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Unit tests for the Mattermost Web app

Unit Tests for Component and Utility Files 

The last required step in building a web app component is to test it. Jest and React Testing Library are the main frameworks/testing utilities used in unit testing components and utility files in our web app. Please visit their respective documentation for detailed information on how to get started, best practices, and updates. React Testing Library is used to render and interact with React components in a way that closely mirrors how users interact with them in the browser, while Jest is used to perform snapshot testing against these components.

With React Testing Library, you can focus on testing the behavior of your components, rather than their implementation details. This means that your tests will be more resilient to changes in your codebase, and will give you more confidence that your application is working as expected. If you need to unit test something related to Redux, please check out Redux Unit and E2E Testing.

Writing unit tests 

Below is a brief guide on how to do component testing:

  1. Use our testing library helpers to render a component and its child components. Use screen to interact with the rendered component and assert on the expected results. Match snapshots using default or expected props. Note that while using snapshots is convenient, do not rely solely on this for every test case, as changes can be easily overlooked when using the command jest --updateSnapshot to update multiple snapshots at once. For example:

    import {render, screen, userEvent} from 'tests/react_testing_utils';
    const baseProps = {
        active: true,
        onSubmit: jest.fn(),
    test('should match snapshot, not send email notifications', () => {
        // For components that use Redux, React Intl, or Reach Router, you can also use the renderWithFullContext helper
        const {container} = render(<EmailNotificationSetting {...baseProps}/>);
        // Assert on a specific element rendered by the component
        const submitButton = screen.getByRole('button', { name: 'Submit' });
  2. Use screen from React Testing Library to query for elements by text or role and use assertions to verify their existence and properties.

    expect(screen.getByRole('checkbox', { id: 'emailNotificationImmediately' })).toBeInTheDocument();
    expect(screen.getByRole('heading', { name: props.siteName })).toBeInTheDocument();
    expect(screen.getByRole('heading', { name: props.customDescriptionText })).toBeInTheDocument();
  3. Use toHaveClass matcher from Jest DOM to check CSS classes.

  4. Simulate events using userEvent and verify state changes using expect.

    test('should pass handleChange', () => {
        const {getByRole} = render(<EmailNotificationSetting {...baseProps}/>);
        const emailNotificationImmediately = getByRole('checkbox', { id: 'emailNotificationImmediately' });
  5. Ensure that all functions of a component are tested. This can be done via events, state changes, or just calling it directly.

    const baseProps = {
        updateSection: jest.fn(),
    test('should call updateSection on handleExpand', () => {
        const {getByRole} = render(<EmailNotificationSetting {...baseProps}/>);
        const button = getByRole('button', { name: /expand/i });
  6. When a function is passed to a component via props, make sure to test if it gets called for a particular event call or its state changes.

    const baseProps = {
        onSubmit: jest.fn(),
        updateSection: jest.fn(),
    test('should call functions on handleSubmit', () => {
        const {getByTestId} = render(<EmailNotificationSetting {...baseProps}/>);
        const submitButton = getByTestId('submit-button');
        const emailNotificationNever = getByTestId('email-notification-never');
        expect(baseProps.onSubmit).toBeCalledWith({enableEmail: 'false'});
  7. Provide a mock for a single function imported from another file while keeping the original version of the rest of that file’s exports.

    jest.mock('utils/utils', () => {
        const original = jest.requireActual('utils/utils');
        return {
            isMobileView: jest.fn(() => true),
  8. Mock async redux actions as necessary while providing a readable action type and having them pass their arguments.

    jest.mock('mattermost-redux/actions/channels', () => {
        const original = jest.requireActual('mattermost-redux/actions/channels');
        return {
            fetchMyChannelsAndMembers: (...args) => ({type: 'MOCK_FETCH_CHANNELS_AND_MEMBERS', args}),
    // Then compare the dispatched actions
    await testStore.dispatch(Actions.loadChannelsForCurrentUser());
  9. For utility functions, list all test cases with test description, input and output.

    describe('stripMarkdown | RemoveMarkdown', () => {
        const testCases = [{
            description: 'emoji: same',
            inputText: 'Hey 😄 👍 :)',
            outputText: 'Hey 😄 👍 :)',
            description: 'at-mention: same',
            inputText: 'Hey @user and @test',
            outputText: 'Hey @user and @test',
        testCases.forEach((testCase) => {
            test(testCase.description, () => {

Running unit tests 

  1. To run all tests that have been modified or added since the last commit, run Jest in watch mode with the command: npm run test:watch.
    • If there are new tests in the first run, test snapshots will be generated and placed in the __snapshots__ directory in the folder where the test suite is located.

    • In watch mode, you can also run specific sets of tests:

      › Press a to run all tests.
      › Press f to run only failed tests.
      › Press o to only run tests related to changed files.
      › Press q to quit watch mode.
      › Press p to filter by a filename regex pattern.
      › Press t to filter by a test name regex pattern.
      › Press Enter to trigger a test run.
      • To see if a component test passes, select p in watch mode and enter the filename for the component test.


  • If you get an error similar to UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'filter' of undefined:

    • Check if the code being tested uses native timer functions (i.e., setTimeout, setInterval, clearTimeout, clearInterval). You can mock the timers and/or run fake timers (e.g. jest.useFakeTimers()) if necessary. Note that jest.useFakeTimers() is already used in the Jest global setup, but there are cases where it needs to run specifically depending on how the component uses the native timer functions.

  • If you get an error similar to UnhandledPromiseRejectionWarning: TypeError: (0 , \_fff.hhh) is not a function:

    • Check if you’re mocking part of an imported module without providing other exports which are used. You can use jest.requireActual to get the un-mocked version of the file.

      // DO NOT partially mock the module
      jest.mock('actions/storage', () => ({
          setGlobalItem: (...args) => ({type: 'MOCK_SET_GLOBAL_ITEM', args}),
      // DO fully mock the module
      jest.mock('actions/storage', () => {
          const original = jest.requireActual('actions/storage');
          return {
              setGlobalItem: (...args) => ({type: 'MOCK_SET_GLOBAL_ITEM', args}),
  • If you get an error similar to UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'data' of undefined:

    • Use async mock functions with resolved values. The property in question that cannot be read can be error, data, exists, match, or whatever else the resolved value(s) contains. For example, if the file being tested contains a line where it awaits on an async value like:

      const {data} = await this.props.actions.addUsersToTeam(this.props.currentTeamId, userIds);

      The mocked function should return a Promise by using mockResolvedValue:

      // DO NOT assign a regular mock function.
      const addUsersToTeam = jest.fn();
      // DO NOT forget to provide a resolved value.
      const addUsersToTeam: jest.fn(() => {
          return new Promise((resolve) => {
              process.nextTick(() => resolve());
      // DO mock async function with resolved value. `mockResolvedValue` is the easiest way to do this.
      const addUsersToTeam = jest.fn().mockResolvedValue({data: true})
      // DO mock async function with several resolved values for repeated calls.
      const addUsersToTeam = jest.fn().
      mockResolvedValueOnce({error: true}).
      mockResolvedValue({data: true});

      Remember to make individual test cases async when testing async functions:

      // DO NOT forget to wait for the async function to complete.
      test('should match state when handleSubmit is called', () => {
      // DO remember to wait on the async function and to make the entire test case async.
      test('should match state when handleSubmit is called', async () => {
          await wrapper.instance().handleSubmit();

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