[NTARelix]

This is correct, but I think the point nickdandakis was trying to make is that useEffect does not directly correlate with componentDidUnmount because useEffect's returned callback could be called in the middle of the component's lifecycle, not only when the component unmounts.

[pcthrowaway]

Err.. no, you pass a callback (let's call it the effect callback) that returns a callback (the cleanup callback) to useEffect. The effect callback gets called when the component mounts, and when dependencies (if any) change. The cleanup callback gets called when the dependency unmounts (and will not be called at other times in the lifecycle)

edit: I'm wrong, the cleanup is also called when dependencies change, just not on mount.. eg. it gets called before the effect callback being called again.

In other words, the cleanup function is called on unmount when useEffect is called with an empty array, and if useEffect has dependencies, it will also be called when those dependencies change