Glossary of basic terms in effector, along with their type signatures. The types are documented using TypeScript notation.


Event is a function you can subscribe to. It can be an intention to change the store, indication of something happening in the application, a command to be executed, aggregated analytics trigger and so on.

Event in api documentation

function createEvent<E>(eventName?: string): Event<E>
type Event<Payload> = {
(payload: Payload): Payload
watch(watcher: (payload: Payload) => any): Subscription
map<T>(fn: (payload: Payload) => T): Event<T>
filter(options: {fn(payload: Payload): boolean}): Event<Payload>
filterMap<T>(fn: (payload: Payload) => T | void): Event<T>
prepend<Before>(fn: (params: Before) => Payload): Event<Before>
  • (payload) calls Event with payload
  • watch(watcher) listens to the event and calls provided watcher
  • map(fn) creates a new event, which will be trigger after the original event was triggered, transforming the payload with provided fn
  • filter({fn}) creates a new event that will tigger only if provided fn function returns true
  • filterMap(fn) creates a new event that will be triggered with the result of fn applied to the payload, but only if the result is not undefined. Use cases: extract value from react's refs; statically typed filters;
  • prepend(fn) creates a new event that will trigger the original event with a payload transformed by fn


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

Store in api documentation

function createStore<State>(defaultState: State): Store<State>
type Store<State> = {
trigger: Event<E> | Effect<E, any, any> | Store<E>,
reducer: (state: State, payload: E) => State | void,
): this
map<T>(fn: (_: State) => T): Store<T>
...triggers: Array<Event<any> | Effect<any, any, any> | Store<any>>
): this
watch<E>(watcher: (state: State) => any): Subscription
updates: Event<State>
defaultState: State


Effect is a container for (possibly async) side effects. It exposes special events and stores, such as pending, done, fail, finally, etc...

It can be safely used in place of the original async function.

It returns promise with result of a function call

The only requirement for the function:

  • Must have zero or one argument

Effect in api documentation

function createEffect<Params, Done, Fail>(config?: {
handler?: (params: Params) => Promise<Done> | Done
}): Effect<Params, Done, Fail>
type Effect<Params, Done, Fail = Error> = {
(payload: Params): Promise<Done>
doneData: Event<Done>
failData: Event<Fail>
done: Event<{params: Params; result: Done}>
fail: Event<{params: Params; error: Fail}>
pending: Store<boolean>
inFlight: Store<number>
use: {
(asyncFunction: (params: Params) => Promise<Done>): this
getCurrent(): (params: Params) => Promise<Done>
watch(watcher: (payload: Params) => any): Subscription
  • (payload) starts the Effect with payload and returns a Promise
  • use(function) replaces the handler in the effect (can be called multiple times)
  • watch(watcher) listens to the effect and calls provided watcher when effect starts


Domain is a namespace for your events, stores and effects.

Domains are notified when events, stores, effects, or nested domains are created via onCreateEvent, onCreateStore, onCreateEffect, onCreateDomain methods.

It is useful for logging or other side effects.

Domain in api documentation

function createDomain(domainName?: string): Domain
type Domain = {
onCreateEvent(hook: (newEvent: Event<unknown>) => any): Subscription
hook: (newEffect: Effect<unknown, unknown, unknown>) => any,
): Subscription
onCreateStore(hook: (newStore: Store<unknown>) => any): Subscription
onCreateDomain(hook: (newDomain: Domain) => any): Subscription
createEvent<Payload>(name?: string): Event<Payload>
createEffect<Params, Done, Fail>(name?: string): Effect<Params, Done, Fail>
createStore<State>(defaultState: State): Store<State>
createDomain(name?: string): Domain


Data type used to describe business logic of applications. Most of the effector methods deal with unit processing. Three are four units types: store, event, effect and domain

Common unit

Common units can be used to trigger updates of other units. There are three common unit types: store, event and effect. When a method accepts units, it means that it accepts events, effects, and stores a source of reactive updates


Most of functions in api must not call other events or effects: it's easier to reason about application's data flow when imperative triggers are grouped inside watchers and effect handlers rather than spread across entire business logic.

Correct, imperative:

import {createStore, createEvent} from 'effector'
const login = createStore('guest')
const loginSize = => login.length)
const submitLoginSize = createEvent() => {

Try it in docs in docs

Correct, declarative:

import {createStore, createEvent, forward} from 'effector'
const login = createStore('guest')
const loginSize = => login.length)
const submitLoginSize = createEvent()
from: loginSize,
to: submitLoginSize,

Try it

forward in docs


import {createStore, createEvent, forward} from 'effector'
const submitLoginSize = createEvent()
const login = createStore('guest')
const loginSize = => {
// no! use forward or watch instead
return login.length


type StoreReducer<State, E> = (state: State, payload: E) => State | void
type EventOrEffectReducer<T, E> = (state: T, payload: E) => T

Reducer calculates a new state given the previous state and an event's payload. For stores, if reducer returns undefined or the same state (===), then there will be no update for a given store.


type Watcher<T> = (update: T) => any

Watcher is used for side effects. Accepted by, and domain.onCreate* hooks. Return value of a watcher is ignored.


type Subscription = {
(): void
unsubscribe(): void

Function, returned by forward,, and some others methods. Used for cancelling a subscription. After first call, subscription will do nothing

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