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Version: 3.x

Testing with Jest

Reanimated provides testing API, based on Jest. It allows user to mock web-based animations.

Reference​

test('reference', () => {
  // some styles

  const { getByTestId } = render(<AnimatedComponent />);
  const view = getByTestId('view');
  const button = getByTestId('button');

  expect(view).toHaveAnimatedStyle(style);

  fireEvent.press(button);
  jest.advanceTimersByTime(250); // if whole animation duration is a 500ms

  style.width = 50; // value of component width after 250ms of animation
  expect(view).toHaveAnimatedStyle(style);
});

Setup​

Add the following line to your jest-setup.js file:

require('react-native-reanimated').setUpTests();
  • setUpTests() can take optional config argument. Default config is { fps: 60 }.

To be sure, check if your jest.config.js file contains:

...
preset: 'react-native',
setupFilesAfterEnv: ['./jest-setup.js'],
...
caution

If you use Jest in a version older than 28, you should set setupFiles property instead of setupFilesAfterEnv

API​

Style checker​

expect(component).toHaveAnimatedStyle(expectedStyle)​

Checking equality of selected styles with current component styles.

  • component - tested component.
  • expectedStyle - contains expected styles of testing component, for example { width: 100 }.

expect(component).toHaveAnimatedStyle(expectedStyle, {exact: true})​

Checking equality of all current component styles with expected styles.

getDefaultStyle(component)​

Gets all styles of tested component.

Timers​

You can use Jest's fake timers to control animation progress. Check the full guide about mocking timers on Jest documentation website.

jest.useFakeTimers();
// call animation
jest.runAllTimers();

If you want more control over animation, you can use jest.advanceTimersByTime to move to a certain point in the animation:

jest.useFakeTimers();
// call animation
jest.advanceTimersByTime(250);
// make assertions on what you expect the styles of a component should be after 250ms

Example​

  test('withTiming animation', () => {
    const style = getDefaultStyle();

    const { getByTestId } = render(<AnimatedComponent />);
    const view = getByTestId('view');
    const button = getByTestId('button');

    expect(view.props.style.width).toBe(0);
    expect(view).toHaveAnimatedStyle(style);

    fireEvent.press(button);
    jest.advanceTimersByTime(600);

    style.width = 100;
    expect(view).toHaveAnimatedStyle(style);
  });
});

More examples from react-native-reanimated repository:

Remarks​

  • Tests must run with Node 16 or newer.
  • If you have custom babel configuration for testing, make sure that Reanimated's babel plugin is enabled in that environment.

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