Effect
jotai-effect is a utility package for reactive side effects.
install
npm i jotai-effect
atomEffect
atomEffect
is a utility function for declaring side effects and synchronizing atoms in Jotai. It is useful for observing and reacting to state changes.
Parameters
type CleanupFn = () => voidtype EffectFn = (get: Getter & { peek: Getter },set: Setter & { recurse: Setter },) => CleanupFn | voiddeclare function atomEffect(effectFn: EffectFn): Atom<void>
effectFn (required): A function for listening to state updates with get
and writing state updates with set
. The effectFn
is useful for creating side effects that interact with other Jotai atoms. You can cleanup these side effects by returning a cleanup function.
Usage
Subscribe to Atom Changes
import { atomEffect } from 'jotai-effect'const loggingEffect = atomEffect((get, set) => {// runs on mount or whenever someAtom changesconst value = get(someAtom)loggingService.setValue(value)})
Setup and Teardown Side Effects
import { atomEffect } from 'jotai-effect'const subscriptionEffect = atomEffect((get, set) => {const unsubscribe = subscribe((value) => {set(valueAtom, value)})return unsubscribe})
Mounting with Atoms or Hooks
After defining an effect using atomEffect
, it can be integrated within another atom's read function or passed to Jotai hooks.
const anAtom = atom((get) => {// mounts the atomEffect when anAtom mountsget(loggingEffect)// ...})// mounts the atomEffect when the component mountsfunction MyComponent() {useAtom(subscriptionEffect)// ...}
The atomEffect
behavior
Cleanup Function: The cleanup function is invoked on unmount or before re-evaluation.
Example
atomEffect((get, set) => {const intervalId = setInterval(() => set(clockAtom, Date.now()))return () => clearInterval(intervalId)})Resistant To Infinite Loops:
atomEffect
does not rerun when it changes a value withset
that it is watching.Example
const countAtom = atom(0)atomEffect((get, set) => {// this will not infinite loopget(countAtom) // after mount, count will be 1set(countAtom, increment)})Supports Recursion: Recursion is supported with
set.recurse
for both sync and async use cases, but is not supported in the cleanup function.Example
const countAtom = atom(0)atomEffect((get, set) => {// increments count once per secondconst count = get(countAtom)const timeoutId = setTimeout(() => {set.recurse(countAtom, increment)}, 1000)return () => clearTimeout(timeoutId)})Supports Peek: Read atom data without subscribing to changes with
get.peek
.Example
const countAtom = atom(0)atomEffect((get, set) => {// will not rerun when countAtom changesconst count = get.peek(countAtom)})Executes In The Next Microtask:
effectFn
runs in the next available microtask, after all Jotai synchronous read evaluations have completed.Example
const countAtom = atom(0)const logAtom = atom([])const logCounts = atomEffect((get, set) => {set(logAtom, (curr) => [...curr, get(countAtom)])})const setCountAndReadLog = atom(null, async (get, set) => {get(logAtom) // [0]set(countAtom, increment) // effect runs in next microtaskget(logAtom) // [0]await Promise.resolve().then()get(logAtom) // [0, 1]})store.set(setCountAndReadLog)Batches Synchronous Updates (Atomic Transactions): Multiple synchronous updates to
atomEffect
atom dependencies are batched. The effect is run with the final values as a single atomic transaction.Example
const enabledAtom = atom(false)const countAtom = atom(0)const updateEnabledAndCount = atom(null, (get, set) => {set(enabledAtom, (value) => !value)set(countAtom, (value) => value + 1)})const combos = atom([])const combosEffect = atomEffect((get, set) => {set(combos, (arr) => [...arr, [get(enabledAtom), get(countAtom)]])})store.set(updateEnabledAndCount)store.get(combos) // [[false, 0], [true, 1]]Conditionally Running atomEffect:
atomEffect
is active only when it is mounted within the application. This prevents unnecessary computations and side effects when they are not needed. You can disable the effect by unmounting it.Example
atom((get) => {if (get(isEnabledAtom)) {get(effectAtom)}})Idempotent:
atomEffect
runs once when state changes regardless of how many times it is mounted.Example
let i = 0const effectAtom = atomEffect(() => {get(countAtom)i++})const mountTwice = atom(() => {get(effectAtom)get(effectAtom)})store.set(countAtom, increment)Promise.resolve.then(() => {console.log(i) // 1})
Dependency Management
Aside from mount events, the effect runs when any of its dependencies change value.
Sync: All atoms accessed with
get
during the synchronous evaluation of the effect are added to the atom's internal dependency map.Example
atomEffect((get, set) => {// updates whenever `anAtom` changes value but not when `anotherAtom` changes valueget(anAtom)setTimeout(() => {get(anotherAtom)}, 5000)})Async: For async effects, you should use an abort controller to cancel pending fetch requests and promises.
Example
atomEffect((get, set) => {const count = get(countAtom) // countAtom is an atom dependencyconst abortController = new AbortController();(async () => {try {await delay(1000)abortController.signal.throwIfAborted()get(dataAtom) // dataAtom is not an atom dependency} catch (e) {if (e instanceof AbortError) {// async cleanup logic here} else {console.error(e)}}})()return () => {// abort when countAtom changesabortController.abort(new AbortError())}})Cleanup: Accessing atoms with
get
in the cleanup function does not add them to the atom's internal dependency map.Example
atomEffect((get, set) => {// runs once on mount// does not update when `idAtom` changesconst unsubscribe = subscribe((valueAtom) => {const value = get(valueAtom)// ...})return () => {const id = get(idAtom)unsubscribe(id)}})Recalculation of Dependency Map: The dependency map is recalculated on every run. If an atom was not watched during the current run, it will not be in the current run's dependency map. Only actively watched atoms are considered dependencies.
Example
const isEnabledAtom = atom(true)atomEffect((get, set) => {// if `isEnabledAtom` is true, runs when `isEnabledAtom` or `anAtom` changes value// otherwise runs when `isEnabledAtom` or `anotherAtom` changes valueif (get(isEnabledAtom)) {const aValue = get(anAtom)} else {const anotherValue = get(anotherAtom)}})
Comparison with useEffect
Component Side Effects
useEffect is a React Hook that lets you synchronize a component with an external system.
Hooks are functions that let you “hook into” React state and lifecycle features from function components. They are a way to reuse, but not centralize, stateful logic. Each call to a hook has a completely isolated state. This isolation can be referred to as component-scoped. For synchronizing component props and state with a Jotai atom, you should use the useEffect hook.
Global Side Effects
For setting up global side-effects, deciding between useEffect and atomEffect comes down to developer preference. Whether you prefer to build this logic directly into the component or build this logic into the Jotai state model depends on what mental model you adopt.
atomEffects are more appropriate for modeling behavior in atoms. They are scoped to the store context rather than the component. This guarantees that a single effect will be used regardless of how many calls they have.
The same guarantee can be achieved with the useEffect hook if you ensure that the useEffect is idempotent.
atomEffects are distinguished from useEffect in a few other ways. They can directly react to atom state changes, are resistant to infinite loops, and can be mounted conditionally.
It's up to you
Both useEffect and atomEffect have their own advantages and applications. Your project’s specific needs and your comfort level should guide your selection. Always lean towards an approach that gives you a smoother, more intuitive development experience. Happy coding!