Store

Store is an object that holds the state value. There can be multiple stores.

Store Methods

map(fn: (state: State, lastState?: T) => T)

Creates a derived store. It will call a provided function with the state, when the original store updates, and will use the result to update the derived store

Formulae

const $second = $first.map(fn)

• When $first store is updated, call fn with new state and previous state
• Next update $second store with result of fn() call and trigger all subscribers

Arguments

1. fn (Function): Function that receives state and lastState? and returns a new state for the derived store

If the function returns an old state or if it returns undefined, the new store will not be updated.

Should be pure

Returns

Store: New store

Example

import {createEvent, createStore} from 'effector'

const changed = createEvent()
const title = createStore('').on(changed, (_, newTitle) => newTitle)
const length = title.map(title => title.length)

length.watch(length => {
  console.log('new length', length)
})
// => new length 0

changed('hello')
// => new length 5

changed('world')
// no reaction

changed('hello world')
// => new length 11

Try it

---

on(trigger, handler)

Updates state when trigger is triggered by using handler. For each trigger, last installed handler will override previous handlers (useful for dynamic behavior).

Formulae

$store.on(trigger, handler)

• When trigger is triggered, call handler with payload of the trigger and data of $store
• Next update $store with result of handler() call and trigger all subscribers

Arguments

1. trigger Event, Effect or Store
2. handler (Function): Reducer function that receives state and params and returns a new state, should be pure. A store cannot hold an undefined value. If a reducer function returns undefined, the store will not be updated.
   • state: Current state of store
   • params: Parameters passed to event call

Returns

Store: Current store

Example

import {createEvent, createStore} from 'effector'

const store = createStore(0)
const changed = createEvent()

store.on(changed, (state, params) => state + params)

store.watch(value => {
  console.log('updated', value)
})

changed(2)
// => updated 2

changed(2)
// => updated 4

Try it

---

on(triggers[], handler)

Updates state when any from triggers is triggered by using handler.

Formulae

$store.on([triggerA, triggerB, ...], handler)

• When triggerA or triggerB is triggered, call handler with payload of the triggerA or triggerB and data of $store
• Next update $store with result of handler() call and trigger all subscribers
• Any count of triggers can be passed to triggers

Arguments

1. triggers array of Event, Effect or Store
2. handler (Function): Reducer function that receives state and params and returns a new state, should be pure. A store cannot hold an undefined value. If a reducer function returns undefined, the store will not be updated.
   • state: Current state of store
   • payload: Value passed to event/effect call, or source if it passed as trigger

Returns

Store: Current store

Example

import {createEvent, createStore} from 'effector'

const store = createStore(0)
const changedA = createEvent()
const changedB = createEvent()

store.on([changedA, changedB], (state, params) => state + params)

store.watch(value => {
  console.log('updated', value)
})

changedA(2)
// => updated 2

changedB(2)
// => updated 4

// You can unsubscribe from any trigger
store.off(changedA)

Unsubscribe example

import {createEvent, createStore} from 'effector'

const store = createStore(0)
const changedA = createEvent()
const changedB = createEvent()

// If you want to unsubscribe from all triggers simultaneously, better to manually merge
const changed = merge([changedA, changedB])

store.on(changed, (state, params) => state + params)

store.off(changed)

Try it

---

watch(watcher)

Call watcher function each time when store is updated.
If trigger not passed, run watcher on each event that linked to the store.

Formulae

const unwatch = $store.watch(watcher)

• On initialize and each $store update, call watcher with the new state of $store
• When unwatch is called, stop calling watcher

Arguments

1. watcher (Watcher): Watcher function that receives current store state as the first argument

Returns

Subscription: Unsubscribe function

Example

const add = createEvent()
const store = createStore(0).on(add, (state, payload) => state + payload)

store.watch(value => console.log(`current value: ${value}`))
// => current value: 0
add(4)
// => current value: 4
add(3)
// => current value: 7

---

watch(trigger, watcher)

Run watcher only when trigger event triggered.

Formulae

const unwatch = $store.watch(trigger, watcher)

• On each $store update with passed trigger, call watcher with the new state of $store and payload from trigger
• When unwatch is called, stop calling watcher

Arguments

1. trigger Event, Effect or Store: Trigger, which leads to call of watcher
2. watcher (Function): Function that receives current store state as the first argument and payload of trigger as the second argument.

Returns

Subscription: Unsubscribe function

Example 1

.watch trigger watcher when foo is executed, because foo is explicitly passed to watch.
First argument of watcher is a state value, second is an event value.

import {createEvent, createStore} from 'effector'

const foo = createEvent()
const bar = createEvent()

const store = createStore(0)

store.watch(foo, (storeValue, eventValue) => {
  console.log(`triggered ${storeValue}, ${eventValue}`)
})

foo(1)
// => triggered 0, 1

bar(2)

foo(3)
// => triggered 0, 3

Try it

Example 2

Here .on(bar, ...) changes the state between foo executions. But .watch reacts only to foo event

import {createEvent, createStore} from 'effector'

const foo = createEvent()
const bar = createEvent()

const store = createStore(0).on(bar, (state, value) => value)

store.watch(foo, value => {
  console.log(`triggered ${value}`)
})

foo(1)
// => triggered 0

bar(2)

foo(3)
// => triggered 2

Try it

Example 3

Here watch reacts only to incr and decr events, because it is explicitly used in .on calls. But doesn't react to any other event.

import {createEvent, createStore} from 'effector'

const incr = createEvent()
const decr = createEvent()
const another = createEvent()

const store = createStore(0)
  .on(incr, (state, value) => state + value)
  .on(decr, (state, value) => state - value)

store.watch(value => console.log(`triggered ${value}`))

another(100)
incr(1) // 0 + 1 = 1
incr(2) // 1 + 2 = 3
decr(3) // 3 - 3 = 0
another(200)

Try it

Example with Effect

Effect is an Event with 2 additional events: fail and done.
You can subscribe to fail and done events of the effect.

import {createEffect, createStore} from 'effector'

const effectFx = createEffect().use(
  value => new Promise(res => setTimeout(res, 200, value)),
)

const store = createStore('initial')

store.watch(effectFx, (state, params) => console.log(`executed with ${params}`))

store.watch(effectFx.done, (state, {params, result}) =>
  console.log(`executed with ${params}, resolved with ${result}`),
)

store.watch(effectFx.fail, (state, {params, result}) =>
  console.log(`rejected with ${params}, resolved with ${result}`),
)

effectFx(100)

Try it

Example with another Store

One store can subscribe to updates of another store.

import {createEvent, createStore} from 'effector'

const change = createEvent()

const first = createStore(0).on(change, (state, value) => state + value)

const second = createStore(100)

second.watch(first, (secondState, firstState) =>
  console.log(secondState * firstState),
)

// Change first store and trigger watch in second
change(20)

Output

> 0
> 2000

Try it

Example with watcher

import {createEvent, createStore} from 'effector'

const foo = createEvent()

const store = createStore(0)

store.watch(foo, (storeValue, eventValue) => {
  console.log(`store: ${storeValue}, event: ${eventValue}`)
})

foo(1)

Output

> store: 0, event: 1

Try it

---

reset(...triggers)

Resets store state to the default value.

A state is reset when Event or Effect is called or another Store is changed.

Formulae

$store.reset(...triggers)

• When any unit from triggers list is triggered, update $store with its default state, from createStore(defaultState)

Arguments

1. triggers ((Event | Effect | Store)[]): any number of Events, Effects or Stores

Returns

Store: Current store

Example

import {createEvent, createStore} from 'effector'

const store = createStore(0)
const increment = createEvent()
const reset = createEvent()

store.on(increment, state => state + 1).reset(reset)

store.watch(state => console.log('changed', state))
// changed 0
// watch method calls its function immediately

increment() // changed 1
increment() // changed 2
reset() // changed 0

Try it

---

off(trigger)

$store.off(trigger)

• Removes handler for given trigger, which was installed via $store.on
• If there was no handler for that trigger, this method will do nothing

Arguments

1. trigger: Event, Effect or Store

Returns

Store: Current store

---

getState()

Returns current state of store

Returns

(State): Current state of the store

Example

import {createEvent, createStore} from 'effector'

const store = createStore(0)
const updated = createEvent()

store.on(updated, (state, value) => state + value)

updated(2)
updated(3)

store.watch(console.log) // => 5

Try it

---

thru(fn)

Creates a new store. This method calls a function that receives Store. Other words it's an "escape hatch" for creating composed functions, and also making chains. For example, you want to make multiple, summary and divide operations. You can create these functions and use them in .thru.

Formulae

const $new = $store.thru(fn)

• Call fn with $store as argument
• Set result of the fn() call to $new

Arguments

1. fn (Function): Function that receives Store and returns some value

Returns

(any): Value, returned by fn

Example

import {createStore} from 'effector'

const sum = value => value + 10
const square = value => value * value
const divide = value => value / 2

const enhance = fn => store => store.map(fn)

const store = createStore(0)
const newStore = store
  .thru(enhance(sum))
  .thru(enhance(square))
  .thru(enhance(divide))
  .watch(state => console.log(`newStore: ${state}`))

Output

// sum: 10
// square: 100
// divide: 50

> newStore: 50

Try it

---

Store Properties

updates

Formulae

$store.updates

• When $store is changed trigger updates event with the new state

Returns

Event: Event that represents updates of the given store.

Use case: watchers, which will not trigger immediately after creation (unlike store.watch)

import {createStore, is} from 'effector'

const clicksAmount = createStore(0)
is.event(clicksAmount.updates) // => true

clicksAmount.watch(amount => {
  console.log('will be triggered with current state, immediately, sync', amount)
})

clicksAmount.updates.watch(amount => {
  console.log('will not be triggered unless store value is changed', amount)
})

Try it

---

shortName

Returns

(string): ID or short name of the store

---

defaultState

Returns

(State): Default state of the store

Example

const $store = createStore('DEFAULT')

console.log($store.defaultState === 'DEFAULT')
// => true