effect.js

import { createStore, createEffect } from 'effector'

const getPost = createEffect('Fetch posts')

const $isLoading = createStore(false)
const $post = createStore(null)
const $error = createStore(null)

$isLoading
  .on(getPost, () => true)
  .on(getPost.done, () => false)
  .on(getPost.fail, () => false)

$post
  .on(getPost, () => null)
  .on(getPost.done, (state, { result }) => result)

$error
  .on(getPost, () => null)
  .on(getPost.fail, (state, { error }) => error)

// Fetch post on effect call
getPost.use(({ id }) => fetch(`/posts/${id}`).then(r => r.json()))

// When effect is called
getPost.watch(params => {
  console.log(params) // { id }
})

// When effect is resolved
getPost.done.watch(({ params, result }) => {
  console.log(params)  // { id }
  console.log(result)  // Response JSON
})

// When effect is rejected
getPost.fail.watch(({ params, error }) => {
  console.log(params)  // { id }
  console.log(error)   // Error 
})

// Just call getPost with payload
getPost({ id: 1 })