fix(middleware/persist): ensure argument for onRehydrateStorage and onHydrate is defined on first hydration

[coffeebeats]

Related Issues or Discussions

Fixes #1691

Summary

This PR ensures that the argument passed to onRehydrateStorage and onHydrate is defined during the first hydration of a store. Instead of passing the result of get(), which will always be undefined before the first hydration, get() ?? configResult is passed. I also added tests validating that onRehydrateStorage is always passed a defined value for state.

One caveat is that it's actually not possible to add an onHydrate listener which will be invoked prior to first hydration. As such, I can't test this case, but I've documented it in the test I added.

Alternative Solution

An alternative solution would be to update the argument type signature of onRehydrateStorage and onHydrate and to state?: S instead of state: S, which reflects the potential for state to be undefined. This is probably not the right solution, however, as it both changes a public API and makes the persist API less useful.

Check List

• yarn run prettier for formatting code and docs

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zustand-4.3.X-bug (forked)	Issue #1691

[dai-shi]

I think it's the right fix as merge() does the same.

Can you make changes only for newImpl?

[coffeebeats]

Done!

[dai-shi]

LGTM