combine

This method allows you to get state from each passed store and combine it to single value and save to single store, that updates every time like each passed store.

Caution

Combine returns not just a common store. Instead, it returns DerivedStore, it cannot be modified by the events or used as target in sample.

combine(...stores, fn)

Formulae

$result = combine(
  $first, $second, $third, ...,
  (first, second, third, ...) => result
)

• After call combine, state of each store is extracted and passed to function arguments, result of a function call will be state of store $result
• Any amount of stores can be passed to combine, but latest argument always should be function-reducer that returns new state
• If function returned the same result as previous, store $result will not be triggered
• If several stores updated at the same time (during one tick) there will be single call of function and single update of $result store

Returns

DerivedStore: New derived store

Example

import {createStore, combine} from 'effector'

const $balance = createStore(0)
const $username = createStore('zerobias')

const $greeting = combine($balance, $username, (balance, username) => {
  return `Hello, ${username}. Your balance is ${balance}`
})

$greeting.watch(data => console.log(data)) // => Hello, zerobias. Your balance is 0

const $arrStores = combine([$balance, $username])
$arrStores.watch(console.log) // => [0, 'zerobias']

Try it

combine({ A, B, C }, fn?)

note

Formerly known as createStoreObject

Formulae

$result = combine({a: $first, b: $second, c: $third})

• Read state from stores $first, $second, $third and assign it to properties a, b, c accordingly, that object will be saved to $result store
• Store $result contain object {a, b, c} and will be updated on each update of passed stores
• If several stores updated at the same time (during one tick) there will be single update of $result store

Formulae with fn

$result = combine({a: $first, b: $second, c: $third}, ({a, b, c}) => result)

• Read state from stores $first, $second, $third and assign it to properties a, b, c accordingly, calls function with that object
• The result of the function call saved in $result store
• If function returned the same result as previous, store $result will not be triggered
• If several stores updated at the same time (during one tick) there will be single call of function and single update of $result store

Returns

DerivedStore: New derived store

Example

import {createStore, combine} from 'effector'

const $r = createStore(255)
const $g = createStore(0)
const $b = createStore(255)

const $color = combine({r: $r, g: $g, b: $b})
$color.watch(console.log) // => {r: 255, b: 0, b: 255}

const $sum = combine({r: $r, g: $g, b: $b}, ({r, g, b}) => r + g + b)
$sum.watch(console.log) // => 510

Try it

combine([A, B, C], fn?)

Formulae

$result = combine([$first, $second, $third], ([A, B, C]) => result)

• Read state from stores $first, $second, $third and assign it to array with the same order as passed stores, calls function with that array
• The result of the function call saved in $result store
• If function returned the same result as previous, store $result will not be triggered
• If several stores updated at the same time (during one tick) there will be single call of function and single update of $result store

Formulae without fn

$result = combine([$first, $second, $third])

• Read state from stores $first, $second, $third and assign it to array with the same order as passed stores, that array will be saved to $result store
• Store $result will be updated on each update of passed stores
• If several stores updated at the same time (during one tick) there will be single update of $result store

Returns

DerivedStore: New derived store

Example

import {createStore, combine} from 'effector'

const $r = createStore(255)
const $g = createStore(0)
const $b = createStore(255)

const $color = combine([$r, $g, $b])
$color.watch(console.log)
// => [255, 0, 255]

const $sum = combine([r, g, b], ([r, g, b]) => r + g + b)
$sum.watch(console.log)
// => 510

Try it

combine with primitives and objects

It works the same as before. Now primitives and objects can be used in combine, and combine will not be triggered. Effector will not track mutations of objects and primitives.

Example

const $a = createStore('a')
const b = 2
const c = [false]
const d = {value: 1}

const $resultUsingComa = combine($a, b, c, d)
const $resultUsingArray = combine([$a, b, c, d])
const $resultUsingObject = combine({$a, b, c, d})

const $withFn = combine($a, b, c, d, (a, b) => ({a, b}))

$resultUsingComa.watch(console.log)
// => ["a", 2, [false], {value: 1}]
$resultUsingArray.watch(console.log)
// => ["a", 2, [false], {value: 1}]
$resultUsingObject.watch(console.log)
// => {$a: "a", b: 2, c: [false], d: {value: 1}}
$withFn.watch(console.log)
// => {a: "a", b: 2}

// will not trigger combine, but object and array will be changed because of reference
// uncomment the code below to see changes
// c.push(true)
// d.value = 2

Try it