How to Determine a JavaScript Promise’s Status

By  on  

Promises have changed the landscape of JavaScript. Many old APIs have been reincarnated to use Promises (XHR to fetch, Battery API), while new APIs trend toward Promises. Developers can use async/await to handle promises, or then/catch/finally with callbacks, but what Promises don't tell you is their status. Wouldn't it be great if the Promise.prototype provided developers a status property to know whether a promise is rejected, resolved, or just done?

My research led me to this gist which I found quite clever. I took some time to modify a bit of code and add comments. The following solution provides helper methods for determining a Promise's status:

// Uses setTimeout with Promise to create an arbitrary delay time
// In these examples, a 0 millisecond delay is 
// an instantly resolving promise that we can jude status against
async function delay(milliseconds = 0, returnValue) {
  return new Promise(done => setTimeout((() => done(returnValue)), milliseconds));

// Promise.race in all of these functions uses delay of 0 to
// instantly resolve.  If the promise is resolved or rejected,
// returning that value will beat the setTimeout in the race

async function isResolved(promise) {
  return await Promise.race([delay(0, false), promise.then(() => true, () => false)]);

async function isRejected(promise) {
  return await Promise.race([delay(0, false), promise.then(() => false, () => true)]);

async function isFinished(promise) {
  return await Promise.race([delay(0, false), promise.then(() => true, () => true)]);

A few examples of usage:

// Testing isResolved
await isResolved(new Promise(resolve => resolve())); // true
await isResolved(new Promise((_, reject) => reject()));  // false

// Testing isRejected
await isRejected(new Promise((_, reject) => reject())); // true

// We done yet?
await isFinished(new Promise(resolve => resolve())); // true
await isFinished(new Promise((_, reject) => reject()));  // true

Developers can always add another await or then to a Promise to execute something but it is interesting to figure out the status of a given Promise. Is there an easier way to know a Promise's status? Let me know!

Recent Features

  • By
    CSS Filters

    CSS filter support recently landed within WebKit nightlies. CSS filters provide a method for modifying the rendering of a basic DOM element, image, or video. CSS filters allow for blurring, warping, and modifying the color intensity of elements. Let's have...

  • By
    Serving Fonts from CDN

    For maximum performance, we all know we must put our assets on CDN (another domain).  Along with those assets are custom web fonts.  Unfortunately custom web fonts via CDN (or any cross-domain font request) don't work in Firefox or Internet Explorer (correctly so, by spec) though...

Incredible Demos

  • By
    Morphing Elements Using MooTools and CSS

    Morphing an element between CSS classes is another great trick the MooTools JavaScript library enables you to do. Morphing isn't the most practical use of MooTools, but it's still a trick at your disposal. Step 1: The XHTML The block of content that will change is...

  • By
    MooTools Gone Wild: Element Flashing

    If you're like me and lay awake in bed at night, you've flipped on the TV and seen the commercials: misguided, attention-starved college girls fueled by alcohol ruining their futures by flashing lame camera-men on Spring Break. Why do they do it? Attention...


  1. Gordon
    function state(promise: Promise) {
        let state = "pending";
        promise.then(() => state = "fulfilled", () => state = "rejected");
        return () => state;
    const p = Promise.resolve(42);
    const s = state(p);
    console.log(s());   //  pending
    setTimeout(() => {
        console.log(s());  //  fulfilled
    }, 0);

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!