Persistence

Jotai has an atomWithStorage function in the utils bundle for persistance that supports persisting state in sessionStorage, localStorage, AsyncStorage, or the URL hash.

There are also several alternate implementations here:

A simple pattern with localStorage

const strAtom = atom(localStorage.getItem('myKey') ?? 'foo')

const strAtomWithPersistence = atom(
  (get) => get(strAtom),
  (get, set, newStr) => {
    set(strAtom, newStr)
    localStorage.setItem('myKey', newStr)
  }
)

A helper function with localStorage and JSON parse

const atomWithLocalStorage = (key, initialValue) => {
  const getInitialValue = () => {
    const item = localStorage.getItem(key)
    if (item !== null) {
      return JSON.parse(item)
    }
    return initialValue
  }
  const baseAtom = atom(getInitialValue())
  const derivedAtom = atom(
    (get) => get(baseAtom),
    (get, set, update) => {
      const nextValue = typeof update === 'function' ? update(get(baseAtom)) : update
      set(baseAtom, nextValue)
      localStorage.setItem(key, JSON.stringify(nextValue))
    }
  )
  return derivedAtom
}

(Error handling should be added.)

A helper function with AsyncStorage and JSON parse

This requires onMount.

const atomWithAsyncStorage = (key, initialValue) => {
  const baseAtom = atom(initialValue)
  baseAtom.onMount = (setValue) => {
    ;(async () => {
      const item = await AsyncStorage.getItem(key)
      setValue(JSON.parse(item))
    })()
  }
  const derivedAtom = atom(
    (get) => get(baseAtom),
    (get, set, update) => {
      const nextValue = typeof update === 'function' ? update(get(baseAtom)) : update
      set(baseAtom, nextValue)
      AsyncStorage.setItem(key, JSON.stringify(nextValue))
    }
  )
  return derivedAtom
}

A serialize atom pattern

const serializeAtom = atom<
  null,
  { type: 'serialize'; callback: (value: string) => void } | { type: 'deserialize'; value: string }
>(null, (get, set, action) => {
  if (action.type === 'serialize') {
    const obj = {
      todos: get(todosAtom).map(get),
    }
    action.callback(JSON.stringify(obj))
  } else if (action.type === 'deserialize') {
    const obj = JSON.parse(action.value)
    // needs error handling and type checking
    set(
      todosAtom,
      obj.todos.map((todo: Todo) => atom(todo))
    )
  }
})

const Persist: React.FC = () => {
  const [, dispatch] = useAtom(serializeAtom)
  const save = () => {
    dispatch({
      type: 'serialize',
      callback: (value) => {
        localStorage.setItem('serializedTodos', value)
      },
    })
  }
  const load = () => {
    const value = localStorage.getItem('serializedTodos')
    if (value) {
      dispatch({ type: 'deserialize', value })
    }
  }
  return (
    <div>
      <button onClick={save}>Save to localStorage</button>
      <button onClick={load}>Load from localStorage</button>
    </div>
  )
}

Examples

A pattern with atomFamily

const serializeAtom = atom<
  null,
  { type: 'serialize'; callback: (value: string) => void } | { type: 'deserialize'; value: string }
>(null, (get, set, action) => {
  if (action.type === 'serialize') {
    const todos = get(todosAtom)
    const todoMap: Record<string, { title: string; completed: boolean }> = {}
    todos.forEach((id) => {
      todoMap[id] = get(todoAtomFamily({ id }))
    })
    const obj = {
      todos,
      todoMap,
      filter: get(filterAtom),
    }
    action.callback(JSON.stringify(obj))
  } else if (action.type === 'deserialize') {
    const obj = JSON.parse(action.value)
    // needs error handling and type checking
    set(filterAtom, obj.filter)
    obj.todos.forEach((id: string) => {
      const todo = obj.todoMap[id]
      set(todoAtomFamily({ id, ...todo }), todo)
    })
    set(todosAtom, obj.todos)
  }
})

const Persist: React.FC = () => {
  const [, dispatch] = useAtom(serializeAtom)
  const save = () => {
    dispatch({
      type: 'serialize',
      callback: (value) => {
        localStorage.setItem('serializedTodos', value)
      },
    })
  }
  const load = () => {
    const value = localStorage.getItem('serializedTodos')
    if (value) {
      dispatch({ type: 'deserialize', value })
    }
  }
  return (
    <div>
      <button onClick={save}>Save to localStorage</button>
      <button onClick={load}>Load from localStorage</button>
    </div>
  )
}

Examples