introduction.mdx

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Introduction	How to use Jotai	0

npm install jotai or yarn add jotai

Jotai is pronounced "joe-tie" and means "state" in Japanese.

You can try live demos here: Demo 1 | Demo 2.

How does Jotai differ from Recoil?

• Minimalistic API
• No string keys
• TypeScript oriented

First create a primitive atom

An atom represents a piece of state. All you need is to specify an initial value, which can be a primitive value like a string, number, object or array. You can create as many primitive atoms as you like.

import { atom } from 'jotai'

const countAtom = atom(0)
const countryAtom = atom('Japan')
const citiesAtom = atom(['Tokyo', 'Kyoto', 'Osaka'])
const mangaAtom = atom({ 'Dragon Ball': 1984, 'One Piece': 1997, Naruto: 1999 })

Use the atom in your components

Use an atom like you'd use React.useState:

import { useAtom } from 'jotai'

function Counter() {
  const [count, setCount] = useAtom(countAtom)
  return (
    <h1>
      {count}
      <button onClick={() => setCount(c => c + 1)}>one up</button>

Create derived atoms with computed values

A new read-only atom can be created from existing atoms by passing a read function as the first argument. get allows you to fetch the contextual value of any atom.

const doubledCountAtom = atom((get) => get(countAtom) * 2)

function DoubleCounter() {
  const [doubledCount] = useAtom(doubledCountAtom)
  return <h2>{doubledCount}</h2>

Recipes

Creating an atom from atoms

You can combine atoms to create a derived atom.

const count1 = atom(1)
const count2 = atom(2)
const count3 = atom(3)

const sum = atom((get) => get(count1) + get(count2) + get(count3))

Or if you like fp patterns ...

const atoms = [count1, count2, count3, ...otherAtoms]
const sum = atom((get) => atoms.map(get).reduce((acc, count) => acc + count))

Derived async atoms

You can make the read function an async function.

const urlAtom = atom("https://json.host.com")
const fetchUrlAtom = atom(
  async (get) => {
    const response = await fetch(get(urlAtom))
    return await response.json()
  }
)

function Status() {
  // Re-renders the component after urlAtom changed and the async function above concludes
  const [json] = useAtom(fetchUrlAtom)

You can create a writable derived atom

Specify a write function at the second argument. get will return the current value of an atom. set will update an atom's value.

const decrementCountAtom = atom(
  (get) => get(countAtom),
  (get, set, _arg) => set(countAtom, get(countAtom) - 1),
)

function Counter() {
  const [count, decrement] = useAtom(decrementCountAtom)
  return (
    <h1>
      {count}
      <button onClick={decrement}>Decrease</button>

Write only atoms

To make a write-only atom, don't define a read function by passing null as the first argument.

const multiplyCountAtom = atom(null, (get, set, by) => set(countAtom, get(countAtom) * by))

function Controls() {
  const [, multiply] = useAtom(multiplyCountAtom)
  return <button onClick={() => multiply(3)}>triple</button>

Async actions

Make the write function an async function, and call set when you're ready.

const fetchCountAtom = atom(
  (get) => get(countAtom),
  async (_get, set, url) => {
    const response = await fetch(url)
    set(countAtom, (await response.json()).count)
  }
)

function Controls() {
  const [count, compute] = useAtom(fetchCountAtom)
  return <button onClick={() => compute("http://count.host.com")}>compute</button>