Event

Event is an intention to change the state. Let's imagine life situation, you enter a shop and, according to etiquette, you have to say "hello" - intention, then you say "hello" - event.

Event calls always return its payload:

import {createEvent} from 'effector'

const event = createEvent()

console.log(event(1))
// => 1
console.log(event())
// => undefined

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Event Methods

watch(watcher)

It is a function which allows you to watch the event or to create side effects.

Formulae

const unwatch = event.watch(fn)

• Call fn on each event trigger, pass payload of event as argument to fn
• When unwatch is called, stop calling fn on each event trigger

Arguments

1. watcher (Watcher): A function that receives payload.

Returns

Subscription: Unsubscribe function.

Example

import {createEvent} from 'effector'

const sayHi = createEvent()
const unwatch = sayHi.watch(name => console.log(`${name}, hi there!`))

sayHi('Peter') // => Peter, hi there!
unwatch()

sayHi('Drew') // => nothing happened

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---

map(fn)

Creates a new event, which will be called after the original event is called, applying the result of a fn as a payload. It is special function which allows you to decompose dataflow, extract or transform data.

Formulae

const second = first.map(fn)

• When first is triggered, pass payload from first to fn
• Trigger second with the result of the fn() call as payload

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

Example

import {createEvent} from 'effector'

const userUpdated = createEvent()
const userNameUpdated = userUpdated.map(({name}) => name) // you may decompose dataflow with .map() method
const userRoleUpdated = userUpdated.map(({role}) => role.toUpperCase()) // either way you can transform data

userNameUpdated.watch(name => console.log(`User's name is [${name}] now`))
userRoleUpdated.watch(role => console.log(`User's role is [${role}] now`))

userUpdated({name: 'john', role: 'admin'})
// => User's name is [john] now
// => User's role is [ADMIN] now

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---

prepend(fn)

Creates an event, upon trigger it sends transformed data into the source event. Works kind of like reverse .map. In case of .prepend data transforms before the original event occurs and in the case of .map, data transforms after original event occurred.

If original event belongs to some domain then new event will belong to it as well

Formulae

const second = first.prepend(fn)

• When second event is triggered
• Call fn with payload from second
• Trigger first with result of fn()

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

Example

import {createEvent} from 'effector'

const userPropertyChanged = createEvent()

userPropertyChanged.watch(({field, value}) => {
  console.log(`User property "${field}" changed to ${value}`)
})

const changeName = userPropertyChanged.prepend(name => ({
  field: 'name',
  value: name,
}))
const changeRole = userPropertyChanged.prepend(role => ({
  field: 'role',
  value: role.toUpperCase(),
}))

changeName('john')
// => User property "name" changed to john

changeRole('admin')
// => User property "role" changed to ADMIN

changeName('alice')
// => User property "name" changed to alice

Try it

---

filterMap(fn)

since

effector 20.0.0

Creates a new event, which will be called after the original event is called if fn returns a value other than undefined.
Consider a scenario where you walk into a grocery store with a specific task: you need to purchase 10 apples, but only if they're red. If they're not red, you're out of luck. Let's consider by steps:

1. Take one apple;
2. Have a look, is it red(put in a pack) or not(take another).

And you repeat this until you complete the task. Now think about it in the effector terms, and we consider the positive case:

1. Take an apple - event;
2. Have a look, red or no - filter;
3. You keep it - map;
4. Put in pack - event.
5. Pack - store

You may see that we united filter() and map() methods, the reason for creating was an impossibility to event filtering. The method is useful with JavaScript APIs those returns undefined.

Formulae

const second = first.filterMap(fn)

• When first is triggered, call fn with payload from first
  • If fn() returned undefined do not trigger second
  • If fn() returned some data, trigger second with data from fn()

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

Example

const listReceived = createEvent<string[]>()
const effectorFound = listReceived.filterMap(list => list.find(name => name === 'effector'))

effectorFound.watch(name => console.info("found", name))
listReceived(["redux", "effector", "mobx"]) // found effector
listReceived(["redux", "mobx"])

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---

filter({fn})

Creates a new event, which will be called after the original event is called if fn returns true.

Let's assume a standard situation when you want to buy sneakers in the shop, but there is no size. You subscribe to a particular size of the sneakers model, and in addition, you want to receive a notification if they have it, and ignore any other notification. Therefore, filtering can be helpful for that. Event filtering works in the same way. If filter returns true, the event will be called.

Formulae

const second = first.filter({fn})

• When first is triggered, pass payload from first to fn
• If fn() returns true, second will be triggered with payload from first

Arguments

1. fn (Function): A function that receives payload, should be pure.

Returns

Event: New event.

note

Object form is used because event.filter(fn) was an alias for event.filterMap

note

guard method is the preferred filtering method

Example

import {createEvent, createStore} from 'effector'

const numbers = createEvent()
const positiveNumbers = numbers.filter({
  fn: ({x}) => x > 0,
})

const $lastPositive = createStore(0).on(positiveNumbers, (n, {x}) => x)

$lastPositive.watch(x => {
  console.log('last positive:', x)
})

// => last positive: 0

numbers({x: 0})
// no reaction

numbers({x: -10})
// no reaction

numbers({x: 10})
// => last positive: 10

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