The function given to useEffect will be fired after a successful _commit_ (aka React actually manipulates the DOM after a render).

Here's a helpful diagram that shows the difference between render and commit phases in terms of the class lifecycle methods: http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram...

So when a render executes useEffect with a callback (and values in its dependency array has changed), it will schedule the callback to be run once React has committed the DOM changes based on the current render.