Implement useIntersection hook.

[kevinnorris]

As described in #604.

[wardoost]

Looks great 👏

I have a few small comments + would you be able to add tests?

[kevinnorris]

I'm not sure how to test the main functionality of the hook, as it requires a ref and elements intersecting via scrolling. It's likely possible by using a library to fully render the DOM and then force scroll events. Any thoughts on a test approach would be appreciated. 😃

[peter-mouland]

@kevinnorris I recently created a hook very similar to this and used shopify mocks for the testing

import { intersectionObserver } from '@shopify/jest-dom-mocks';

[wardoost]

@kevinnorris I think you could either

• mock intersection observer, see example tests, or
• use something like puppeteer to simulate a browser, see example tests

I'm not sure what is best in this case, @Belco90 might have a suggestion on this.

[Belco90]

Never done this before for intersectionObserver but I'd go for mocking it and just assume it will do its thing in the real world. How to do it is up to you I guess: mocking it manually or using a third party library for it (that @shopify/jest-dom-mocks that @peter-mouland mentioned looks really interesting).

[streamich]

🎉 This PR is included in version 12.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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