper.findByTestId('my-foo-id'); // with shallowMountExtended or mountExtended – check below
|
||
wrapper.find({ ref: 'foo'});
|
||
|
||
// Bad
|
||
wrapper.find('.js-foo');
|
||
wrapper.find('.btn-primary');
|
||
wrapper.find('.qa-foo-component');
|
||
wrapper.find('[data-qa-selector="foo"]');
|
||
});
|
||
```
|
||
|
||
It is recommended to use `kebab-case` for `data-testid` attribute.
|
||
|
||
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.
|
||
|
||
### Querying for child components
|
||
|
||
When testing Vue components with `@vue/test-utils` another possible approach is querying for child
|
||
components instead of querying for DOM nodes. This assumes that implementation details of behavior
|
||
under test should be covered by that component's individual unit test. There is no strong preference
|
||
in writing DOM or component queries as long as your tests reliably cover expected behavior for the
|
||
component under test.
|
||
|
||
Example:
|
||
|
||
```javascript
|
||
it('exists', () => {
|
||
wrapper.findComponent(FooComponent);
|
||
});
|
||
```
|
||
|
||
### Naming unit tests
|
||
|
||
When writing describe test blocks to test specific functions/methods,
|
||
use the method name as the describe block name.
|
||
|
||
**Bad**:
|
||
|
||
```javascript
|
||
describe('#methodName', () => {
|
||
it('passes', () => {
|
||
expect(true).toEqual(true);
|
||
});
|
||
});
|
||
|
||
describe('.methodName', () => {
|
||
it('passes', () => {
|
||
expect(true).toEqual(true);
|
||
});
|
||
});
|
||
```
|
||
|
||
**Good**:
|
||
|
||
```javascript
|
||
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 () => {
|
||
await expect(user.getUserName(1)).rejects.toThrow('User with 1 not found.');
|
||
});
|
||
```
|
||
|
||
You can also return a promise from the test function.
|
||
|
||
Using the `done` and `done.fail` callbacks is discouraged when working with
|
||
promises. They should not be used.
|
||
|
||
**Bad**:
|
||
|
||
```javascript
|
||
// missing return
|
||
it('tests a promise', () => {
|
||
promise.then(data => {
|
||
expect(data).toBe(asExpected);
|
||
});
|
||
});
|
||
|
||
// uses done/done.fail
|
||
it('tests a promise', done => {
|
||
promise
|
||
.then(data => {
|
||
expect(data).toBe(asExpected);
|
||
})
|
||
.then(done)
|
||
.catch(done.fail);
|
||
});
|
||
```
|
||
|
||
**Good**:
|
||
|
||
```javascript
|
||
// verifying a resolved promise
|
||
it('tests a promise', () => {
|
||
return promise
|
||
.then(data => {
|
||
expect(data).toBe(asExpected);
|
||
});
|
||
});
|
||
|
||
// verifying a resolved promise using Jest's `resolves` matcher
|
||
it('tests a promise', () => {
|
||
return expect(promise).resolves.toBe(asExpected);
|
||
});
|
||
|
||
// verifying a rejected promise using Jest's `rejects` matcher
|
||
it('tests a promise rejection', () => {
|
||
return expect(promise).rejects.toThrow(expectedError);
|
||
});
|
||
```
|
||
|
||
### Manipulating Time
|
||
|
||
Sometimes we have to test time-sensitive code. For example, recurring events that run every X amount of seconds or similar. Here are 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/timer-mocks)).
|
||
|
||
```javascript
|
||
const doSomethingLater = () => {
|
||
setTimeout(() => {
|
||
// do something
|
||
}, 4000);
|
||
};
|
||
```
|
||
|
||
**in Jest:**
|
||
|
||
```javascript
|
||
it('does something', () => {
|
||
doSomethingLater();
|
||
jest.runAllTimers();
|
||
|
||
expect(something).toBe('done');
|
||
});
|
||
```
|
||
|
||
### Mocking the current location in Jest
|
||
|
||
NOTE:
|
||
The value of `window.location.href` is reset before every test to avoid earlier
|
||
tests affecting later ones.
|
||
|
||
If your tests require `window.location.href` to take a particular value, use
|
||
the `setWindowLocation` helper:
|
||
|
||
```javascript
|
||
import setWindowLocation from 'helpers/set_window_location';
|
||
|
||
it('passes', () => {
|
||
setWindowLocation('https://gitlab.test/foo?bar=true');
|
||
|
||
expect(window.location).toMatchObject({
|
||
hostname: 'gitlab.test',
|
||
pathname: '/foo',
|
||
search: '?bar=true',
|
||
});
|
||
});
|
||
```
|
||
|
||
To modify only the hash, use either the `setWindowLocation` helper, or assign
|
||
directly to `window.location.hash`, for example:
|
||
|
||
```javascript
|
||
it('passes', () => {
|
||
window.location.hash = '#foo';
|
||
|
||
expect(window.location.href).toBe('http://test.host/#foo');
|
||
});
|
||
```
|
||
|
||
If your tests need to assert that certain `window.location` methods were
|
||
called, use the `useMockLocationHelper` helper:
|
||
|
||
```javascript
|
||
import { useMockLocationHelper } from 'helpers/mock_window_location_helper';
|
||
|
||
useMockLocationHelper();
|
||
|
||
it('passes', () => {
|
||
window.location.reload();
|
||
|
||
expect(window.location.reload).toHaveBeenCalled();
|
||
});
|
||
```
|
||
|
||
### Waiting in tests
|
||
|
||
Sometimes a test needs to wait for something to happen in the application before it continues.
|
||
|
||
You should try to avoid:
|
||
|
||
- [`setTimeout`](https://developer.mozilla.org/en-US/docs/Web/API/setTimeout) because it makes the reason for waiting unclear. Additionally, it is faked in our tests so its usage is tricky.
|
||
- [`setImmediate`](https://developer.mozilla.org/en-US/docs/Web/API/Window/setImmediate) because it is no longer supported in Jest 27 and later. See [this epic](https://gitlab.com/groups/gitlab-org/-/epics/7002) for details.
|
||
|
||
#### 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');
|
||
});
|
||
```
|
||
|
||
If you cannot register handlers to the `Promise`, for example because it is executed in a synchronous Vue lifecycle hook, take a look at the [`waitFor`](#wait-until-axios-requests-finish) helpers or flush all pending `Promise`s with:
|
||
|
||
**in Jest:**
|
||
|
||
```javascript
|
||
it('waits for an Ajax call', async () => {
|
||
synchronousFunction();
|
||
|
||
await waitForPromises();
|
||
|
||
expect(something).toBe('done');
|
||
});
|
||
```
|
||
|
||
#### Vue rendering
|
||
|
||
Use [`nextTick()`](https://v2.vuejs.org/v2/api/#Vue-nextTick) to wait until a Vue component is
|
||
re-rendered.
|
||
|
||
**in Jest:**
|
||
|
||
```javascript
|
||
import { nextTick } from 'vue';
|
||
|
||
// ...
|
||
|
||
it('renders something', async () => {
|
||
wrapper.setProps({ value: 'new value' });
|
||
|
||
await nextTick();
|
||
|
||
expect(wrapper.text()).toBe('new value');
|
||
});
|
||
```
|
||
|
||
#### 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', () => {
|
||
eventHub.$once('someEvent', eventHandler);
|
||
|
||
someFunction();
|
||
|
||
return new Promise((resolve) => {
|
||
function expectEventHandler() {
|
||
expect(something).toBe('done');
|
||
resolve();
|
||
}
|
||
});
|
||
});
|
||
```
|
||
|
||
In Jest you can also use a `Promise` for this:
|
||
|
||
```javascript
|
||
it('waits for an event', () => {
|
||
const eventTriggered = new Promise(resolve => eventHub.$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.
|
||
|
||
### Jest best practices
|
||
|
||
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34209) in GitLab 13.2.
|
||
|
||
#### Prefer `toBe` over `toEqual` when comparing primitive values
|
||
|
||
Jest has [`toBe`](https://jestjs.io/docs/expect#tobevalue) and
|
||
[`toEqual`](https://jestjs.io/docs/expect#toequalvalue) matchers.
|
||
As [`toBe`](https://jestjs.io/docs/expect#tobevalue) uses
|
||
[`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is)
|
||
to compare values, it's faster (by default) than using `toEqual`.
|
||
While the latter eventually falls back to leverage [`Object.is`](https://github.com/facebook/jest/blob/master/packages/expect/src/jasmineUtils.ts#L91),
|
||
for primitive values, it should only be used when complex objects need a comparison.
|
||
|
||
Examples:
|
||
|
||
```javascript
|
||
const foo = 1;
|
||
|
||
// Bad
|
||
expect(foo).toEqual(1);
|
||
|
||
// Good
|
||
expect(foo).toBe(1);
|
||
```
|
||
|
||
#### Prefer more befitting matchers
|
||
|
||
Jest provides useful matchers like `toHaveLength` or `toBeUndefined` to make your tests more
|
||
readable and to produce more understandable error messages. Check their docs for the
|
||
[full list of matchers](https://jestjs.io/docs/expect#methods).
|
||
|
||
Examples:
|
||
|
||
```javascript
|
||
const arr = [1, 2];
|
||
|
||
// prints:
|
||
// Expected length: 1
|
||
// Received length: 2
|
||
expect(arr).toHaveLength(1);
|
||
|
||
// prints:
|
||
// Expected: 1
|
||
// Received: 2
|
||
expect(arr.length).toBe(1);
|
||
|
||
// prints:
|
||
// expect(received).toBe(expected) // Object.is equality
|
||
// Expected: undefined
|
||
// Received: "bar"
|
||
const foo = 'bar';
|
||
expect(foo).toBe(undefined);
|
||
|
||
// prints:
|
||
// expect(received).toBeUndefined()
|
||
// Received: "bar"
|
||
const foo = 'bar';
|
||
expect(foo).toBeUndefined();
|
||
```
|
||
|
||
#### Avoid using `toBeTruthy` or `toBeFalsy`
|
||
|
||
Jest also provides following matchers: `toBeTruthy` and `toBeFalsy`. We should not use them because
|
||
they make tests weaker and produce false-positive results.
|
||
|
||
For example, `expect(someBoolean).toBeFalsy()` passes when `someBoolean === null`, and when
|
||
`someBoolean === false`.
|
||
|
||
#### Tricky `toBeDefined` matcher
|
||
|
||
Jest has the tricky `toBeDefined` matcher that can produce false positive test. Because it
|
||
[validates](https://github.com/facebook/jest/blob/master/packages/expect/src/matchers.ts#L204)
|
||
the given value for `undefined` only.
|
||
|
||
```javascript
|
||
// Bad: if finder returns null, the test will pass
|
||
expect(wrapper.find('foo')).toBeDefined();
|
||
|
||
// Good
|
||
expect(wrapper.find('foo').exists()).toBe(true);
|
||
```
|
||
|
||
#### Avoid using `setImmediate`
|
||
|
||
Try to avoid using `setImmediate`. `setImmediate` is an ad-hoc solution to run your callback after
|
||
the I/O completes. And it's not part of the Web API, hence, we target NodeJS environments in our
|
||
unit tests.
|
||
|
||
Instead of `setImmediate`, use `jest.runAllTimers` or `jest.runOnlyPendingTimers` to run pending timers.
|
||
The latter is useful when you have `setInterval` in the code. **Remember:** our Jest configuration uses fake timers.
|
||
|
||
## Avoid non-deterministic specs
|
||
|
||
Non-determinism is the breeding ground for flaky and brittle specs. Such specs end up breaking the CI pipeline, interrupting the work flow of other contributors.
|
||
|
||
1. Make sure your test subject's collaborators (for example, Axios, Apollo, Lodash helpers) and test environment (for example, Date) behave consistently across systems and over time.
|
||
1. Make sure tests are focused and not doing "extra work" (for example, needlessly creating the test subject more than once in an individual test).
|
||
|
||
### Faking `Date` for determinism
|
||
|
||
`Date` is faked by default in our Jest environment. This means every call to `Date()` or `Date.now()` returns a fixed deterministic value.
|
||
|
||
If you really need to change the default fake date, you can call `useFakeDate` within any `describe` block, and
|
||
the date will be replaced for that specs within that `describe` context only:
|
||
|
||
```javascript
|
||
import { useFakeDate } from 'helpers/fake_date';
|
||
|
||
describe('cool/component', () => {
|
||
// Default fake `Date`
|
||
const TODAY = new Date();
|
||
|
||
// NOTE: `useFakeDate` cannot be called during test execution (that is, inside `it`, `beforeEach`, `beforeAll`, etc.).
|
||
describe("on Ada Lovelace's Birthday", () => {
|
||
useFakeDate(1815, 11, 10)
|
||
|
||
it('Date is no longer default', () => {
|
||
expect(new Date()).not.toEqual(TODAY);
|
||
});
|
||
});
|
||
|
||
it('Date is still default in this scope', () => {
|
||
expect(new Date()).toEqual(TODAY)
|
||
});
|
||
})
|
||
```
|
||
|
||
Similarly, if you really need to use the real `Date` class, then you can import and call `useRealDate` within any `describe` block:
|
||
|
||
```javascript
|
||
import { useRealDate } from 'helpers/fake_date';
|
||
|
||
// NOTE: `useRealDate` cannot be called during test execution (that is, inside `it`, `beforeEach`, `beforeAll`, etc.).
|
||
describe('with real date', () => {
|
||
useRealDate();
|
||
});
|
||
```
|
||
|
||
### Faking `Math.random` for determinism
|
||
|
||
Consider replacing `Math.random` with a fake when the test subject depends on it.
|
||
|
||
```javascript
|
||
beforeEach(() => {
|
||
// https://xkcd.com/221/
|
||
jest.spyOn(Math, 'random').mockReturnValue(0.4);
|
||
});
|
||
```
|
||
|
||
## Factories
|
||
|
||
TBU
|
||
|
||
## Mocking Strategies with Jest
|
||
|
||
### Stubbing and Mocking
|
||
|
||
Stubs or spies are often used synonymously. In Jest it's quite easy thanks to the `.spyOn` method.
|
||
[Official docs](https://jestjs.io/docs/jest-object#jestspyonobject-methodname)
|
||
The more challenging part are mocks, which can be used for functions or even dependencies.
|
||
|
||
### Manual module mocks
|
||
|
||
Manual mocks are used to mock modules across the entire Jest environment. This is a very powerful testing tool that helps simplify
|
||
unit testing by mocking out modules that cannot be easily consumed in our test environment.
|
||
|
||
> **WARNING:** Do not use manual mocks if a mock should not be consistently applied in every spec (that is, it's only needed by a few specs).
|
||
> Instead, consider using [`jest.mock(..)`](https://jestjs.io/docs/jest-object#jestmockmodulename-factory-options)
|
||
> (or a similar mocking function) in the relevant spec file.
|
||
|
||
#### Where should I put manual mocks?
|
||
|
||
Jest supports [manual module mocks](https://jestjs.io/docs/manual-mocks) by placing a mock in a `__mocks__/` directory next to the source module
|
||
(for example, `app/assets/javascripts/ide/__mocks__`). **Don't do this.** We want to keep all of our test-related code in one place (the `spec/` folder).
|
||
|
||
If a manual mock is needed for a `node_modules` package, use the `spec/frontend/__mocks__` folder. Here's an example of
|
||
a [Jest mock for the package `monaco-editor`](https://gitlab.com/gitlab-org/gitlab/-/blob/b7f914cddec9fc5971238cdf12766e79fa1629d7/spec/frontend/__mocks__/monaco-editor/index.js#L1).
|
||
|
||
If a manual mock is needed for a CE module, place the implementation in
|
||
`spec/frontend/__helpers__/mocks` and add a line to the `frontend/test_setup`
|
||
(or the `frontend/shared_test_setup`) that looks something like:
|
||
|
||
```javascript
|
||
// "~/lib/utils/axios_utils" is the path to the real module
|
||
// "helpers/mocks/axios_utils" is the path to the mocked implementation
|
||
jest.mock('~/lib/utils/axios_utils', () => jest.requireActual('helpers/mocks/axios_utils'));
|
||
```
|
||
|
||
#### Manual mock examples
|
||
|
||
- [`__helpers__/mocks/axios_utils`](https://gitlab.com/gitlab-org/gitlab/-/blob/a50edd12b3b1531389624086b6381a042c8143ef/spec/frontend/__helpers__/mocks/axios_utils.js#L1) -
|
||
This mock is helpful because we don't want any unmocked requests to pass any tests. Also, we are able to inject some test helpers such as `axios.waitForAll`.
|
||
- [`__mocks__/mousetrap/index.js`](https://gitlab.com/gitlab-org/gitlab/-/blob/cd4c086d894226445be9d18294a060ba46572435/spec/frontend/__mocks__/mousetrap/index.js#L1) -
|
||
This mock is helpful because the module itself uses AMD format which webpack understands, but is incompatible with the jest environment. This mock doesn't remove
|
||
any behavior, only provides a nice es6 compatible wrapper.
|
||
- [`__mocks__/monaco-editor/index.js`](https://gitlab.com/gitlab-org/gitlab/-/blob/b7f914cddec9fc5971238cdf12766e79fa1629d7/spec/frontend/__mocks__/monaco-editor/index.js) -
|
||
This mock is helpful because the Monaco package is completely incompatible in a Jest environment. In fact, webpack requires a special loader to make it work. This mock
|
||
makes this package consumable by Jest.
|
||
|
||
### Keep mocks light
|
||
|
||
Global mocks introduce magic and technically can reduce test coverage. When mocking is deemed profitable:
|
||
|
||
- Keep the mock short and focused.
|
||
- Leave a top-level comment in the mock on why it is necessary.
|
||
|
||
### Additional mocking techniques
|
||
|
||
Consult the [official Jest docs](https://jestjs.io/docs/jest-object#mock-modules) for a full overview of the available mocking features.
|
||
|
||
## Running Frontend Tests
|
||
|
||
Before generating fixtures, make sure you have a running GDK instance.
|
||
|
||
For running the frontend tests, you need the following commands:
|
||
|
||
- `rake frontend:fixtures` (re-)generates [fixtures](#frontend-test-fixtures). Make sure that
|
||
fixtures are up-to-date before running tests that require them.
|
||
- `yarn jest` runs Jest tests.
|
||
|
||
### Live testing and focused testing -- Jest
|
||
|
||
While you work on a test suite, 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 specific 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
|
||
```
|
||
|
||
## Frontend test fixtures
|
||
|
||
Frontend fixtures are files containing responses from backend controllers. These responses can be either HTML
|
||
generated from HAML templates or JSON payloads. Frontend tests that rely on these responses are
|
||
often using fixtures to validate correct integration with the backend code.
|
||
|
||
### Use fixtures
|
||
|
||
To import a JSON fixture, `import` it using the `test_fixtures` alias.
|
||
|
||
```javascript
|
||
import responseBody from 'test_fixtures/some/fixture.json' // loads spec/frontend/fixtures/some/fixture.json
|
||
|
||
it('makes a request', () => {
|
||
axiosMock.onGet(endpoint).reply(200, responseBody);
|
||
|
||
myButton.click();
|
||
|
||
// ...
|
||
});
|
||
```
|
||
|
||
For other fixtures, Jest uses `spec/frontend/__helpers__/fixtures.js` to import them in tests.
|
||
|
||
The following are examples of tests that work for Jest:
|
||
|
||
```javascript
|
||
it('uses some HTML element', () => {
|
||
loadHTMLFixture('some/page.html'); // loads spec/frontend/fixtures/some/page.html and adds it to the DOM
|
||
|
||
const element = document.getElementById('#my-id');
|
||
|
||
// ...
|
||
|
||
// Jest does not clean up the DOM automatically
|
||
resetHTMLFixture();
|
||
});
|
||
```
|
||
|
||
### Generate fixtures
|
||
|
||
You can find code to generate test fixtures in:
|
||
|
||
- `spec/frontend/fixtures/`, for running tests in CE.
|
||
- `ee/spec/frontend/fixtures/`, for running tests in EE.
|
||
|
||
You can generate fixtures by running:
|
||
|
||
- `bin/rake frontend:fixtures` to generate all fixtures
|
||
- `bin/rspec spec/frontend/fixtures/merge_requests.rb` to generate specific fixtures (in this case for `merge_request.rb`)
|
||
|
||
You can find generated fixtures are in `tmp/tests/frontend/fixtures-ee`.
|
||
|
||
#### Creating new fixtures
|
||
|
||
For each fixture, you can find the content of the `response` variable in the output file.
|
||
For example, a test named `"merge_requests/diff_discussion.json"` in `spec/frontend/fixtures/merge_requests.rb`
|
||
produces an output file `tmp/tests/frontend/fixtures-ee/merge_requests/diff_discussion.json`.
|
||
The `response` variable gets automatically set if the test is marked as `type: :request` or `type: :controller`.
|
||
|
||
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/`.
|
||
|
||
##### GraphQL query fixtures
|
||
|
||
You can create a fixture that represents the result of a GraphQL query using the `get_graphql_query_as_string`
|
||
helper method. For example:
|
||
|
||
```ruby
|
||
# spec/frontend/fixtures/releases.rb
|
||
|
||
describe GraphQL::Query, type: :request do
|
||
include GraphqlHelpers
|
||
|
||
all_releases_query_path = 'releases/graphql/queries/all_releases.query.graphql'
|
||
|
||
it "graphql/#{all_releases_query_path}.json" do
|
||
query = get_graphql_query_as_string(all_releases_query_path)
|
||
|
||
post_graphql(query, current_user: admin, variables: { fullPath: project.full_path })
|
||
|
||
expect_graphql_errors_to_be_empty
|
||
end
|
||
end
|
||
```
|
||
|
||
This will create a new fixture located at
|
||
`tmp/tests/frontend/fixtures-ee/graphql/releases/graphql/queries/all_releases.query.graphql.json`.
|
||
|
||
You can import the JSON fixture in a Jest test using the `test_fixtures` alias
|
||
[as described previously](#use-fixtures).
|
||
|
||
## 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/api#testeachtable-name-fn-timeout) (aliased to `it.each`).
|
||
- Groups of tests using [`describe.each`](https://jestjs.io/docs/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)
|
||
}
|
||
);
|
||
```
|
||
|
||
NOTE:
|
||
Only use template literal block if pretty print is not needed for spec output. For example, empty strings, nested objects etc.
|
||
|
||
For example, when testing the difference between an empty search string and a non-empty search string, the use of the array block syntax with the pretty print option would be preferred. That way the differences between an empty string (`''`) and a non-empty string (`'search string'`) would be visible in the spec output. Whereas with a template literal block, the empty string would be shown as a space, which could lead to a confusing developer experience.
|
||
|
||
```javascript
|
||
// bad
|
||
it.each`
|
||
searchTerm | expected
|
||
${''} | ${{ issue: { users: { nodes: [] } } }}
|
||
${'search term'} | ${{ issue: { other: { nested: [] } } }}
|
||
`('when search term is $searchTerm, it returns $expected', ({ searchTerm, expected }) => {
|
||
expect(search(searchTerm)).toEqual(expected)
|
||
});
|
||
|
||
// good
|
||
it.each([
|
||
['', { issue: { users: { nodes: [] } } }],
|
||
['search term', { issue: { other: { nested: [] } } }],
|
||
])('when search term is %p, expect to return %p',
|
||
(searchTerm, expected) => {
|
||
expect(search(searchTerm)).toEqual(expected)
|
||
}
|
||
);
|
||
|
||
```
|
||
|
||
```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)
|
||
})
|
||
});
|
||
```
|
||
|
||
## Gotchas
|
||
|
||
### RSpec errors due to JavaScript
|
||
|
||
By default RSpec unit tests don't run JavaScript in the headless browser
|
||
and 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, the spec runner displays vague error messages.
|
||
|
||
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:
|
||
|
||
```ruby
|
||
# 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
|
||
```
|
||
|
||
### Jest test timeout due to asynchronous imports
|
||
|
||
If a module asynchronously imports some other modules at runtime, these modules must be
|
||
transpiled by the Jest loaders at runtime. It's possible that this can cause [Jest to timeout](https://gitlab.com/gitlab-org/gitlab/-/issues/280809).
|
||
|
||
If you run into this issue, consider eager importing the module so that Jest compiles
|
||
and caches it at compile-time, fixing the runtime timeout.
|
||
|
||
Consider the following example:
|
||
|
||
```javascript
|
||
// the_subject.js
|
||
|
||
export default {
|
||
components: {
|
||
// Async import Thing because it is large and isn't always needed.
|
||
Thing: () => import(/* webpackChunkName: 'thing' */ './path/to/thing.vue'),
|
||
}
|
||
};
|
||
```
|
||
|
||
Jest doesn't automatically transpile the `thing.vue` module, and depending on its size, could
|
||
cause Jest to time out. We can force Jest to transpile and cache this module by eagerly importing
|
||
it like so:
|
||
|
||
```javascript
|
||
// the_subject_spec.js
|
||
|
||
import Subject from '~/feature/the_subject.vue';
|
||
|
||
// Force Jest to transpile and cache
|
||
// eslint-disable-next-line no-unused-vars
|
||
import _Thing from '~/feature/path/to/thing.vue';
|
||
```
|
||
|
||
NOTE:
|
||
Do not disregard test timeouts. This could be a sign that there's
|
||
actually a production problem. Use this opportunity to analyze the production webpack bundles and
|
||
chunks and confirm that there is not a production issue with the asynchronous imports.
|
||
|
||
## Overview of Frontend Testing Levels
|
||
|
||
Main information on frontend testing levels can be found in the [Testing Levels page](testing_levels.md).
|
||
|
||
Tests relevant for frontend development can be found at the following places:
|
||
|
||
- `spec/frontend/`, for Jest tests
|
||
- `spec/features/`, for RSpec tests
|
||
|
||
RSpec runs complete [feature tests](testing_levels.md#frontend-feature-tests), while the Jest directories contain [frontend unit tests](testing_levels.md#frontend-unit-tests), [frontend component tests](testing_levels.md#frontend-component-tests), and [frontend integration tests](testing_levels.md#frontend-integration-tests).
|
||
|
||
Before May 2018, `features/` also contained feature tests run by Spinach. These tests were removed from the codebase in May 2018 ([#23036](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/23036)).
|
||
|
||
See also [Notes on testing Vue components](../fe_guide/vue.md#testing-vue-components).
|
||
|
||
## Test helpers
|
||
|
||
Test helpers can be found in [`spec/frontend/__helpers__`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/frontend/__helpers__).
|
||
If you introduce new helpers, place them in that directory.
|
||
|
||
### Vuex Helper: `testAction`
|
||
|
||
We have a helper available to make testing actions easier, as per [official documentation](https://vuex.vuejs.org/guide/testing.html):
|
||
|
||
```javascript
|
||
// prefer using like this, a single object argument so parameters are obvious from reading the test
|
||
await testAction({
|
||
action: actions.actionName,
|
||
payload: { deleteListId: 1 },
|
||
state: { lists: [1, 2, 3] },
|
||
expectedMutations: [ { type: types.MUTATION} ],
|
||
expectedActions: [],
|
||
});
|
||
|
||
// old way, don't do this for new tests
|
||
testAction(
|
||
actions.actionName, // action
|
||
{ }, // params to be passed to action
|
||
state, // state
|
||
[
|
||
{ type: types.MUTATION},
|
||
{ type: types.MUTATION_1, payload: {}},
|
||
], // mutations committed
|
||
[
|
||
{ type: 'actionName', payload: {}},
|
||
{ type: 'actionName1', payload: {}},
|
||
] // actions dispatched
|
||
done,
|
||
);
|
||
```
|
||
|
||
### Wait until Axios requests finish
|
||
|
||
<!-- vale gitlab.Spelling = NO -->
|
||
|
||
The Axios Utils mock module located in `spec/frontend/__helpers__/mocks/axios_utils.js` contains two helper methods for Jest tests that spawn HTTP requests.
|
||
These are very useful if you don't have a handle to the request's Promise, for example when a Vue component does a request as part of its life cycle.
|
||
|
||
<!-- vale gitlab.Spelling = YES -->
|
||
|
||
- `waitFor(url, callback)`: Runs `callback` after a request to `url` finishes (either successfully or unsuccessfully).
|
||
- `waitForAll(callback)`: Runs `callback` once all pending requests have finished. If no requests are pending, runs `callback` on the next tick.
|
||
|
||
Both functions run `callback` on the next tick after the requests finish (using `setImmediate()`), to allow any `.then()` or `.catch()` handlers to run.
|
||
|
||
### `shallowMountExtended` and `mountExtended`
|
||
|
||
The `shallowMountExtended` and `mountExtended` utilities provide you with the ability to perform
|
||
any of the available [DOM Testing Library queries](https://testing-library.com/docs/queries/about/)
|
||
by prefixing them with `find` or `findAll`.
|
||
|
||
```javascript
|
||
import { shallowMountExtended } from 'helpers/vue_test_utils_helper';
|
||
|
||
describe('FooComponent', () => {
|
||
const wrapper = shallowMountExtended({
|
||
template: `
|
||
<div data-testid="gitlab-frontend-stack">
|
||
<p>GitLab frontend stack</p>
|
||
<div role="tablist">
|
||
<button role="tab" aria-selected="true">Vue.js</button>
|
||
<button role="tab" aria-selected="false">GraphQL</button>
|
||
<button role="tab" aria-selected="false">SCSS</button>
|
||
</div>
|
||
</div>
|
||
`,
|
||
});
|
||
|
||
it('finds elements with `findByTestId`', () => {
|
||
expect(wrapper.findByTestId('gitlab-frontend-stack').exists()).toBe(true);
|
||
});
|
||
|
||
it('finds elements with `findByText`', () => {
|
||
expect(wrapper.findByText('GitLab frontend stack').exists()).toBe(true);
|
||
expect(wrapper.findByText('TypeScript').exists()).toBe(false);
|
||
});
|
||
|
||
it('finds elements with `findAllByRole`', () => {
|
||
expect(wrapper.findAllByRole('tab').length).toBe(3);
|
||
});
|
||
});
|
||
```
|
||
|
||
Check an example in [`spec/frontend/alert_management/components/alert_details_spec.js`](https://gitlab.com/gitlab-org/gitlab/-/blob/ac1c9fa4c5b3b45f9566147b1c88fd1339cd7c25/spec/frontend/alert_management/components/alert_details_spec.js#L32).
|
||
|
||
## Testing with older browsers
|
||
|
||
Some regressions only affect a specific browser version. We can install and test in particular browsers with either Firefox or BrowserStack using the following steps:
|
||
|
||
### BrowserStack
|
||
|
||
[BrowserStack](https://www.browserstack.com/) allows you to test more than 1200 mobile devices and browsers.
|
||
You can use it directly through the [live app](https://www.browserstack.com/live) or you can install the [chrome extension](https://chrome.google.com/webstore/detail/browserstack/nkihdmlheodkdfojglpcjjmioefjahjb) for easy access.
|
||
Sign in to BrowserStack with the credentials saved in the **Engineering** vault of the GitLab
|
||
[shared 1Password account](https://about.gitlab.com/handbook/security/#1password-guide).
|
||
|
||
### Firefox
|
||
|
||
#### macOS
|
||
|
||
You can download any older version of Firefox from the releases FTP server, <https://ftp.mozilla.org/pub/firefox/releases/>:
|
||
|
||
1. From the website, select a version, in this case `50.0.1`.
|
||
1. Go to the mac folder.
|
||
1. Select your preferred language. The DMG package is inside. Download it.
|
||
1. Drag and drop the application to any other folder but the `Applications` folder.
|
||
1. Rename the application to something like `Firefox_Old`.
|
||
1. Move the application to the `Applications` folder.
|
||
1. Open up a terminal and run `/Applications/Firefox_Old.app/Contents/MacOS/firefox-bin -profilemanager` to create a new profile specific to that Firefox version.
|
||
1. Once the profile has been created, quit the app, and run it again like normal. You now have a working older Firefox version.
|
||
|
||
## Snapshots
|
||
|
||
By now you've probably heard of [Jest snapshot tests](https://jestjs.io/docs/snapshot-testing) and why they are useful for various reasons.
|
||
To use them within GitLab, there are a few guidelines that should be highlighted:
|
||
|
||
- Treat snapshots as code
|
||
- Don't think of a snapshot file as a Blackbox
|
||
- Care for the output of the snapshot, otherwise, it's not providing any real value. This will usually involve reading the generated snapshot file as you would read any other piece of code
|
||
|
||
Think of a snapshot test as a simple way to store a raw `String` representation of what you've put into the item being tested. This can be used to evaluate changes in a component, a store, a complex piece of generated output, etc. You can see more in the list below for some recommended `Do's and Don'ts`.
|
||
While snapshot tests can be a very powerful tool. They are meant to supplement, not to replace unit tests.
|
||
|
||
Jest provides a great set of docs on [best practices](https://jestjs.io/docs/snapshot-testing#best-practices) that we should keep in mind when creating snapshots.
|
||
|
||
### How does a snapshot work?
|
||
|
||
A snapshot is purely a stringified version of what you ask to be tested on the lefthand side of the function call. This means any kind of changes you make to the formatting of the string has an impact on the outcome. This process is done by leveraging serializers for an automatic transform step. For Vue this is already taken care of by leveraging the `vue-jest` package, which offers the proper serializer.
|
||
|
||
Should the outcome of your spec be different from what is in the generated snapshot file, you'll be notified about it by a failing test in your test suite.
|
||
|
||
Find all the details in Jests official documentation [https://jestjs.io/docs/snapshot-testing](https://jestjs.io/docs/snapshot-testing)
|
||
|
||
### How to take a snapshot
|
||
|
||
```javascript
|
||
it('makes the name look pretty', () => {
|
||
expect(prettifyName('Homer Simpson')).toMatchSnapshot()
|
||
})
|
||
```
|
||
|
||
When this test runs the first time a fresh `.snap` file will be created. It will look something like this:
|
||
|
||
```txt
|
||
// Jest Snapshot v1, https://goo.gl/fbAQLP
|
||
|
||
exports[`makes the name look pretty`] = `
|
||
Sir Homer Simpson the Third
|
||
`
|
||
```
|
||
|
||
Now, every time you call this test, the new snapshot will be evaluated against the previously created version. This should highlight the fact that it's important to understand the content of your snapshot file and treat it with care. Snapshots will lose their value if the output of the snapshot is too big or complex to read, this means keeping snapshots isolated to human-readable items that can be either evaluated in a merge request review or are guaranteed to never change.
|
||
The same can be done for `wrappers` or `elements`
|
||
|
||
```javascript
|
||
it('renders the component correctly', () => {
|
||
expect(wrapper).toMatchSnapshot()
|
||
expect(wrapper.element).toMatchSnapshot();
|
||
})
|
||
```
|
||
|
||
The above test will create two snapshots. It's important to decide which of the snapshots provide more value for codebase safety. That is, if one of these snapshots changes, does that highlight a possible break in the codebase? This can help catch unexpected changes if something in an underlying dependency changes without our knowledge.
|
||
|
||
### Pros and Cons
|
||
|
||
**Pros**
|
||
|
||
- Speed up the creation of unit tests
|
||
- Easy to maintain
|
||
- Provides a good safety net to protect against accidental breakage of important HTML structures
|
||
|
||
**Cons**
|
||
|
||
- Is not a catch-all solution that replaces the work of integration or unit tests
|
||
- No meaningful assertions or expectations within snapshots
|
||
- When carelessly used with [GitLab UI](https://gitlab.com/gitlab-org/gitlab-ui) it can create fragility in tests when the underlying library changes the HTML of a component we are testing
|
||
|
||
A good guideline to follow: the more complex the component you may want to steer away from just snapshot testing. But that's not to say you can't still snapshot test and test your component as normal.
|
||
|
||
### When to use
|
||
|
||
**Use snapshots when**
|
||
|
||
- to capture a components rendered output
|
||
- to fully or partially match templates
|
||
- to match readable data structures
|
||
- to verify correctly composed native HTML elements
|
||
- as a safety net for critical structures so others don't break it by accident
|
||
- Template heavy component
|
||
- Not a lot of logic in the component
|
||
- Composed of native HTML elements
|
||
|
||
### When not to use
|
||
|
||
**Don't use snapshots when**
|
||
|
||
- To capture large data structures just to have something
|
||
- To just have some kind of test written
|
||
- To capture highly volatile UI elements without stubbing them (Think of GitLab UI version updates)
|
||
|
||
---
|
||
|
||
[Return to Testing documentation](index.md)
|