Do ion's implicitly memoize?

[Aleksion]

I've been using selectors to control my renders a bit more as of late, and outside of the #83 bug, it's been working well.
But some of the selectors iterate pretty long lists, which isn't a problem yet but easily could be... which @bowheart addressed in the docs:

https://omnistac.github.io/zedux/docs/walkthrough/selectors#ions

Only... it talks about injectMemo, but then doesn't go about using it. Is that an oversight or because there's some implicit use here?

[bowheart]

Those docs should probably have an injectMemo example, you're right @Aleksion.

Ions are just atoms. All atoms and selectors only rerun when one of their dependencies updates. In the example in the docs:

const sortedUsersAtom = ion('sortedUsers', ({ get }) =>
  get(usersAtom).sort((userA, userB) => userA.name.localeCompare(userB.name))
)

This atom will only run when usersAtom updates. It needs to re-sort the users every time the usersAtom updates. injectMemo could be used:

const sortedUsersAtom = ion('sortedUsers', ({ get }) => {
  const users = get(usersAtom)

  const sortedUsers = injectMemo(
    () => users.sort((userA, userB) => userA.name.localeCompare(userB.name)),
    [users]
  )

  return sortedUsers
})

But this is unnecessary since injectMemo will rerun every time users changes which is the only thing that could cause this particular atom itself to reevaluate.

EDIT: Better Example & Some Context:

But if the atom had more dependencies or its own store, injectMemo would be necessary to prevent the users from being sorted again unnecessarily:

const sortedUsersAtom = ion('sortedUsers', ({ get }) => {
  const isFiltering = get(isFilteringUsersAtom)
  const users = get(isFiltering ? filteredUsersAtom : usersAtom)

  const sortedUsers = injectMemo(
    () => users.sort((userA, userB) => userA.name.localeCompare(userB.name)),
    [users]
  )

  return sortedUsers
})

In this example, the atom now has multiple dependencies that could all make this atom reevaluate. But we only care about re-sorting the users if the actual users array has changed.

This is the difference between selectors and atoms. Selectors can't control whether any of the inner operations run when a particular dependency updates. The whole selector re-runs every time. Atoms give you fine-grained control via injectMemo.