Glossary of basic terms in effector.


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


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

Store in api documentation


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 the result of a function call.

The only requirement for the function:

  • Must have zero or one argument

Effect in api documentation


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


Data type used to describe business logic of applications. Most of the effector methods deal with unit processing. There are five unit types: Store, Event, Effect, Domain and Scope.

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 as a source of reactive updates.


Most of the 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 submitLoginSize = createEvent();

const $login = createStore("guest");
const $loginSize = $ => login.length);

$ => {

Try it


Better, declarative:

import { createStore, createEvent, sample } from "effector";

const submitLoginSize = createEvent();

const $login = createStore("guest");
const $loginSize = $ => login.length);

  clock: $loginSize,
  target: submitLoginSize,

Try it

Reference: sample


import { createStore, createEvent } from "effector";

const submitLoginSize = createEvent();

const $login = createStore("guest");
const $loginSize = $ => {
  // no! use `sample` 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.


import { type Subscription } from "effector";

Looks like:

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

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


Managing subscriptions manually distracts from business logic improvements.

Effector provides a wide range of features to minimize the need to remove subscriptions. This sets it apart from most other reactive libraries.