There's really no need to use createSelector here, since what it returns is a primitive, and primitives won't have issues on equality checks.

Memoizing and things like createSelector in React can have two benefits:

1. Reducing computation
2. Making sure we return the same object reference if the inputs don't change

#2 is the main one you generally want to be thinking about, but #1 comes up sometimes. I don't think either of them are relevant here.

I think we can just replace this with a function that takes state and returns currentUserFID. The result of calling createSelector here has the same function signature, but it just has extra memoization we don't need

I think it'd be better to move this function call into useSelector. See the docs:

When an action is dispatched, useSelector() will do a reference comparison of the previous selector result value and the current result value. If they are different, the component will be forced to re-render. If they are the same, the component will not re-render. useSelector() uses strict === reference equality checks by default, not shallow equality (see the following section for more details).

As you've written this, the component will re-render whenever anything in syncedMetadata changes. If you move the function call into useSelector, then the component will only rerender when the result of getCurrentUserFID changes.

Given that every location you use this, you also call useSelector, I wonder if we should replace this with a hook that does this:

function useCurrentUserFID(): ?string {
  return useSelector(
    state => state.syncedMetadataStore.syncedMetadata[syncedMetadataNames.CURRENT_USER_FID] ?? null,
  );
}