attach

since

effector 20.13.0

Wrapper for effect, which allows to map effect arguments and use data from stores.

Use cases: declarative way to pass values from stores to effects and argument preprocessing.

Formulae

attach({ effect, mapParams?, source?, name? }): newEffect

When newEffect is called, call mapParams with params of the newEffect and data from source, then call original effect

• If attach called without source, mapParams will be called only with params of the newEffect
• attach() always returns new Effect

Short example

import {createEffect, attach} from 'effector'

const original = createEffect(params => {

console.log('Original effect called with', params)

})

const created = attach({

effect: original,

mapParams: params => {

console.log('Created effect called with', params)

return {wrapped: params}

},

})

await created('HELLO')

// => Created effect called with "HELLO"

// => Original effect called with { wrapped: "HELLO" }

Try it

Short example with source

import {createEffect, attach} from 'effector'

const original = createEffect(params => {

console.log('Original effect called with', params)

})

const data = createStore(8900)

const created = attach({

effect: original,

source: data,

mapParams: (params, data) => {

console.log('Created effect called with', params, 'and data', data)

return {wrapped: params, data}

},

})

await created('HELLO')

// => Created effect called with "HELLO" and data 8900

// => Original effect called with {wrapped: "HELLO", data: 8900}

Try it

Long example

import {createEffect, attach, createStore} from 'effector'

const backendRequest = createEffect(async ({token, data, resource}) => {

return fetch(`https://example.com/api${resource}`, {

method: 'POST',

headers: {

Authorization: `Bearer ${token}`,

},

body: JSON.stringify(data),

})

})

const requestsSend = createStore(0).on(backendRequest, total => total + 1)

requestsSend.watch(total => {

console.log(`client analytics: sent ${total} requests`)

})

const token = createStore('guest_token')

const authorizedRequest = attach({

effect: backendRequest,

source: token,

mapParams: ({data, resource}, token) => ({data, resource, token}),

})

const createRequest = resource =>

attach({

effect: authorizedRequest,

mapParams: data => ({data, resource}),

})

const getUser = createRequest('/user')

const getPosts = createRequest('/posts')

const user = await getUser({name: 'alice'})

/*

POST https://example.com/api/user

{"name": "alice"}

Authorization: Bearer guest_token

*/

// => client analytics: sent 1 requests

const posts = await getPosts({user: user.id})

/*

POST https://example.com/api/posts

{"user": 18329}

Authorization: Bearer guest_token

*/

// => client analytics: sent 2 requests

attach({effect, source})

Create effect which will trigger given one with values from source stores

Arguments

• effect (Effect): Wrapped effect
• source (Store | {[key: string]: Store}): Store or object with stores, values of which will be passed to the second argument of mapParams

Returns

Effect: New effect

attach({effect, mapParams})

Create effect which will trigger given one by transforming params by mapParams function

Arguments

• effect (Effect): Wrapped effect
• mapParams ((newParams) => effectParams): Function which receives new params and maps them to the params of the wrapped effect. Works like event.prepend. Errors happened in mapParams function will force attached effect to fail

Returns

Effect: New effect

attach({effect, mapParams, source})

Create effect which will read values from source stores, pass them with params to mapParams function and call effect with result

Arguments

• effect (Effect): Wrapped effect
• mapParams ((newParams, values) => effectParams): Function which receives new params and current value of source and combines them to the params of the wrapped effect. Errors happened in mapParams function will force attached effect to fail
• source (Store | {[key: string]: Store}): Store or object with stores, values of which will be passed to the second argument of mapParams

Returns

Effect: New effect

note

If mapParams throw an error, it will trigger fail event and nested effect will not be called at all

attach({effect})

Create effect which will call effect with params as it is. That allow to create separate effects with shared behavior.

Arguments

• effect (Effect): Wrapped effect

Returns

Effect: New effect

since

effector 21.5.0