This doc describes jotai/utils bundle.

useUpdateAtom

Ref: https://github.com/pmndrs/jotai/issues/26

import { atom, useAtom } from 'jotai'
import { useUpdateAtom } from 'jotai/utils'

const countAtom = atom(0)

const Counter = () => {
  const [count] = useAtom(countAtom)
  return <div>count: {count}</div>
}

const Controls = () => {
  const setCount = useUpdateAtom(countAtom)
  const inc = () => setCount((c) => c + 1)
  return <button onClick={inc}>+1</button>
}

https://codesandbox.io/s/react-typescript-forked-3q11k

useAtomValue

Ref: https://github.com/pmndrs/jotai/issues/212

import { atom, Provider, useAtom } from 'jotai'
import { useAtomValue } from 'jotai/utils'

const countAtom = atom(0)

const Counter = () => {
  const setCount = useUpdateAtom(countAtom)
  const count = useAtomValue(countAtom)
  return (
    <>
      <div>count: {count}</div>
      <button onClick={() => setCount(count + 1)}>+1</button>
    </>
  )
}

https://codesandbox.io/s/react-typescript-forked-1x90m

atomWithReset

Ref: https://github.com/pmndrs/jotai/issues/41

function atomWithReset<Value>(
  initialValue: Value
): WritableAtom<Value, SetStateAction<Value> | typeof RESET>

Creates an atom that could be reset to its initialValue with useResetAtom hook. It works exactly the same way as primitive atom would, but you are also able to set it to a special value RESET. See examples in Resettable atoms.

Example

import { atomWithReset } from 'jotai/utils'

const dollarsAtom = atomWithReset(0)
const todoListAtom = atomWithReset([
  { description: 'Add a todo', checked: false },
])

useResetAtom

function useResetAtom<Value>(
  anAtom: WritableAtom<Value, typeof RESET>
): () => void | Promise<void>

Resets a Resettable atom to its initial value.

Example

import { useResetAtom } from 'jotai/utils'
import { todoListAtom } from './store'

const TodoResetButton = () => {
  const resetTodoList = useResetAtom(todoListAtom)
  return <button onClick={resetTodoList}>Reset</button>
}

RESET

Ref: https://github.com/pmndrs/jotai/issues/217

const RESET: unique symbol

Special value that is accepted by Resettable atoms created with atomWithReset or writable atom created with atom if it accepts RESET symbol.

Example

import { atom } from 'jotai'
import { atomWithReset, useResetAtom, RESET } from 'jotai/utils'

const dollarsAtom = atomWithReset(0)
const centsAtom = atom(
  (get) => get(dollarsAtom) * 100,
  (get, set, newValue: number | typeof RESET) =>
    set(dollarsAtom, newValue === RESET ? newValue : newValue / 100)
)

const ResetExample: React.FC = () => {
  const setDollars = useUpdateAtom(dollarsAtom)
  const resetCents = useResetAtom(centsAtom)

  return (
    <>
      <button onClick={() => setDollars(RESET)}>Reset dollars</button>
      <button onClick={resetCents}>Reset cents</button>
    </>
  )
}

useReducerAtom

import { atom } from 'jotai'
import { useReducerAtom } from 'jotai/utils'

const countReducer = (prev, action) => {
  if (action.type === 'inc') return prev + 1
  if (action.type === 'dec') return prev - 1
  throw new Error('unknown action type')
}

const countAtom = atom(0)

const Counter = () => {
  const [count, dispatch] = useReducerAtom(countAtom, countReducer)
  return (
    <div>
      {count}
      <button onClick={() => dispatch({ type: 'inc' })}>+1</button>
      <button onClick={() => dispatch({ type: 'dec' })}>-1</button>
    </div>
  )
}

https://codesandbox.io/s/react-typescript-forked-eg0mw

atomWithReducer

Ref: https://github.com/pmndrs/jotai/issues/38

import { atomWithReducer } from 'jotai/utils'

const countReducer = (prev, action) => {
  if (action.type === 'inc') return prev + 1
  if (action.type === 'dec') return prev - 1
  throw new Error('unknown action type')
}

const countReducerAtom = atomWithReducer(0, countReducer)

https://codesandbox.io/s/react-typescript-forked-g3tsx

atomWithDefault

Ref: https://github.com/pmndrs/jotai/issues/352

This is a function to create a primitive atom. Its default value can be specified with a read function instead of a static initial value.

import { atomWithDefault } from 'jotai/utils'

const count1Atom = atom(1)
const count2Atom = atomWithDefault((get) => get(count1Atom) * 2)

https://codesandbox.io/s/react-typescript-forked-unfro

atomFamily

Ref: https://github.com/pmndrs/jotai/issues/23

Usage

atomFamily(initializeAtom, areEqual): (param) => Atom

This will create a function that takes param and returns an atom. If it's already created, it will return it from the cache. initializeAtom is function that can return any kind of atom (atom(), atomWithDefault(), ...). Note that areEqual is optional, which tell if two params are equal (defaults to Object.is).

To reproduce the similar behavior to Recoil's atomFamily/selectorFamily, specify a deepEqual function to areEqual. For example:

import { atom } from 'jotai'
import deepEqual from 'fast-deep-equal'

const fooFamily = atomFamily((param) => atom(param), deepEqual)

Examples

import { atom } from 'jotai'
import { atomFamily } from 'jotai/utils'

const todoFamily = atomFamily((name) => atom(name))

todoFamily('foo')
// this will create a new atom('foo'), or return the one if already created

import { atom } from 'jotai'
import { atomFamily } from 'jotai/utils'

const todoFamily = atomFamily((name) =>
  atom(
    (get) => get(todosAtom)[name],
    (get, set, arg) => {
      const prev = get(todosAtom)
      return { ...prev, [name]: { ...prev[name], ...arg } }
    }
  )
)

import { atom } from 'jotai'
import { atomFamily } from 'jotai/utils'

const todoFamily = atomFamily(
  ({ id, name }) => atom({ name }),
  (a, b) => a.id === b.id
)

Codesandbox

https://codesandbox.io/s/react-typescript-forked-8zfrn

selectAtom

Ref: https://github.com/pmndrs/jotai/issues/36

function selectAtom<Value, Slice>(
  anAtom: Atom<Value>,
  selector: (v: Value) => Slice,
  equalityFn: (a: Slice, b: Slice) => boolean = Object.is
): Atom<Slice>

This function creates a derived atom whose value is a function of the original atom's value, determined by selector. The selector function runs whenever the original atom changes; it updates the derived atom only if equalityFn reports that the derived value has changed. By default, equalityFn is reference equality, but you can supply your favorite deep-equals function to stabilize the derived value where necessary.

Examples

const defaultPerson = {
  name: {
    first: 'Jane',
    last: 'Doe',
  },
  birth: {
    year: 2000,
    month: 'Jan',
    day: 1,
    time: {
      hour: number,
      minute: number,
    },
  },
}

// Original atom.
const personAtom = atom(defaultPerson)

// Tracks person.name. Updated when person.name object changes, even
// if neither name.first nor name.last actually change.
const nameAtom = selectAtom(personAtom, (person) => person.name)

// Tracks person.birth. Updated when year, month, day, hour, or minute changes.
// Use of deepEquals means that this atom doesn't update if birth field is
// replaced with a new object containing the same data. E.g., if person is re-read
// from a database.
const birthAtom = selectAtom(personAtom, (person) => person.birth, deepEquals)

Ref: https://codesandbox.io/s/react-typescript-forked-8czek

useAtomCallback

Ref: https://github.com/pmndrs/jotai/issues/60

Usage

useAtomCallback(
  callback: (get: Getter, set: Setter, arg: Arg) => Result
): (arg: Arg) => Promise<Result>

This hook allows to interact with atoms imperatively. It takes a callback function that works like atom write function, and returns a function that returns a promise.

The callback to pass in the hook must be stable (should be wrapped with useCallback).

Examples

import { useEffect, useState, useCallback } from 'react'
import { Provider, atom, useAtom } from 'jotai'
import { useAtomCallback } from 'jotai/utils'

const countAtom = atom(0)

const Counter = () => {
  const [count, setCount] = useAtom(countAtom)
  return (
    <>
      {count} <button onClick={() => setCount((c) => c + 1)}>+1</button>
    </>
  )
}

const Monitor = () => {
  const [count, setCount] = useState(0)
  const readCount = useAtomCallback(
    useCallback((get) => {
      const currCount = get(countAtom)
      setCount(currCount)
      return currCount
    }, [])
  )
  useEffect(() => {
    const timer = setInterval(async () => {
      console.log(await readCount())
    }, 1000)
    return () => {
      clearInterval(timer)
    }
  }, [readCount])
  return <div>current count: {count}</div>
}

Codesandbox

https://codesandbox.io/s/react-typescript-forked-6ur43

freezeAtom

import { atom } from 'jotai'
import { freezeAtom } from 'jotai/utils'

const countAtom = freezeAtom(atom(0))

freezeAtom take an existing atom and return a new derived atom. The returned atom is "frozen" which means when you use the atom with useAtom in components or get in other atoms, the atom value will be deeply freezed with Object.freeze. It would be useful to find bugs where you accidentally tried to mutate objects which can lead to unexpected behavior.

atomFrozenInDev

import { atomFrozenInDev as atom } from 'jotai/utils'

const countAtom = atom(0)

atomFrozenInDev is another function to create a frozen atom. The atom is frozen only in the development mode. In production, it works as the normal atom.

splitAtom

The splitAtom utility is useful for when you want to get an atom for each element in a list. It works for read/write atoms that contain a list. When used on such an atom, it returns an atom which itself contains a list of atoms, each corresponding to the respective item in the original list.

A simplified type signature would be:

type SplitAtom = <Item>(arrayAtom: PrimitiveAtom<Array<Item>>): Atom<Array<PrimitiveAtom<Item>>>

Additionally, the atom returned by splitAtom contains a removal function in the write direction, this is useful for when you want a simple way to remove each element in the original atom.

See the below example for usage.

codesandbox

https://codesandbox.io/s/react-typescript-forked-7nir9?file=/src/App.tsx

import * as React from 'react'
import { Provider, atom, useAtom, PrimitiveAtom } from 'jotai'
import { splitAtom } from 'jotai/utils'
import './styles.css'

const initialState = [
  {
    task: 'help the town',
    done: false,
  },
  {
    task: 'feed the dragon',
    done: false,
  },
]

const todosAtom = atom(initialState)
const todoAtomsAtom = splitAtom(todosAtom)

const TodoList = () => {
  const [todoAtoms, removeTodoAtom] = useAtom(todoAtomsAtom)
  return (
    <ul>
      {todoAtoms.map((todoAtom) => (
        <TodoItem todoAtom={todoAtom} remove={() => removeTodoAtom(todoAtom)} />
      ))}
    </ul>
  )
}

type TodoType = typeof initialState[number]

const TodoItem = ({
  todoAtom,
  remove,
}: {
  todoAtom: PrimitiveAtom<TodoType>
  remove: () => void
}) => {
  const [todo, setTodo] = useAtom(todoAtom)
  return (
    <div>
      <input
        value={todo.task}
        onChange={(e) => {
          setTodo((oldValue) => ({ ...oldValue, task: e.target.value }))
        }}
      />
      <input
        type="checkbox"
        checked={todo.done}
        onChange={() => {
          setTodo((oldValue) => ({ ...oldValue, done: !oldValue.done }))
        }}
      />
      <button onClick={remove}>remove</button>
    </div>
  )
}

const App = () => (
  <Provider>
    <TodoList />
  </Provider>
)

export default App

waitForAll

Sometimes you have multiple async atoms in your components:

const dogsAtom = atom(async (get) => {
  const response = await fetch('/dogs')
  return await response.json()
})
const catsAtom = atom(async (get) => {
  const response = await fetch('/cats')
  return await response.json()
})

const App = () => {
  const [dogs] = useAtom(dogsAtom)
  const [cats] = useAtom(catsAtom)
  // ...
}

However, this will start fetching one at the time, which is not optimal - It would be better if we can start fetching both as soon as possible.

The waitForAll utility is a concurrency helper, which allows us to evaluate multiple async atoms:

const dogsAtom = atom(async (get) => {
  const response = await fetch('/dogs')
  return await response.json()
})
const catsAtom = atom(async (get) => {
  const response = await fetch('/cats')
  return await response.json()
})

const App = () => {
  const [[dogs, cats]] = useAtom(waitForAll([dogsAtom, catsAtom]))
  // or ...
  const [dogs, cats] = useAtomValue(waitForAll([dogsAtom, catsAtom]))
  // ...
}

You can also use waitForAll inside an atom - It's also possible to name them for readability:

const dogsAtom = atom(async (get) => {
  const response = await fetch('/dogs')
  return await response.json()
})
const catsAtom = atom(async (get) => {
  const response = await fetch('/cats')
  return await response.json()
})

const animalsAtom = atom((get) => {
  return get(
    waitForAll({
      dogs: dogsAtom,
      cats: catsAtom,
    })
  )
})

const App = () => {
  const [{ dogs, cats }] = useAtom(animalsAtom)
  // or ...
  const { dogs, cats } = useAtomValue(animalsAtom)
  // ...
}

Codesandbox

https://codesandbox.io/s/react-typescript-forked-krwsv?file=/src/App.tsx