Measuring the test coverage of Jest for evaluating the useEffect that modifies the styling of an HTML element

Question

Measuring the test coverage of Jest for evaluating the useEffect that modifies the styling of an HTML element

As a newcomer to Jest and React, I am currently working on developing a scroll button component in React using the useEffect function. Here's a snippet of my code:

useEffect(() => {
            square && (square.style.transform = `translateX(-${scroll}px)`);
        });

In this code snippet, scroll is a component state that gets updated whenever a button is clicked.

While trying to figure out how to test my code coverage with Jest, I have made some progress by writing the following test:

 it('test transformX', () => {
    const squareMock = jest.fn().mockReturnValue({
        style: { transform: jest.fn() } 
    });
    const {getByLabelText} = render(
        <ScrollButton square={ squareMock } />
    );
    const rightButton = getByLabelText('rightButton');
    fireEvent.click(rightButton);
    });

I would appreciate any guidance on what steps I should take next. Thank you for your help!

java css reactjs jestjs react-testing-library

Answer 1

Answer №1

It's generally recommended not to directly manipulate the DOM within a useEffect hook in React. Instead, you should allow React to handle rendering to the DOM for you. Here's an example of how your code could be structured:

const ScrollButton() {
  const [scroll] = useState(0);
  const [transform, setTransform] = useState(undefined);
  useEffect(() => {
    setTransform(`translateX(-${scroll}px)`);
  }, [scroll]);

  return (
    <div style={{ transform }}>my square</div>
  )
}

When it comes to testing useEffect(), you can use the waitFor function from the Testing Library to await a style change or any other DOM alteration:

// inside test
// render component
// then wait for useEffect to run
await waitFor(() => {
  expect(queryByText('my square'))
    .toHaveStyle({transform: `translateX(-10px)`});
});
// continue with the rest of the test

If your useEffect hook has dependencies, you can test these by simulating user interactions that would cause changes in those dependencies. For instance, triggering a button click event to update a state variable like scroll which is used as a dependency in your effect.

Avoid mocking useEffect() or directly testing the effect itself. And remember, there's no equivalent of runAllTimers() like runAllEffects() for testing effects; focus on testing the expected side effects when dependencies change.

So, test effects by verifying their intended consequences when their dependent variables are altered.

Answer 2

It's generally recommended not to directly manipulate the DOM within a useEffect hook in React. Instead, you should allow React to handle rendering to the DOM for you. Here's an example of how your code could be structured:

const ScrollButton() {
  const [scroll] = useState(0);
  const [transform, setTransform] = useState(undefined);
  useEffect(() => {
    setTransform(`translateX(-${scroll}px)`);
  }, [scroll]);

  return (
    <div style={{ transform }}>my square</div>
  )
}

When it comes to testing useEffect(), you can use the waitFor function from the Testing Library to await a style change or any other DOM alteration:

// inside test
// render component
// then wait for useEffect to run
await waitFor(() => {
  expect(queryByText('my square'))
    .toHaveStyle({transform: `translateX(-10px)`});
});
// continue with the rest of the test

If your useEffect hook has dependencies, you can test these by simulating user interactions that would cause changes in those dependencies. For instance, triggering a button click event to update a state variable like scroll which is used as a dependency in your effect.

Avoid mocking useEffect() or directly testing the effect itself. And remember, there's no equivalent of runAllTimers() like runAllEffects() for testing effects; focus on testing the expected side effects when dependencies change.

So, test effects by verifying their intended consequences when their dependent variables are altered.

Measuring the test coverage of Jest for evaluating the useEffect that modifies the styling of an HTML element

Answer №1

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