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Easily manage promises in reactive JavaScript libraries and frameworks such as React.js, Vue.js, Angular, Svelte, Alpine.js, etc.


npm i promistate


Comparison (example with Vue.js)

Without promistate

  • Need to manage promise result, error and pending status all as separate state
  • Need to manage a computed field to determine whether the result is empty or not
  • Need to write custom logic to ignore stale promises / cancel promises
  • results in bloated code as all of this is repeated in many components
    <div v-if="error">Whoops!</div>
    <div v-else-if="isPending">loading...</div>
    <div v-else-if="users.length === 0">no results...</div>
    <UserList v-else :users="users" />

export default {
    data() {
        return { users: [], isPending: true, error: null }
    async created() {
        const groupId = this.$route.params.groupId
        this.isPending = true
        try {
            this.users = await fetch(`/api/${groupId}/users`).then(res => res.json())
        } catch(error) {
            this.error = error
        this.isPending = false

With promistate

All state changes handled internally and exposed through a single object 👍.

    <div v-if="userPromise.error">Whoops!</div>
    <div v-else-if="userPromise.isPending">loading...</div>
    <div v-else-if="userPromise.isEmpty">no results...</div>
    <UserList v-else :users="userPromise.value" />

import promistate from 'promistate'

export default {
    data: ({
        userPromise: promistate(groupId => fetch(`/api/${groupId}/users`).then(res => res.json()))
    created() {


import promistate from 'promistate'

const userPromise = promistate(async function callback(id) {
    return fetch('/api/user/' + id).then(res => res.json()) // any promise

// later...
console.log(userPromise.value) // null
await userPromise.load(1)
console.log(userPromise.value) // { id: 1, name: '...' }

The callback passed into promistate gets executed once you call the "load" method.

Calling "promistate()" immediately returns an object that has the following properties.

(See below for react hook example)

field description
load A method to call the previously passed in callback. Arguments get propogated to callback
value Holds the resolved promise result
isPending Defines if promise is currently pending
isDelayOver Defines if promise is currently pending and the given delay (see configurations) has elapsed.
timesSettled counts how many times a promise was settled. Sometimes you want to wait until a promise was settled
isEmpty Defines if there is a result. Conveniently switches to false when promise is pending. isEmpty is true when the result is an empty array, empty object, null or undefined
error Error object in case promise was rejected
reset A method to reset all state (value, isEmpty, error, isPending)


You can pass in arguments as needed

const calculator = promistate(async function callback(num1, num2) {
    return num1 + num2

await calculator.load(1, 2)

"load" returns a status message about the promise. This can be either

  • IGNORED (see configurations below)

This can be useful if you have to do more work after loading a promise. Note how there is no need to reach for this in the example at the top.

To avoid hardcoding these, you can import "PromistateStatus" from the library

import promistate, { PromistateStatus } from 'promistate'

const userPromise = promistate(() => fetch('...'))

if (await userPromise.load() === PromistateStatus.RESOLVED) {
    console.log("It's resolved!", userPromise.value)


Pass configurations as the second argument

import promistate from 'promistate'

promistate(async function callback() {
    return somethingAsync()
}, { catchErrors: false, defaultValue: 42 })
key type default use case
catchErrors boolean true You already use something like an ErrorBoundary component for catching errors
defaultValue any null You already have a value at hand, or want to default it to an empty array, object, etc.
ignoreStaleLoad boolean false If you "load" while there is already a promise pending, this will ignore any stale promise results. By calling "reset" you can also cancel promises this way.
ignoreLoadWhenPending boolean false Prevent an event being fired twice e.g. when clicking a button. With this boolean set, while the first promise is still pending, subsequent loads would be ignored (not deferred!). When a subsequent load gets ignored, the "load" method returns the status "IGNORED"
delay number 200 Specifies after how many ms isDelayOver will be set to true. This is useful to avoid flashing loading spinners for fast requests.
isEmpty Function undefined Say, the result is { page: 1, items: [] }, the default "isEmpty" would always evaluate to false since a filled object is considered not empty. You can tweak the check like this: { isEmpty: value => value.items.length < 1 }
listen Function undefined Listen to any state changes. Useful for integrations in libraries like Svelte or React.js

usage with React.js

Usage with react differs in two ways

  1. Import usePromistate from promistate/lib/react
  2. usePromistate returns a tuple with the first value holding the state, and the second value holding all methods to update the state
import React from "react";
import { usePromistate } from "promistate/lib/react";

export default function App() {
  const [todosPromise, todoActions] = usePromistate(somePromise);
  // todosPromise.value, todosPromise.isPending, ...
  // todoActions.load(), todoActions.reset(), todoActions.setValue('new Value')

You can pass promistate options as the second argument usePromistate(somePromise, { defaultValue: "" }), and promise depedencies as the third argument usePromistate(somePromise, { }, [dep1, dep2]).


To type the result of the promise you can make use of generics.

import promistate from 'promistate'

promistate<string>(async function callback() {
    return 'updated'
}, { defaultValue: 'initial' })


Can I use this for POST/PUT/DELETE requests as well?


an API call returns a page token which I need the next time I make a request to fetch the next page, I also need to append the data to the previous result

As long as you don't use arrow functions you can access the state using this.

import promistate from 'promistate'

promistate(async function callback() {
    const result = await fetchItems(this.value.pageToken)
    return { pageToken: result.pageToken, items: this.items.concat(result.items) }
}, {
    defaultValue: { items: [], pageToken: null },
    isEmpty: value => value.items.length < 1,

I need to manually change the value of a promise

Often times you want to reset the promise to its initial state. For this you can use the "reset" method.

But of course you can still mutate the value directly.

import promistate from 'promistate'

const promise = promistate(() => fetch('...'))

promise.value // null
promise.isEmpty // true

promise.value = 2
promise.isEmpty // false

If you use the react hook, do it like this instead:

import { usePromistate } from 'promistate/lib/react'

const [promise, actions] = usePromistate(() => fetch('...'))


The same way you can also set the error programmatically.

import promistate from 'promistate'

const promise = promistate(() => fetch('...'))

promise.error = new Error('internal error')

or with the react hook:

import { usePromistate } from 'promistate/lib/react'

const [promise, actions] = usePromistate(() => fetch('...'))

actions.setError(new Error('internal error'))

I need to make a lot of ajax requests, is there something better than regular fetch?

Sure, I personally use fetch-me-json.

import promistate from 'promistate'
import JSONFetch from 'fetch-me-json'

const promise = promistate(() => JSONFetch.get('/api/...'))


Compile project

npm run build

Run tests

npm test


Manage promises in UI libraries such as React.js, Vue.js, Angular, Svelte, Alpine.js, etc.






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