Short reminder what React Hooks are, and here useState and useEffect hooks in detail.
This blog article is about how to fetch data with React hook...
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i always use empty array : useEffect(() => {}, []) it similars componentDidMount()
@psyduck , it really depends what your use case is. A cleanup should be always provided, you can also read more about it how-to-use-useeffect-hook-in-react/.