useCallback doesn't memoize as expected

[cdoublev]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

useCallback doesn't return a memoized function when used as a "higher order ref callback".

https://codesandbox.io/s/snowy-tdd-wb8gu

1. Click "Toggle" button to unmount the second div node
2. Open console and look at the output, described by the following quote from the doc

If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.

https://reactjs.org/docs/refs-and-the-dom.html#caveats-with-callback-refs

What is the expected behavior?

useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed.

https://reactjs.org/docs/hooks-reference.html#usecallback

It doesn't work with useMemo as well, even if #15774 looks similar. It works with a "standard" memoize function, like _.memoize.

As indicated in #17204, I was looking for a way to identify the unmounted node.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

React 16.11.0

[miturostislav]

Hi @cdoublev ,
Actually "useCallback" returns a memoized function but when you call that function ("setRefs"), it creates and returns a new function every time.
The inner function is not memoized.
Hope it helps :).

[cdoublev]

I should stop creating issue at the end of the day. Thank you for enlighting me a little longer @miturostislav. :)