Async

Using async atoms, you gain access to real-world data while still managing them directly from your atoms and with incredible ease.

We can separate them in two main categories:

• Async read atoms: async request is started instantly as soon as you try to get its value. You could relate to them as "smart getters".
• Async write atoms: async request is started at a specific moment. You could relate to them as "actions".

Async read atom

The read function of an atom can return a promise.

const countAtom = atom(1)
const asyncAtom = atom(async (get) => get(countAtom) * 2)

Jotai is inherently leveraging Suspense to handle asynchronous flows.

const ComponentUsingAsyncAtoms = () => {
  const [num] = useAtom(asyncAtom)
  // here `num` is always `number` even though asyncAtom returns a Promise
}
const App = () => {
  return (
    <Suspense fallback=>
      <ComponentUsingAsyncAtoms />
    </Suspense>
  )
}

Alternatively, you could avoid the inherent suspending that Jotai does for you, by wrapping your atoms with the loadable API.

If another atom uses an async atom, it will return a promise. So, we need to make the atom also async.

const anotherAtom = atom(async (get) => (await get(asyncAtom)) / 2)

This also applies to an atom with write function.

const asyncAtom = atom(async (get) => ...)
const writeAtom = atom(null, async (get, set, payload) => {
  await get(asyncAtom)
  // ...
})

Async write atom

Async write atoms are another kind of async atom. When the write function of atom returns a promise.

const countAtom = atom(1)
const asyncIncrementAtom = atom(null, async (get, set) => {
  // await something
  set(countAtom, get(countAtom) + 1)
})

const Component = () => {
  const [, increment] = useAtom(asyncIncrementAtom)
  const handleClick = () => {
    increment()
  }
  // ...
}

Async sometimes

An interesting pattern that can be achieved with Jotai is switching from async to sync to trigger suspending when wanted.

const request = async () =>
  fetch('https://jsonplaceholder.typicode.com/todos/1').then((res) =>
    res.json(),
  )
const baseAtom = atom(0)
const Component = () => {
  const [value, setValue] = useAtom(baseAtom)
  const handleClick = () => {
    setValue(request()) // Will suspend until request resolves
  }
  // ...
}

Usage in TypeScript

In TypeScript atom(0) is inferred as PrimitiveAtom<number>. It cannot accept Promise<number> as a value so preceding code would not typecheck. To accommodate for that you need to type your atom explicitly and add Promise<number> as accepted value.

const baseAtom = atom<number | Promise<number>>(0) // Will accept sync and async values

Async forever

Sometimes you may want to suspend until an unpredetermined moment (or never).

const baseAtom = atom(new Promise(() => {})) // Will be suspend until set otherwise

Suspense

Async support is first class in Jotai. It fully leverages React Suspense at its core.

Technically, Suspense usage other than React.lazy is still unsupported / undocumented in React 17. If this is blocking, so you can still use the loadable API to avoid suspending

To use async atoms, you need to wrap your component tree with <Suspense>.

If you have a <Provider>, place at least one <Suspense> inside said <Provider>; otherwise, it may cause an endless loop while rendering the components.

const App = () => (
  <Provider>
    <Suspense fallback="Loading...">
      <Layout />
    </Suspense>
  </Provider>
)

Having more <Suspense>s in the component tree is also possible and must be considered to profit from Jotai inherent handling at best.