const intervalRef = useRef<number|undefined>();
The thinking behind making the parameter required was the same as the thinking behind making creatContext's argument required.
Dan said back in the day that even though you technically can not give an argument, you still are supposed to and that the docs having an empty argument list example was an oversight.
The thing with the |null is to make it more convenient to use DOM/Component refs, and you will see that all examples of useRef for that purpose have it initialized with null. It's not mutable there because React is the one that should do the mutation, not user code. Explicitly giving null in the generic argument does skip the overload, though.
You could maybe have an overload like useRef<T = undefined>(): MutableRefObject<T | undefined> (template default there to stop it from being {} by default), but I'd like to hear from @gaearon (or someone else involved) whether that's an intended use pattern or a documentation oversight.
Cool, you've obviously got your ear closer to the ground than I have - I hadn't heard that the empty arg list docs example was an oversight. Looking at the source, it does look like the parameter isn't optional.
https://github.com/facebook/react/blob/504576306461a5ff339dc99691842f0f35a8bf4c/packages/react/src/ReactHooks.js#L68
This needs to be revisited. It's possible to use useRef without initial value and it works just fine. You can even find examples in react docs:
https://reactjs.org/docs/hooks-faq.html#is-there-something-like-instance-variables
https://reactjs.org/docs/hooks-faq.html#how-to-get-the-previous-props-or-state
Edit: more examples from the userland:
https://egghead.io/lessons/react-access-and-modify-a-dom-node-with-the-react-useref-and-useeffect-hooks
https://blog.bitsrc.io/react-hooks-beyond-usestate-useeffect-f54dd6df6d1d
https://www.fullstackreact.com/articles/an-introduction-to-hooks-in-react/#useref-hook-example
Edit 2: also, this is the function responsible for creating initial ref:
https://github.com/facebook/react/blob/0e67969cb1ad8c27a72294662e68fa5d7c2c9783/packages/react-reconciler/src/ReactFiberHooks.js#L761-L769
If you don't pass initialValue, it will just be undefined.
There was a twitter thread about this subject yesterday as well https://twitter.com/dan_abramov/status/1097844767956918274
The conclusion was (again) that it is an oversight.
