Intervals are pretty darn tricky in React; if you're not careful, you'll wind up with a "stale" interval, one which can't access current values of state or props.

This hook allows us to sidestep that confusion, and it also gives us a superpower: we can modify the delay without having to worry about stopping and starting the interval.

I learned about this hook from Dan in his blog post on the subject, and he does a fantastic job explaining what all this funkiness is and why you need it. He explains it better than I would be able to, so if you're curious how it works, read his blog post all about it!

I did make one significant change to his version: My version of the hook returns the interval ID, similar to window.setInterval. This allows you to cancel the interval imperatively, if you need to. Note that this is an escape hatch: the primary declarative way to cancel the interval is to set the delay equal to null.

useInterval looks quite a bit like setInterval in its usage:


Here's an example of a "Stopwatch" component. useInterval lets us track how much time has elapsed since the user has started the stopwatch.


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