Why does a react component, tested with React Testing Library has issue with react-intersection-observer? - reactjs

Question

I wrote a component built with react 17.0.2 that uses react-intersection-observer 9.1.0
import { useInView } from 'react-intersection-observer'
...
const [ref, inView] = useInView({
threshold: 0.99,
root: scrollRef.current,
delay: 250,
trackVisibility: true,
onChange: (inView: boolean) => {
onChildInView(index, inView)
}
})
to detect sliding behaviours inside or outside the viewport. And the component works fine.
I wrote some unit tests to make the component safer, using #testing-library/react 12.1.4 and #testing-library/jest-dom 5.16.3.
As soon as I test just the existence or visibility of the above component with the following code
describe('method render', () => {
test('renders correctly', () => {
render(
<MyComponent
props={...}
data-testid="component-id"
>
<div />
<div />
</MyComponent>
)
const componentNode = screen.getByTestId('component-id')
expect(componentNode).toBeInTheDocument()
expect(componentNode).toBeVisible()
})
})
the testing library complains with the message error.
ReferenceError: IntersectionObserver is not defined
I tried to fix it with this suggestion of mocking the library (as linked here) written at the top of the test
const intersectionObserverMock = () => ({
observe: () => null
})
declare global {
interface Window {
IntersectionObserver: typeof IntersectionObserver
}
}
window.IntersectionObserver = jest.fn().mockImplementation(intersectionObserverMock);
but it did not work due to
TypeError: observer.unobserve is not a function
Suggestions? Missing something?

Answer 1

To fix this issue I'd recommend using mockAllIsIntersecting from test-utils.js in react-intersection-observer. This function mocks the IntersectionObserver.
e.g.
import { mockAllIsIntersecting } from 'react-intersection-observer/test-utils';
describe('method render', () => {
test('renders correctly', () => {
render(
<MyComponent
props={...}
data-testid="component-id"
>
<div />
<div />
</MyComponent>
)
mockAllIsIntersecting(true)
const componentNode = screen.getByTestId('component-id')
expect(componentNode).toBeInTheDocument()
expect(componentNode).toBeVisible()
})
})